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Ground settlement induced by tunneling crossing interface of water-bearing mixed ground: A lesson from Changsha, China
Tunnelling and Underground Space Technology 96 (2020) 103224
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Ground settlement induced by tunneling crossing interface of water-bearing mixed ground: A lesson from Changsha, China
T
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Pin Zhanga, Ren-Peng Chena,b,c, Huai-Na Wua,b,c, , Yuan Liua a
College of Civil Engineering, Hunan University, Changsha 410082, China Key Laboratory of Building Safety and Energy Efficiency (Hunan University), Ministry of Education, Changsha 410082, China c National Center for International Research Collaboration in Building Safety and Environment, Hunan University, Changsha 410082, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Shield tunnel Mixed ground Groundwater Settlement Numerical modelling
Tunneling in mixed ground faces great challenges in control of shield machine, and improper operation easily trigger hazards without warning. Recently, an unexpected ground surface settlement of approximately 0.05 m was observed in the mixed ground of Changsha, China. When the shield machine advanced from the lowpermeability ground to the high-permeability ground, the lag in the regulation of chamber pressure allowed the water inflow from the excavation face, hydraulic pressure loss at the tunnel face thus induced large settlement. To further investigate ground responses induced by tunneling crossing the interface of water-bearing mixed ground, a 3D fluid-solid coupling finite element model is established in this study. The results indicate obvious drawdown of the groundwater occurs when the shield machine crosses the interface of low-permeability and high-permeability ground with low chamber pressure, leading to significant ground consolidation settlement with close to the measured results. Three typical settlement development modes that are related to the relative position of the monitoring section to the interface are proposed to describe the tunneling-induced settlement in the water-bearing mixed ground. The experience of this case history provides an important lesson for the future control of shield tunneling in the water-bearing mixed ground.
1. Introduction
tunneling-induced settlement development is not clear and the control of shield machine is more difficult in the mixed ground. In the areas with strong motion of plates, such as the central south area of China, ground condition is complex with heterogeneous geological and geomechanical characteristics (Jin et al., 2018; Zhang et al., 2012; Zhang et al., 2018a). Therefore, mixed ground can be frequently encountered during tunneling process in these areas, with sudden change of geomechanical and hydrological characteristics at both sides of interface (Tóth et al., 2013). Herein, mixed ground is defined as the simultaneous occurrence of two or more geological formations with remarkably different properties in rock/soil mechanics, engineering geology as well as hydro geology, or the same geological formation with different weathering grades (Steingrimsson et al., 2002). When tunneling in mixed ground, the operational parameters of the shield machine may not be regulated timely to adapt to the appearance of mixed ground interface. The lag in the regulation of shield machine operational parameters may lead to large ground settlement or even face instability (Ma et al., 2015). Especially in the water-bearing high-permeability mixed ground, Zhao et al. (2007) reported that the highpressure water results in unstable face conditions, subsequently spoil
Metro tunnels have been densely constructed in large cities of China to alleviate the increasing traffic pressure caused by rapid urbanization (Zhang and Huang, 2014; Zhang, 2019; Chen et al., 2016, 2018; Jiang and Yin, 2014). Shield-driven method, which is characterized by small disturbance, high speeds and safe operation, has been extensively adopted for construction of metro tunnels in variety of ground conditions (Jiang and Yin, 2012; Wu et al., 2017; Zhang and Huang, 2014), e.g. from soft ground to weathered rocks. The ground settlement caused by shield tunneling has drawn much concerns, because large settlement can lead to damages to surrounding structures and infrastructures (Ng et al., 2015; Ye et al., 2015; Yiu et al., 2017; Zheng et al., 2017). Numerous research works based on field observations, numerical analysis, laboratory tests, and machine learning have been conducted to study the ground response to shield tunneling and the control measures (Chen et al., 2019a; Chen et al., 2019b; Hu et al., 2019; Shen et al., 2014; Xie et al., 2016; Zhang et al., 2019a). Nevertheless, these research works mainly focus on the ground response in homogeneous soils (Berthoz et al., 2018; Peck, 1969; Sugiyama et al., 1999). The mechanism of
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Corresponding author at: College of Civil Engineering, Hunan University, Changsha 410082, China. E-mail address: [email protected] (H.-N. Wu).
https://doi.org/10.1016/j.tust.2019.103224 Received 29 April 2019; Received in revised form 29 October 2019; Accepted 28 November 2019 0886-7798/ © 2019 Elsevier Ltd. All rights reserved.
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located on the west bank of Xiang River. The position of the interface of moderately weathered limestone and moderately weathered sandstone is marked. A four-storey concrete structure I and a one-storey bamboo structure are located near the first tunnel. Five surface settlement monitoring points were installed above the tunnel lining rings No. 189, 196, 206, 212 and 219, respectively (hereafter called monitoring points No. 189, 196, 206, 212 and 219, respectively), as shown in Fig. 2. The settlement presented in this paper represents the ground surface settlement if no special explanation is given. The tunnel was constructed by an EPB shield. The diameter of the cutterhead and the length of EPB shield used in this project are 6.28 m and 8.735 m, respectively. The open ratio of the cutterhead is 35%. The outer and inner diameters of the segmental lining are 6 m and 5.4 m, respectively. The segmental ring is 1.5 m in width. The operational parameters of the shield machine were recorded at one-minute interval by the shield data acquisition system. Fig. 3 shows the geological profile of the construction site, together with the relative depth of the tunnel. There is a backfill layer on the top of the ground with a thickness of about 1.3 to 2 m. It is underlain by a silty clay layer with a thickness of 2 to 4 m. Under that is a strongly weathered sandstone layer, with a thickness of 6 to 12 m. The following is a moderately weathered limestone layer and a moderately weathered sandstone layer, with an interface between them. The groundwater table is about 3.6 m below the ground surface. The cover depth of the tunnel is about 18 m. The ground encountered by the tunnel was the moderately weathered limestone from ring No. 180 to No. 199, and the moderately weathered sandstone from ring No. 201 to No. 220. Ring No. 200 was located at the interface of these two weathered rocks. The physical and mechanical properties of the ground are listed in Table 1. Note that the hydraulic conductivity of the moderately weathered sandstone is 10 times that of the moderately weathered limestone based on the results of the pumping test.
Fig. 1. Water inflow from the excavation face.
and water even ran from the screw conveyor. Water inflow can trigger changes of the flow regime, leading to drawdown of the groundwater level if there is no sufficient recharge (Wu et al., 2019; Wu et al., 2018b). Serious groundwater inflow will even cause large ground settlement (Moon and Fernandez, 2010; Zhang et al., 2017). In this study, a typical case history of shield tunneling in the waterbearing mixed ground in Changsha, China, was reported. The shield machine advanced from the moderately weathered limestone to the moderately weathered sandstone. During tunneling in the moderately weathered limestone, the chamber pressure was relatively low because of the good self-stability of the moderately weathered limestone. However, the chamber pressure did not increase immediately when the shield machine entered into the moderately weathered sandstone with relatively high permeability. The shield face with low chamber pressure acted as a drainage boundary that led to groundwater inflow (Dammyr et al., 2017; Hu et al., 2006; Li et al., 2017), as shown in Fig. 1. Because of the high permeability of the moderately weathered sandstone, a significant drawdown of the groundwater near the excavation face was rapidly formed, leading to significant ground settlement of approximately 50 mm. Although large settlement caused by the groundwater inflow through the empty chamber has been occasionally reported by some researchers (Dammyr et al., 2017; Hu et al., 2006), few attention has been paid to the potential risk on the groundwater inflow and the associated ground settlement caused by sudden change of the permeability in the water-bearing mixed ground. This case history in Changsha will provide lessons and experience for the control of shield tunneling in the water-bearing mixed ground. To further understand the response of settlement and pore water pressure to EPB shield tunneling in the water-bearing mixed ground, a coupled hydro-mechanical 3D FE analysis was established, compared with the measured results observed from Changsha Metro Line 4. This paper is organized as follows: Section 2 introduces the field investigation of tunneling-induced large settlement in water-bearing mixed ground from Changsha Metro Line 4 project, thereafter analyzing the mechanism of this phenomenon. In Section 3, a 3D FE model based on PLAXIS 3D was established considering coupled hydro-mechanical effect. Two flow regimes that are free phreatic surface and no seepage, two relative permeability ratio of weathered rocks at both sides of the interface were taken into consideration. In Section 4, the results of FE simulation were compared with measured results, and the responses of ground settlement, pore water pressure and ground stress were presented comprehensively. In the last section, three typical settlement patterns in the water-bearing mixed ground were proposed.
2.2. Shield machine operation Fig. 4 presents the EPB shield advance timeline from ring No. 180 to No. 220. It can be seen from Fig. 4 that the shield machine advanced at 0.50 m/h from ring No. 180 to 203 and from ring No. 210 to 220. The advance rate decreased to 0.23 m/h from ring No. 204 to 209. The advance rate also takes the suspension of shield machine for installation of segmental lining into consideration. The variation of the chamber pressure and thrust from ring No. 180 to No. 220 is illustrated in Fig. 5. It can be observed that the chamber pressure was relatively low with the minimum value of 0.5 bar when the shield machine advanced in the moderately weathered limestone. After the shield machine crossing the interface, the chamber pressure kept low until the face was 7.5 m away from the interface, where the chamber pressure started to increase due to the decrease in the speed of screw conveyor controlled by field workers. The variation of thrust showed an opposite trend, which was higher in the moderately weathered limestone, and started to decrease after 7.5 m from the interface. It is worthy noted that the chamber pressure did not rise immediately when the EPB shield entered into the moderately weathered sandstone with high permeability and lower stiffness. This allowed the development of settlement when the shield machine was crossing the interface of low-permeability and high-permeability weathered rocks in the water-bearing ground, since the empty of working chamber with low pressure allows the groundwater to easily inflow from the excavation face. Overall, the value of the chamber pressure was much less than the average horizontal earth pressure at the tunnel face (269 kPa in the moderately weathered limestone and 391 kPa in the moderately weathered sandstone).
2. Field investigation 2.1. Project overview
2.3. Measured ground settlement Fig. 2 gives a plan view of the construction site with the tunnel arrangements. The investigated section of Changsha Metro Line 4 is
Fig. 6 presents the development of longitudinal settlement profile 2
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Fig. 2. Plan view of construction site.
Fig. 4. EPB shield advance timeline.
where EPB shield passed through the interface of the moderately weathered limestone and the moderately weathered sandstone, the settlement of three monitoring points No. 196, 206 and 212 increased synchronically. The ground settlement above ring No. 206 increased dramatically from 10 mm to 35 mm when Compared with ring No. 196, the settlement above the ring No. 206 was obviously larger, although the distance of the monitoring point at ring No. 196 to the interface is less than the monitoring point at ring No. 206. The settlement at five monitoring points is plotted against the distance from cutterhead to the monitoring section, as shown in Fig. 7. The settlement started to develop when the cutterhead was about 40 m or 6.67 D (D is the outer diameter of the lining ring) in front of the instrument line. Ground surface heave only occurred at the monitoring point No. 189 while the settlement of remaining four monitoring points increased gradually before the shield machine reached the monitoring section. When the shield machine was directly beneath the monitoring
Fig. 3. Geological profiles of construction site.
along the tunnel alignment as the advance of EPB shield machine. It is evident that the settlement of all monitoring points was small before the shield machine reached the interface of weathered limestone and sandstone. A maximum settlement of 10 mm was observed at ring No. 206. When the EPB shield advanced from ring No. 196 to No. 204,
Table 1 Physical and mechanical properties of the ground. Parameter γ γsat c′ Φ′ k
Description (unit) 3
Unit weight (kN/m ) Saturated unit weight (kN/m3) Cohesion (kPa) Internal friction angle (°) Hydraulic conductivity (m/day)
Backfill
Silty clay
Strongly weathered sandstone
Moderately weathered sandstone
Moderately weathered limestone
19.0 19.2 46.0 18.5 3.00
19.9 20.1 51.8 16.3 0.005
25.9 25.9 45.0 18.0 1.5
26.3 26.3 53.0 22.0 1.0
26.8 26.8 21.0 29.4 0.1
3
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settlement tends to reach a maximum when the shield machine reaches the monitoring sections (x = 0). Settlement development essentially ceased when the shield machine passed 20 m (3.33 D) from the instrument line. It is noteworthy that the settlement of the monitoring point No. 206 rebounded slightly when the shield machine crossed the monitoring section. 3. Three-dimensional FE model Numerical analyses were conducted to provide a deeper insight on three phenomena mentioned above: (i) large settlement when the shield machine was passing through the interface of low-permeability and high-permeability ground; (ii) slight rebound of the settlement when the shield machine reached ring No. 204; (iii) significant difference in the development pattern of settlement at five monitoring points. The FE software PLAXIS 3D was used for the establishment of the numerical model in this study.
Fig. 5. Measured chamber pressure and thrust.
3.1. Formulation of finite element model Fig. 8 shows the established 3D finite element (FE) model in this study. It should be note that this case is modelled within the framework of saturated soil, considering numerous research works were implemented to investigate ground responses induced by groundwater fluctuation based on the framework of saturated soil (You et al., 2018; Zeng et al., 2019; Zhang et al., 2018b), and the difficulties of applying unsaturated soil constitutive models to 3-dimentional modelling (Johari and Gholampour, 2018; Zhang et al., 2019b). Half section of the construction site was modelled since tunnel excavation is a symmetry problem. The tunnel axis runs along the x-direction (from 0 to 120 m) and the model laterally extends to a distance of 60 m from the tunnel centerline. The depth of the model is 50 m, extending a distance of 3D from the bottom of the tunnel. The complicated stratigraphy was simplified by assuming a uniform thickness for each soil layer. The thickness of each soil layer in the FE model is determined based on the average value of each soil layer thickness at all boreholes. It can be seen from Fig. 3 that the actual thickness of each layer does not change dramatically, therefore, the application of average thickness of each soil layer in the FE model is reasonable and it can also reduce computational cost. The upper ground is consisted of a 1.9 m-thick backfill layer, a 2 m-thick silty clay layer and a 9.3 m-thick strongly weathered sandstone layer. Under that is a 14.4 m-thick moderately weathered sandstone layer and a 36.8 m-thick moderately weathered limestone with a 14.4 m-high interface between the two rocks. The cover depth of the tunnel is 18 m and the phreatic water table is 3.6 m below the ground surface. The ring No. 200 is located at the interface of the moderately weathered limestone and the strongly weathered sandstone. The model consists of 48,691 elements and 74,041 nodes. The soil layers and lining rings were modelled using 10-noded solid elements. Shell elements were used for modelling the EPB shield machine. The cone-shaped shield machine with 9 m length was adopted to simulate the volume loss, as shown in the zoom area of Fig. 8. Herein, the volume loss was simulated by assigning the contraction ratio on the shell elements. After determine the values of hydraulic conductivity and consolidation time, the consolidation-induced settlement is fixed (Yoo, 2005; Yoo and Kim, 2008). Thereafter back analysis of contraction ratio achieves the agreement between measured and predicted settlement. The value of contraction ratio is thus determined.
Fig. 6. Longitudinal settlement profile during shield tunneling.
Fig. 7. Settlement development plotted against the distance from the cutterhead to monitoring section.
sections, a settlement of about 9 mm was measured at ring No. 196, which was about 41% of the final settlement, and a settlement of 38 mm was measured at ring No. 206, which was about 83% of the final settlement. In contrast, 100% of the total settlement was measured at monitoring points No. 212 and No. 219 due to the appearance of interface in front of these two sections. In regard to the monitoring points that were installed farther behind the interface, the tunneling-induced
3.2. Simulation procedure The simulation process comprises of the following five steps: excavation with chamber pressure at the shield face, tail grouting, installation of segmental rings, ground volume loss, and consolidation that is a steady seepage analysis. In FE model, the tunnel was excavated and supported at 3 m per step, instead of 1.5 m in the practical 4
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Fig. 8. FE mesh used for the 3D analysis.
the effective rigidity ratio of the lining is 0.7 in the circumferential direction (Huang et al., 2013), and the effective rigidity ratio is 0.15 in the longitudinal direction (Wu et al., 2018a). Shield machine also complied with isotropic linear elastic behavior. The parameters of concrete lining and shield machine are presented in Table 3. The hydraulic conductivity was determined by the in-situ pumping test and isotropic permeability was adopted for each soil (see Table 1).
engineering to save computational time. On the basis of advance rate presented in Fig. 4, the time for excavation and supporting per ring (3 m) is 0.25 day from ring No.180 to 203 and from ring No. 210 to 220. From ring No. 204 to 209, the time extends to 0.54 day per lining ring in the FE model. The excavation, installation and consolidation steps were preceeded synchronically. Therefore, the tunneling progress in the numerical model totally complied with the measured progress. The tunnel was excavated with the face pressure of 150 kPa at the tunnel crown with increasing gradient of 10 kPa/m in vertical direction. Note that the face pressure in the FE model considered the combination of chamber and cutterhead pressure, thereby was larger than the measured chamber pressure with the average value of 95 kPa (see Fig. 5). Tail grouting pressure was set as 180 kPa at the tunnel crown and increased with the gradient of 12 kPa/m in vertical direction. This combination of face pressure and tail grouting pressure is the lower bound to excavate tunnel in FE analysis, which can simulate the low chamber pressure in the practical tunneling process. The tunneling process was modelled as follows: (1) K0 generation of initial effective stresses, achieving the equilibrium of ground stress; (2) activating the EPB shield, face pressure and grouting pressure; (3) activating excavation step, including freezing current face pressure and grouting pressure, excavating 3 m span of soil along the tunnel alignment, installing concrete lining on the rear of the shield machine, moving shield machine, face pressure and grouting pressure to the next position, (4) activating the consolidation step; (5) repeating steps (3) and (4) until the completion of tunnel.
3.4. Boundary conditions The boundary conditions are as follows. The displacement perpendicular to lateral boundaries was restrained while the vertical displacement was allowable. There was no vertical or horizontal displacement along the bottom boundary, and the top boundary was not constrained. No-flow condition was assigned to the vertical boundary corresponding to the plane of symmetry and the model bottom boundary. The water table at the boundaries (x = 120 m and y = 60 m) that are far from tunnel excavation area was maintained at a constant level. Zero pore pressure was assigned as the boundary condition of the excavation face to enable the water inflow occur at the excavation face. As mentioned above, due to the good self-stability of rocks, the chamber was virtually empty during tunneling in this section for accelerating penetration rate, the chamber with low pressure thus allowed the groundwater to easily inflow from the excavation face. Therefore, zero pore pressure was assigned as the boundary condition of the excavation face to comply with measured results. Assuming that the segmental lining keeps intact and watertight after installation, the lining was set as impermeable.
3.3. Material constitutive models The weathered limestone and sandstone were modelled as linear elastic-perfectly plastic with Mohr-Coulomb (MC) yield surface. The behaviors of the backfill and silty clay were described using Hardening Soil Small (HSS) model. The parameters used in these constructive models are shown in Table 2, which refer to Lin et al. (2019)’s research, because soils are from a same tunnel project. Continuous concrete lining was modelled in accordance with isotropic linear elastic behavior. Considering the weakening effect of joint on the stiffness of lining,
3.5. Modelling plans The following three cases were investigated in this study: Case I: The permeability of both the moderately weathered sandstone and the moderately weathered limestone was consistent with the results of the pumping test. That is, free phreatic surface with ks/ kl = 10, where ks is the hydraulic conductivity coefficient of 5
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Table 2 Parameters used in constitutive models for the all soil layers. Parameter
E ν′ Eref 50 Eref oed Eref ur Gref 0 pref v′ ur γ0.7 m Ψ K0 Rf
Description (unit)
Backfill
Silty clay
Strongly weathered sandstone
Moderately weathered sandstone
Moderately weathered limestone
Constitutive model Young’s modulus (kPa) Poisson’s ratio Reference secant stiffness in triaxial test (kPa) Reference tangent stiffness (kPa) Reference unloading/reloading stiffness (kPa) Reference small strain stiffness (ε < 10–6) (kPa) Reference stress (kPa) Poisson ratio of unloading/reloading Strain at which the secant shear modulus Gs = 0.7G0 Power for stress-level dependency of stiffness Dilatancy angle (°) Coefficient of lateral earth pressure Failure ratio
HSS / / 4750 4750 19,000 38,000
HSS / / 7420 7420 22,260 47,500
MC 31,940 0.3 / / / /
MC 37,140 0.3 / / / /
MC 82,000 0.25 / / / /
100 0.2 0.0001
100 0.2 0.0001
/ / /
/ / /
/ / /
0.7 0 0.68 0.9
0.9 0 0.72 0.9
/ 0 0.69 0.9
/ 0 0.63 0.9
/ 0 0.51 0.9
Table 3 Parameters of shield machine and concrete lining. Parameter
Description
Shield machine
Concrete lining
E v γ
Elastic stiffness (kPa) Poisson’s ratio Unit weight (kN/m3)
23E7 0.2 49.5
31E6 0.2 25
moderately weathered sandstone, kl is the hydraulic conductivity coefficient of moderately weathered limestone. It is a steady seepage analysis and totally complies with actual condition; Case II: In order to investigate the effect of the relative permeability of the ground at both sides of interface on the ground response, the hydraulic conductivity coefficient of the moderately weathered sandstone was changed to the same value of the moderately weathered limestone, and other physical and mechanical characteristics of stiffness and shear resistance remains unchanged. That is, a steady seepage analysis with free phreatic surface was conducted, and ks/kl = 1 was set in the model; Case III: In order to investigate the effect of the hydraulic pressure loss at the tunnel face on the ground response, the water pressure is equal to initial hydrostatic pressure throughout the analysis and the boundary condition of the excavation face is changed into impermeable boundary. Meanwhile, other physical and mechanical characteristics of stiffness and shear resistance remains unchanged. That is, no seepage occurs throughout the analysis.
Fig. 9. Predicted settlement evolution using FE in comparison with measured settlement.
reaches the monitoring section, the calculated settlement reaches 92.2% of the total settlement in the FE analysis while the measured settlement reaches the maximum value of 45.48 mm. The calculated maximum settlement in the FE analysis is 42.38 mm. The consistency of the calculated results and the measured results verifies the reliability of the FE analysis, providing a basis for further investigation. In the Case II, all of the calculation parameters are not changed except the permeability of the moderately weathered sandstone, which is assumed to equal to kl. As presented in Fig. 9, one obvious difference in Case II is that sudden increase in settlement does not occur when the shield machine passes through the interface. Meanwhile, the rebound of the settlement is not observed when the cutterhead crosses the monitoring section. The ultimate settlement in Case II is 31.8 mm, which is much smaller than that of the Case I. It can be concluded that large settlement can occur when the shield machine advances from the low-permeability ground to the high-permeability ground with the zero pore water pressure on the excavation face. The development of settlement in the no seepage case (Case III) is similar to that in Case II, as shown in Fig. 9. In comparison with the case I, the ultimate settlement decreased by 15.18 mm to 27.2 mm. The results indicate the lag in the increase of chamber pressure causes large increase of settlement after the shield machine enter into the ground with lower stiffness, even no seepage occurred. Another noteworthy feature is that the settlement in the Case III is insignificant before the shield machine reaches the interface, while the settlement is about
4. Results of FEM simulation 4.1. Ground settlement 4.1.1. Longitudinal settlement curve Fig. 9 presents the development of settlement at the monitoring point No. 206 in three cases calculated by FE model, compared with the measured settlement. X donates the distance from the cutterhead to the monitoring section. It can be seen from Fig. 9 that the calculated settlement with ks/kl = 10 shows great agreement with the measured one. In the FE analysis, the dramatic increase of settlement is also observed when the shield machine passes through the interface. The settlement increases continuously until the cutterhead crosses the monitoring section, a rebound of 2.9 mm is observed according to the simulated results. Such phenomenon of rebound is in accordance with the measured one, which was 3.7 mm. When the cutterhead is beneath the monitoring section, 77.9% and 82.3% of the total settlement complete in the simulated and measured results, respectively. After the shield tail 6
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Fig. 11. Calculated transverse settlement troughs using FE analysis: (a) free phreatic surface with ks/kl = 10; (b) free phreatic surface with ks/kl = 1; (c) no seepage.
Fig. 10. Calculated subsurface longitudinal settlement using FE analysis: (a) free phreatic surface with ks/kl = 10; (b) free phreatic surface with ks/kl = 1; (c) no seepage.
surface settlement. These four surface monitoring points at −1, −3, −8, −16 m, respectively, are derived from four soil layers. It can be observed that the settlement increases with the decreasing distance between the monitoring points and the tunnel crown. The sudden increase in the subsurface settlement as the shield crossing the interface and the rebound of subsurface settlement after the shield crossing the monitoring section can also be observed in the Case I (see Fig. 10[a]), whereas they do not occurred in the Cases II and III (see Fig. 10[b and
6 mm in Cases I and II. This is due to the fact that only the volume loss caused the settlement in the Case III, while in the former two cases groundwater inflow to the shield machine causes additional settlement. Further discussion will be presented in the following sections. The calculated subsurface longitudinal settlement curves below the monitoring point No. 206 are presented in Fig. 10, compared with the 7
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c]). In Fig. 10(a), it should be note that the value of rebound settlement decreases with the decreasing distance between the monitoring point and the tunnel crown, and the rebound of settlement even does not occur in the monitoring point e that is only 2 m from the tunnel crown. This is attributed to the tradeoff between the ground volume loss and the recharge of groundwater, which will be clearly explained in the next section.
reduction in the settlement trough width in the no seepage case is more notable than those in other two cases. This is attributed to the seepage in Case I and Case II, which leads to a wide range of ground consolidation. The ultimately steady settlement trough width in Case I is nearly 1.5 times the width for Case III. The settlement trough width for Case II is between these two cases, lying much closer to the latter than to the former.
4.1.2. Transverse settlement trough Fig. 11 presents the transverse settlement troughs of the monitoring section No. 206 when the EPB shield is −9 m, −6 m, 0, 9 m, and 27 m away from the section, respectively. y donates the distance from the tunnel centerline. It can be observed that the transverse settlement trough develops gradually as the shield machine advances in the FE analysis. The width of the settlement trough increases dramatically in Case I when the shield machine passes through the interface (from −9 m to −6 m, note that the interface is at x = −7.5 m). In contrast, the change of the settlement trough can be negligible in Case II and Case III during this period. When the shield machine is crossing the monitoring section (from 0 to 9 m), an obvious development of settlement troughs in Case II and Case III are observed while the change in Case I is insignificant. This is consistent with the longitudinal development of settlement in this period as shown in Fig. 9. The development of the transverse settlement trough virtually ceases when the shield machine is 27 m from the monitoring section. The ultimate width and the depth of the settlement trough in Case I are larger than those in Case II and Case III. In the case of a single tunnel, Peck (1969) pointed out that the tunneling-induced transverse settlement trough can be well-described by an inverted Gaussian distribution curve:
4.2. Pore water pressure
Sv = Smax exp(−y 2 /2i 2)
Fig. 13 shows the distribution of pore water pressure of the monitoring section No.206 when the EPB shield is −9 m, −6 m, 0 m, 9 m, and 27 m away from the section in Case I (ks/kl = 10) and Case II (ks/ kl = 1), respectively. The influential zone is defined as the lateral range of any pore water pressure differing from the hydrostatic pressure. It is evident that the influential zone of Case I is much larger than that of Case II in all four phases. (i). When the cutter head is 9 m ahead from the monitoring section No. 206, the area within 40 m extent from the tunnel centerline starts to drawdown in Case I, while the pore water pressure around tunnel decreases slightly in Case II. (ii). When the shield machine pass through the interface, the phreatic surface lowers by 4.5 m and the influential zone increases dramatically in Case I. The pore water pressure in the vicinity of tunnel decreases and the influential zone increases to 15 m from tunnel centerline in Case II. The variation of pore water pressure at this phase in two cases can explain the sudden change of settlement as shown in Figs. 9–11. The large drawdown in Case I leads to the sudden increase in settlement and the expansion of settlement trough when the EPB shield is crossing the interface. In Case II, the small variation of pore water pressure around the tunnel cannot cause the sudden increase in the settlement. (iii). When the cutterhead is beneath the monitoring section, a drawdown of 7.5 m is observed in Case I. The influential zone in Case II is also expanded, but the phreatic surface still holds steady. (iv). When the cutterhead is 27 m from the monitoring section. A recovery of pore water pressure can be observed in two cases. Note that the phreatic surface in Case I rises to 1.7 m below the original level, whereas the pore water pressure in Case II totally recovers to the original state, because larger drawdown occurred in the case I, resulting in lower groundwater recovery speed.
(1)
where, Smax = maximum settlement above the tunnel centerline; i = horizontal distance from the tunnel centerline to the inflexion point of the settlement trough; y = distance from the tunnel centerline. The fitted results using Eq. (1) are shown in Fig. 11. It can be observed that tunneling-induced settlement with no seepage (Case III) can be well described by the inverted Gaussian curve with the coefficient of determination up to 0.98. However, in the drawdown cases, the deviation of fitted and predicted settlement troughs is discernable and the deviation is largest in the area close to the tunnel centerline. The evolution of the settlement trough width is summarized in Fig. 12. Throughout the tunneling process, the settlement trough width i in three cases decreases gradually with the advance of the EPB shield machine. As the EPB shield is approaching the monitoring section, the
Fig. 14 presents the variation of pore water pressure surrounding the tunnel in the section No. 206 in both Case I and Case II. The pore water at three positions which are located at the tunnel crown, the springline, and the bottom, are investigated. The variation of the pore water pressure at the three positions is essentially similar to each other. Before the cutterhead reaches the interface, the pore water pressure decrease gradually. When the shield machine is crossing the interface, the pore water pressure decreases by about 60 kPa at all three positions in Case I. In contrast, the reduction in pore water pressure in Case II is small at this phase. When the cutterhead reaches the monitoring section, pore water pressure in Case I and Case II decreases to the minimum values, which are 24 and 60 kPa, respectively. The drawdown of phreatic water table causes much smaller pore water pressure in Case I. Note that the pore water pressure around tunnel is essentially identical at this phase due to the zero pore water pressure at the excavation face. This result is consistent with the contour of pore water pressure around tunnel presented in the Fig. 13. After the EPB shield passes through the monitoring section, the immediate recovery of pore water pressure is observed. It can be observed that the recovery speed in Case I is faster than in Case II, since the process of hydraulic equilibrium depends greatly on the ground permeability. However, the pore water in Case I is smaller than in Case II at each step since large drawdown occurs before the EPB shield
Fig. 12. Development of settlement trough width in three cases. 8
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Fig. 13. Pore water pressure distributions: (a) free phreatic surface with ks/kl = 10; (b) free phreatic surface with ks/kl = 1.
pressure and vertical effective stress along the vertical axis line above the tunnel crown at the monitoring section No. 206. The stress distributions with the EPB shield at five positions (−9 m, −6 m, 0 m, 9 m, and 27 m away from the section, respectively) are investigated. It can be seen that the response of ground stress in Case I and Case II is greatly different. The vertical total stress is unchanged in both cases when the EPB shield is –9 m ahead of the monitoring section. In Case I, the pore water pressure decreases because of the decrease in the phreatic line as presented in Fig. 13(a). As a consequence, the effective stress increases slightly. In Case II, the pore water pressure at the 13 m below the ground surface decreases, where the corresponding vertical effective stress increases. It verifies that only stress status in the vicinity of tunnel changes, showing agreement with the results in the Fig. 13(b). When the EPB shield is 6 m ahead from the monitoring section, the ground vertical total stress decreases slightly in Case I while it is still identical to the initial distribution in Case II. The phreatic line decreases to 8.1 m from the ground surface in Case I, showing agreement with the illustration in Fig. 13(a). The pore water pressure in the vicinity of tunnel decreases continuously in Case II, but the inflection point depth of the pore water pressure distribution is virtually unchanged. The corresponding vertical effective stress further increases in both cases. When the EPB shield is beneath the monitoring section, the vertical total stress above the tunnel crown increases in both cases, since the face pressure imposes on the excavation face. The corresponding vertical effective stress above the tunnel crown also increases in two cases. The phreatic line decreases to 10.6 m below the ground surface in Case I and the inflection point depth of the pore water pressure distribution is still at −13 m in Case II. The largest decrease in pore water pressure occurs at the tunnel crown. The corresponding minimum pore water pressure in Case I and Case II are 24 and 60 kPa, respectively. When the EPB shield is 9 m away from the monitoring section, the vertical total stress and effective stress first decreases dramatically in both cases. This is attributed to the volume loss during the shield tail passing through the monitoring section. The inflection point of the vertical total stress and effective stress is located at 8 m below the ground surface, which is related to the development of soil arching (Chen et al., 2011). The pore water pressure around the tunnel recovers dramatically in Case II and the phreatic line recovers to –6 m in Case I. At this phase, the volume loss causes ground settlement, but the dramatic recharge of the groundwater leads to soil swelling, causing the rebound of settlement. The tradeoff of these two effects ultimately causes a slight rebound of settlement when the shield machine is
Fig. 14. Pore water pressure around tunnel: (a) free phreatic surface with ks/ kl = 10; (b) free phreatic surface with ks/kl = 1.
reaches the monitoring section. This characteristic will also affect the evolution of the total stress and settlement at this phase. 4.3. Ground stress Fig. 15 shows the distributions of vertical total stress, pore water 9
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Fig. 15. Stress distribution along the tunnel centerline above the tunnel crown: (a) free phreatic surface with ks/kl = 10; (b) free phreatic surface with ks/kl = 1.
crossing the monitoring section in Case I, as shown in Fig. 9. The effect of volume loss on the settlement decreases with the increasing distance from the tunnel crown, while the effect of recharge of groundwater on the settlement increases with the increasing distance from the tunnel crown. Hence, the value of rebound settlement decreases with the decreasing distance between the monitoring point and the tunnel crown, and the rebound of settlement does not occur if the monitoring point is much closer to the tunnel crown, as shown in Fig. 10(a). However, in Case II, the rebound of pore water pressure merely occurs in the vicinity of tunnel. The development of settlement mainly depends on the volume loss, and thus no settlement recovery occurs. When the EPB shield is 27 m away from the monitoring section, the continuous recharge of the groundwater results in the increase of the vertical total stress in both cases. The pore water pressure essentially recovers to the initial hydrostatic pressure condition in Case II. In Case I, the phreatic line recovers to 1.7 m below the ground surface. It can be observed that the vertical effective stress is relatively low in both cases and the proportion of pore water pressure in the vertical total stress is nearly 90%. This result is consistent with the field record presented by Inokuma and Ishimura (1995). They recorded the total soil and water pressure on a concrete segmental lining in the loose clayey silty sand and pointed out the ground loading was primarily caused by the pore water pressure while the effective stress was hardly discernable. Fig. 16 presents the variation of vertical total stress surrounding the tunnel at the monitoring section No. 206. The vertical total stress at three positions, at the tunnel crown, the springline and the bottom, are investigated. Before the cutterhead reaches the interface, the variation of the vertical total stress in three cases presents the same trend. Meanwhile, the vertical total stress increases synchronically as the cutterhead approaches the monitoring section due to the face pressure
Fig. 16. Vertical total stress around tunnel in three cases.
imposed on the excavation face. The vertical total stress decreases dramatically when the shield machine is crossing the monitoring section. After the cutterhead crosses the interface, a slight reduction in the vertical total stress around tunnel is observed in both Case I and Case II, while the vertical total stress around tunnel increases slightly in Case III. When the cutterhead drives away from the monitoring section, the continuous recovery of the vertical total stress around tunnel is observed in Case I and Case II, while the vertical total stress around tunnel continuously decreases in Case III. Moreover, the recovery speed of 10
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recovers immediately. At the S1, groundwater has recovered before the EPB shield passes through the interface. Then the drawdown caused by crossing the interface occurs. Therefore, the maximum settlement is observed in S3, followed by S2 and S1. Another obvious characteristic among three cases is that a slight rebound of settlement is observed at the phase II of S3. The magnitude of settlement rebound Δμ1 depends on the tradeoff of the volume loss at the shield tail and the groundwater recovery as mentioned above. The rebound of settlement is not observed in the remaining two cases. Stress relief dominates the settlement development at phase II of S2 since the slight recovery of groundwater in the vicinity of the tunnel cannot cause large rebound of settlement. At S1, similar to the settlement development of the monitoring point ring No. 196 in Figs. 6 and 7, the effect of groundwater recovery is lowest. One reason is that the drawdown is lowest in this mode, thereby the recovery of groundwater is also limited. Another reason is that the S1 is far away from the cutterhead when it is crossing the interface, the effect of groundwater recovery on the settlement is insignificant. This is also the reason that tunneling-induced settlement is the lowest (15.99 mm) at the S1 when the shield machine crosses the interface. The last characteristic is observed at phase III. It can be seen from Fig. 16 that the continuous increase in the settlement is observed at S3 while a slight rebound of settlement occurs in S1 and S2. This is related to the time of groundwater recovery. At the S3, the groundwater recovers when the cutterhead is beneath the monitoring section. At the S2, the groundwater recovers after the cutterhead crosses the monitoring section. In the S1, the second groundwater rebounds after the EPB shield tail drives away from the monitoring section. Therefore, in the S3, the magnitude of groundwater recovery is lowest at phase III among three modes since the large recovery of groundwater has completed after the EPB shield tail crosses the monitoring section. Stress relief dominates the settlement development at phase III, thereby the continuous increase in settlement is observed in S3. However, in the remaining two modes, the recovery of groundwater dominates the settlement development at phase III, therefore, the rebound of settlement at phase III is observed especially in the S1.
Table 4 Percentage of settlement at three phases. Case
I
II
III
ks/kl = 10 ks/kl = 1 No seepage
77.88 41.19 32.46
14.32 39.81 43.17
7.80 19.00 24.37
Note: I = the period of EPB shield ahead of the monitoring section; II = the period of the EPB shield body passing the monitoring section; III = the remaining tunneling process.
vertical total stress in Case I is faster than in Case II. This is related to the recovery speed of pore water pressure. As shown in Fig. 14, the recovery speed of pore water pressure in Case I is faster than Case II. The recovery of pore water pressure also limits the development of settlement after the EPB shield tail drives away from the monitoring section. Therefore, only 7.8% of the total settlement in the ks/kl = 10 case is developed at this phase, in comparison with the 19.0% and 24.4% in ks/kl = 1 and no seepage cases, respectively. The proportion of settlement at each phase is summarized in Table 4.
5. Discussion As mentioned above, the phreatic water table will decline dramatically when the EPB shield machine passes through the interface of low-permeability and high-permeability ground, leading to large ground settlement. However, as the relative position of the monitoring section to the interface varies, different pattern of the settlement development will be presented, as shown in Figs. 6 and 7. The settlement development of three points, S1, S2 and S3 in Case I is summarized in Fig. 17. S1, S2, S3 are located at − 12, 0 and 6 m from the interface, respectively. The whole tunneling process can be divided into three phases. Phase I represents the period that the EPB shield is in front of the monitoring section. Phase II represents the whole period of the EPB shield body passing the monitoring section. The remaining tunneling process is represented by phase III. The settlement development of these three points presents three typical modes when EPB shield crosses from the low-permeability ground to the high-permeability ground. The largest settlement is observed in the S3 corresponding to the monitoring point ring No. 206, since the magnitude of drawdown is the largest in this mode. Crossing the interface will lead to groundwater inflow, and the phreatic water table will continuously decline before the cutterhead reaches the monitoring section. At the S2, the drawdown caused by crossing the interface occurs when the EPB shield is beneath the monitoring section, after that the groundwater
6. Conclusions Field records obtained from Changsha Metro Line 4 project indicate that large settlement tends to occur when the EPB shield advances from the low-permeability ground to the high-permeability ground. This paper investigated the interaction of tunneling and groundwater in the water-bearing mixed ground using coupled hydro-mechanical 3D finite element (FE) analysis. The responses of settlement, pore water pressure and ground stress to tunneling were investigated comprehensively. The following conclusions can be drawn: (1) In the practice engineering, the chamber pressure is recommended to increase before the shield machine enters from the low-permeability ground to the high-permeability ground, even if the selfstability of the ground is good enough, because the lag in the increase of chamber pressure when the shield machine advances from low-permeability ground to the high-permeability ground allows groundwater easily inflows from the excavation face. Therefore, the large drawdown occurs, causing large ground surface settlement and wide settlement trough. (2) When shield machine advances in the water-bearing mixed ground, the ground settlement and pore water pressure responses can be categorized into five phases: (i) approaching the interface––a slight decrease and increase in pore water pressure and settlement, respectively; (ii) reaching the interface––a sudden decrease and increase in pore water pressure and settlement, respectively; (iii) reaching the monitoring section––minimum pore water pressure and continuous increase in settlement; (iv) shield machine body passing––rapid recovery of pore water pressure and slight rebound
Fig. 17. Three settlement development modes. 11
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of settlement (depending on the trade-off of stress relief and recovery of pore water pressure); (v) shield machine driving away––slight recovery of pore water pressure and slight increase in settlement. (3) The settlement development pattern is related to the relative position of the monitoring points to the interface. When the shield machine advances towards the interface, the maximum settlement of the monitoring points that are located behind the interface is larger than that of the monitoring points above the interface or ahead of the interface.
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The significance of mixed-face conditions for TBM performance. World Tunnell. 15, 435–441. Sugiyama, T., Hagiwara, T., Nomoto, T., Nomoto, M., Ano, Y., Mair, R.J., Bolton, M.D., Soga, K., 1999. Observations of ground movements during tunnel construction by slurry shield method at the Docklands light railway Lewisham Extention-East London. Soils Found. 39, 99–112. Tóth, Á., Gong, Q., Zhao, J., 2013. Case studies of TBM tunneling performance in rock–soil interface mixed ground. Tunnell. Undergr. Space Technol. 38, 140–150. Wu, H.N., Shen, S.L., Yang, J., 2017. Identification of tunnel settlement caused by land subsidence in soft deposit of Shanghai. J. Perform. Constr. Fac. 31, 04017092. Wu, H.N., Shen, S.L., Yang, J., Zhou, A.N., 2018a. Soil-tunnel interaction modelling for shield tunnels considering shearing dislocation in longitudinal joints. Tunnell. Undergr. Space Technol. 78, 168–177. Wu, Y.X., Lyu, H.M., Han, J., Shen, S.L., 2019. 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Interaction between Tunneling and Groundwater—Numerical Investigation Using Three Dimensional Stress-Pore Pressure Coupled Analysis. J. Geotech. Geoenviron. Eng. 131, 240–250. Yoo, C., Kim, S.-B., 2008. Three-dimensional numerical investigation of multifaced tunneling in water-bearing soft ground. Can. Geotech. J. 45, 1467–1486. Zhang, C.Q., Feng, X.T., Zhou, H., 2012. Estimation of in situ stress along deep tunnels buried in complex geological conditions. Int. J Rock Mech. Min. 52, 139–162. Zhang, D.M., Huang, Z.K., Yin, Z.Y., Ran, L.Z., Huang, H.W., 2017. Predicting the grouting effect on leakage-induced tunnels and ground response in saturated soils. Tunnell. Undergr. Space Technol. 65, 76–90. You, Y., Yan, C., Xu, B., Liu, S., Che, C., 2018. Optimization of dewatering schemes for a deep foundation pit near the Yangtze River, China. J. Rock Mech. Geotech. Eng. 10, 555–566. Zeng, C.-F., Zheng, G., Zhou, X.-F., Xue, X.-L., Zhou, H.-Z., 2019. 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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement The present work was carried out with the support of National Natural Science Foundation of China (No. 51878267, No. 51938005), the Research Program of Changsha Science and Technology Bureau (cskq1703051). References Berthoz, N., Branque, D., Wong, H., Subrin, D., 2018. TBM soft ground interaction: experimental study on a 1 g reduced-scale EPBS model. Tunnell. Undergr. Space Technol. 72, 189–209. Chen, C.N., Huang, W.Y., Tseng, C.T., 2011. Stress redistribution and ground arch development during tunneling. Tunnell. Undergr. Space Technol. 26, 228–235. Chen, R.P., Lin, X.T., Kang, X., Zhong, Z.Q., Liu, Y., Zhang, P., Wu, H.N., 2018. Deformation and stress characteristics of existing twin tunnels induced by closedistance EPBS under-crossing. Tunnell. Undergr. Space Technol. 82, 468–481. Chen, R.P., Meng, F.Y., Li, Z.C., Ye, Y.H., Ye, J.N., 2016. Investigation of response of metro tunnels due to adjacent large excavation and protective measures in soft soils. Tunnell. Undergr. Space Technol. 58, 224–235. Chen, R.P., Zhang, P., Kang, X., Zhong, Z.Q., Liu, Y., Wu, H.N., 2019a. Prediction of maximum surface settlement caused by EPB shield tunneling with ANN methods. Soils Found. 59, 284–295. Chen, R.P., Zhang, P., Wu, H.N., Wang, Z.T., Zhong, Z.Q., 2019b. Prediction of shield tunneling-induced ground settlement using machine learning techniques. Front. Struct. Civ. Eng. 13, 1363–1378. Dammyr, Ø., Nilsen, B., Gollegger, J., 2017. Feasibility of tunnel boring through weakness zones in deep Norwegian subsea tunnels. Tunnell. Undergr. Space Technol. 69, 133–146. Hu, X., He, C., Peng, Z., Yang, W., 2019. Analysis of ground settlement induced by Earth pressure balance shield tunneling in sandy soils with different water contents. Sustain. Cities Soc. 45, 296–306. Hu, X.P., Sun, M., Li, J.H., Dong, X.P., 2006. Computing of supporting pressure of working chamber for EPB shield applied in metro. Chin. J. Undergr. Space Eng. 2, 1413–1417. Huang, X., Schweiger, H.F., Huang, H., 2013. Influence of deep excavations on nearby existing tunnels. Int. J. Geomech. 13, 170–180. Inokuma, A., Ishimura, T., 1995. Earth pressure acting on shield driven tunnels in soft ground. In: Balkema, A.A. (Ed.), International Conference on Underground Construction in Soft Ground. New Delhi, India, pp. 221–224. Jiang, M., Yin, Z.Y., 2012. Analysis of stress redistribution in soil and earth pressure on tunnel lining using the discrete element method. Tunnell. Undergr. Space Technol. 32, 251–259. Jiang, M., Yin, Z.Y., 2014. Influence of soil conditioning on ground deformation during longitudinal tunneling. Comptes Rendus Mecanique 342, 189–197. Jin, D., Yuan, D., Li, X., Zheng, H., 2018. Analysis of the settlement of an existing tunnel induced by shield tunneling underneath. Tunnell. Undergr. Space Technol. 81, 209–220. Johari, A., Gholampour, A., 2018. A practical approach for reliability analysis of unsaturated slope by conditional random finite element method. Comput. Geotech. 102, 79–91. Li, S., Liu, B., Xu, X., Nie, L., Liu, Z., Song, J., Sun, H., Chen, L., Fan, K., 2017. An overview of ahead geological prospecting in tunneling. Tunnell. Undergr. Space Technol. 63, 69–94. Lin, X.-T., Chen, R.-P., Wu, H.-N., Cheng, H.-Z., 2019. Deformation behaviors of existing
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Ground settlement induced by tunneling crossing interface of water-bearing mixed ground: A lesson from Changsha, China
T
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Pin Zhanga, Ren-Peng Chena,b,c, Huai-Na Wua,b,c, , Yuan Liua a
College of Civil Engineering, Hunan University, Changsha 410082, China Key Laboratory of Building Safety and Energy Efficiency (Hunan University), Ministry of Education, Changsha 410082, China c National Center for International Research Collaboration in Building Safety and Environment, Hunan University, Changsha 410082, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Shield tunnel Mixed ground Groundwater Settlement Numerical modelling
Tunneling in mixed ground faces great challenges in control of shield machine, and improper operation easily trigger hazards without warning. Recently, an unexpected ground surface settlement of approximately 0.05 m was observed in the mixed ground of Changsha, China. When the shield machine advanced from the lowpermeability ground to the high-permeability ground, the lag in the regulation of chamber pressure allowed the water inflow from the excavation face, hydraulic pressure loss at the tunnel face thus induced large settlement. To further investigate ground responses induced by tunneling crossing the interface of water-bearing mixed ground, a 3D fluid-solid coupling finite element model is established in this study. The results indicate obvious drawdown of the groundwater occurs when the shield machine crosses the interface of low-permeability and high-permeability ground with low chamber pressure, leading to significant ground consolidation settlement with close to the measured results. Three typical settlement development modes that are related to the relative position of the monitoring section to the interface are proposed to describe the tunneling-induced settlement in the water-bearing mixed ground. The experience of this case history provides an important lesson for the future control of shield tunneling in the water-bearing mixed ground.
1. Introduction
tunneling-induced settlement development is not clear and the control of shield machine is more difficult in the mixed ground. In the areas with strong motion of plates, such as the central south area of China, ground condition is complex with heterogeneous geological and geomechanical characteristics (Jin et al., 2018; Zhang et al., 2012; Zhang et al., 2018a). Therefore, mixed ground can be frequently encountered during tunneling process in these areas, with sudden change of geomechanical and hydrological characteristics at both sides of interface (Tóth et al., 2013). Herein, mixed ground is defined as the simultaneous occurrence of two or more geological formations with remarkably different properties in rock/soil mechanics, engineering geology as well as hydro geology, or the same geological formation with different weathering grades (Steingrimsson et al., 2002). When tunneling in mixed ground, the operational parameters of the shield machine may not be regulated timely to adapt to the appearance of mixed ground interface. The lag in the regulation of shield machine operational parameters may lead to large ground settlement or even face instability (Ma et al., 2015). Especially in the water-bearing high-permeability mixed ground, Zhao et al. (2007) reported that the highpressure water results in unstable face conditions, subsequently spoil
Metro tunnels have been densely constructed in large cities of China to alleviate the increasing traffic pressure caused by rapid urbanization (Zhang and Huang, 2014; Zhang, 2019; Chen et al., 2016, 2018; Jiang and Yin, 2014). Shield-driven method, which is characterized by small disturbance, high speeds and safe operation, has been extensively adopted for construction of metro tunnels in variety of ground conditions (Jiang and Yin, 2012; Wu et al., 2017; Zhang and Huang, 2014), e.g. from soft ground to weathered rocks. The ground settlement caused by shield tunneling has drawn much concerns, because large settlement can lead to damages to surrounding structures and infrastructures (Ng et al., 2015; Ye et al., 2015; Yiu et al., 2017; Zheng et al., 2017). Numerous research works based on field observations, numerical analysis, laboratory tests, and machine learning have been conducted to study the ground response to shield tunneling and the control measures (Chen et al., 2019a; Chen et al., 2019b; Hu et al., 2019; Shen et al., 2014; Xie et al., 2016; Zhang et al., 2019a). Nevertheless, these research works mainly focus on the ground response in homogeneous soils (Berthoz et al., 2018; Peck, 1969; Sugiyama et al., 1999). The mechanism of
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Corresponding author at: College of Civil Engineering, Hunan University, Changsha 410082, China. E-mail address: [email protected] (H.-N. Wu).
https://doi.org/10.1016/j.tust.2019.103224 Received 29 April 2019; Received in revised form 29 October 2019; Accepted 28 November 2019 0886-7798/ © 2019 Elsevier Ltd. All rights reserved.
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located on the west bank of Xiang River. The position of the interface of moderately weathered limestone and moderately weathered sandstone is marked. A four-storey concrete structure I and a one-storey bamboo structure are located near the first tunnel. Five surface settlement monitoring points were installed above the tunnel lining rings No. 189, 196, 206, 212 and 219, respectively (hereafter called monitoring points No. 189, 196, 206, 212 and 219, respectively), as shown in Fig. 2. The settlement presented in this paper represents the ground surface settlement if no special explanation is given. The tunnel was constructed by an EPB shield. The diameter of the cutterhead and the length of EPB shield used in this project are 6.28 m and 8.735 m, respectively. The open ratio of the cutterhead is 35%. The outer and inner diameters of the segmental lining are 6 m and 5.4 m, respectively. The segmental ring is 1.5 m in width. The operational parameters of the shield machine were recorded at one-minute interval by the shield data acquisition system. Fig. 3 shows the geological profile of the construction site, together with the relative depth of the tunnel. There is a backfill layer on the top of the ground with a thickness of about 1.3 to 2 m. It is underlain by a silty clay layer with a thickness of 2 to 4 m. Under that is a strongly weathered sandstone layer, with a thickness of 6 to 12 m. The following is a moderately weathered limestone layer and a moderately weathered sandstone layer, with an interface between them. The groundwater table is about 3.6 m below the ground surface. The cover depth of the tunnel is about 18 m. The ground encountered by the tunnel was the moderately weathered limestone from ring No. 180 to No. 199, and the moderately weathered sandstone from ring No. 201 to No. 220. Ring No. 200 was located at the interface of these two weathered rocks. The physical and mechanical properties of the ground are listed in Table 1. Note that the hydraulic conductivity of the moderately weathered sandstone is 10 times that of the moderately weathered limestone based on the results of the pumping test.
Fig. 1. Water inflow from the excavation face.
and water even ran from the screw conveyor. Water inflow can trigger changes of the flow regime, leading to drawdown of the groundwater level if there is no sufficient recharge (Wu et al., 2019; Wu et al., 2018b). Serious groundwater inflow will even cause large ground settlement (Moon and Fernandez, 2010; Zhang et al., 2017). In this study, a typical case history of shield tunneling in the waterbearing mixed ground in Changsha, China, was reported. The shield machine advanced from the moderately weathered limestone to the moderately weathered sandstone. During tunneling in the moderately weathered limestone, the chamber pressure was relatively low because of the good self-stability of the moderately weathered limestone. However, the chamber pressure did not increase immediately when the shield machine entered into the moderately weathered sandstone with relatively high permeability. The shield face with low chamber pressure acted as a drainage boundary that led to groundwater inflow (Dammyr et al., 2017; Hu et al., 2006; Li et al., 2017), as shown in Fig. 1. Because of the high permeability of the moderately weathered sandstone, a significant drawdown of the groundwater near the excavation face was rapidly formed, leading to significant ground settlement of approximately 50 mm. Although large settlement caused by the groundwater inflow through the empty chamber has been occasionally reported by some researchers (Dammyr et al., 2017; Hu et al., 2006), few attention has been paid to the potential risk on the groundwater inflow and the associated ground settlement caused by sudden change of the permeability in the water-bearing mixed ground. This case history in Changsha will provide lessons and experience for the control of shield tunneling in the water-bearing mixed ground. To further understand the response of settlement and pore water pressure to EPB shield tunneling in the water-bearing mixed ground, a coupled hydro-mechanical 3D FE analysis was established, compared with the measured results observed from Changsha Metro Line 4. This paper is organized as follows: Section 2 introduces the field investigation of tunneling-induced large settlement in water-bearing mixed ground from Changsha Metro Line 4 project, thereafter analyzing the mechanism of this phenomenon. In Section 3, a 3D FE model based on PLAXIS 3D was established considering coupled hydro-mechanical effect. Two flow regimes that are free phreatic surface and no seepage, two relative permeability ratio of weathered rocks at both sides of the interface were taken into consideration. In Section 4, the results of FE simulation were compared with measured results, and the responses of ground settlement, pore water pressure and ground stress were presented comprehensively. In the last section, three typical settlement patterns in the water-bearing mixed ground were proposed.
2.2. Shield machine operation Fig. 4 presents the EPB shield advance timeline from ring No. 180 to No. 220. It can be seen from Fig. 4 that the shield machine advanced at 0.50 m/h from ring No. 180 to 203 and from ring No. 210 to 220. The advance rate decreased to 0.23 m/h from ring No. 204 to 209. The advance rate also takes the suspension of shield machine for installation of segmental lining into consideration. The variation of the chamber pressure and thrust from ring No. 180 to No. 220 is illustrated in Fig. 5. It can be observed that the chamber pressure was relatively low with the minimum value of 0.5 bar when the shield machine advanced in the moderately weathered limestone. After the shield machine crossing the interface, the chamber pressure kept low until the face was 7.5 m away from the interface, where the chamber pressure started to increase due to the decrease in the speed of screw conveyor controlled by field workers. The variation of thrust showed an opposite trend, which was higher in the moderately weathered limestone, and started to decrease after 7.5 m from the interface. It is worthy noted that the chamber pressure did not rise immediately when the EPB shield entered into the moderately weathered sandstone with high permeability and lower stiffness. This allowed the development of settlement when the shield machine was crossing the interface of low-permeability and high-permeability weathered rocks in the water-bearing ground, since the empty of working chamber with low pressure allows the groundwater to easily inflow from the excavation face. Overall, the value of the chamber pressure was much less than the average horizontal earth pressure at the tunnel face (269 kPa in the moderately weathered limestone and 391 kPa in the moderately weathered sandstone).
2. Field investigation 2.1. Project overview
2.3. Measured ground settlement Fig. 2 gives a plan view of the construction site with the tunnel arrangements. The investigated section of Changsha Metro Line 4 is
Fig. 6 presents the development of longitudinal settlement profile 2
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Fig. 2. Plan view of construction site.
Fig. 4. EPB shield advance timeline.
where EPB shield passed through the interface of the moderately weathered limestone and the moderately weathered sandstone, the settlement of three monitoring points No. 196, 206 and 212 increased synchronically. The ground settlement above ring No. 206 increased dramatically from 10 mm to 35 mm when Compared with ring No. 196, the settlement above the ring No. 206 was obviously larger, although the distance of the monitoring point at ring No. 196 to the interface is less than the monitoring point at ring No. 206. The settlement at five monitoring points is plotted against the distance from cutterhead to the monitoring section, as shown in Fig. 7. The settlement started to develop when the cutterhead was about 40 m or 6.67 D (D is the outer diameter of the lining ring) in front of the instrument line. Ground surface heave only occurred at the monitoring point No. 189 while the settlement of remaining four monitoring points increased gradually before the shield machine reached the monitoring section. When the shield machine was directly beneath the monitoring
Fig. 3. Geological profiles of construction site.
along the tunnel alignment as the advance of EPB shield machine. It is evident that the settlement of all monitoring points was small before the shield machine reached the interface of weathered limestone and sandstone. A maximum settlement of 10 mm was observed at ring No. 206. When the EPB shield advanced from ring No. 196 to No. 204,
Table 1 Physical and mechanical properties of the ground. Parameter γ γsat c′ Φ′ k
Description (unit) 3
Unit weight (kN/m ) Saturated unit weight (kN/m3) Cohesion (kPa) Internal friction angle (°) Hydraulic conductivity (m/day)
Backfill
Silty clay
Strongly weathered sandstone
Moderately weathered sandstone
Moderately weathered limestone
19.0 19.2 46.0 18.5 3.00
19.9 20.1 51.8 16.3 0.005
25.9 25.9 45.0 18.0 1.5
26.3 26.3 53.0 22.0 1.0
26.8 26.8 21.0 29.4 0.1
3
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settlement tends to reach a maximum when the shield machine reaches the monitoring sections (x = 0). Settlement development essentially ceased when the shield machine passed 20 m (3.33 D) from the instrument line. It is noteworthy that the settlement of the monitoring point No. 206 rebounded slightly when the shield machine crossed the monitoring section. 3. Three-dimensional FE model Numerical analyses were conducted to provide a deeper insight on three phenomena mentioned above: (i) large settlement when the shield machine was passing through the interface of low-permeability and high-permeability ground; (ii) slight rebound of the settlement when the shield machine reached ring No. 204; (iii) significant difference in the development pattern of settlement at five monitoring points. The FE software PLAXIS 3D was used for the establishment of the numerical model in this study.
Fig. 5. Measured chamber pressure and thrust.
3.1. Formulation of finite element model Fig. 8 shows the established 3D finite element (FE) model in this study. It should be note that this case is modelled within the framework of saturated soil, considering numerous research works were implemented to investigate ground responses induced by groundwater fluctuation based on the framework of saturated soil (You et al., 2018; Zeng et al., 2019; Zhang et al., 2018b), and the difficulties of applying unsaturated soil constitutive models to 3-dimentional modelling (Johari and Gholampour, 2018; Zhang et al., 2019b). Half section of the construction site was modelled since tunnel excavation is a symmetry problem. The tunnel axis runs along the x-direction (from 0 to 120 m) and the model laterally extends to a distance of 60 m from the tunnel centerline. The depth of the model is 50 m, extending a distance of 3D from the bottom of the tunnel. The complicated stratigraphy was simplified by assuming a uniform thickness for each soil layer. The thickness of each soil layer in the FE model is determined based on the average value of each soil layer thickness at all boreholes. It can be seen from Fig. 3 that the actual thickness of each layer does not change dramatically, therefore, the application of average thickness of each soil layer in the FE model is reasonable and it can also reduce computational cost. The upper ground is consisted of a 1.9 m-thick backfill layer, a 2 m-thick silty clay layer and a 9.3 m-thick strongly weathered sandstone layer. Under that is a 14.4 m-thick moderately weathered sandstone layer and a 36.8 m-thick moderately weathered limestone with a 14.4 m-high interface between the two rocks. The cover depth of the tunnel is 18 m and the phreatic water table is 3.6 m below the ground surface. The ring No. 200 is located at the interface of the moderately weathered limestone and the strongly weathered sandstone. The model consists of 48,691 elements and 74,041 nodes. The soil layers and lining rings were modelled using 10-noded solid elements. Shell elements were used for modelling the EPB shield machine. The cone-shaped shield machine with 9 m length was adopted to simulate the volume loss, as shown in the zoom area of Fig. 8. Herein, the volume loss was simulated by assigning the contraction ratio on the shell elements. After determine the values of hydraulic conductivity and consolidation time, the consolidation-induced settlement is fixed (Yoo, 2005; Yoo and Kim, 2008). Thereafter back analysis of contraction ratio achieves the agreement between measured and predicted settlement. The value of contraction ratio is thus determined.
Fig. 6. Longitudinal settlement profile during shield tunneling.
Fig. 7. Settlement development plotted against the distance from the cutterhead to monitoring section.
sections, a settlement of about 9 mm was measured at ring No. 196, which was about 41% of the final settlement, and a settlement of 38 mm was measured at ring No. 206, which was about 83% of the final settlement. In contrast, 100% of the total settlement was measured at monitoring points No. 212 and No. 219 due to the appearance of interface in front of these two sections. In regard to the monitoring points that were installed farther behind the interface, the tunneling-induced
3.2. Simulation procedure The simulation process comprises of the following five steps: excavation with chamber pressure at the shield face, tail grouting, installation of segmental rings, ground volume loss, and consolidation that is a steady seepage analysis. In FE model, the tunnel was excavated and supported at 3 m per step, instead of 1.5 m in the practical 4
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Fig. 8. FE mesh used for the 3D analysis.
the effective rigidity ratio of the lining is 0.7 in the circumferential direction (Huang et al., 2013), and the effective rigidity ratio is 0.15 in the longitudinal direction (Wu et al., 2018a). Shield machine also complied with isotropic linear elastic behavior. The parameters of concrete lining and shield machine are presented in Table 3. The hydraulic conductivity was determined by the in-situ pumping test and isotropic permeability was adopted for each soil (see Table 1).
engineering to save computational time. On the basis of advance rate presented in Fig. 4, the time for excavation and supporting per ring (3 m) is 0.25 day from ring No.180 to 203 and from ring No. 210 to 220. From ring No. 204 to 209, the time extends to 0.54 day per lining ring in the FE model. The excavation, installation and consolidation steps were preceeded synchronically. Therefore, the tunneling progress in the numerical model totally complied with the measured progress. The tunnel was excavated with the face pressure of 150 kPa at the tunnel crown with increasing gradient of 10 kPa/m in vertical direction. Note that the face pressure in the FE model considered the combination of chamber and cutterhead pressure, thereby was larger than the measured chamber pressure with the average value of 95 kPa (see Fig. 5). Tail grouting pressure was set as 180 kPa at the tunnel crown and increased with the gradient of 12 kPa/m in vertical direction. This combination of face pressure and tail grouting pressure is the lower bound to excavate tunnel in FE analysis, which can simulate the low chamber pressure in the practical tunneling process. The tunneling process was modelled as follows: (1) K0 generation of initial effective stresses, achieving the equilibrium of ground stress; (2) activating the EPB shield, face pressure and grouting pressure; (3) activating excavation step, including freezing current face pressure and grouting pressure, excavating 3 m span of soil along the tunnel alignment, installing concrete lining on the rear of the shield machine, moving shield machine, face pressure and grouting pressure to the next position, (4) activating the consolidation step; (5) repeating steps (3) and (4) until the completion of tunnel.
3.4. Boundary conditions The boundary conditions are as follows. The displacement perpendicular to lateral boundaries was restrained while the vertical displacement was allowable. There was no vertical or horizontal displacement along the bottom boundary, and the top boundary was not constrained. No-flow condition was assigned to the vertical boundary corresponding to the plane of symmetry and the model bottom boundary. The water table at the boundaries (x = 120 m and y = 60 m) that are far from tunnel excavation area was maintained at a constant level. Zero pore pressure was assigned as the boundary condition of the excavation face to enable the water inflow occur at the excavation face. As mentioned above, due to the good self-stability of rocks, the chamber was virtually empty during tunneling in this section for accelerating penetration rate, the chamber with low pressure thus allowed the groundwater to easily inflow from the excavation face. Therefore, zero pore pressure was assigned as the boundary condition of the excavation face to comply with measured results. Assuming that the segmental lining keeps intact and watertight after installation, the lining was set as impermeable.
3.3. Material constitutive models The weathered limestone and sandstone were modelled as linear elastic-perfectly plastic with Mohr-Coulomb (MC) yield surface. The behaviors of the backfill and silty clay were described using Hardening Soil Small (HSS) model. The parameters used in these constructive models are shown in Table 2, which refer to Lin et al. (2019)’s research, because soils are from a same tunnel project. Continuous concrete lining was modelled in accordance with isotropic linear elastic behavior. Considering the weakening effect of joint on the stiffness of lining,
3.5. Modelling plans The following three cases were investigated in this study: Case I: The permeability of both the moderately weathered sandstone and the moderately weathered limestone was consistent with the results of the pumping test. That is, free phreatic surface with ks/ kl = 10, where ks is the hydraulic conductivity coefficient of 5
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Table 2 Parameters used in constitutive models for the all soil layers. Parameter
E ν′ Eref 50 Eref oed Eref ur Gref 0 pref v′ ur γ0.7 m Ψ K0 Rf
Description (unit)
Backfill
Silty clay
Strongly weathered sandstone
Moderately weathered sandstone
Moderately weathered limestone
Constitutive model Young’s modulus (kPa) Poisson’s ratio Reference secant stiffness in triaxial test (kPa) Reference tangent stiffness (kPa) Reference unloading/reloading stiffness (kPa) Reference small strain stiffness (ε < 10–6) (kPa) Reference stress (kPa) Poisson ratio of unloading/reloading Strain at which the secant shear modulus Gs = 0.7G0 Power for stress-level dependency of stiffness Dilatancy angle (°) Coefficient of lateral earth pressure Failure ratio
HSS / / 4750 4750 19,000 38,000
HSS / / 7420 7420 22,260 47,500
MC 31,940 0.3 / / / /
MC 37,140 0.3 / / / /
MC 82,000 0.25 / / / /
100 0.2 0.0001
100 0.2 0.0001
/ / /
/ / /
/ / /
0.7 0 0.68 0.9
0.9 0 0.72 0.9
/ 0 0.69 0.9
/ 0 0.63 0.9
/ 0 0.51 0.9
Table 3 Parameters of shield machine and concrete lining. Parameter
Description
Shield machine
Concrete lining
E v γ
Elastic stiffness (kPa) Poisson’s ratio Unit weight (kN/m3)
23E7 0.2 49.5
31E6 0.2 25
moderately weathered sandstone, kl is the hydraulic conductivity coefficient of moderately weathered limestone. It is a steady seepage analysis and totally complies with actual condition; Case II: In order to investigate the effect of the relative permeability of the ground at both sides of interface on the ground response, the hydraulic conductivity coefficient of the moderately weathered sandstone was changed to the same value of the moderately weathered limestone, and other physical and mechanical characteristics of stiffness and shear resistance remains unchanged. That is, a steady seepage analysis with free phreatic surface was conducted, and ks/kl = 1 was set in the model; Case III: In order to investigate the effect of the hydraulic pressure loss at the tunnel face on the ground response, the water pressure is equal to initial hydrostatic pressure throughout the analysis and the boundary condition of the excavation face is changed into impermeable boundary. Meanwhile, other physical and mechanical characteristics of stiffness and shear resistance remains unchanged. That is, no seepage occurs throughout the analysis.
Fig. 9. Predicted settlement evolution using FE in comparison with measured settlement.
reaches the monitoring section, the calculated settlement reaches 92.2% of the total settlement in the FE analysis while the measured settlement reaches the maximum value of 45.48 mm. The calculated maximum settlement in the FE analysis is 42.38 mm. The consistency of the calculated results and the measured results verifies the reliability of the FE analysis, providing a basis for further investigation. In the Case II, all of the calculation parameters are not changed except the permeability of the moderately weathered sandstone, which is assumed to equal to kl. As presented in Fig. 9, one obvious difference in Case II is that sudden increase in settlement does not occur when the shield machine passes through the interface. Meanwhile, the rebound of the settlement is not observed when the cutterhead crosses the monitoring section. The ultimate settlement in Case II is 31.8 mm, which is much smaller than that of the Case I. It can be concluded that large settlement can occur when the shield machine advances from the low-permeability ground to the high-permeability ground with the zero pore water pressure on the excavation face. The development of settlement in the no seepage case (Case III) is similar to that in Case II, as shown in Fig. 9. In comparison with the case I, the ultimate settlement decreased by 15.18 mm to 27.2 mm. The results indicate the lag in the increase of chamber pressure causes large increase of settlement after the shield machine enter into the ground with lower stiffness, even no seepage occurred. Another noteworthy feature is that the settlement in the Case III is insignificant before the shield machine reaches the interface, while the settlement is about
4. Results of FEM simulation 4.1. Ground settlement 4.1.1. Longitudinal settlement curve Fig. 9 presents the development of settlement at the monitoring point No. 206 in three cases calculated by FE model, compared with the measured settlement. X donates the distance from the cutterhead to the monitoring section. It can be seen from Fig. 9 that the calculated settlement with ks/kl = 10 shows great agreement with the measured one. In the FE analysis, the dramatic increase of settlement is also observed when the shield machine passes through the interface. The settlement increases continuously until the cutterhead crosses the monitoring section, a rebound of 2.9 mm is observed according to the simulated results. Such phenomenon of rebound is in accordance with the measured one, which was 3.7 mm. When the cutterhead is beneath the monitoring section, 77.9% and 82.3% of the total settlement complete in the simulated and measured results, respectively. After the shield tail 6
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Fig. 11. Calculated transverse settlement troughs using FE analysis: (a) free phreatic surface with ks/kl = 10; (b) free phreatic surface with ks/kl = 1; (c) no seepage.
Fig. 10. Calculated subsurface longitudinal settlement using FE analysis: (a) free phreatic surface with ks/kl = 10; (b) free phreatic surface with ks/kl = 1; (c) no seepage.
surface settlement. These four surface monitoring points at −1, −3, −8, −16 m, respectively, are derived from four soil layers. It can be observed that the settlement increases with the decreasing distance between the monitoring points and the tunnel crown. The sudden increase in the subsurface settlement as the shield crossing the interface and the rebound of subsurface settlement after the shield crossing the monitoring section can also be observed in the Case I (see Fig. 10[a]), whereas they do not occurred in the Cases II and III (see Fig. 10[b and
6 mm in Cases I and II. This is due to the fact that only the volume loss caused the settlement in the Case III, while in the former two cases groundwater inflow to the shield machine causes additional settlement. Further discussion will be presented in the following sections. The calculated subsurface longitudinal settlement curves below the monitoring point No. 206 are presented in Fig. 10, compared with the 7
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c]). In Fig. 10(a), it should be note that the value of rebound settlement decreases with the decreasing distance between the monitoring point and the tunnel crown, and the rebound of settlement even does not occur in the monitoring point e that is only 2 m from the tunnel crown. This is attributed to the tradeoff between the ground volume loss and the recharge of groundwater, which will be clearly explained in the next section.
reduction in the settlement trough width in the no seepage case is more notable than those in other two cases. This is attributed to the seepage in Case I and Case II, which leads to a wide range of ground consolidation. The ultimately steady settlement trough width in Case I is nearly 1.5 times the width for Case III. The settlement trough width for Case II is between these two cases, lying much closer to the latter than to the former.
4.1.2. Transverse settlement trough Fig. 11 presents the transverse settlement troughs of the monitoring section No. 206 when the EPB shield is −9 m, −6 m, 0, 9 m, and 27 m away from the section, respectively. y donates the distance from the tunnel centerline. It can be observed that the transverse settlement trough develops gradually as the shield machine advances in the FE analysis. The width of the settlement trough increases dramatically in Case I when the shield machine passes through the interface (from −9 m to −6 m, note that the interface is at x = −7.5 m). In contrast, the change of the settlement trough can be negligible in Case II and Case III during this period. When the shield machine is crossing the monitoring section (from 0 to 9 m), an obvious development of settlement troughs in Case II and Case III are observed while the change in Case I is insignificant. This is consistent with the longitudinal development of settlement in this period as shown in Fig. 9. The development of the transverse settlement trough virtually ceases when the shield machine is 27 m from the monitoring section. The ultimate width and the depth of the settlement trough in Case I are larger than those in Case II and Case III. In the case of a single tunnel, Peck (1969) pointed out that the tunneling-induced transverse settlement trough can be well-described by an inverted Gaussian distribution curve:
4.2. Pore water pressure
Sv = Smax exp(−y 2 /2i 2)
Fig. 13 shows the distribution of pore water pressure of the monitoring section No.206 when the EPB shield is −9 m, −6 m, 0 m, 9 m, and 27 m away from the section in Case I (ks/kl = 10) and Case II (ks/ kl = 1), respectively. The influential zone is defined as the lateral range of any pore water pressure differing from the hydrostatic pressure. It is evident that the influential zone of Case I is much larger than that of Case II in all four phases. (i). When the cutter head is 9 m ahead from the monitoring section No. 206, the area within 40 m extent from the tunnel centerline starts to drawdown in Case I, while the pore water pressure around tunnel decreases slightly in Case II. (ii). When the shield machine pass through the interface, the phreatic surface lowers by 4.5 m and the influential zone increases dramatically in Case I. The pore water pressure in the vicinity of tunnel decreases and the influential zone increases to 15 m from tunnel centerline in Case II. The variation of pore water pressure at this phase in two cases can explain the sudden change of settlement as shown in Figs. 9–11. The large drawdown in Case I leads to the sudden increase in settlement and the expansion of settlement trough when the EPB shield is crossing the interface. In Case II, the small variation of pore water pressure around the tunnel cannot cause the sudden increase in the settlement. (iii). When the cutterhead is beneath the monitoring section, a drawdown of 7.5 m is observed in Case I. The influential zone in Case II is also expanded, but the phreatic surface still holds steady. (iv). When the cutterhead is 27 m from the monitoring section. A recovery of pore water pressure can be observed in two cases. Note that the phreatic surface in Case I rises to 1.7 m below the original level, whereas the pore water pressure in Case II totally recovers to the original state, because larger drawdown occurred in the case I, resulting in lower groundwater recovery speed.
(1)
where, Smax = maximum settlement above the tunnel centerline; i = horizontal distance from the tunnel centerline to the inflexion point of the settlement trough; y = distance from the tunnel centerline. The fitted results using Eq. (1) are shown in Fig. 11. It can be observed that tunneling-induced settlement with no seepage (Case III) can be well described by the inverted Gaussian curve with the coefficient of determination up to 0.98. However, in the drawdown cases, the deviation of fitted and predicted settlement troughs is discernable and the deviation is largest in the area close to the tunnel centerline. The evolution of the settlement trough width is summarized in Fig. 12. Throughout the tunneling process, the settlement trough width i in three cases decreases gradually with the advance of the EPB shield machine. As the EPB shield is approaching the monitoring section, the
Fig. 14 presents the variation of pore water pressure surrounding the tunnel in the section No. 206 in both Case I and Case II. The pore water at three positions which are located at the tunnel crown, the springline, and the bottom, are investigated. The variation of the pore water pressure at the three positions is essentially similar to each other. Before the cutterhead reaches the interface, the pore water pressure decrease gradually. When the shield machine is crossing the interface, the pore water pressure decreases by about 60 kPa at all three positions in Case I. In contrast, the reduction in pore water pressure in Case II is small at this phase. When the cutterhead reaches the monitoring section, pore water pressure in Case I and Case II decreases to the minimum values, which are 24 and 60 kPa, respectively. The drawdown of phreatic water table causes much smaller pore water pressure in Case I. Note that the pore water pressure around tunnel is essentially identical at this phase due to the zero pore water pressure at the excavation face. This result is consistent with the contour of pore water pressure around tunnel presented in the Fig. 13. After the EPB shield passes through the monitoring section, the immediate recovery of pore water pressure is observed. It can be observed that the recovery speed in Case I is faster than in Case II, since the process of hydraulic equilibrium depends greatly on the ground permeability. However, the pore water in Case I is smaller than in Case II at each step since large drawdown occurs before the EPB shield
Fig. 12. Development of settlement trough width in three cases. 8
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Fig. 13. Pore water pressure distributions: (a) free phreatic surface with ks/kl = 10; (b) free phreatic surface with ks/kl = 1.
pressure and vertical effective stress along the vertical axis line above the tunnel crown at the monitoring section No. 206. The stress distributions with the EPB shield at five positions (−9 m, −6 m, 0 m, 9 m, and 27 m away from the section, respectively) are investigated. It can be seen that the response of ground stress in Case I and Case II is greatly different. The vertical total stress is unchanged in both cases when the EPB shield is –9 m ahead of the monitoring section. In Case I, the pore water pressure decreases because of the decrease in the phreatic line as presented in Fig. 13(a). As a consequence, the effective stress increases slightly. In Case II, the pore water pressure at the 13 m below the ground surface decreases, where the corresponding vertical effective stress increases. It verifies that only stress status in the vicinity of tunnel changes, showing agreement with the results in the Fig. 13(b). When the EPB shield is 6 m ahead from the monitoring section, the ground vertical total stress decreases slightly in Case I while it is still identical to the initial distribution in Case II. The phreatic line decreases to 8.1 m from the ground surface in Case I, showing agreement with the illustration in Fig. 13(a). The pore water pressure in the vicinity of tunnel decreases continuously in Case II, but the inflection point depth of the pore water pressure distribution is virtually unchanged. The corresponding vertical effective stress further increases in both cases. When the EPB shield is beneath the monitoring section, the vertical total stress above the tunnel crown increases in both cases, since the face pressure imposes on the excavation face. The corresponding vertical effective stress above the tunnel crown also increases in two cases. The phreatic line decreases to 10.6 m below the ground surface in Case I and the inflection point depth of the pore water pressure distribution is still at −13 m in Case II. The largest decrease in pore water pressure occurs at the tunnel crown. The corresponding minimum pore water pressure in Case I and Case II are 24 and 60 kPa, respectively. When the EPB shield is 9 m away from the monitoring section, the vertical total stress and effective stress first decreases dramatically in both cases. This is attributed to the volume loss during the shield tail passing through the monitoring section. The inflection point of the vertical total stress and effective stress is located at 8 m below the ground surface, which is related to the development of soil arching (Chen et al., 2011). The pore water pressure around the tunnel recovers dramatically in Case II and the phreatic line recovers to –6 m in Case I. At this phase, the volume loss causes ground settlement, but the dramatic recharge of the groundwater leads to soil swelling, causing the rebound of settlement. The tradeoff of these two effects ultimately causes a slight rebound of settlement when the shield machine is
Fig. 14. Pore water pressure around tunnel: (a) free phreatic surface with ks/ kl = 10; (b) free phreatic surface with ks/kl = 1.
reaches the monitoring section. This characteristic will also affect the evolution of the total stress and settlement at this phase. 4.3. Ground stress Fig. 15 shows the distributions of vertical total stress, pore water 9
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Fig. 15. Stress distribution along the tunnel centerline above the tunnel crown: (a) free phreatic surface with ks/kl = 10; (b) free phreatic surface with ks/kl = 1.
crossing the monitoring section in Case I, as shown in Fig. 9. The effect of volume loss on the settlement decreases with the increasing distance from the tunnel crown, while the effect of recharge of groundwater on the settlement increases with the increasing distance from the tunnel crown. Hence, the value of rebound settlement decreases with the decreasing distance between the monitoring point and the tunnel crown, and the rebound of settlement does not occur if the monitoring point is much closer to the tunnel crown, as shown in Fig. 10(a). However, in Case II, the rebound of pore water pressure merely occurs in the vicinity of tunnel. The development of settlement mainly depends on the volume loss, and thus no settlement recovery occurs. When the EPB shield is 27 m away from the monitoring section, the continuous recharge of the groundwater results in the increase of the vertical total stress in both cases. The pore water pressure essentially recovers to the initial hydrostatic pressure condition in Case II. In Case I, the phreatic line recovers to 1.7 m below the ground surface. It can be observed that the vertical effective stress is relatively low in both cases and the proportion of pore water pressure in the vertical total stress is nearly 90%. This result is consistent with the field record presented by Inokuma and Ishimura (1995). They recorded the total soil and water pressure on a concrete segmental lining in the loose clayey silty sand and pointed out the ground loading was primarily caused by the pore water pressure while the effective stress was hardly discernable. Fig. 16 presents the variation of vertical total stress surrounding the tunnel at the monitoring section No. 206. The vertical total stress at three positions, at the tunnel crown, the springline and the bottom, are investigated. Before the cutterhead reaches the interface, the variation of the vertical total stress in three cases presents the same trend. Meanwhile, the vertical total stress increases synchronically as the cutterhead approaches the monitoring section due to the face pressure
Fig. 16. Vertical total stress around tunnel in three cases.
imposed on the excavation face. The vertical total stress decreases dramatically when the shield machine is crossing the monitoring section. After the cutterhead crosses the interface, a slight reduction in the vertical total stress around tunnel is observed in both Case I and Case II, while the vertical total stress around tunnel increases slightly in Case III. When the cutterhead drives away from the monitoring section, the continuous recovery of the vertical total stress around tunnel is observed in Case I and Case II, while the vertical total stress around tunnel continuously decreases in Case III. Moreover, the recovery speed of 10
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recovers immediately. At the S1, groundwater has recovered before the EPB shield passes through the interface. Then the drawdown caused by crossing the interface occurs. Therefore, the maximum settlement is observed in S3, followed by S2 and S1. Another obvious characteristic among three cases is that a slight rebound of settlement is observed at the phase II of S3. The magnitude of settlement rebound Δμ1 depends on the tradeoff of the volume loss at the shield tail and the groundwater recovery as mentioned above. The rebound of settlement is not observed in the remaining two cases. Stress relief dominates the settlement development at phase II of S2 since the slight recovery of groundwater in the vicinity of the tunnel cannot cause large rebound of settlement. At S1, similar to the settlement development of the monitoring point ring No. 196 in Figs. 6 and 7, the effect of groundwater recovery is lowest. One reason is that the drawdown is lowest in this mode, thereby the recovery of groundwater is also limited. Another reason is that the S1 is far away from the cutterhead when it is crossing the interface, the effect of groundwater recovery on the settlement is insignificant. This is also the reason that tunneling-induced settlement is the lowest (15.99 mm) at the S1 when the shield machine crosses the interface. The last characteristic is observed at phase III. It can be seen from Fig. 16 that the continuous increase in the settlement is observed at S3 while a slight rebound of settlement occurs in S1 and S2. This is related to the time of groundwater recovery. At the S3, the groundwater recovers when the cutterhead is beneath the monitoring section. At the S2, the groundwater recovers after the cutterhead crosses the monitoring section. In the S1, the second groundwater rebounds after the EPB shield tail drives away from the monitoring section. Therefore, in the S3, the magnitude of groundwater recovery is lowest at phase III among three modes since the large recovery of groundwater has completed after the EPB shield tail crosses the monitoring section. Stress relief dominates the settlement development at phase III, thereby the continuous increase in settlement is observed in S3. However, in the remaining two modes, the recovery of groundwater dominates the settlement development at phase III, therefore, the rebound of settlement at phase III is observed especially in the S1.
Table 4 Percentage of settlement at three phases. Case
I
II
III
ks/kl = 10 ks/kl = 1 No seepage
77.88 41.19 32.46
14.32 39.81 43.17
7.80 19.00 24.37
Note: I = the period of EPB shield ahead of the monitoring section; II = the period of the EPB shield body passing the monitoring section; III = the remaining tunneling process.
vertical total stress in Case I is faster than in Case II. This is related to the recovery speed of pore water pressure. As shown in Fig. 14, the recovery speed of pore water pressure in Case I is faster than Case II. The recovery of pore water pressure also limits the development of settlement after the EPB shield tail drives away from the monitoring section. Therefore, only 7.8% of the total settlement in the ks/kl = 10 case is developed at this phase, in comparison with the 19.0% and 24.4% in ks/kl = 1 and no seepage cases, respectively. The proportion of settlement at each phase is summarized in Table 4.
5. Discussion As mentioned above, the phreatic water table will decline dramatically when the EPB shield machine passes through the interface of low-permeability and high-permeability ground, leading to large ground settlement. However, as the relative position of the monitoring section to the interface varies, different pattern of the settlement development will be presented, as shown in Figs. 6 and 7. The settlement development of three points, S1, S2 and S3 in Case I is summarized in Fig. 17. S1, S2, S3 are located at − 12, 0 and 6 m from the interface, respectively. The whole tunneling process can be divided into three phases. Phase I represents the period that the EPB shield is in front of the monitoring section. Phase II represents the whole period of the EPB shield body passing the monitoring section. The remaining tunneling process is represented by phase III. The settlement development of these three points presents three typical modes when EPB shield crosses from the low-permeability ground to the high-permeability ground. The largest settlement is observed in the S3 corresponding to the monitoring point ring No. 206, since the magnitude of drawdown is the largest in this mode. Crossing the interface will lead to groundwater inflow, and the phreatic water table will continuously decline before the cutterhead reaches the monitoring section. At the S2, the drawdown caused by crossing the interface occurs when the EPB shield is beneath the monitoring section, after that the groundwater
6. Conclusions Field records obtained from Changsha Metro Line 4 project indicate that large settlement tends to occur when the EPB shield advances from the low-permeability ground to the high-permeability ground. This paper investigated the interaction of tunneling and groundwater in the water-bearing mixed ground using coupled hydro-mechanical 3D finite element (FE) analysis. The responses of settlement, pore water pressure and ground stress to tunneling were investigated comprehensively. The following conclusions can be drawn: (1) In the practice engineering, the chamber pressure is recommended to increase before the shield machine enters from the low-permeability ground to the high-permeability ground, even if the selfstability of the ground is good enough, because the lag in the increase of chamber pressure when the shield machine advances from low-permeability ground to the high-permeability ground allows groundwater easily inflows from the excavation face. Therefore, the large drawdown occurs, causing large ground surface settlement and wide settlement trough. (2) When shield machine advances in the water-bearing mixed ground, the ground settlement and pore water pressure responses can be categorized into five phases: (i) approaching the interface––a slight decrease and increase in pore water pressure and settlement, respectively; (ii) reaching the interface––a sudden decrease and increase in pore water pressure and settlement, respectively; (iii) reaching the monitoring section––minimum pore water pressure and continuous increase in settlement; (iv) shield machine body passing––rapid recovery of pore water pressure and slight rebound
Fig. 17. Three settlement development modes. 11
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of settlement (depending on the trade-off of stress relief and recovery of pore water pressure); (v) shield machine driving away––slight recovery of pore water pressure and slight increase in settlement. (3) The settlement development pattern is related to the relative position of the monitoring points to the interface. When the shield machine advances towards the interface, the maximum settlement of the monitoring points that are located behind the interface is larger than that of the monitoring points above the interface or ahead of the interface.
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