Tunnelling-induced ground settlements in a groundwater drawdown environment – A case history

Tunnelling and Underground Space Technology 29 (2012) 69–77

Contents lists available at SciVerse ScienceDirect

Tunnelling and Underground Space Technology journal homepage: www.elsevier.com/locate/tust

Tunnelling-induced ground settlements in a groundwater drawdown environment – A case history Chungsik Yoo a,⇑, YongJoo Lee b, Sang-Hwan Kim c, Hong-Taek Kim d a

Dept. of Civil and Envir. Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea Dept. of Civil Engineering, Seoul National University of Science & Technology, Seoul 139-743, Republic of Korea c Dept. of Civil Engineering, Hoseo University, Asan 336-795, Republic of Korea d School of Urban & Civil Engineering, Hongik University, Seoul 121-791, Republic of Korea b

a r t i c l e

i n f o

Article history: Received 24 December 2010 Received in revised form 4 January 2012 Accepted 11 January 2012 Available online 12 February 2012 Keywords: Tunnelling Ground settlement Groundwater NATM Pre-grouting Stress–pore pressure coupled analysis

a b s t r a c t In this paper a case history of conventional tunnelling in which tunnelling-induced groundwater drawdown caused excessive surface settlements is presented. The measured ground surface settlements were first analyzed to identify the settlement characteristics at the site in relation to the tunnelling-induced groundwater drawdown. The measured settlement data revealed a considerably larger settlement affected zone than for cases with no groundwater drawdown, with a tendency for slow settlement stabilization. Also revealed in the measured piezometric data is that a significant portion of the total groundwater drawdown, around 65%, was completed prior to the top heading arrival. This suggests that pregrouting is of paramount importance in controlling groundwater inflow in tunnelling situations with highly permeable water bearing ground. In addition a three-dimensional stress–pore pressure coupled finite element analysis was performed which confirmed the direct link between groundwater drawdown and excessive settlements. The practical implications of the findings from this study are also further discussed. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction The conventional tunnelling method, known as the new Austrian tunnelling method (NATM), is often executed in highly permeable water bearing ground. The pre-grouting technique is usually adopted in such tunnelling situations to create a watertight shell around the tunnel in order to control the groundwater inflow and to maintain stability during tunnelling. Even with the pre-grouting however, the tunnelling work inevitably causes water inflow into the excavated area if the quality of the pre-grouting is not properly controlled. A direct environmental consequence of this water inflow during tunnelling is a drawdown of the groundwater in the surrounding aquifer, which may cause settlements as a result of the reduction in pore water pressures in soil layers, damaging nearby structures and utilities (Yoo, 2005; Yoo and Kim, 2006). Groundwater control in such tunnelling conditions is crucial to minimize the cost to society of handling disturbances and damages due to unwanted groundwater drawdown. The Norwegian Soil and Rock Engineering Association (NSREA, 1995) reported two case histories in which groundwater drawdown during tunnelling caused excessive ground surface settlements. The first case history involves the 2 km long Holmenkollbannen subway ⇑ Corresponding author. Tel.: +82 31 290 7518; fax: +82 31 290 7549. E-mail address: [email protected] (C. Yoo). 0886-7798/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.tust.2012.01.002

tunnel construction in Oslo in which up to 30–40 cm of settlement was recorded, damaging residential and office buildings along the tunnel route. The second case history reported is that of the Romeriksporten tunnel project, completed in 1999, where the tunnelling caused a few meters of subsidence in valleys and the drying up of a creek. More recently, Yoo et al. (2009) briefly reported a case history of an urban tunnelling case in Korea in which tunnelling-induced groundwater drawdown caused excessive ground surface settlements; this raised significant concerns over the operational safety of facilities situated in the settlement area. Although not directly related to tunnelling, Lei (1999), Shen et al. (2006), Qiao and Liu (2006) and Xu et al. (2006) have also reported case histories of ground subsidence caused by groundwater pumping from an aquifer. More recently, Wu et al. (2010) reported a case history of land subsidence due to groundwater overdrafting in Shanghai and investigated possible subsidence for two groundwater pumping scenarios using a coupled model consisting of a three-dimensional groundwater flow model and a one-dimensional vertical deformation model. These case histories highlighted the significant contribution of groundwater drawdown to the development of ground settlement during tunnelling or groundwater pumping. Although numerous studies have been conducted in the area of ground settlement due to tunnelling (Peck, 1969; Attewell et al., 1986; New and O’Reilly, 1991), studies concerning ground settlement during tunnelling in a groundwater drawdown environment

70

C. Yoo et al. / Tunnelling and Underground Space Technology 29 (2012) 69–77

are limited. Yoo (2005) conducted a 3D numerical investigation on the interaction between groundwater and tunnelling and identified the characteristics of ground settlement during tunnelling in water bearing ground. Altinbilek (2006) conducted a study for the estimation of consolidation settlement caused by groundwater drainage at the Ulus-Kecioren subway project and identified the potential effects of groundwater drainage on the ground settlement development. Later, Yoo and Kim (2008) performed a numerical investigation into the effect of multi-faced tunnelling in water bearing soft ground with an emphasis on the groundwater and tunnelling interaction. In these studies, the importance of controlling the groundwater drawdown in ground settlement sensitive areas, especially in urban tunnelling situations was highlighted. Although not directly related to the issue of settlement development due to tunnelling-induced groundwater drawdown, Shin et al. (2002) performed a study concerning the effect of groundwater on long-term tunnel behavior. Most recently, Chiocchini and Castaldi (2011) reported case histories concerning the impact of groundwater on the excavation of tunnels in two different hydrogeological settings in central Italy. This study highlighted the importance of correct interpretation of the hydrogeological condition of a project site in minimizing the groundwater ingress. In this paper a case history of conventional tunnelling during which excessive ground surface settlements were induced by a tunnelling-induced groundwater drawdown is presented, extending the previous work by Yoo et al. (2009). The dynamics of the measured ground surface settlement as well as the piezometric data during tunnel driving were first analyzed in a systematic way such that the characteristics of the groundwater drawdowninduced surface settlements can be identified. A three-dimensional (3D) finite element analysis was subsequently carried out for the tunnelling condition within the framework of a stress–pore pressure coupled formulation in order to further examine a possible link between the groundwater drawdown and excessive settlements. The following sections present the site condition, the tunnel design, the measured settlement, the piezometric data, and results of the 3D finite-element analysis and the practical implications of the findings. 2. Description of the project site 2.1. General The project site is located 20 km west of the city of Seoul, the capital of Korea, near to a busy domestic airport (Incheon Airport Railway Construction Authority, 2002). The Han River is located approximately 4 km away from the project site (Fig. 1). The ground at the site is characterized by thick highly permeable water bearing soil layers with a relatively high groundwater table at 4–6 m from the ground surface. The tunnel is a double track tunnel, approximately 1700 m in length, which was constructed using the drill and blast technique. In one stretch of tunnel, a section approximately 460 m long was constructed directly under the airport apron. The owner of the airport imposed strict settlement control limits to ensure the operational safety of the airport facilities. Considerable ground surface settlements occurred during the tunnel excavation under the airport apron however and prompted this investigation. 2.2. Geological and hydrological conditions The geological setting of the tunnelling site consists of a Precambrian Gyeonggi Gneiss rock complex overlain by fill and Quaternary alluvium layers. The base rock is biotite gneiss, a foliated metamorphic rock, characterized by its anisotropic stress–strain–

strength behavior. No major fault is present in the project area. Figs. 2 and 3 show typical transverse and longitudinal geological cross sections of the tunnel route constructed based on borehole data. As shown, the top layer is a 4–10 m thick miscellaneous fill material formed of gravelly, silty sand with an SPT blow count of N = 15–30. Underlying the fill layer is a 2–4 m thick clayey sand alluvial deposit having N = 30–35 followed by a 12–21 m thick decomposed gneissic soil layer having an N value ranging from 10 to 50. The decomposed soil layer is underlain by a 10 m thick weathered gneissic rock exhibiting 9–30 cm of penetration for 50 blows. The weathered gneissic rock layer is followed by a soft to hard gneissic rock layer with a maximum RQD of 40. Geological mapping during tunnel driving however revealed that the decomposed weathered soil layer extends further down to the tunnel springline. The geotechnical and hydraulic properties of each layer are summarized in Table 1. It should be noted that the hydraulic conductivities of the soil layers were obtained from field permeability tests while those for the weathered and solid rocks were obtained from Lugeon tests. The coefficients of specific storage were inferred from Eq. (1) given by Bear (1979).

Ss ¼ cw ðmv þ nbÞ

ð1Þ

where cw is the unit weight of water, mv is the coefficient of compressibility, n is the porosity and b is the coefficient of compressibility of water. Note that in the calculation, the coefficient of compressibility mv was related to the deformation modulus of each layer E as mv = 1/E with b = 0 assuming negligible compressibility of the water. The general hydrogeological model of the study area can be described as an unconfined aquifer consisting of fill, alluvium, and weathered soil layers resting on a base rock. The groundwater level, which was measured by a borehole, was quite high, at approximately 4–6 m below the ground surface. No detailed hydrogeological information was available at the time of this investigation as no detailed hydrogeological study was carried out during the design stage. According to a study conducted by Kim (2010) in a site near the study area, the direction of natural seepage for the site near the study area is approximately 300– 350° from north with flow velocities in the range of 0.05–0.07 m/ day. The average annual precipitation in the project area is 1200 mm/year with the highest precipitation during the monsoon season from June to August. In fact, approximately 70% of the total annual precipitation is in the 3-month period of the monsoon season. 2.3. Tunnel design The tunnel section excavated under the airport apron has an excavation width and height of approximately 10.5 m and 8.7 m, respectively, with an average cover depth of 25 m. Fig. 4 shows the tunnel cross section together with the support pattern adopted. The tunnel was excavated using the bench-cut method with a primary support system consisting of a 0.2 m thick steel fiber reinforced shotcrete (SFRS) layer with 3 m long system rock bolts installed at 0.8 and 1.2 m longitudinal and transverse spacings respectively. The pipe umbrella technique using 800 mm diameter, grout injected 12 m long steel pipes was additionally implemented to cover the upper ±60° of the tunnel crown to promote face stability through improving the load carrying capacity of the ground ahead of the face. Additionally, trumpet shaped micro-silica pregrouting (MSG) was implemented around the upper tunnel periphery and the face to create a 5 m thick, 12 m long watertight shell as shown in Fig. 5. Details of the tunnel support pattern adopted in this section are summarized in Table 2.

C. Yoo et al. / Tunnelling and Underground Space Technology 29 (2012) 69–77

71

Fig. 1. Bird’s eye view of project site.

Fig. 2. Sectional profile (typical).

2.4. Tunnel construction and instrumentation layout The east and west bound tunnel excavations commenced concurrently from STA22km500 and STA22km960, respectively, as

shown in Fig. 3. After advancing each tunnel heading to STA22km513 and STA22km937 respectively, significant surface settlements in the region ahead of and behind the west bound tunnel heading developed. In order to monitor surface settlement, the contractor installed a grid of settlement points at 10 m transverse and longitudinal spacings along the tunnel route, with approximately 300 settlement points in total. At each transverse section, perpendicular to the tunnel driving, seven settlement points were installed at 10 m spacing. Such a layout was later found to be insufficient to cover the entire settlement affected zone in the transverse sections. In addition to the settlement points, conventional porous tube type piezometers were installed at two locations at STA22km780 and STA22km840 to monitor changes in the groundwater level during tunnelling. It should be noted that the piezometer at STA22km780 (W1) was installed prior to the tunnel driving, while that at STA22km840 (W2) was installed at the time the settlement grid was constructed, i.e. sometime after the initial tunnel driving. Therefore, the piezometric data from W2 did not reflect the groundwater drawdown caused by the initial tunnel driving, if any occurred. The readings from W1 however provided complete data concerning the change in the groundwater level during tunnel driving.

Fig. 3. Longitudinal sectional profile.

72

C. Yoo et al. / Tunnelling and Underground Space Technology 29 (2012) 69–77

Table 1 Geotechnical properties of soil/rock layers.

c (kN/m3)

Type Fill Alluvial Weathered soil Weathered rock Solid rock

18 20 25 25 26

c0 (kPa) 0 15 15 60 100

/0 (deg) 27 30 30 35 35

E (MPa) 5 10 50 120 200

m 0.40 0.40 0.33 0.30 0.25

e

K (cm/s) 4

0.4 0.4 0.6 – –

3.8  10 3.8  104 2.4  104 8.8  105 5.0  105

Ss (103 m1) 2 1 0.2 0.08 0.005

Note: c = unit weight, c0 = cohesion, /0 = internal friction angle, E = Young’s modulus, m = Poisson’s ratio, e = void ratio, K = coefficient of permeability, Ss = coefficient of specific storage.

Table 2 Summary of support patterns. Ground condition

0.5D above crown

Decomposed soil

Upper

Weathered soil

Lower

Weathered rock

Excavation method Length of one-round excavation (m) Shot’c thickness (m) Rock bolt Length (m) Longitudinal spacing (m) Transverse spacing (m) Auxiliary Crown area method reinforcement Fig. 4. Support pattern for STA22km800–890.

3. Measured settlements The surface settlements were measured using the settlement grid points over an 8 month period from March to October 2005. The measured data only represents the settlements that developed after the installation of the settlement points. The settlements that had occurred prior to the settlement points installation, i.e. those developed while advancing from STA22km960 to STA22km937 are therefore not included. 3.1. Surface settlement development The longitudinal surface settlement profiles along the tunnel center line at different top heading locations are given in Fig. 6. As can be observed in this figure, the tunnel advancement gradually increased the longitudinal settlements both ahead of and behind

Bench-cut 0.8 0.2 3.0 0.8 1.2  Pipe umbrella (L = 12.0 m, £800 mm), Cover zone: 120°, overlap: 4 m  MSG pre-grouting

the heading with a maximum settlement of approximately 160 mm occurring at STA22km880 upon completion of the tunnel excavation. Also noted in this figure is that for a given top heading location, the settlement affected area extended up to approximately 4–5D ahead of the top heading with a tendency to stabilize 6–7D behind the tunnel heading. Such an extent of the settlement zone is in fact considerably larger than that of tunnelling cases with no groundwater drawdown as will be discussed later in this paper. Settlement history plots for selected monitoring points in the region between STA22km800–STA22km960 are shown in Fig. 7, from which the trend of progressive development of settlements in relation to the top heading location can be identified. Note that the settlement at each monitoring point is plotted against the top heading location relative to the respective monitoring point for ease of comparison. Due to the remoteness of the east bound tunnel heading starting from STA22km500, the measured settlements in this region

Fig. 5. Schematic view of pre-grouting.

73

C. Yoo et al. / Tunnelling and Underground Space Technology 29 (2012) 69–77

0

STA22km960

top heading at STA22km+

80

924 887 852 814 772 757

120 160 200

700

top heading location (West bound tunnel)

750

800

22km+

850

900

950

22km+

Station

0

Settlement, Sv (mm)

Station 22km880 22km870 22km860

40 80 120 160 700

750

800

200 -10

850

Station

-5

900

950

22km+

0

5

10

15

Distance to Top Heading (times D)

(a) STA22km830~880 0

Settlement (mm)

40 700

80

22km+

750

800

850

Station

900

22km920 22km910 22km900

-5

0

5

850

-120

900

-160 -200

0

50

100

150

200

950 250

10

15

Fig. 8. Development of surface settlement with time.

monitoring section. Considering that a tunnel driving typically affects the zone 2–3D ahead and behind the tunnel heading when no groundwater drawdown is involved, such an extent of the settlement zone can be considered to be larger than typical. Such a trend implies that groundwater drawdown may have contributed to settlement development. Yoo (2005) and Yoo and Kim (2007) have also reported wider settlement zones for tunnelling cases with groundwater drawdown than those with no groundwater drawdown. In Fig. 8 the settlement data for STA22km860–STA22km880 are re-plotted against the time elapsed from the start of excavation. Of interest here is the trend shown of the continuation of settlement development during the period that the tunnel advancement was interrupted over a considerable time, from 1 week or up to 1 month. Such a trend also demonstrates that tunnelling activity independent settlements may have been involved, which are thought to be associated with the groundwater drawdown. This will be further discussed in the following section. 3.2. Settlement correction

950

Station

200

800

Time vs. Top heading

-80

22km+

120 160

750

22km860 22km870 22km880

-40

Time elaspsed from start of exca. (days)

Fig. 6. Longitudinal surface settlement profiles during tunnelling.

22km+

Settlement, Sv (mm)

40

Monitoring Station

Top heading (STA.22km+)

Settlement (mm)

0

20

Distance to Top Heading (times D)

(b) STA22km900~920 Fig. 7. Progressive development surface settlements at various monitoring stations.

were thought to be mainly caused by the west bound tunnel driving starting from STA22km960. One feature of interest noted in these figures is that the settlement curves for STA22km860, STA22km870 and STA22km880 in Fig. 7a are almost identical while those for STA22km900, STA22km910, and STA22km920 in Fig. 7b show some disparities. There is a trend of smaller settlements occurring at the monitoring points closer to the west bound tunnel portal at STA22km960. This trend may be an indication that the initial driving of the west bound tunnel from STA22km960 to STA22km937 that took place before the installation of the settlement points induced some settlements at the stations STA22km900 and STA22km910. Further inspection of the measured settlement history plots shown in Fig. 7 revealed that the settlements started to develop when the top heading was 5D away from the monitoring point. The settlements then continued to increase with the tunnel advancement until the tunnel heading advanced 6D beyond the

As discussed in the previous section, the settlement data in the region STA22km900–920 in Fig. 7b did not seem to represent the whole settlements induced by the entire tunnelling process. A further correction to this settlement data to include the settlements that had occurred prior to the installation of the settlement grid (hereafter termed pre-settlement) was therefore necessary in order to properly assess the operational safety of the airport facilities. A settlement characteristic curve was first constructed by fitting the settlement data for STA22km860, STA22km870 and STA22km880 as they did not seem to be affected by the initial tunnel driving prior to monitoring. A cumulative probability function (New and O’Reilly, 1991) was used to construct the settlement characteristic curve, given as:

  x  x o Vs y2 n x  xi  f w ¼ pffiffiffiffiffiffiffi exp  2 G G i i 2pi 2i

ð2Þ

where Vs is the volume of the settlement trough per unit distance of tunnel advance and i is the inflection point defining the form and span of the settlement trough on the assumption that the semitransverse (y-axis) settlement profile can be described by a normal probability equation. xi and xf are the tunnel start point and the tunnel face position along the tunnel axis (y = 0) respectively. G(a) can be determined from a standard probability table. As shown in Fig. 9, the cumulative probability function provided an excellent fit to the measured settlement data with the location of the inflection point (i) at 2.5D. Note that the location of the inflection point i = 2.5D is five times larger than that computed using Eq. (3) by Clough and Schmidt (1981), which has been

74

C. Yoo et al. / Tunnelling and Underground Space Technology 29 (2012) 69–77

40

80

measured 22km880 22km870 22km860

120 160 200 -10

0

top heading direction of tunnelling

Settlement (mm)

Settlement (mm)

40

fitted cumulative probability function

-5

0

10

15

direction of tunnelling

-40 -80 measured data

-160 -15

20

top heading

Projected settlements

fitted cumulative probability function

-120

5

Station 22km900

15 mm

0

Distance to Top Heading (times D)

-10

-5

0

5

10

15

20

Distance to Top Heading (times D)

(a) STA22km900

Fig. 9. Construction of characteristic line using settlement data for STA22km860– 880.

40

where D and zo are the tunnel diameter and cover depth respectively. The settlement characteristic curve was then used to fit the settlement curves for STA22km900, STA22km910, and STA22km920 with an assumption that the settlement immediately above the top heading is approximately 45 mm as in the case for the unaffected settlement data for STA22km860, STA22km870, and STA22km880. As shown in Fig. 10, the settlement characteristic curve provided a reasonably good fit to the settlement data for STA22km900, STA22km910, and STA22km920, suggesting that the settlement characteristics in this region are in fact similar to those for STA22km860, STA22km870, and STA22km880. The presettlements were then inferred directly from the plots as 15, 25, and 40 mm for STA22km900, STA22km910, and STA22km920, respectively. The fitted cumulative probability function was later used to assess the operational safety of the airport facilities. Assessment of the operational safety of the airport facilities is beyond the scope of this study and can be found in Kim et al. (2005). 3.3. Piezometric data

25 mm

Station 22km910 top heading direction of tunnelling

Projected settlements

-40 -80 measured data

-120 -160 -15

fitted cumulative probability function

-10

-5

0

5

10

15

20

Distance to Top Heading (times D)

(b) STA22km910 40 0

40 mm

ð3Þ

Settlement (mm)

D zo 0:8 i¼ 2 D

0

Settlement (mm)

proven to be applicable for tunnelling cases in Korea (Yoo, 2001). A larger i value in fact implies a wider settlement affected zone, again due possibly to the groundwater drawdown.

Station 22km920 top heading direction of tunnelling

Projected settlements

-40 -80 measured data fitted cumulative probability function

-120 -160 -15

-10

-5

0

5

10

15

20

Distance to Top Heading (times D) Fig. 11 shows the progressive changes in the piezometric levels for W1 and W2, which were installed in the region of excessive settlements. As mentioned, the piezometer W2 at STA 22k840 was installed after the excessive settlements became an issue and therefore the earlier portion of the groundwater drawdown was not reflected in the data. The readings taken from W1 at STA 22k780 on the other hand provide a complete trend of the effect of tunnelling on the groundwater drawdown as W1 had been installed prior to the initial tunnel driving. As shown in Fig. 11a, the data from W1 shows that the groundwater level, originally at ground level (GL) 4 m, started to decrease when the top heading was approximately 5D away from W1. The groundwater level then sharply decreased to GL 20 m at the time the top heading arrived at the monitoring section, yielding a drawdown of 16 m. Further tunnel advancement continued to decrease the groundwater level to GL 29 m until the top heading advanced 2D beyond the monitoring section, resulting in a total drawdown of 25 m. Of particular importance is the observation that approximately 65% of the total groundwater drawdown had been completed when the top heading arrived at the section. If there had been any post-grouting executed during tunnelling, the major portion of the drawdown would not have been

(c) STA22km920 Fig. 10. Corrected settlement curves for STA22km900–920.

prevented, suggesting that the post-grouting is not a suitable alternative to pre-grouting, as discussed by Yoo (2005). The groundwater drawdown of 25 m in fact implies that the pre-grouting failed to meet the intended water tightness requirement, possibly due to poor quality control. The pre-grouting only seemed to decrease the rate of groundwater drawdown, although the reasons for this were not immediately clear at the time of investigation. Fig. 11b illustrates the time variation of the groundwater level with tunnel advancement. Also plotted in this figure is the location of the top heading with respect to the time elapsed from the start of tunnel driving. Of interest here is the trend shown of the continuation of groundwater drawdown during interrupted periods of the tunnel advancement, even when the top heading advanced beyond 3D from the monitoring section. This may be an indication of continued water inflow into the tunnel during the interrupted period of tunnelling activity. Such a trend is in accordance with the settlement data, in which a tendency for continued settlement development during the interrupted period of tunnel advancement

C. Yoo et al. / Tunnelling and Underground Space Technology 29 (2012) 69–77

75

4.1. Stress–pore pressure coupled finite element model

0

Piezometric Level (m)

W1 (STA22km780) 14 m

10

20

W2* (STA22km840) *installed in 28/5/05 top heading at STA22km878

30

40 -12

-8

-4

0

4

8

12

Distance to Top Heading (times D)

0

Piezometric Level (m)

750 10

W2 (STA22km840)

800

Time vs. Top heading

20

850

30

900 *installed 28/5/05/ when top heading at STA22km878

40

Top heading (STA.22km+)

W1 (STA22km780)

950 0

50

100

150

200

250

Time elaspsed from start of exca. (days)

Fig. 11. Progressive change in groundwater level.

was observed, suggesting a strong tie between groundwater drawdown and settlement development. The discharge of groundwater measured at a location near the eastern portal at STA23km167 was approximately 3.2 m3/day/m. No other water inflow data was available at the time of investigation. Note however that the discharge measurement started 3 month after the tunnel driving. Considering the time variation of water inflow during tunnel excavation and the highly permeable nature of the ground, the measured value of 3.2 m3/day/m appears to represent the steady state water inflow. 4. Stress–pore pressure coupled finite element analysis A 3D stress–pore pressure coupled finite element analysis was carried out in order to confirm that the excessive settlements were in fact mainly associated with the groundwater drawdown. The following sections describe the finite element model adopted and the results.

The stress–pore pressure coupled effective formulation was adopted in order to realistically capture the interaction mechanism between the tunnelling and the groundwater. A commercial finite element package Abaqus (Abaqus, 2006) was used for the analysis. Fundamentals of the stress–pore pressure coupled formulation adopted in Abaqus can be found in the Abaqus user’s manual (Abaqus, 2006). The transverse cross section given in Fig. 2, including the ground profile, was used to create a 3D finite element model. As shown in Fig. 12, the finite element model extends to a depth of three times the tunnel diameter (D) below the tunnel invert. The lateral boundaries in the x and y directions extend to a distance of 15D and 21D respectively from the tunnel center. At the vertical boundaries, displacements perpendicular to the boundaries were restrained whereas pin supports were placed at the bottom boundary. With reference to Fig. 12, a no-flow condition was assigned to the bottom boundary while the groundwater level at the vertical boundaries was assumed to be constant throughout the analysis at the original groundwater level of GL 4 m. The locations of the lateral and bottom boundaries were selected based on the results of a preliminary study so that the presence of the artificial boundaries does not significantly influence the stress–strain–pore pressure field in the domain. In the 3D model, the pre-grouting and the shotcrete lining adopted in the actual construction were explicitly modeled. The system rock bolts however were not considered for the sake of modeling simplicity. The ground, as well as the shotcrete lining, were discretized using eight node displacement and pore pressure elements with reduced integration (C3D8RP), resulting in 49,976 nodes and 49,115 elements. With regards to the material constitutive modeling, the soil and rock layers were assumed to be an elasto-plastic material conforming to the Mohr–Coulomb failure criterion together with the nonassociated flow rule proposed by Davis (1968). The shotcrete lining was assumed to behave in a linear elastic manner having an average value of Young’s modulus of 1 GPa, representing green and hard shotcrete conditions reported in the literature (Queiroz et al., 2006). The geotechnical properties given in Table 1 for the ground layers were used for the analysis. For the pre-grouting zone, shear strength parameters of c0 = 100 kPa, u0 = 35° were assigned together with a hydraulic conductivity of k = 1  104 cm/ s. Note that these values were calibrated to some extent, with due consideration of the recommendation of Spriano (1997), due to the lack of relevant information for the pre-grouting zone. In simulating the tunnelling process, the actual tunnelling sequence adopted at the site was closely followed. Prior to the tunnel excavation, the pore pressure below the ground water table was assumed to be hydrostatic. After establishing the initial stress

Fig. 12. Finite element model adopted.

C. Yoo et al. / Tunnelling and Underground Space Technology 29 (2012) 69–77

0

Piezometric Level (m)

and pore pressure conditions with appropriate boundary conditions, the tunnelling process consisting of a series of pre-grouting, top and bench excavation and the lining installation stages was then simulated by adding and removing corresponding elements at designated steps. For the excavation boundary, i.e. the tunnel walls and face, a zero pore pressure flow boundary condition is assigned to allow for water inflow to occur. Based on the information available in the construction record, it was assumed that a total length of 170 m of tunnel section is excavated in approximately 250 days, giving an average rate of 0.6 m/day. Also considered in the analysis were the interrupted periods of the tunnel advancement, as identified in the construction record, for realistic simulation of the tunnel advancement. Due to the lack of hydrogeological data, no consideration of the natural seepage or the Han River effect was made in the modeling. Furthermore, possible infiltration of precipitation as a source of recharge was not considered, although one can argue that in reality such a condition is only valid during a very dry season. Although such simplifications can be considered as limitations of the current modeling and may have some impact on the results of the analysis, it should be noted that the current numerical modeling was intended to provide a qualitative assessment of the mechanical and hydrological coupled effect of the tunneling and groundwater interaction on the ground settlement during the tunnel excavation. A comprehensive simulation of the groundwater due to the tunnel excavation needs more detailed hydrogeological modeling together with relevant input data.

10

20 measured 30

0

Settlement (mm)

40 80

measured computed

160 200

0

5

10

15

Distance from tunnel center (times D)

(a) transverse settlement 0 measured

Settlement (mm)

40

computed

80 120

Direction of tunneling

FD

160 200 -10

-5

0

5

-4

0

4

8

Fig. 14. Computed vs. measured piezometric level.

8.0

Computed water inflow

6.0 *measured water inflow at steady state = 3.2 m3 /day/m

4.0

0

200

400

600

800

Time elaspsed after exca. (days)

Fig. 13 compares the computed and the measured settlement data. Note that the monitored settlement data measured at STA22km870 were used for comparison. As shown in Fig. 13a, the computed transverse settlement trough tends to show a good

120

-8

Distance to Top Heading (times D)

2.0

4.2. Comparison between measured and computed

computed

40 -12

Water inflow (m3/day/m)

76

10

Distance to Top Heading, FD (times D)

(b) settlement vs. top heading Fig. 13. Computed vs. measured settlements.

15

Fig. 15. Time variation of computed water inflow.

agreement with the measured data. The trend of progressive development of settlement with respect to the top heading location is shown in Fig. 13b. As shown, a good agreement can be observed between the computed and the measured data in terms of the general trend as well as the maximum settlement value, although the settlement from the analysis tends to start earlier than the measured, resulting in the settlement curve located to the left of the measured one (see Fig. 13b). The predicted total groundwater drawdown level also matches fairly well with the piezometric data measured at STA 22km780 in terms of the maximum drawdown as shown in Fig. 14. In terms of the time development, however, the computed piezometric level tends to decrease in a delayed fashion compared to the measured data. Also shown in Fig. 15 is the plot of the time variation of water inflow based on the computed results. As shown, the initial water inflow is maximum at 7.6 m3/day/m. The water inflow value then decreases to the steady state flow of 3.4 m3/day/m. Although the direct comparison between the computed and the measured values may not be straight forward, the computed and the measured values at the steady state flow condition were found to be in reasonably good agreement. Although the exact cause(s) for the disparities between the computed and the measured data in terms of the time of development shown above are not immediately clear and were not further investigated, the limitations of the current numerical model mentioned previously may be responsible. Another possible reason may be the mechanical and hydraulic properties of the pre-grouting zone assumed in the analysis which may not represent actual values. Although a comprehensive hydrogeological study and a field investigation are required to further improve the numerical model, it is beyond the scope the current investigation. Nevertheless the results of the 3D stress–pore pressure coupled FE analysis confirmed that the tunnelling-induced groundwater drawdown was indeed the primary cause for the excessive settlements.

C. Yoo et al. / Tunnelling and Underground Space Technology 29 (2012) 69–77

5. Conclusions A case history of conventional tunnelling during which excessive ground surface settlements were induced by the tunnellinginduced groundwater drawdown is presented here. Measured data is thoroughly analyzed such that the characteristics of the groundwater drawdown-induced surface settlements can be identified. A three-dimensional (3D) stress–pore water pressure coupled finite element analysis was carried out to further examine the possible link between the excessive settlements and the groundwater drawdown. The following conclusions can be drawn: (1) The measured settlement data indicated that for a given top heading location, the settlement affected area extended to approximately 4–5D ahead of the top heading with a tendency of stabilizing 6–7D behind the tunnel heading, resulting in a larger settlement affected area than a typical case with no groundwater drawdown. (2) The cumulative probability function provided an excellent fit to the measured settlement history curves with a location of influence point at 2.5D. The location of the inflection point i = 2.5D was found to be five times larger than that computed for a case without groundwater drawdown using an empirical formula. (3) A total of 25 m of groundwater drawdown occurred despite the fact that systematic pre-grouting was implemented, indicating that the quality of the pre-grouting was not as intended. The data indicated that approximately 65% of the total groundwater drawdown had been completed when the top heading arrived at the monitoring section. Had any post-grouting been executed during tunnelling, the major portion of the drawdown would not have been prevented, suggesting that the post-grouting is not an alternative to pre-grouting. (4) Although limited, the results of the 3D stress–pore pressure coupled finite element analysis confirmed that the groundwater drawdown was indeed the primary cause for the excessive settlements that occurred at the site. Acknowledgements This research is supported by Grant No. 20100008227 from the Basic Research Program of the Korea Science & Engineering Foundation, by Grant No. 10CCTI-E09 from the Ministry of Land, Transport and Maritime Affairs, Korea, and by Hyundai Cooperation Ltd. The financial support is gratefully acknowledged. References Abaqus users manual, Version 6.7., 2006. Hibbitt, Karlsson, and Sorensen, Inc., Pawtucket, Providence, R.I. Altinbilek, M.E., 2006. Estimation of Consolidation Settlements Caused by Groundwater Drainage at Ulus-Kecioren Subway Project, MS. Thesis, Middle East technical University, pp. 189.

77

Attewell, P.B., Yeates, J., Selby, A.R., 1986. Soil Movements Induced by Tunnelling and Their Effects on Pipelines and Structures. Blackie, Glasgow, London. Bear, J., 1979. Hydraulics of Groundwater. McGraw-Hill, New York. Chiocchini, U., Castaldi, F., 2011. The impact of groundwater on the excavation of tunnels in two different hydrogeological settings in central Italy. Hydrogeology Journal 18, 651–669. Clough, G.W., Schmidt, B., 1981. Design and Performance of Excavations and Tunnels in Soft Clay, Soft Clay Engineering. Elsevier, pp. 569–634. Davis, E.H., 1968. Theories of plasticity and the failure of soil masses. Soil Mechanics: Selected Topics, Butterworth’s London, UK, pp. 341–380. Incheon Airport Railway Construction Authority 2002. Contract 1-1 Design Report. Incheon Airport Railway Construction Authority, Korea. Kim, C.H., 2010. A study on delineation of the influential range in groundwater system near construction sites along cutting faces, MS. Thesis, City University of Seoul, p. 93. Kim, H.T., Yoo, C., Seo, Y.C., Kim, S.H., Lee, Y.J., 2005. Stability Evaluation of Contract 1-1 Tunnel Section in Incheon Airport Railway Construction, KGS, Report to Hyundai Construction Ltd. Lei, S., 1999. An analytical solution for steady flow into a tunnel. Ground Water 37 (1), 23–26. New, B.M. O’Reilly, M.P., 1991. Tunnelling induced ground movements: predicting their magnitude and effects. In: 4th Int. Conf. On Ground Movements & Structures, pp. 671–697. Norwegian Soil and Rock Engineering Association (NSREA), 1995. Norwegian Urban Tunnelling, Publication No. 10. Peck, R.B., 1969. Deep excavations and tunnelling in soft ground. In: Proc. 7th Int. Conf. on Soil Mech. and Found. Engrg, pp. 225–290. Qiao, S., Liu, B., 2006. Prediction of ground displacement and deformation induced by dewatering of groundwater. Underground Construction and Ground Movement, pp. 73–79. Queiroz, P.I.B., Roure, R.N., and Negro, A. 2006, Bayesian updating of tunnel performance for Ko estimate of Santiago gravel. In: Proceedings, International Symposium on Geotechnical Aspects of Underground Construction in Soft Ground, London, 15–17 June. Taylor and Francis/Balkema, Leiden, The Netherlands. pp. 211–217. Shen, S.L., Tang C.P. Bai, Y., Xu, Y.S., 2006. Analysis of settlement due to withdrawal of groundwater around an unexcavated foundation pit. Underground Construction and Ground Movement, pp. 377–384. Shin, J., Addenbrooke, T., Potts, D., 2002. A numerical stud of the effect of groundwater movement on long-term tunnel behavior. Geotechnique 52 (6), 391–403. Spriano, S., 1997. Design Issues of the Concrete Lining of the Waterway Tunnel – Report to Korea Resources Corporation, Elecktrowatt Engineering Ltd. Wu, J., Shi, X., Ye, S., Xue, Y., Zhang, Y., Wei, Z., Fang, Z., 2010. Numerical simulation of viscoelastoplastic land subsidence due to groundwater overdrafting in Shangahi, China. Journal of Hydrologic Engineering 15 (3), 223–236. Xu, Y.S., Shen, S.-L., Bai, Y., 2006. State-of-art of land subsidence prediction due to groundwater withdrawal in China. Underground Construction and Ground Movement, 58–65. Yoo, C., 2001. Damage assessment of buried pipelines due to tunnelling-induced ground movements. Korean Journal of Geotechnical Engineering, KGS 17 (4), 71–86. Yoo, C., 2005. Interaction between tunnelling and groundwater-numerical investigation using three dimensional stress–pore pressure coupled analysis. Journal of Geotechnical and Geoenvironmental Engineering, ASCE 131 (2), 240– 250. Yoo, C., Kim, S.B., 2006. Stability analysis of an urban tunnelling in difficult ground condition. Tunnelling and Underground Space Technology, Elsevier, vol. 21 (3– 4), pp. 351–352. Yoo, C., Kim, S.B., 2007. A study on ground surface settlement due to groundwater drawdown during tunnelling. Korean Journal of Geotechnical Engineering, KGS 9 (4), 1–15. Yoo, C., Kim, S.B., 2008. Three-dimensional numerical investigation of multifaced tunneling in water-bearing soft ground. Canadian Geotechnical Journal 45 (10), 1467–1486. Yoo, C., Kim, J.W., Jung, H.S., 2009. Lessons learned from case histories of tunnellinginduced groundwater drawdown and associated settlement. In: Proc. World Tunnelling Congress 2009, Budapest, Hungary, pp. 12–16.