- Email: [email protected]
Underground excavation in Xiaolangdi project in Yellow River
Engineering Geology 76 (2004) 129 – 139 www.elsevier.com/locate/enggeo
Underground excavation in Xiaolangdi project in Yellow River Guogang Xu *, Guoyan Ma, Qingbo Li , Zhifang Cui The Reconnaissance, Plan, Design and Research Institute, YRCC, MWR, Beijing, PR China Received 3 December 2002; accepted 7 June 2004 Available online
Abstract The paper summarizes the excavation methodology, based on analysis of the characteristics of the surrounding rock mass, which has been involved in Xiaolangdi multipurpose dam project. Some special support and excavation methods, such as twin side drift and lattice girder, moderate blasting and double-layer mesh to collapsed tunnel and fault zone, are presented. D 2004 Published by Elsevier B.V. Keywords: Underground tunnels; Stability of surrounding rock mass; Excavation; Deformation monitoring
1. Introduction Xiaolangdi Multipurpose Dam Project, located at the outlet of the last gorge, consists of main dam, complicated tunnels system and power system. It is one of the most challenging projects in the world due to its special geographical locality, large scale and complicated geological condition. The reservoir, with a capacity of 12.65 billion m3 at the elevation of 275 m, accounting 92.3% of the Yellow River catchment basin, extends to the dam toe of Sanmenxia, another dam 130 km upstream of Xiaolangdi in the middle reaches of the Yellow River. The downstream area is the Huang-Huai-Hai plain (see Fig. 1). As one of the strategic project, its functions include flood control, ice flood control, sediment
settling, water supply, irrigation and power generating, etc. Considering high silt content of the Yellow River and the engineering geological condition at the dam area, the flood and sediment discharge facility and the power system are concentrated on the left bank to prevent silt from accumulation at the water intake and outlet. Over 100 underground tunnels and caves form a complicated and closely distributed system, including three diversion tunnels, three sediment tunnels and three free flow tunnels, six power-generating tunnels (Table 1). This is the most densely distributed tunnel and cave system in the world, involving an excavation volume of 2.7 million m3. Valuable experience has been gained in excavation of underground tunnels.
2. Geotechnical conditions * Corresponding author. Tel.: +86-371-6024947; fax: +86-3715959236. E-mail address: [email protected] (G. Xu). 0013-7952/$ - see front matter D 2004 Published by Elsevier B.V. doi:10.1016/j.enggeo.2004.06.010
Rocks on the left bank are mainly lower Triassic purplish red, thick and medium bedded or mega-
130
G. Xu et al. / Engineering Geology 76 (2004) 129–139
Fig. 1. Location plan of the Xiaolangdi project.
bedded calca-siliceous or calcareous sandstone and calca-argillaceous, or argillaceous siltstone intercalated with thin-bedded claystone, with a total thickness of 350 m. Based on the characteristics, the rocks in the tunnel area were classified into six subgroups, namely T11, T12, T13, T14, T15 and T16 (shown in Table 2). The rock exposed at the dam site dips to the east at an angle of 8– 12j. Several faults, F28, F236 and F238 have been found in the intake area. F28 is in front of the intake tower of the diversion and flood tunnels and has no effect on the underground buildings. F236 and F238, sub-parallel faults, with wide broken zones, intersect with tunnels at acute angle. These two faults, together with the branch faults and fracture rocks,
compose the adverse condition to the stability of the tunnels (see Fig. 2). Argillized intercalation, another discontinuity, is found distributed along the bedding plane and repeatedly occurs in the direction vertical to the bedding plane, but discontinuous or thinned out along the bedding plane. Because of its high clay content, low shear strength and swelling capacity, the argillized intercalation has been suggested to form a weak sliding plane or a detached plane, which will affect the stability of rock mass. According to the rock mass classification used in water conservancy and hydropower field in China, the rock masses in the tunnel area are generally categorized into grades IV and V, some place dissected by the main faults fall into grades IV or V. The in-site stress in the Xiaolangdi project is measured by stress release method and the hydraulic-fracturing method, supplemented with the Kaiser method and fabric analysis. Regression analysis of the in situ stress field has been carried out on the basis of the measurement result. It is concluded that, in the tunnel area, the self-gravity is dominant and the tectonic stress is minor. The lateral compression coefficient is 0.7– 0.9. The maximum horizontal in situ stress is 5 MPa, directing to 20jNE and making an angle of 30j with the axis of the underground powerhouse and an angle of 83j with the axis of the flood discharge tunnels. The stress – strength ratio of the surrounding rock is S>4. The results show that the influence of in situ stress upon
Table 1 Data of underground openings Item
Number
Typical cross-section (m)
Typical lining section (m)
Length for each (m)
Diversion (orifice) tunnel
3
F14.5
1100
Free flow tunnel Sediment tunnel Power tunnel Tailrace tunnel Power house Transformer chamber Tailrace gate chamber Irrigation tunnel
3 3 6 3 1 1 1 1
F18.5 – 23.0 (mid-gate chamber: 23.4 22.0) \ 13.7 16.2 F8.3 F9.6 \ 12.8 19 \ 26.2 61.44 \ 15.2 17.8 \ 10.6 20.65 F4.5
\ 10.5 13.0 F6.5 F7.8 \ 12.8 19.0 \ 26.2 61.44 \ 15.2 17.8 \ 10.6 20.65 F3.5
1000 1000 360 – 420 900 251 174.7 175.8
G. Xu et al. / Engineering Geology 76 (2004) 129–139
131
Table 2 Formations in the tunnel area Strata Thickness Quantity Percentage of single bed thickness (m) (m) of layers Massive Thick Medium Thin (>2) (2 – 0.5) thick ( < 0.1) (0.5 – 0.1)
Percentage of each rock type (%) Siliceous Calcareous Argillaceous T61 calcaSiltstone Mudstone sandstone silty fine silty fine argillaceous sandstone sandstone siltstone Strength classification (MPa) Extremely Very Sound sound sound (100 – 60) (>150) (150 – 100)
T61 – 3 T61 – 2 T61 – 1 T51 – 3 T51 – 2 T51 – 1 T41 T31 – 2 T31 – 1 T21 T11
34.5 8.00 50.72 29.88 22.92 9.22 62.30 30.21 28.78 33.11 28.87
17 4 69 42 27 13 46 41 55 32 23
71.4 43.75 37.16 30.20 46.13 71.30 31.77 16.54 51.80 62.52
24.72 56.25 46.73 52.40 38.90 72.80 19.35 52.61 63.55 42.59 31.52
3.88 15.14 15.60 12.90 26.70 4.90 15.25 18.55 5.01 5.96
0.97 1.80 2.07 0.50 4.45 0.37 1.36 0.6
the stability of the rock mass of the underground house is negligible.
3. Excavation and support of underground tunnels 3.1. Excavation The underground openings, most of them with a large diameter and a considerable length, are excavated by means of NATM, cooperated with peripheral smooth blasting to avoid overbreak and underbreak and to avoid vibration of the rock mass. The supporting pattern is revised at due time according to the encountered geological conditions to make fully use of bearing capacity of the surrounding rockmass completely (Xu et al., 1985). The benching method has been introduced to excavate the tunnels with a large diameter and a considerable length. For examples, the diversion tunnels, with a face area of 220– 320 m2 were excavated in four steps of five phases (Fig. 3). Major faults, F236, F238 and F240, all intersect with the diversion tunnels, sediment tunnels at an acute angle (Fig. 4). The excavation geological records
4.91 21.78 74.20 19.10 82.03 16.10 47.81
24.15 91.30 15.50 57.57 14.42 31.26 14.96 50.43 39.58 74.33 86.66
3.07 8.70 10.75 17.41 9.38 9.70 1.39 24.94 10.56 23.80 10.92
Fairly sound (60 – 30)
Soft Very (30 – 15) soft (15 – 5) 72.78
6.41
62.43 3.24 2.00 39.94 1.62 8.53 2.05 1.87 2.42
show that where the three major faults passed, joints are extremely developed, and rocks are fractured, filled with argillized materials. Such rocks are graded as IV or V, which can stand itself only for a short time, and are not easily tunneled. Furthermore, because the surrounding rocks are gently dipped and intercalated with argillized layers, and cut by steeply dipped structural discontinuity, the rock at the crown is prone to detach along the bedding plane because of excavation relaxation and blasting vibration, and collapsing might occur in case the tunnel is undersupported. The emulsion explosive and millisecond non-electric blasting cap is applied, with average explosive consumption of 0.6 kg/m3. The holes for peripheral smooth surface blasting have a diameter of 45 mm, a depth of 4.9 m and a spacing of 30 –40 cm. Explosive is loaded in a mode of uncoupling alternate, with a line density of 10 –80 g/m. 3.2. Support Shotcrete is used to support large diameter tunnels such as diversion, sediment discharge and free flow tunnels and underground powerhouse.
132
G. Xu et al. / Engineering Geology 76 (2004) 129–139
Fig. 2. Geological plan in underground tunnel area.
Shotcrete combined with wiremesh and rock bolt to support the tunnels is generally used. In adverse geological conditions, for example, faults or collapse, not only blasting parameters and supporting like shotcrete thickness and mesh layers, need to be adjusted, some other supporting measures, like forepoles, lattice girder, and anchorage cable can also be applied. The guiding principle for excavation and supporting of faulted tunnel sections is short advance, moderating blasting, strong supporting and frequent monitoring, which is detailed in the following. (1) Before blasting, a circle of rock bolts is installed in the periphery of the tunnel to strengthen the surrounding rock in front of the working face.
(2) The advance of each batch blasting is controlled within 2 m. (3) Explosive (TNT) charge is strictly controlled. The holes spacing for perimeter smooth blasting is controlled in a range of 35– 45 cm, the explosive-loading line density is 0.1 – 0.15 kg/ m, and the explosive consumption is below 0.6 kg/m3. Excavation is mostly done by machine to leave the rock as undisturbed as possible. (4) After mucking and clearing the rock face, 5 cm plain shotcrete is immediately used to cover the excavation face. (5) Such supporting systems as double-layer wire mesh, shotcrete and rock bolt is immediately installed for the finished sections of the tunnels. Even lattice girder is applied where necessary, which is used in 270 m faulted section of the
G. Xu et al. / Engineering Geology 76 (2004) 129–139
133
Fig. 3. Benching of diversion tunnels.
diversion tunnels and in 141 m of no. 1 free flow tunnel. The supporting above combined with the rock mass itself forms an arch to provide enough resistance against relaxation deformation.
4. Typical collapses Although supporting measures were applied, collapses took place unavoidably, especially where unfavorable geological conditions were encountered. The excavation of the diversion tunnel started at the end of 1994. During the enlargement excavation in phase 2, 16 times of collapsing occurred from 11th, April 1995 to 4th, April 1996. The volume of each collapse ranged from 10 to 2700 m3. The total collapse volume was more than 8000 m3. Among them, collapse nos. 3, 4 and 8 were larger. Most of the collapses occurred near fault F236, F238 and F240. Small collapses were treated with conventional anchorage support, while large ones were specially treated.
4.1. Collapse no. 3 Collapse no. 3 occurred between Sta. no. 0 + 710 – 0 + 680 of diversion tunnel no. 2, with a collapse volume of 2147 m3 (see Fig. 5). The collapsing, after initial occurrence, was gradually developed and extended up to the ground surface. For the collapsed material is mostly clay and clastic rock blocks, consolidation grouting was carried out from the ground surface and inside the tunnel to concrete the collapsed material, and concrete R28 = 15 MPa was used to fill the cavity from the ground surface. Downstream of the collapse, the ‘‘twin side drift’’ was applied to treat the collapse. After the twin side drift was firstly excavated and supported, the crown arch was excavated and supported. Finally, the central pillar was excavated and the lattice girder inside the drift was dismantled. The advance of a single cycle was 1.0 –2.0 m. Plain shotcrete was used to cover the fresh face, and the forepole and lattice girders were used. Each lattice girder, consisting of three sections, was jointed on the jobsite through bolts and fixed through radial bolts. The
G. Xu et al. / Engineering Geology 76 (2004) 129–139
lattice girders are of ‘‘I’’ shaped steel, with a spacing of 1.0 – 1.2 m. They were longitudinally jointed through a F8 @ 20 cm 20 cm wire mesh. A 15– 25 cm thick shotcrete was gunited to cover the lattice girders. 4.2. Treatment of collapse no. 8
Fig. 4. Longitudinal geological profile of diversion tunnel no. 1.
134
Collapse no. 8 occurred in diversion tunnel no. 1, as high as 10 m, blocked the tunnel completely. Since the collapse cavity was only 40 m away from the ground surface, it was firstly filled with concrete from ground surface through drilling hole. Full-face lattice girders were used to support the tunnel. The advance of a single excavation cycle was 1 – 2 m, and forepoles were installed along the excavation line, and lattice girders were used in a scope of 5 m in front of the collapse to strengthen the tunnel. Then, partial mucking was done to dispose the tunnel crest. In case supporting failed and a cavity was left behind, ‘‘I’’ shaped steel was ribbed, and wire mesh, shotcrete and textile bags were used to completely fill the cavity. Finally, concrete was gradually filled into the cavity through guide hole from the surface. The two sides were firstly excavated and supported, and then collapsed broken bits were excavated with machinery, like backhoe or loader. Rock bolts were installed mainly by human beings through drillings by multiboom drill. Wire meshes were also installed by men through the climbing platform. Crane was used for shotcreting and erecting lattice girders. In general, three methods, conventional shotcrete anchorage, lattice girder combined with concrete filling from the ground surface and twin side drift combined with concrete filling from the ground surface, were used to support the collapses. The first one is used for the small collapse where shotcrete is possible, and the latter two were used to support large collapsing.
5. Deformation monitoring of surrounding rock mass Deformation monitoring, which not only provided deformation of the surrounding rock mass, a basis for adjusting the supporting design data, but also ensured the construction safety, was carried out in the whole
G. Xu et al. / Engineering Geology 76 (2004) 129–139
Fig. 5. Collapse no. 3 of diversion tunnel no. 2.
process of tunneling. Furthermore, post-supporting monitoring was also carried out to monitor the deformation characteristics of the tunnels. 5.1. Deformation monitoring during the enlargement of the diversion tunnel The instruments installed during the enlargement of the diversion tunnel are shown in Table 3. The monitoring results demonstrate that the deformation is non-uniform. In most of the monitoring
135
sections, convergence during enlargement excavation, crown settlement is within 10 mm. Among sections with deformation more than 10 mm, crown subsidence was observed in 68 sections, accounting for 16% of all, and convergence in 37 sections, making up 16.1% of all. There were nine sets of multiple point displacement meters installed, holding 24.3% of all the measured instruments (Fig. 6). Also, there are several sections with deformation of more than 30 mm. In section nos. 0 + 605 and 0 + 610 of diversion tunnel no. 2 and section 0 + 945 of diversion tunnel no. 3, for example, their convergence of sidewall was 69.43, 58.65 and 53.75 mm, respectively. The non-uniform spatial distribution of deformation of the diversion tunnel showed a good correlation to various geologic conditions in different locations of the tunnel. The measured sections with major faults (F236, F238 and F240) usually showed a deformation of more than 10 cm even though they were monitored and reinforced during the construction. Guided by such deformation monitoring and by taking full advantage of the bearing capacity of the surrounding rock mass itself, enlargement excavation and supporting methods were timely adjusted based on the encountered geological condition, the diversion tunnel was completed efficiently and safely.
Table 3 List of monitoring instruments installed in diversion tunnel Items
Tunnel no.
Quantity
Total
Crown settlement
1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3
122 151 156 34 31 58 7 5 7 30 34 23 13 7 17 13 12 5
429
Convergence
Multi-point extensometer Monitoring of rock bolt stress
Optical measurement (3 points) Optical measurement (5 points) Ruler measurement
123
19
87
37
30
229
136
G. Xu et al. / Engineering Geology 76 (2004) 129–139
Fig. 6. Displacement monitoring curve for diversion tunnels. (a) Division tunnel no. 3, (b) division tunnel no. 2, (c) division tunnel no. 1.
The data of deformation show that during the enlargement excavation of the diversion tunnel, the spatial effect of deformation is principal and the time effect is minor. Generally speaking, deformation tends to have no change when the measured point is 20 m away from the working face, which is equivalent to 1.0– 2.0 D. The supporting pattern, in addition to geological condition, contributed to the deformation of the surrounding rock mass. Of measured sections at Sta. no. 0 + 945, 0 + 949, 0 + 960, 0 + 976 in the fault zone of F238, the former two had a convergence deformation of 53.75 and 49.06 mm, respectively, without support of lattice girder, while the latter two had a deformation of 31.04 and 34.81 mm, respectively, with lattice girder, reduced by 40% compared with the former, and most of the
deformation took place before creation of the lattice girder. To measure the effectiveness of double-layer mesh supporting applied in the faulted section of the diversion tunnel, a monitoring section was built up and the result is shown in Fig. 7. In area A, since monitoring points were far from the working face, the deformation measured was little and negligible. In area B, monitoring points were close to the working face, so the deformation increased substantially because of relaxation of enlargement excavation; while in area C, with the double-layer mesh, both crown settlement and convergence deformation showed a stable trend. The upper half circle of the diversion tunnel was successfully excavated by means of the doublelayer mesh support, which demonstrates that the
G. Xu et al. / Engineering Geology 76 (2004) 129–139
137
Fig. 7. Deformation curves of rock around diversion tunnel.
supporting of a fault zone can be done by a completely flexible supporting arch of double-layer mesh during enlargement excavation of the diversion tunnel. 5.2. Deformation monitoring ground powerhouse
of
the
under-
Convergence monitoring began shortly after the excavation of the middle pilot tunnel in the crown of the powerhouse. Multiple point extensometers, single point extensometers and inclinometers, were installed in 29 convergence monitoring sections covering the crown and sidewalls during the construction, as listed in Table 4. The layout of monitoring sections and lines are shown in Fig. 8, and the extensometers and inclinometers installed are shown in Fig. 9. 5.2.1. Deformation of the crown of the power house The monitoring results show that the settlement of the crown was constantly increased with the progress of excavation. Benching method was introduced to excavate the powerhouse. For the crown arch, top heading was firstly excavated, and then enlargement excavation was excavated in two sides of the top heading. When the enlargement excavation was done, the deformation convergence reached the maximum of 30 and 17 mm,
respectively, at Sta. no. 0 + 020 (distance from the entry of a tunnel, 0 in ‘‘km’’ and 020 in ‘‘m’’, the same below) and 0 + 135. The deformation rate of the crown and sidewall evidently dropped down after anchorage cables were installed in the crown, which demonstrated that the installed prestressed anchorage cable contributed to stabilizing the surrounding rock. Most of the measured sections with considerable deformation are at and around Sta. no. 0 + 020 and 0 + 135, where joints are developed. Comparing the convergence with the excavation process, it is found that the increase of convergence is mainly due to the spatial effect of excavation but not the time effect, which indicates that the surrounding rock has a good stability. Nine multi-point extensometers were installed in three sections in the crown of the main powerhouse. Two years (starting from 1995) of monitoring showed that the displacement was very small. The displacement was relatively uniform within a depth of 10 m, and not more than 3 mm and averaged about 1.39 mm at the monitoring hole surface. When excavation reached EL.152 m, the downward deformation rate was slowed down, with a deformation of less than 3 mm, and tended to stop when excavation reached EL.146 m. As excavation continued, due to the deformation increase of the sidewall towards the free face, the rebound deformation of the crown got more than its settlement, causing the surrounding
138
G. Xu et al. / Engineering Geology 76 (2004) 129–139
Table 4 Convergence monitoring sections of the power house Monitoring line
Quantity of sections
Sta. no.
2 – 3, 1 – 3, 1 – 2
2
4 – 5, 1 – 5, 1 – 4
9
6–7 8–9
1 9
2 – 3, 1 – 3, 1 – 2
8
0 + 020, 0 + 037 0 + 020, 0 + 037, 0 + 099, 0 + 124, 0 + 100 0 + 020, 0 + 073, 0 + 100, 0 + 150, 0 + 022, 0 + 050, 0 + 030, 0 + 012,
0 + 023, 0 + 073 0 + 100, 0 + 135, 0 + 247 0 + 037, 0 + 240 0 + 124, 0 + 175, 0 + 215 0 + 035, 0 + 140 0 + 067, 0 + 026
rock mass to displace upwards. In a word, displacement increasing mainly occurred in the initial period of the excavation, and the displacement rate tended to zero or was just zero (in 1997), which means that the crown of the powerhouse is stable. 5.2.2. Deformation of sidewalls of the power house Fifteen multi-point extensometers were installed in three sections of the sidewalls of the powerhouse. With excavating downward, especially after the last two benches were excavated, the deformation rate of the sidewall distinctly accelerated, but soon tended to a stable state. The maximum convergence measured was 23.15 mm, and the relative convergence was 0.103%, which was smaller than the value of 0.2 – 0.5% for grade III rock mass specified in relative Chinese standard. The average deformation was also smaller than the specified 0.15 mm/day. Fifteen multipoint extensometers installed in three sections of the sidewall showed that the sidewalls between EL.147 –154 m had various displacements towards the free face, which was within 5 mm, averaged 2.94 mm at the surface of the monitoring holes. The displacement was about the same from the monitoring surface to a depth of 4 m, showing that the surrounding rock behaved as a whole-reinforced shell. The gradient of the displacement curve was larger from 4 to 9 m of the holes.
6. Conclusions
(1) Most of rock masses involved in the underground tunnels of Xiaolangdi are multi-fractured and stratified, falling into grades II and III in terms of the classification generally accepted in the water conservancy and hydroelectric field in China, in which large size tunnel can be excavated. (2) Since the two major faults, F236 and F238, intersected with the tunnels at an acute angle and the rock mass cut by them was fallen into grades IV and V in terms of the classification, strong supporting was applied. (3) The underground openings, most of them with a large diameter and a considerable length, are excavated by means of NATM, cooperated with peripheral smooth blasting to avoid overbreak and underbreak and to avoid vibration of the rock mass. The supporting pattern is revised at due time according to deformation monitoring and the
Fig. 8. A convergence monitoring section of the power house.
G. Xu et al. / Engineering Geology 76 (2004) 129–139
139
Fig. 9. Monitoring instruments installed in the power house.
encountered geological conditions to ensure safe and efficient construction of the underground openings. (4) In faulted tunnel section, short headway, moderate blasting, strong supporting and frequent monitoring are guiding principle and has been proved effective.
Reference Xu, F., Gao, G., Wu, Y., 1985. The monitoring of surrounding rock stability and supporting structure in a large span experimental tunnel of Xiaolangdi Project, Yellow River. Water Conservancy Journal October, 39 – 47 (in Chinese).
Underground excavation in Xiaolangdi project in Yellow River Guogang Xu *, Guoyan Ma, Qingbo Li , Zhifang Cui The Reconnaissance, Plan, Design and Research Institute, YRCC, MWR, Beijing, PR China Received 3 December 2002; accepted 7 June 2004 Available online
Abstract The paper summarizes the excavation methodology, based on analysis of the characteristics of the surrounding rock mass, which has been involved in Xiaolangdi multipurpose dam project. Some special support and excavation methods, such as twin side drift and lattice girder, moderate blasting and double-layer mesh to collapsed tunnel and fault zone, are presented. D 2004 Published by Elsevier B.V. Keywords: Underground tunnels; Stability of surrounding rock mass; Excavation; Deformation monitoring
1. Introduction Xiaolangdi Multipurpose Dam Project, located at the outlet of the last gorge, consists of main dam, complicated tunnels system and power system. It is one of the most challenging projects in the world due to its special geographical locality, large scale and complicated geological condition. The reservoir, with a capacity of 12.65 billion m3 at the elevation of 275 m, accounting 92.3% of the Yellow River catchment basin, extends to the dam toe of Sanmenxia, another dam 130 km upstream of Xiaolangdi in the middle reaches of the Yellow River. The downstream area is the Huang-Huai-Hai plain (see Fig. 1). As one of the strategic project, its functions include flood control, ice flood control, sediment
settling, water supply, irrigation and power generating, etc. Considering high silt content of the Yellow River and the engineering geological condition at the dam area, the flood and sediment discharge facility and the power system are concentrated on the left bank to prevent silt from accumulation at the water intake and outlet. Over 100 underground tunnels and caves form a complicated and closely distributed system, including three diversion tunnels, three sediment tunnels and three free flow tunnels, six power-generating tunnels (Table 1). This is the most densely distributed tunnel and cave system in the world, involving an excavation volume of 2.7 million m3. Valuable experience has been gained in excavation of underground tunnels.
2. Geotechnical conditions * Corresponding author. Tel.: +86-371-6024947; fax: +86-3715959236. E-mail address: [email protected] (G. Xu). 0013-7952/$ - see front matter D 2004 Published by Elsevier B.V. doi:10.1016/j.enggeo.2004.06.010
Rocks on the left bank are mainly lower Triassic purplish red, thick and medium bedded or mega-
130
G. Xu et al. / Engineering Geology 76 (2004) 129–139
Fig. 1. Location plan of the Xiaolangdi project.
bedded calca-siliceous or calcareous sandstone and calca-argillaceous, or argillaceous siltstone intercalated with thin-bedded claystone, with a total thickness of 350 m. Based on the characteristics, the rocks in the tunnel area were classified into six subgroups, namely T11, T12, T13, T14, T15 and T16 (shown in Table 2). The rock exposed at the dam site dips to the east at an angle of 8– 12j. Several faults, F28, F236 and F238 have been found in the intake area. F28 is in front of the intake tower of the diversion and flood tunnels and has no effect on the underground buildings. F236 and F238, sub-parallel faults, with wide broken zones, intersect with tunnels at acute angle. These two faults, together with the branch faults and fracture rocks,
compose the adverse condition to the stability of the tunnels (see Fig. 2). Argillized intercalation, another discontinuity, is found distributed along the bedding plane and repeatedly occurs in the direction vertical to the bedding plane, but discontinuous or thinned out along the bedding plane. Because of its high clay content, low shear strength and swelling capacity, the argillized intercalation has been suggested to form a weak sliding plane or a detached plane, which will affect the stability of rock mass. According to the rock mass classification used in water conservancy and hydropower field in China, the rock masses in the tunnel area are generally categorized into grades IV and V, some place dissected by the main faults fall into grades IV or V. The in-site stress in the Xiaolangdi project is measured by stress release method and the hydraulic-fracturing method, supplemented with the Kaiser method and fabric analysis. Regression analysis of the in situ stress field has been carried out on the basis of the measurement result. It is concluded that, in the tunnel area, the self-gravity is dominant and the tectonic stress is minor. The lateral compression coefficient is 0.7– 0.9. The maximum horizontal in situ stress is 5 MPa, directing to 20jNE and making an angle of 30j with the axis of the underground powerhouse and an angle of 83j with the axis of the flood discharge tunnels. The stress – strength ratio of the surrounding rock is S>4. The results show that the influence of in situ stress upon
Table 1 Data of underground openings Item
Number
Typical cross-section (m)
Typical lining section (m)
Length for each (m)
Diversion (orifice) tunnel
3
F14.5
1100
Free flow tunnel Sediment tunnel Power tunnel Tailrace tunnel Power house Transformer chamber Tailrace gate chamber Irrigation tunnel
3 3 6 3 1 1 1 1
F18.5 – 23.0 (mid-gate chamber: 23.4 22.0) \ 13.7 16.2 F8.3 F9.6 \ 12.8 19 \ 26.2 61.44 \ 15.2 17.8 \ 10.6 20.65 F4.5
\ 10.5 13.0 F6.5 F7.8 \ 12.8 19.0 \ 26.2 61.44 \ 15.2 17.8 \ 10.6 20.65 F3.5
1000 1000 360 – 420 900 251 174.7 175.8
G. Xu et al. / Engineering Geology 76 (2004) 129–139
131
Table 2 Formations in the tunnel area Strata Thickness Quantity Percentage of single bed thickness (m) (m) of layers Massive Thick Medium Thin (>2) (2 – 0.5) thick ( < 0.1) (0.5 – 0.1)
Percentage of each rock type (%) Siliceous Calcareous Argillaceous T61 calcaSiltstone Mudstone sandstone silty fine silty fine argillaceous sandstone sandstone siltstone Strength classification (MPa) Extremely Very Sound sound sound (100 – 60) (>150) (150 – 100)
T61 – 3 T61 – 2 T61 – 1 T51 – 3 T51 – 2 T51 – 1 T41 T31 – 2 T31 – 1 T21 T11
34.5 8.00 50.72 29.88 22.92 9.22 62.30 30.21 28.78 33.11 28.87
17 4 69 42 27 13 46 41 55 32 23
71.4 43.75 37.16 30.20 46.13 71.30 31.77 16.54 51.80 62.52
24.72 56.25 46.73 52.40 38.90 72.80 19.35 52.61 63.55 42.59 31.52
3.88 15.14 15.60 12.90 26.70 4.90 15.25 18.55 5.01 5.96
0.97 1.80 2.07 0.50 4.45 0.37 1.36 0.6
the stability of the rock mass of the underground house is negligible.
3. Excavation and support of underground tunnels 3.1. Excavation The underground openings, most of them with a large diameter and a considerable length, are excavated by means of NATM, cooperated with peripheral smooth blasting to avoid overbreak and underbreak and to avoid vibration of the rock mass. The supporting pattern is revised at due time according to the encountered geological conditions to make fully use of bearing capacity of the surrounding rockmass completely (Xu et al., 1985). The benching method has been introduced to excavate the tunnels with a large diameter and a considerable length. For examples, the diversion tunnels, with a face area of 220– 320 m2 were excavated in four steps of five phases (Fig. 3). Major faults, F236, F238 and F240, all intersect with the diversion tunnels, sediment tunnels at an acute angle (Fig. 4). The excavation geological records
4.91 21.78 74.20 19.10 82.03 16.10 47.81
24.15 91.30 15.50 57.57 14.42 31.26 14.96 50.43 39.58 74.33 86.66
3.07 8.70 10.75 17.41 9.38 9.70 1.39 24.94 10.56 23.80 10.92
Fairly sound (60 – 30)
Soft Very (30 – 15) soft (15 – 5) 72.78
6.41
62.43 3.24 2.00 39.94 1.62 8.53 2.05 1.87 2.42
show that where the three major faults passed, joints are extremely developed, and rocks are fractured, filled with argillized materials. Such rocks are graded as IV or V, which can stand itself only for a short time, and are not easily tunneled. Furthermore, because the surrounding rocks are gently dipped and intercalated with argillized layers, and cut by steeply dipped structural discontinuity, the rock at the crown is prone to detach along the bedding plane because of excavation relaxation and blasting vibration, and collapsing might occur in case the tunnel is undersupported. The emulsion explosive and millisecond non-electric blasting cap is applied, with average explosive consumption of 0.6 kg/m3. The holes for peripheral smooth surface blasting have a diameter of 45 mm, a depth of 4.9 m and a spacing of 30 –40 cm. Explosive is loaded in a mode of uncoupling alternate, with a line density of 10 –80 g/m. 3.2. Support Shotcrete is used to support large diameter tunnels such as diversion, sediment discharge and free flow tunnels and underground powerhouse.
132
G. Xu et al. / Engineering Geology 76 (2004) 129–139
Fig. 2. Geological plan in underground tunnel area.
Shotcrete combined with wiremesh and rock bolt to support the tunnels is generally used. In adverse geological conditions, for example, faults or collapse, not only blasting parameters and supporting like shotcrete thickness and mesh layers, need to be adjusted, some other supporting measures, like forepoles, lattice girder, and anchorage cable can also be applied. The guiding principle for excavation and supporting of faulted tunnel sections is short advance, moderating blasting, strong supporting and frequent monitoring, which is detailed in the following. (1) Before blasting, a circle of rock bolts is installed in the periphery of the tunnel to strengthen the surrounding rock in front of the working face.
(2) The advance of each batch blasting is controlled within 2 m. (3) Explosive (TNT) charge is strictly controlled. The holes spacing for perimeter smooth blasting is controlled in a range of 35– 45 cm, the explosive-loading line density is 0.1 – 0.15 kg/ m, and the explosive consumption is below 0.6 kg/m3. Excavation is mostly done by machine to leave the rock as undisturbed as possible. (4) After mucking and clearing the rock face, 5 cm plain shotcrete is immediately used to cover the excavation face. (5) Such supporting systems as double-layer wire mesh, shotcrete and rock bolt is immediately installed for the finished sections of the tunnels. Even lattice girder is applied where necessary, which is used in 270 m faulted section of the
G. Xu et al. / Engineering Geology 76 (2004) 129–139
133
Fig. 3. Benching of diversion tunnels.
diversion tunnels and in 141 m of no. 1 free flow tunnel. The supporting above combined with the rock mass itself forms an arch to provide enough resistance against relaxation deformation.
4. Typical collapses Although supporting measures were applied, collapses took place unavoidably, especially where unfavorable geological conditions were encountered. The excavation of the diversion tunnel started at the end of 1994. During the enlargement excavation in phase 2, 16 times of collapsing occurred from 11th, April 1995 to 4th, April 1996. The volume of each collapse ranged from 10 to 2700 m3. The total collapse volume was more than 8000 m3. Among them, collapse nos. 3, 4 and 8 were larger. Most of the collapses occurred near fault F236, F238 and F240. Small collapses were treated with conventional anchorage support, while large ones were specially treated.
4.1. Collapse no. 3 Collapse no. 3 occurred between Sta. no. 0 + 710 – 0 + 680 of diversion tunnel no. 2, with a collapse volume of 2147 m3 (see Fig. 5). The collapsing, after initial occurrence, was gradually developed and extended up to the ground surface. For the collapsed material is mostly clay and clastic rock blocks, consolidation grouting was carried out from the ground surface and inside the tunnel to concrete the collapsed material, and concrete R28 = 15 MPa was used to fill the cavity from the ground surface. Downstream of the collapse, the ‘‘twin side drift’’ was applied to treat the collapse. After the twin side drift was firstly excavated and supported, the crown arch was excavated and supported. Finally, the central pillar was excavated and the lattice girder inside the drift was dismantled. The advance of a single cycle was 1.0 –2.0 m. Plain shotcrete was used to cover the fresh face, and the forepole and lattice girders were used. Each lattice girder, consisting of three sections, was jointed on the jobsite through bolts and fixed through radial bolts. The
G. Xu et al. / Engineering Geology 76 (2004) 129–139
lattice girders are of ‘‘I’’ shaped steel, with a spacing of 1.0 – 1.2 m. They were longitudinally jointed through a F8 @ 20 cm 20 cm wire mesh. A 15– 25 cm thick shotcrete was gunited to cover the lattice girders. 4.2. Treatment of collapse no. 8
Fig. 4. Longitudinal geological profile of diversion tunnel no. 1.
134
Collapse no. 8 occurred in diversion tunnel no. 1, as high as 10 m, blocked the tunnel completely. Since the collapse cavity was only 40 m away from the ground surface, it was firstly filled with concrete from ground surface through drilling hole. Full-face lattice girders were used to support the tunnel. The advance of a single excavation cycle was 1 – 2 m, and forepoles were installed along the excavation line, and lattice girders were used in a scope of 5 m in front of the collapse to strengthen the tunnel. Then, partial mucking was done to dispose the tunnel crest. In case supporting failed and a cavity was left behind, ‘‘I’’ shaped steel was ribbed, and wire mesh, shotcrete and textile bags were used to completely fill the cavity. Finally, concrete was gradually filled into the cavity through guide hole from the surface. The two sides were firstly excavated and supported, and then collapsed broken bits were excavated with machinery, like backhoe or loader. Rock bolts were installed mainly by human beings through drillings by multiboom drill. Wire meshes were also installed by men through the climbing platform. Crane was used for shotcreting and erecting lattice girders. In general, three methods, conventional shotcrete anchorage, lattice girder combined with concrete filling from the ground surface and twin side drift combined with concrete filling from the ground surface, were used to support the collapses. The first one is used for the small collapse where shotcrete is possible, and the latter two were used to support large collapsing.
5. Deformation monitoring of surrounding rock mass Deformation monitoring, which not only provided deformation of the surrounding rock mass, a basis for adjusting the supporting design data, but also ensured the construction safety, was carried out in the whole
G. Xu et al. / Engineering Geology 76 (2004) 129–139
Fig. 5. Collapse no. 3 of diversion tunnel no. 2.
process of tunneling. Furthermore, post-supporting monitoring was also carried out to monitor the deformation characteristics of the tunnels. 5.1. Deformation monitoring during the enlargement of the diversion tunnel The instruments installed during the enlargement of the diversion tunnel are shown in Table 3. The monitoring results demonstrate that the deformation is non-uniform. In most of the monitoring
135
sections, convergence during enlargement excavation, crown settlement is within 10 mm. Among sections with deformation more than 10 mm, crown subsidence was observed in 68 sections, accounting for 16% of all, and convergence in 37 sections, making up 16.1% of all. There were nine sets of multiple point displacement meters installed, holding 24.3% of all the measured instruments (Fig. 6). Also, there are several sections with deformation of more than 30 mm. In section nos. 0 + 605 and 0 + 610 of diversion tunnel no. 2 and section 0 + 945 of diversion tunnel no. 3, for example, their convergence of sidewall was 69.43, 58.65 and 53.75 mm, respectively. The non-uniform spatial distribution of deformation of the diversion tunnel showed a good correlation to various geologic conditions in different locations of the tunnel. The measured sections with major faults (F236, F238 and F240) usually showed a deformation of more than 10 cm even though they were monitored and reinforced during the construction. Guided by such deformation monitoring and by taking full advantage of the bearing capacity of the surrounding rock mass itself, enlargement excavation and supporting methods were timely adjusted based on the encountered geological condition, the diversion tunnel was completed efficiently and safely.
Table 3 List of monitoring instruments installed in diversion tunnel Items
Tunnel no.
Quantity
Total
Crown settlement
1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3
122 151 156 34 31 58 7 5 7 30 34 23 13 7 17 13 12 5
429
Convergence
Multi-point extensometer Monitoring of rock bolt stress
Optical measurement (3 points) Optical measurement (5 points) Ruler measurement
123
19
87
37
30
229
136
G. Xu et al. / Engineering Geology 76 (2004) 129–139
Fig. 6. Displacement monitoring curve for diversion tunnels. (a) Division tunnel no. 3, (b) division tunnel no. 2, (c) division tunnel no. 1.
The data of deformation show that during the enlargement excavation of the diversion tunnel, the spatial effect of deformation is principal and the time effect is minor. Generally speaking, deformation tends to have no change when the measured point is 20 m away from the working face, which is equivalent to 1.0– 2.0 D. The supporting pattern, in addition to geological condition, contributed to the deformation of the surrounding rock mass. Of measured sections at Sta. no. 0 + 945, 0 + 949, 0 + 960, 0 + 976 in the fault zone of F238, the former two had a convergence deformation of 53.75 and 49.06 mm, respectively, without support of lattice girder, while the latter two had a deformation of 31.04 and 34.81 mm, respectively, with lattice girder, reduced by 40% compared with the former, and most of the
deformation took place before creation of the lattice girder. To measure the effectiveness of double-layer mesh supporting applied in the faulted section of the diversion tunnel, a monitoring section was built up and the result is shown in Fig. 7. In area A, since monitoring points were far from the working face, the deformation measured was little and negligible. In area B, monitoring points were close to the working face, so the deformation increased substantially because of relaxation of enlargement excavation; while in area C, with the double-layer mesh, both crown settlement and convergence deformation showed a stable trend. The upper half circle of the diversion tunnel was successfully excavated by means of the doublelayer mesh support, which demonstrates that the
G. Xu et al. / Engineering Geology 76 (2004) 129–139
137
Fig. 7. Deformation curves of rock around diversion tunnel.
supporting of a fault zone can be done by a completely flexible supporting arch of double-layer mesh during enlargement excavation of the diversion tunnel. 5.2. Deformation monitoring ground powerhouse
of
the
under-
Convergence monitoring began shortly after the excavation of the middle pilot tunnel in the crown of the powerhouse. Multiple point extensometers, single point extensometers and inclinometers, were installed in 29 convergence monitoring sections covering the crown and sidewalls during the construction, as listed in Table 4. The layout of monitoring sections and lines are shown in Fig. 8, and the extensometers and inclinometers installed are shown in Fig. 9. 5.2.1. Deformation of the crown of the power house The monitoring results show that the settlement of the crown was constantly increased with the progress of excavation. Benching method was introduced to excavate the powerhouse. For the crown arch, top heading was firstly excavated, and then enlargement excavation was excavated in two sides of the top heading. When the enlargement excavation was done, the deformation convergence reached the maximum of 30 and 17 mm,
respectively, at Sta. no. 0 + 020 (distance from the entry of a tunnel, 0 in ‘‘km’’ and 020 in ‘‘m’’, the same below) and 0 + 135. The deformation rate of the crown and sidewall evidently dropped down after anchorage cables were installed in the crown, which demonstrated that the installed prestressed anchorage cable contributed to stabilizing the surrounding rock. Most of the measured sections with considerable deformation are at and around Sta. no. 0 + 020 and 0 + 135, where joints are developed. Comparing the convergence with the excavation process, it is found that the increase of convergence is mainly due to the spatial effect of excavation but not the time effect, which indicates that the surrounding rock has a good stability. Nine multi-point extensometers were installed in three sections in the crown of the main powerhouse. Two years (starting from 1995) of monitoring showed that the displacement was very small. The displacement was relatively uniform within a depth of 10 m, and not more than 3 mm and averaged about 1.39 mm at the monitoring hole surface. When excavation reached EL.152 m, the downward deformation rate was slowed down, with a deformation of less than 3 mm, and tended to stop when excavation reached EL.146 m. As excavation continued, due to the deformation increase of the sidewall towards the free face, the rebound deformation of the crown got more than its settlement, causing the surrounding
138
G. Xu et al. / Engineering Geology 76 (2004) 129–139
Table 4 Convergence monitoring sections of the power house Monitoring line
Quantity of sections
Sta. no.
2 – 3, 1 – 3, 1 – 2
2
4 – 5, 1 – 5, 1 – 4
9
6–7 8–9
1 9
2 – 3, 1 – 3, 1 – 2
8
0 + 020, 0 + 037 0 + 020, 0 + 037, 0 + 099, 0 + 124, 0 + 100 0 + 020, 0 + 073, 0 + 100, 0 + 150, 0 + 022, 0 + 050, 0 + 030, 0 + 012,
0 + 023, 0 + 073 0 + 100, 0 + 135, 0 + 247 0 + 037, 0 + 240 0 + 124, 0 + 175, 0 + 215 0 + 035, 0 + 140 0 + 067, 0 + 026
rock mass to displace upwards. In a word, displacement increasing mainly occurred in the initial period of the excavation, and the displacement rate tended to zero or was just zero (in 1997), which means that the crown of the powerhouse is stable. 5.2.2. Deformation of sidewalls of the power house Fifteen multi-point extensometers were installed in three sections of the sidewalls of the powerhouse. With excavating downward, especially after the last two benches were excavated, the deformation rate of the sidewall distinctly accelerated, but soon tended to a stable state. The maximum convergence measured was 23.15 mm, and the relative convergence was 0.103%, which was smaller than the value of 0.2 – 0.5% for grade III rock mass specified in relative Chinese standard. The average deformation was also smaller than the specified 0.15 mm/day. Fifteen multipoint extensometers installed in three sections of the sidewall showed that the sidewalls between EL.147 –154 m had various displacements towards the free face, which was within 5 mm, averaged 2.94 mm at the surface of the monitoring holes. The displacement was about the same from the monitoring surface to a depth of 4 m, showing that the surrounding rock behaved as a whole-reinforced shell. The gradient of the displacement curve was larger from 4 to 9 m of the holes.
6. Conclusions
(1) Most of rock masses involved in the underground tunnels of Xiaolangdi are multi-fractured and stratified, falling into grades II and III in terms of the classification generally accepted in the water conservancy and hydroelectric field in China, in which large size tunnel can be excavated. (2) Since the two major faults, F236 and F238, intersected with the tunnels at an acute angle and the rock mass cut by them was fallen into grades IV and V in terms of the classification, strong supporting was applied. (3) The underground openings, most of them with a large diameter and a considerable length, are excavated by means of NATM, cooperated with peripheral smooth blasting to avoid overbreak and underbreak and to avoid vibration of the rock mass. The supporting pattern is revised at due time according to deformation monitoring and the
Fig. 8. A convergence monitoring section of the power house.
G. Xu et al. / Engineering Geology 76 (2004) 129–139
139
Fig. 9. Monitoring instruments installed in the power house.
encountered geological conditions to ensure safe and efficient construction of the underground openings. (4) In faulted tunnel section, short headway, moderate blasting, strong supporting and frequent monitoring are guiding principle and has been proved effective.
Reference Xu, F., Gao, G., Wu, Y., 1985. The monitoring of surrounding rock stability and supporting structure in a large span experimental tunnel of Xiaolangdi Project, Yellow River. Water Conservancy Journal October, 39 – 47 (in Chinese).