Strata consolidation subsidence induced by metro tunneling in saturated soft clay strata

Journal of Modern Transportation, Mar 2011

To choose the optimum construction method of metro tunneling, we conducted research with numerical simulation on strata consolidation subsidence by dewatering, dynamic dewatering, and non-dewatering construction method, taking the integrated effects of fluid-solid coupling and tunneling mechanics into account. We obtained the curved surfaces of ground surface subsidence and strata consolidation subsidence. The results show that the quantity of ground surface subsidence is 31 mm for the non-dewatering method, 39 mm for the dynamic dewatering method, and 105 mm for the dewatering method. Their ratio is 1:1.3:3.4; and the percentages of strata consolidation subsidence to whole ground surface subsidence of each construction method is 27% (no-dewatering), 50% (dynamic dewatering), and 79% (dewatering). It is obvious that the non-dewatering construction method is the most effective method to control the strata consolidation subsidence induced by metro tunneling in saturated soft clay strata, and it has been successfully applied to the construction of the Shenzhen metro line 1.

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Strata consolidation subsidence induced by metro tunneling in saturated soft clay strata

Journal of Modern Transportation Strata consolidation subsidence induced by metro tunneling in saturated soft clay strata Taiyue QI 0 Bo GAO 0 0 School of Civil Engineering, Southwest Jiaotong University , Chengdu 610031 , China To choose the optimum construction method of metro tunneling, we conducted research with numerical simulation on strata consolidation subsidence by dewatering, dynamic dewatering, and non-dewatering construction method, taking the integrated effects of fluid-solid coupling and tunneling mechanics into account. We obtained the curved surfaces of ground surface subsidence and strata consolidation subsidence. The results show that the quantity of ground surface subsidence is 31 mm for the non-dewatering method, 39 mm for the dynamic dewatering method, and 105 mm for the dewatering method. Their ratio is 1:1.3:3.4; and the percentages of strata consolidation subsidence to whole ground surface subsidence of each construction method is 27% (no-dewatering), 50% (dynamic dewatering), and 79% (dewatering). It is obvious that the non-dewatering construction method is the most effective method to control the strata consolidation subsidence induced by metro tunneling in saturated soft clay strata, and it has been successfully applied to the construction of the Shenzhen metro line 1. - 1. Introduction E sidence, which is composed by two parts, namely xcavation of a subway leads to ground surface subground loss subsidence and strata consolidation subsidence. In watery and soft soil strata, the subway tunnel excavation will result in the loss of underground water, destroying the intrinsic balance of water-earth pressure, and causing the re-consolidation of strata, which will lead to the overall settlement of strata. In watery and soft soil strata, studies have demonstrated that strata consolidation settlement caused by subway tunnel excavation accounts for a large proportion of the whole ground surface settlement, such that strata consolidation subsidence is a problem that cannot be ignored [ 1-3 ]. Fluid-solid coupling problem has always been a hot issue [ 4-10 ]. Specifically considering the ground surface subsidence problem caused by construction of a metro tunnel, we can obtain a numerical analysis conclusion which more exactly agrees with the actual situation if fluid-solid coupling effect and construction mechanics effect are considered simultaneously [ 2,7-8,11 ]. In this paper, ground surface subsidence caused by dewatering, dynamic dewatering and non-dewatering are studied and compared. Dewatering method involves the dewatering well being laid at two sides of the tunnel to pump ground water before the tunnel excavation. When ground water drops below the elevation of the tunnel floor, the anhydrous construction environment is formed in the tunnel face, making the stratum disclosed by the tunnel face stable. As a result, the working environment is improved to make ensure safe construction. Dynamic dewatering method involves local dewatering treatment, performed in the tunnel face, with advance strengthening carried out. Dewatering points will move along with tunnel face advancement, making the water table drop only in the tunnel face. Subsequently, the water table will return to its original position to minimize the disturbance of strata caused by dewatering. The non-dewatering method involves horizontal rotary jet grouting piles to reinforce the soil around and in front of the tunnel face in order to form an annular waterproof wall (Fig. 1). Then the tunnel is excavated. An advance reinforcement cycle formed by the non-dewatering method not only prevents ground water from seeping from strata into the tunnel, it also reinforces strata around and in front of the tunnel face. As a result, mechanical of the tunnel face and perimeter are improved. DOI: 10.1007/BF033257G33routing pipes Rotary jet grouting piles 3 m 2. Numerical model 2.1. Engineering geology and hydrological conditions This paper takes the running tunnel engineering of Da juyuan -Kejiguan in Sh enzh en a s an exam pl e, studying the ground surface subsidence problem caused by construction of a metro tunnel in watery strata. Char a ct er i sti cs of t h e en gin eer in g geol og y a n d hydrological conditions in the Shenzhen metro line 1 are watery and soft soil strata, high groundwater (groundwater level is at -2 m, and the level of ground surface is 0 m), poor engineering geological conditions, high control of ground surface subsidence, and also some areas of tunnel being invaded by a thick sand layer with a confined aquifer. The columnar section and running tunnel layout is shown in Fig. 2. The area of the tunnel that is invaded by a 3 m thick sand layer with a confined aquifer, the most dangerous area of this tunnel construction, is also shown in Fig. 2. 2.2. Model parameters In the model, we assume strata are strain-softening materials and the reinforced cycle of horizontal rotary jet grouting piles is mohr-coulomb material. The details are as shown in Table 1. The seepage parameter of the reinforced cycle of the horizontal rotary-jet grouting piles is shown in Table 2. Pump well Ground surface 5 6 2 Plain fill Gravel cohesive soil Sand layer Fully weathered granite Strongly weathered granite Moderately weathered granite Weakly weathered granite Sequence number 1 2 3 4 5 6 7 Name of strata Plain fill Gravel cohesive soil Sand layer Fully weathered granite Strong weathered granite Moderately weathered granite Weakly weathered granite Horizontal rotary jet grouting piles 8.9 10.9 20 13.9 30.4 83.3 194 Table 2 Seepage properties of strata and horizontal rotary jet grouting piles [ 1,12-16 ] DOI: 10.1007/BF03325733 Strata Sand layer Chemical churning pile Ground water 2.3. Model size and scale Model size is: width×thickness×highness = X×Y×Z 100×50×40 m3. The tunnel section is horseshoe shaped with a width of 6.3 m and a height of 6.6 m. The distance between the right and left tunnels is 14.6 m, the distance from the left (right) line tunnel to model boundary is 36.4 m, the distance from the tunnel vault to ground surface is 15.2 m, and the distance between tunnel floor and model floor is 18.2 m. The thickness of horizontal rotary-jet grouting pile is 2 m, the prereinforcement length is 6–16 m, the distance between dewatering well and the left (or right) line tunnel boundary is 5 m, the distance between dewatering wells is 10 m, and their height is 26 m. The model is made up of 45 150 solid elements (see Fig. 3(a)). A shell element is used to simulate the primary support, and its parameters are shown in Table 3. In the process of construction engineering, the short bench cut method is used to excavate the left and right line tunnels and the length of bench is 4 m. The simulation figure is shown in Fig. 3(b). 3. Advance distance between left and right line tunnels Simulation results show that when the left and right tunnels are excavated along the left and right lines synchronously (namely, advanced distance is 0 m), the stresses around the left and right lines superpose with each other. As a result, the effect on each other is maximal, which is the most unfavorable. When the right line exceeds the left line by 16 m or more, the effect caused by construction stress is not quite clear. Therefore, the starting position of excavation of the right line exceeding that of left line by 16 m is recommended. 4. Deformation of water table and difference of ground surface subsidence According to the research results above, the excavating and driving mode of the right line exceeding the left line by 16 m is adopted. The upper bench of the right line is at the position of y=34 m, and the upper bench of (a) Horizontal section for dewatering method (y=14 m) (b) Vertical section for dewatering method (x=57.3 m) (c) Horizontal section for dynamic dewatering method (y=14 m) (d) Vertical section for dynamic dewatering method (x=57.3 m) (e) Horizontal section for nondewatering method (y=14 m) (f) Vertical section for non-dewatering method (x=57.3 m) left line is at y=18 m. The length of the bench is 4 m. Fig. 4 shows the distribution of pore water pressure for dewatering, dynamic dewatering, and nondewatering methods. Fig. 5 and Fig. 6 show the curved surface of ground surface subsidence and strata consolidation subsidence for the three methods. From Fig. 4 to Fig. 6, we see that: There are great differences of position at 0 MPa of pore water pressure for the three methods (Fig. 4). Ground water drops below the elevation of the tunnel floor as a result of dewatering. The whole water level dropped between from -8 to -26 m. Pore water pressure on the floor of tunnel face dropped to the position of -6 m. Among these three construction methods, dewatering results in the greatest descent of the ground water level, and non-dewatering caused the least. There are great differences among these curved surfaces of ground surface subsidence caused by the three methods. The maximum deformation of the ground surface subsidence occurred at different locations for the three methods (Fig. 5). In the case of the dewatering method, the curved surfaces of ground surface subsidence are quite symmetrical, and the maximum deformation of the ground surface subsidence is located at the position of x=54.476 2 m and y=6 m, while the settlement is the largest, up to -105 mm. In the case of dynamic dewatering, the curved surfaces of ground surface subsidence have some relation with local dewatering in the tunnel face because the maximum deformation of the ground surface subsidence occurred in the tunnel face, up to -39 mm, located at the position of x=40.468 m and y=6 m (8 m behind the left line tunnel face). In the case of the non-dewatering method, the curved surfaces of ground surface subsidence are also symmetrical. This method can control the loss of ground water quite readily, and its ground surface subsidence is the least. The maximum deformation of the ground surface subsidence is located at the position of x=40.468 m and y=6 m, up to -31 mm. The ratio of the maximum deformation of ground surface subsidence for dewatering, dynamic dewatering, and non-dewatering methods is about 1:1.3:3.4. There are great differences among the ground surface subsidence tanks for three construction methods (Fig. 6). The ground surface subsidence tank in the case of DdeOwI:a1te0r.i1n0g07i/sBtFh0e33sa2m57e33as the classical subsidence tank, but the other two ground surface subsidence tanks are not. It is shown that the shape of subsidence tank can be changed by the different construction methods. The largest absolute subsidence, the maximum relative subsidence, and the width of the subsidence tank for these three construction methods are, respectively, as follows: 105 mm, 59 mm, and 90 m for dewatering, 39 mm, 22 mm, and 70 m for dynamic-dewatering, and 31 mm, 17 mm, and 70 m for non-dewatering (Fig. 6). Measured curves of ground surface subsidence for dewatering and non-dewatering methods are, respectively, shown in Fig. 7 and Fig. 8. Comparing Fig. 6, Fig. 7, and Fig. 8, conclusions can be drawn as follows: (1) In the case of the non-dewatering method, the measured results in practice approximately agree with the numerical simulation results (Compared Fig. 6 with Fig. 8). In practice, after the non-dewatering method is adopted, groundwater which leaks into the tunnel is clear, and the inflow of water decreases significantly. (2) Therefore, surface subsidence was effectively controlled after the non-dewatering method was adopted. To sum up, the non-dewatering construction method is the most effective method for the control of strata consolidation subsidence induced by metro tunneling in saturated soft clay strata. The dewatering method was used in the Shenzhen metro line 1 in the early stages. As a result, the ground surface subsidence reached more than 220 mm, even 450 mm in some areas (Fig. 7). The Shennan Road metro line 1 is located in the center of Shenzhen City, with strict requirements on ground surface subsidence (which should be less than 30 mm). Because surface subsidence could not be effectively controlled, the project was forced to shutdown on one occasion. After the above research was done, the nondewatering construction method was adopted in areas of special and difficult conditions, and success was achieved in practice. 20 40 60 Width X (m) 80 100 0 5. Strata consolidation subsidence by difDOI: 10.1007/BF03325733 ferent construction methods Different construction methods and the same model parameters were used in the simulation of strata consolidation subsidence. In the simulation calculation, two situations were considered for analysis. In the first situation, only construction mechanics was considered, but the effect of fluid-solid coupling was not considered; and in the second situation, the integrated effects of fluid-solid coupling and tunneling mechanics were considered. The strata consolidation subsidence of the different construction methods were obtained as the difference of two simulation results. Fig. 9 shows the transverse strata consolidation subsidence curve at the profile of y=6 m. We can see the maximum consolidation subsidence of these three methods: the value is 8.4mm for the nondewatering method, 19.5 mm for the dynamic dewatering method and 83 mm for the dewatering method. The proportion of the maximum consolidation subsidence to the surface subsidence by each method is: 27% for the non-dewatering method, 50% for the dynamic dewatering method, and 79% for dewatering method. Different methods to control the loss of underground water and the extent of ground disturbance produce absolutely different results. The more the underground water loss, the greater the extent of ground disturbance, and the greater the amount of strata consolidation subsidence. The amount of strata consolidation subsidence caused by the non-dewatering method accounts for the least ratio, and its effect to control the loss of underground water is the best among the methods. Also, the extent of ground disturbance is the least. 0 20 40 60 80 100 6. Conclusions (1) In watery and soft soil strata, the problem of strata consolidation subsidence caused by metro excavation ) -20 m m ( cen -40 e d i s b sun -60 o i t a liod -80 s n o C -100 Non-dewatering Dynamic dewatring Dewatering cannot be ignored. When researching these issues with numerical simulation, the integrated effects of fluidsolid coupling and tunneling mechanics should be taken into account, and then research results which are more likely to conform to practice are obtained. (2) In the case of the construction of the Shenzhen metro line 1, the amounts of ground surface subsidence caused by dewatering, dynamic dewatering, and nondewatering construction methods are 31 mm, 39 mm, and 105 mm respectively, and their ratio is about 1:1.3:3.4. The maximum consolidation subsidences for these three methods are 8.4, 19.5, and 83 mm, respectively. The ratios of strata consolidation subsidence accounting for ground surface subsidence are about 27%, 50%, and 79%, respectively. The more the loss of underground water, the greater the amount of strata consolidation subsidence. The non-dewatering construction method is the most effective method to control the deformation of strata consolidation subsidence, and good results are achieved in practice with this method. [1] T.Y. Qi , B. Gao , L. Ma, Centrifugal model test for ground surface subsidence cause by metro tunneling in saturated soft clay strata , Journal of Southwest Jiaotong University, 2006 , 41 ( 2 ): 184 - 189 (in Chinese). [2] B. Wu , W.N. Liu , X.M. Suo , Seepage and stress coupling analysis of land subsidence induced by dewatering and tunneling , Chinese Journal of Rock Mechanics and Engineering , 2006 , 25 ( sup .1): 2979 - 2984 (in Chinese). [3] Y.Y. Li , B.J. Liu , Y.L. Xie , Influences of permeability and consolidation settlement on structure of soft clay , Chinese Journal of Rock Mechanics and Engineering , 2006 , 25 ( sup .2): 3587 - 3592 (in Chinese). [4] W.C. Zhu , Y.M. Kang , T.H. Yang , et al., Application of digital image-based heterogeneity characterization in coupled hydromechanics of rock , Chinese Journal of Geotechnical Engineering , 2006 , 28 ( 12 ): 2087 - 2091 (in Chinese). [5] Z.H. Wang , R.Q. Xi , G.X. Chen , State of arts on response of bridge of pile foundation under earthquake excitation , Journal of Nanjing University of Technology (Natural Science Edition) , 2009 , 31 ( 1 ): 106 - 112 (in Chinese). [6] X.C. Li , Y.Y. Guo , S.Y. Wu , et al., Mathematical model and numerical simulation of fluid-solid coupled flow of coal-bed gas considering swelling stress of adsorption , Chinese Journal of Rock Mechanics and Engineering , 2007 , 26 ( sup .1): 2743 - 2748 (in Chinese). [7] J.B. Li , J.Y. Chen , J. Li , Coupled fluid-mechanical analysis of settlement and stability of subway tunnel , Journal of Disaster Prevention and Mitigation Engineering , 2008 , 28 ( 4 ): 441 - 446 (in Chinese). [8] X.M. Ji , Y.H. Wang , Z.Y. Yang , Hydromechanical coupling model and numerical simulation of tunnel excavation , Rock and Soil Mechanics , 2007 , 28 (sup.): 379 - 384 (in Chinese). D[O9] I: 1X0 ..10H0e7,/BXF. 0T3 .32F5e7n3g3, D.X. Zhang, Taylor extension stockastic FE simulation of fluid-solid coupling in rock , Chinese Journal of Rock Mechanics and Engineering , 2007 , 26 ( sup .1): 2608 - 2612 (in Chinese). [10] G.M. Zhang , H. Liu, J. Zhang , et al., Simulation of hydraulic fracturing of oil well based on fluid-solid coupling equation and non-linear finite element , Acta Petrolei Sinica , 2009 , 30 ( 1 ): 113 - 116 (in Chinese). [11] B. Zhang , Ground settlement control measures in shallow depth excavation , Geotechnical Engineering Technique , 2007 , 21 ( 5 ): 265 - 268 (in Chinese). [12] J.P. Carter , J.R. Booker , J.C. Small , The analysis of finite elasto-plastic consolidation , International Journal for Numerical and Analytical Methods in Geomechanics , 1979 (2): 107 - 129 . [13] P.A. Vermeer , A. Verruijt , An accuracy condition for consolidation by finite elements , International Journal for Numerical and Analytical Methods in Geomechanics , 1981 (5): 1 - 14 . [14] E.E. Alonso , A. Gens , A. Josa , A constitutive model for partially saturated soils , Geotechnique , 1990 , 40 ( 3 ): 42 - 51 . [15] L. Barden , Consolidation of compacted and unsaturated clays , Geotechnique , 1965 , 15 ( 3 ): 257 - 286 . [16] C.G. Bao , Properties of unsaturated soils and slope stability of expansive soil , In: 2nd International Conference on Unsaturated Soils , Beijing, 1998 ( 2 ). [17] B. Wu , W.N. Liu , B. Gao , et al., 3D simulation and analysis of construction behavior of forked metro tunnel , Chinese Journal of Rock Mechanics and Engineering , 2004 , 23 ( 18 ): 2451 - 2456 (in Chinese). [18] B. Wu , B. Gao , J.J. Luo , Numerical simulation of reinforcing effect of horizontal drilling jet-ground pegs on Shenzhen metro tunnel between stations , Journal of Southwest Jiaotong University, 2004 , 39 ( 5 ): 605 - 608 (in Chinese).


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Taiyue Qi, Bo Gao. Strata consolidation subsidence induced by metro tunneling in saturated soft clay strata, Journal of Modern Transportation, 2011, 35-41, DOI: 10.1007/BF03325738