Deformation And Stability Investigation Of Underground Excavations Of Subway Transit System In Soft Clay

With the rapid development of the economics of China, the development of urban rail transits system has been an important methodology for solving the heavy traffic and the following air pollution problem. Zhejiang Province is located in the East China, and the construction of metro systems of the three big seaboard cities Hangzhou, Ningbo and Wenzhou is at a rapid developing period. Hangzhou, Ningbo and Wenzhou are all built on deep soft soils with high water content, low shear strength and large sensitivity, which made the construction of metro system is faced with many challenges. One of the main challenges is that the shield tunneling induced ground settlement is difficult to predict and control in soft clays and excessive settlements generally do harm to the environment. The other of the main challenges is that it’s not easy to obtain the true factor of stability FOS of deep excavation in soft clay and the risk of failure is very high. Based on these engineering requirements, this paper systematically studies the properties of the typical soft clays in Hangzhou and Ningbo firstly, and then the ground settlements induced by shield tunneling and the methodology for assessing the stability of deep excavations are focused on. The major works and results of this dissertation are as follows:(1) High quality soil samples of typical soft clays in Ningbo and Hangzhou are obtained using thin wall samplers. The stress-strain behavior and strength parameters of the soft clays are studied with the one-dimensional consolidated tests, the triaxial isotropically consolidated undrained shear (CIU) tests, the triaxial isotropically consolidated drained shear (CID) tests, and the triaxial isotropically consolidated unloading-reloading drained shear tests based on GDS system. Parameters of the hardening soil (HS) model are obtained from the testing results. And the relationship between the soil modulus parameters ofedref, E50ref, Eurref and Es,1-2are analyzed. The value of Eurref/E50ref of the clays ranges from3.6to11.6and the value of Eurref/Es,1-2ranges from4.5to14.5, and the weaker clays own a high value of Eurref/E50ref and Eurref/Es,1-2, similar to the Shanghai clays. In addition, the CID tests and CIU tests are modeled using finite element program Plaxis2D, and the HS model parameters are evaluated and calibrated with comparing the predicted stress-strain curves with testing results. The influence of disturbance to the one dimensional behavior of Hangzhou soft clay is investigated specially. The Hangzhou soft clays are slightly structured clay, and the structure strength almost increases linearly with depth. The degree of disturbance of the clays defined from yield stress and shear strength are generally linear with each other.(2) Field monitoring and three dimensional numerical modeling of performance of shield tunneling in the Taodu Road-Gulou interval and the Gulou-Chenghuangmiao interval mainly constructed in soft clay of Ningbo Metro Line are conducted. Field monitoring investigation indicates that ground conditions and driven parameters of the shield influence the ground deformation significantly. As the cover soils of the tunnel are mainly silty sand the settlement trough parameters K ranges from0.3to0.4are relatively smaller than those of soft clays with K ranging from0.45to0.55. Large shield force, back-filling grouting ratio and face support pressure results in great settlements. As the driven parameters during shield tunneling are gently controlled and make the Γ=1.0-1.8, the△p0=15-40kPa, the ps/σv=0.8～1.0, and the δ=200%-220%, the ground volume loss VL will be much smaller. The shield tunneling in the Taodu Road-Gulou interval caused large ground volume loss VL, while the ground in the Gulou-Chenghuangmiao interval experienced large ground heave deformation.The shield influenced areas are the ground besides the alignment with4-6times of diameter of the shield (D), and3D ahead the tunnel face and4D-6D behind the tunnel face. The main reasons leading to the excessive heave of the ground in the Gulou-Chenghuangmiao interval is the driving speed of the shield is too high and the soils around the tunnel is very weak, and resulting in the significant compacting effect of the tunnel face and great disturbance of the soils. After the shield passed for240days, the ground settlements still increased with a speed of about0.2mm/d. (3) Collapse of a15.7m deep excavation in very sensitive organic soft clay in Hangzhou, East China is presented in this paper. The ground disturbance due to the collapse was evaluated with a simplified method based on the strength reduction ratio (SRR) of the in-situ soils. According to the SRR values of the soils at different depths, the lower boundaries of the severely disturbed soils were determined, and a possible slip failure surface passing through the bottom of the severely disturbed soils was justified. The factor of safety (FOS) against basal heave of the excavation calculated from Terzaghi’s method, Bjerrum&Eide’s method and the slip circle method were1.05,0.74and0.89, respectively. These computed values were much smaller than the corresponding recommended values. The overall FOS was studied using the shear strength reduction technique based on a finite element method FEM program Plaxis. The FEM analysis results indicated that the FOS of the excavation was1.02, and the computed slip failure surface fit well with that from the soil disturbance investigation. The failure mechanism of the excavation was then justified to be a basal heave. The main reasons leading to the collapse is the insufficient FOS of the excavation, and over-excavation and imperfections of the construction quality of the supporting system.(4) Various conventional methods and reduced shear strength FEM were used for FOS evaluation of the investigated failed deep excavation in soft clay. By comparing the values of FOS of the excavation from different soil strength parameters and different calculation methods, the applicability of the analyses methods and soil strength parameters was evaluated. The research results indicated that:The values of FOS of the excavation calculated from the conventional methods and the reduced shear strength FEM using total stress indexes, i.e., su,Φ=0, are very close to unity and the FOS from Terzaghi’s method, while the values of FOS of the excavation are much greater than unity using the soil strength parameters c and Φ from triaxial tests. When the reduced shear strength FEM is applied for FOS evaluation, the FOS obtained from the convergence criterion method is much greater than that from the intersection method. The FOS of the excavations using the intersection method and Terzaghi’s method are both very close to unity, which is consistent with the limited equilibrium state of the excavations at failure.This research will improve the cognition of on the ground deformation induced by shield tunneling and factor of stability of deep excavations in soft clay in Zhejiang Province, and provide some practical methodology for solving engineering problems.