Long-term settlement mechanisms of shield tunnels in Shanghai soft clay

Excessive differential tunnel settlements have been observed in Shanghai metro tunnels over the past two decades and they become a major concern for the operational authority and the designers since would affect the serviceability and possibly safety of the entire metro system. Although there are a large number of metro tunnels constructed in Shanghai, the principal causes of settlement for metro tunnels in Shanghai soft clay have yet to be fully understood. The objectives of this research are to investigate the fundamental soil properties of Shanghai soft clay and the principal long-term settlement mechanisms of shield tunnels in Shanghai.;Three interconnected research methodologies, namely field monitoring, laboratory testing and numerical analysis are adopted in this study. The fundamental soil properties of Shanghai soft clay including small-strain shear stiffness, shear behaviour and long-term secondary compressibility (creep) were investigated by carrying out laboratory tests on intact block samples taken from Shanghai. Measured long-term tunnel settlements along Shanghai Metro Lines 1 and 2 (up to 12.5 and 7.5 years, respectively), records of groundwater pumping and subsurface soil deformations at Tangqiao location were carefully interpreted and analysed. Based on field monitoring and laboratory test results, principal causes of large long-term tunnel settlements were investigated. In addition, finite element analyses were carried out to verify the likely principal settlement mechanism.;Large non-uniform tunnel settlements were observed along both lines throughout the monitoring period. The maximum tunnel settlements were 289 and 144 mm for Lines 1 and 2, respectively. The angular distortion has continued to increase without showing any sign of reaching a steady value. The largest angular distortion was 0.22% resulting in leakage patches observed adjacent to some joints of the shield tunnels. Four possible causes were carefully investigated, including the effects of tunnel construction, cyclic train loading, secondary compression of soft clay and groundwater pumping.;Due to horizontal layering structures, intact Shanghai soft clay exhibited stiffness anisotropy. The measured degree of inherent stiffness anisotropy in terms of elastic shear modulus ratio [G0(hh)/ G0(hv)] was 1.2. Based on long-term oedometer tests, the coefficients of secondary compression were found to be less than 0.8%, which is classified to be very low to medium secondary compressibility, according to the classifications defined by Mesri (1973). This finding basically rules out the possibility observed long-term tunnel settlement was attributed to the secondary compressibility of Shanghai clay, as it is always postulated by many researchers and engineers in China.;It is found that the sandy Aquifer IV exhibited considerable secondary compression (creep). The compression of Aquifer IV accounted for about 65% of the measured total tunnel settlement while the compression of soft clay layers contributed less than 20% to the total tunnel settlement. Among the four causes considered, excessive groundwater pumping from Aquifer IV was the principal reason for the observed large long-term settlements of shield tunnels in Shanghai.