Undrained behavior of chalk marl under different stress paths simulating tunneling process

摘要

The stress changes experienced by different soil elements around tunnel excavation are discussed assuming two-dimensional analysis for a homogeneous, isotropic, elastic medium. The stress paths representing undrained stress changes on different soil elements in the vicinity of the tunnel excavation are discussed and presented for isotropic and anisotropic consolidation. To avoid inevitable scatter of experimental results due to material non-homogeneity, a multi stage stress path testing procedure was developed, in which the same specimen was subjected to three stages of consolidation and each stage was subjected to different stress paths of loading and unloading at stress levels well below failure stress. The stress paths followed in each stage were intended stress changes on different soil elements around tunnel excavation to study the short term behavior of soil medium surrounding the tunnel. Three stress paths were considered in this study: ⅰ. SP1 where the vertical stress is constant and the horizontal stress is either increasing or decreasing, ⅱ. SP2 where the horizontal stress is increased and the vertical stress is decreased by the same value, ⅲ. SP3 where the vertical stress is either increased or decreased and the horizontal stress is kept constant. The type of stress path has significant effect on: (1) Stress - Strain relationships, (2) Deformation secant modulus Es, during unloading, values of Es for vertical specimens are much lower than the corresponding values during loading for all stress paths. For horizontal specimens Es for loading and unloading were nearly equal. Values of ( Es )h / ( Es )v during loading for SP1, SP2 and SP3 were 1.14, 1.13, and 1.31 respectively, while the corresponding values during unloading were 1.62, 1.84 and 2.42 respectively, which implies that the degree of anisotropy is highly dependent on the applied stress path. (3) Pore pressure response is entirely dependent on stress path and on effective consolidation stress, which is when increases the pore pressure response become more pronounced, whether increasing or decreasing.