Laboratory Tests

Abstract

Through a series of laboratory tests under cyclic loads on saturated soft clay in Shanghai and Nanjing, the pore water pressure, soil dynamic strength, dynamic stress-strain relation, and dynamic elastic modulus were discussed deeply, taking vibration loading frequency, amplitude, and vibration loading cycles into account. The dynamic properties and microstructure deformation damage mechanism were analyzed. Some important conclusions were drawn as below: 1. The development of pore water pressure can be divided into three stages: fast ramping stage, slowly growth stage, and stabilization stage. The logistic model was proposed to fit the pore water pressure growth curves and coefficients of correlation that were all above 0.99. At the fast ramping stage, the growth rate was not always a constant. The pore water pressure increased sharply, slowed down in a short time, and then got into stabilization. After regression analysis, the growth rate of pore water pressure obeyed in the ExpDecay2-type curve. In addition, the excess pore water pressure declined abruptly once the vibration loading ceased and finally recovered at a marginal higher value compared to static pore water (hydrostatic) pressure. 2. The deformation characteristics of saturated soft clay under cyclic loads were mainly influenced by cyclic stress ratio, confining pressure, vibration loading frequency, and vibration cycle number. There existed a critical cyclic stress ratio, relevant to the soil properties, applied load form and consolidation pressure, etc. When the applied dynamic load was smaller than the critical cyclic stress, the dynamic stress increased with vibration loading time. While the growth rate and amplitude gradually fell into a stable constant, to the contrary, the deformation augmented fast and finally got into failure. Under low loading frequency, the influence of frequency was not very conspicuous and can be neglected. While in the high-frequency circumstance, the soil behaved more elastic properties. The main reason may be that when high-frequency vibration load is applied, the pore water pressure could not dissipate in time, and with longer loading cycle duration, the pore water pressure dissipated fully and the creep effects on soil particles were more obvious. The soil characteristics are more plasticity and viscosity. 3. As for the subway tunnel, the deformation in the soil deep around the interior wall of the subway tunnel was relatively small, while the axial strain curve of soils just beneath the subway tunnel was in a very short resilience stage and got into the plastic deformation stage very quickly, resulting in large lateral deformation. Hence, even if there may be no large deformation occurring around the subway tunnel during a long operation time, significant differential settlement is probably to arise as time lasts. KeywordsPore Water PressureTriaxial TestSoft ClayExcess Pore Water PressureCyclic CreepThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.