| Soft clay, having properties like high water content, high compressibility, low strength and poor engineering characteristic, is widely distributed in the South-East coastal area in China. Evident settlement may occur in the tunnel constructed on soft clay layer ground which is a threat to the utility and safety of the tunnel. With remarkable development of social economy and engineering technologies, more and more subsea tunnel are built to improve transportations and reinforce connections between areas. High hydraulic and pore pressure situation of subsea tunnel is one of the most difference compared with tunnel built on land. Once damage occurs in a subsea tunnel, lost would be beyond expectation.In this article, a series of cyclic loading tests with principle stress rotation simulating saturated soft clay under subsea tunnel subjected to transportation load,with a total cycles of 10000 times followed by post-cyclic undrained triaxial compressing test on soft clay in Hangzhou are conducted by taking advantage of ZJU 5Hz Hollow Cylinder Apparatus(ZJU-HCA) to investigate the mechanics characteristics of saturated soft clay subjected to long-term dynamic loading in order to make references to subsea tunnel project design. The topic is the sub-project of <Geotechnical Design and Solution in Shield Subsea Tunnel> subsidized by National Natural Science Foundation. The main research contents are as follows:1. Discussions of pros and cons of simulating transportation load with ZJU-HCA are made. An experiment plan is proposed.2. A series of cyclic loading tests with a large amount cycles and different conditions are conducted to research characteristics of saturated soft clay under different situations generally from the aspects of excess pore water pressure, strain and modulus. There are 4 influence factors considered in the study:(1)Aiming at the high hydraulic condition in subsea tunnel engineering, the influences of static pore water pressure are mainly focused on.(2)Aiming at the drainage conditions in a subsea tunnel, the influences of drainage conditions are investigated.(3)Aiming at the speed of a vehicle or train, the influences of load frequency are investigated.(4)Aiming at the over-consolidation properties of submarine soils formulating in history, the influences of OCR are investigated. Research indicates that all of the 4 factors have evident effects on the mechanics characteristics of saturated soft clay subjected to dynamic loading and the laws are summoned. Based on the analysis, some suggestions to research and engineering are put forward.3. A formula deriving from experimental data to predict and calculate long-term settlement of saturated soft clay layer subjected to transportation load is suggested on the basis of index formula raised by Monismith.4. Post-cyclic undrained triaxial compressing tests are conducted to be compared with the result of a sample without dynamic loading to study the changes of saturated soft clays after dynamic loading in the aspects of mechanics such as stress-strain relationship and strength. The results are compared with existing surveys of other researchers and some conclusions are made. It indicates that if the residual strain and excess pore water pressure are in a certain range at the end of dynamic loading period, the strength of saturated soft clay is almost uninfluenced after long-term dynamic loading. However, the stress-strain relationship is obviously affected mainly from two ways as stiffness intensifying and strength commencing in advance. In combination with the dynamic characteristic of samples, a phenomena of reinforcing in elastic deformation after long-term dynamic loading is found and efforts to explain the post-cyclic behaviors from the view of microstructure are made. It is also suggested that the experimental conditions expect amplitude and orientation of load during dynamic loading have no effect on post-cyclic behavior in mechanics, and research on post-cyclic prosperities with a large amount of cycles is essential. |
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