Experimental Study On The Relationship Of Dynamic Characteristics And Microstructure Of Intact Soft Clay Under Wave Loading

With the exploitation of marine resources and utilization of ocean space, more marine structures are built on the soft ground of seabed. Since these facilities influenced by the long-term wave load in the process of their construction and use, the issues of geotechnical problems caused by the wave load draw the worldwide attention. A great deal of research has been carried out on the dynamic characteristics of saturation clay under the wave load, however, these research findings are mostly based on dynamic triaxial tests which cannot simulate complex stress-path of continuous cyclic rotation of principal stress axes under the wave load. In addition, the research on soil microscopic structure induced by macroscopic dynamic response are lacking because previous studies on soil characteristics still stay on the perspective of macromechanics. Therefore, in order to fully understand soil characteristics under the complex stress paths, research on the deformation of soil microscopic structure and failure mechanism should be carried out to reveal the relationship between soil microstructural parameter and macro-mechanical property on the basis of macro-mechanical tests.In the paper, actual stress state of the seabed subsoil elements were simulated using ZJU-5Hz hollow cylinder apparatus to analyze the dynamic characteristics of saturation clay. The characteristics of changes of soil microscopic structure in the process of tests were studied qualitatively quantitatively using scanning electron microscopy and PCAS quantitative testing technology. This major tasks and research results are as following:(1) Shear tests of continuous rotation of principal stress axes under different frequency and different cyclic stress ratio were conducted. Results of the trials indicated that cyclic stress ratio and frequency have great influence on pore pressure characteristics, deformation characteristics and dynamic modulus degradation characteristics. The lower the frequency or the higher the cyclic stress ratio is, the faster the pore pressure accumulation, strain development and modulus degradation. Compared with the dynamic triaxial test results, the pore pressure accumulation, strain development and modulus degradation decrease rapidly. The curve of pore pressure had no obvious inflection point and did not reach steady state when the soil samples were destroyed. The required the number of cycles for failure decreased significantly, and the soil samples have much higher softening degree at the same number of load cycles. What is more, the critical cyclic stress ratio of soil decreased from0.5to approximately0.2and dynamic strength reduced by35%-40%.(2) The author carried out one-dimensional consolidation test to study the soil changing law of compression characteristic and micro-porosity character. The transformation rule of microstructural parameter with consolidation pressure was obtained and the correlation between soil compression performance and microstructural parameter was also got, which reveal the microscopic mechanism of the soil deformation.(3) The soil microstructure within and around the shear band was studied before and after cycle loading, which revealed soil microstructure deformation and failure mechanism from the perspective of micro-level. Pores are broking and mergering at the same time under cyclic shear stress, which is microscopic nature of apperence of the wrinkle on shear band, also, the pore’s orientation is consistent with the shear band’s direction. The pore size and distribution for different samples when damage happened were similar, while the levels of critical failure strain are almost the same. Soil microstructure deformation mechanism is different under the static load and dynamic load, and the microstructure deformation mechanism of soil under the dynamic load is more complex than that in static conditions. The influence of cyclic shear stress ratio and frequency on soil microstructure change is different. The microstructural parameters change regularly with the cyclic stress ratio, but no apparent regularity varies with frequency.