Microstructure investigation of soft clay subjected to triaxial loading

The microstructure properties of soft clay in Shanghai were discussed in light of experimental observations from a series of triaxial stress path tests and field emission scanning electron microscope (FESEM) tests performed on undisturbed soft clay specimens. The microstructure changes of soft clay were observed during different stress paths tests. The microstructure parameters were analysed by fractal geometry theory. The results show that the mechanical properties of soft clay are subject to the effect of stress paths, variations in initial confining pressure, which are controlled by its microstructure. Shanghai soft clay is manifested by a typical skeleton microstructure with large pores and flat laminated clay particles. The total volume and number of pores increase, and the distributions of soil aggregated particles and pores become looser due to loading; thus, the void ratio and fractal dimensions of the pores and soil particles are positively correlated with the axial strain. They reveal that cementation between the soil particles decreases and that the microstructure of soft clay is damaged by not only pore enlargement, but also degradation of the soil particles. Damage onset, damage development and failure occur during the entire process of soft clay subjected to triaxial loading. The microstructure parameters affected by reduced triaxial compression (RTC) tests change to a larger degree than those affected by conventional triaxial compression (CTC) tests, indicating that there is weaker cementation between the soil particles in the RTC tests due to the stress path and negative pore pressure generation. The microstructure evolution of natural soft clay during loading can be used to analyse the macro-mechanical behaviours.