Experimental Investigation Into Macroscopic Mechanicaland Microscopic Characteristics Of Shanghai Soft Soil

In recent years,many large-scale projects have been built on huge soft soil foundations.Due to lack of understanding of engineering properties of soft soil,many major engineering accidents have taken place.A lot of research and practice have proved that the engineering properties of soft soil are closely related to the microstructure and its variation of particle shape,distribution,connection mode,porosity and so on.Therefore,it is very necessary to strengthen the research on macroscopical and mechanical properties of soft soil,which is of great guiding significance to engineering practice.The consolidation test,resilience test,equal p compression and tensile undrained shear tests under the condition of K₀ consolidation and FE-SEM electron scanning experiments of the fourth layer of Shanghai soft soil were carried out to study the evolution of macroscopic mechanical properties and microscopic characteristics of the soil under different stress states.The results show that with the increase of consolidation pressure,the void ratio and the consolidation coefficient tend to be constant after the rapid decrease,and the compression index decreases slowly after a certain value.The swelling index first increases and then decreases.Under equivalent shear test,the stress-strain relationship of Shanghai soft soil shows obvious strain hardening shape.The stress at the failure point and the peak value of the pore water pressure increases with the increase of the consolidation pressure,and the stress value at the failure point under the compression stress path is far higher than the stress value at the point offailure under tensile stress path.Compression strength of soil is significantly greater than the tensile strength.Consolidation pressures,stress paths,scanning positions,scanning directions and magnifications all have certain influence on the microscopic parameters of soft soil.In the consolidation and resilience test,as the consolidation pressure increases,the number of pores decreases.Pore ratio,particle and pore distribution fractal dimension and probability entropy of pores decrease,particle and pore shape fractal dimension increase,Soil has obvious anisotropy.Under the equal p shear path,with the increasing of strain,the pore ratio,particle and pore shape fractal dimension,particle distribution fractal dimension and the probability entropy of pores increase,and pore distribution fractal dimension decreases.Under the effect of consolidation pressure,the soil has obvious anisotropy.Compression index,swelling index and consolidation coefficient are closely related to microstructure parameters.The relationship between microstructural parameters and shear stress ratio is divided into three stages: slow development,rapid development and rapid destruction.The development of the microstructure parameters under the tensile path in the rapid destruction phase is faster than the compression path.