The impact of structure on the undrained shear strength of cohesive soils

The relationship between the undrained shear strength and the water content of cohesive soils can be described with a two parameter non-linear function. Parameter a is the water content at the undrained shear strength su = 1 kPa, and b is the slope of the linear function which represents the ratio between the water content logarithm and the undrained shear strength logarithm. These parameters depend mainly on the type and amount of clay minerals in the soils, the particle sizes and shapes, their arrangements, the forces between the clay particles, and the chemical composition of the pore water. The impact of quantity, type and size of clay grains on the water content at a uniform undrained shear strength in different soils has been studied previously, whilst the impact of structure is less understood. Particle associations and arrangements (i.e. fabric), and the interparticle forces determine the structure of soil.This paper focuses on an investigation of the relationship between water content and undrained shear strength in kaolinite samples with different structures. The various arrangements of clay particles that resulted from the changed of interparticle forces were obtained by salt addition (NaCl). Microfabrics of wet kaolinite samples were studied using a field emission scanning electron microscope. The results showed that clay structure exerts a strong influence on the relationship between the quantity of water and undrained shear strength. This was reflected in the values of both soil-dependent parameters, a and b. Values of parameter a varied between 39.16% and 62.96%, and b between 0.100 and 0.139, for well crystallized kaolinite with salt concentrations of 0–175 mmol/l. For poorly crystallized kaolinite with salt concentrations of 0–300 mmol/l, the values of a range between 46.96% and 100.30%, and b between 0.090% and 0.148%.