Quantitative analysis for the effect of microstructure on the mechanical strength of frozen silty clay with different contents of sodium sulfate

Microstructural transformation is known substantially to be the reason for the differences in mechanical behavior and the failure mechanism of saline soil. The mechanical strength of frozen saline soil decreases by 19.0% initially and then increases by 49.3% with the increasing mass content of sodium sulfate, with the threshold value of 1.5%. To explain the mechanism of this phenomenon, mercury intrusion porosity and scanning electron microscope were adopted to obtain the pore size distributions and microstructural characteristics of silty clay with different mass contents of sodium sulfate at 0.0, 1.5, 2.5 and 5.0%, respectively. The microstructural parameters were calculated by the Image-Pro Plus software based on the threshold of 75 and magnification of 1000× and 2000×. The porosity increases firstly and reduces with the rising salt content, and the fractal dimension is affected slightly ranging from 1.18 to 1.23. As the salt content increases from 0.0 to 1.5%, salts dissolve completely and the thickness of liquid film increases, so does the pore volume, and the friction of soil particles and the mechanical strength decreases. When the salt content is beyond the threshold value, the solution in soil is supersaturated, and salts crystallize and fill the pores and deduce the pore volume. The increasing interlocking abilities between crystals and particles enhance the mechanical strength of frozen saline soil. The results can provide a theoretical guidance to predict the frost and salt heave of frozen saline soil engineering.