Impact of cooling on shear strength of high salinity soils

Abstract

Engineering activities have increased in northwestern China where high salinity soils exist extensively. This paper investigates the volumetric behavior and shear strength parameters of high salinity soils during cooling. Fine-grained saline soils with salt content varying from 36% to 2.6% were collected along a highway on the Qinghai-Tibet plateau to conduct laboratory cooling and direct shear tests. Cooling curves of soil specimens were obtained to assess the freezing point depression caused by salinity. Volumetric change data during cooling was presented. Direct shear tests were conducted at temperatures ranging from 20°C to −20°C at an interval of 10°C to obtain the shear stress-strain behavior and evaluate shear strength parameters including the cohesion and friction angle. Additional tests were carried out to investigate the effect of increased moisture content on shear strength parameters. Results showed that saline soils with >0.5% sulfate salt will exhibit large volumetric expansion due to salt crystallization and formation of mirabilite. It was found that, as the temperature was lowered from 20°C to −20°C, the shear stress-strain curves of high salinity soils demonstrated stronger dilative and strain-softening behavior, the friction angle remained constant or showed small but consistent gain, and the cohesion increased. It was interesting to find that the freezing of pore water in high salinity soils did not have nearly as large impact on soil strength as it would have for non-saline soils. In addition, a 3% increase in moisture content decreased the cohesion by 60–90% due to reduced matric suction resulted from increased degree of saturation and decreased tension at the water-soil interface. The results are of significance for highway engineering in cold regions with extensive saline soil distribution.