Pile-soil interface characteristics in ice-poor frozen ground under varying exposure temperature

This study investigated the shear strength behavior of frozen sand and steel-frozen sand interface using the direct shear testing technique for evaluating the roughness factor "m" that correlates the adhesion of the pile-frozen soil interface with the cohesion of the frozen soil. The study also assessed the ratio between the internal friction of the frozen sand and the interface friction of the steel-frozen sand element in an attempt of defining a frictional factor analogous to the roughness factor. The test results showed that the strength and deformation at peak for all test samples increase with decreasing exposure temperature and increasing the normal stress and strain rate. The residual strength of frozen sand was dependent on temperature and normal stress, while for steel-frozen sand interface was dependent on normal stress only. The peak and residual strength of frozen sand and the peak strength of steel-frozen sand interface were resultant of cohesion/adhesion and internal/interface friction. The residual strength of steel-frozen sand interface was mainly due to the residual interface friction. The roughness factor "m" was found to decrease with decreasing the temperature recording a value of 0.68 at -10 degrees C and 0.82 at -1 degrees C. The roughness factor at the residual state was very minimal recording 0.04 indicating very low adhesion strength. A frictional factor was successfully defined and recorded a value of 0.76 which was insensitive of the freezing temperature. At the residual state, the frictional factor was 0.34 indicating a frictional dependency of the residual strength of the steel interface. The study also reports a measurement of thermally induced confining pressure around a model steel pile in frozen sand which is considered the first of its kind. This measurement along with the introduced frictional factor is expected to provide a cost-effective design of pile foundations in frozen ice-poor soils.