Experimental Study On Mechanical Properties For Warm Frozen Soils In Permafrost Regions

A special testing apparatus for undisturbed and remoulded frozen soils from Permafrost Regions, characterized by air pressure loading and temperature control from two ends, was developed. The deformation characteristics and mechanical properties for warm frozen soils were studied comprehensively via adding the digital photographic technique(DPT) and resistivity measuring to the present testing chamber, and the strength mechanism of warm frozen soils was preliminarlily revealed in terms of components of unfrozen water, ice crystals, and soil particle.Step loading triaxial tests on single specimen of undisturbed and remoulded warm frozen soils under-1℃ were firstly performed. The shear strength for undisturbed frozen soils exhibits marked scatter due to the non-uniform distribution of internal ice crystals and soil particles. And shear strength for undisturbed frozen soils was two times of that for remoulded frozen soils at the same dry density and water content.Uniaxial compressive testing results for remoulded warm frozen soils under various temperatures, thermal gradient and strain rates showed that the stress-strain relationship for frozen soils changed from strain softening to strain hardening gradually as the temperature and thermal gradient increased. In addition, peak strength and elastic modulus of warm frozen soils decreased as the temperature increased and the strain rate decreased. Meanwhile, and thermal gradient had marked weakening effect on the peak strength and elastic modulus at the same uniform temperature.Deformation distribution of warm frozen soils under different temperature models were obtained and then analyzed based on DPT. The horizontal strain, vertical strain and volume strain under different specimen section height(SSH) were almost identical. And the failure patterns of warm frozen soils with uniform temperature were governed by plastic shearing mechanicsm. However, the maximal deformation was about 40 times greater than that of the minimal deformation for warm frozen soils with thermal gradient.The deformation of apparent warmer tempeartuers mainly contributes to the total deformation of warm frozen soils with thermal gradient, and the failure patterns were governed by plastic shearing mechanism and axial splitting mechanism.Tensile strength of warm frozen soils almost increased linearly as the temperature decreased. The tensile strength of warm frozen soils decreased as the splitting rate increased under a temperature of-0.57℃,-1.04℃ and-1.52℃, whereas the strength increased as the splitting rate increased under a lower temperature of-2.02℃.The mechanism of strength for warm frozen soils was preliminary studied on the basis of resistivity results Structures of frozen soils changed from "soil enwrap water" and "ice enwrap water" to "water enwrap ice" and then to completely melted state during the warming process, while from completely melted state to "water enwrap ice" and then to "soil enwrap water" and "ice enwrap water" during the cooling process. Temperature-changing process was along with the change of components of ice and unfrozen water.