Mechanical behaviour and grain breakage characteristics of Tibetan glacial tills under triaxial compression

Deglaciation due to warming climate has left a great amount of glacial tills in ravine slopes and channels in southern Tibet, which can lead to destructive debris flows and pose great threats to downstream residents and infrastructures. The mechanical properties and grain size characteristics of glacial tills are key factors that influence the initiation and development of these debris flows. In this work, a series of consolidated undrained (CU) triaxial compression tests under different confining pressures were carried out on Zelongnong glacial tills at various relative densities of 9.2%, 49.5%, 69.0% and 88.7%. The grain size distribution change before and after the CU tests was also determined. Results show that the glacial tills at relative densities of 9.2%, 49.5% and 69.0% exhibited a remarkable strain-softening and contractive behaviour during shearing, while those at a relative density of 88.7% presented a dilatant behaviour. As the relative density increased, the shear strength, elastic modulus and shearing friction angle increased, reducing the liquefaction potential. Based on the increasing rate of mechanical resistance with the increasing relative density, a critical value of 69.0% was identified to appreciate the distinct mechanical responses of the specimens. From the grain size distributions before and after CU tests, the content of grains with a diameter larger than 0.85 mm decreased due to the grain breakage while that of grains smaller than 0.3 mm increased. The higher the specimen relative density, the larger the grain breakage ratio owing to the increasing friction resistance. This grain breakage in turn causes a downward shift of the critical state line on the plane of void ratio and mean effective stress.