Abstract: The very weakly cemented rock has the characteristics of late diagenesis time, poor cementation, low intensity, easy weathering and disintegration. The special diagenetic environment and deposition process lead to its unique physical and mechanical properties. The X-ray diffraction test shows that the clay mineral composition of very weakly cemented rock is mainly kaolinite with the contents as high as 54%-69%. The microstructure of very weakly cemented rocks is honeycomb, porous and well-connected, with a poor overall structure. Hydration is easy to occur when encountering water, which results in an increase in porosity and a decrease in strength. It is easily weathered and cracks after a short period of exposure. Uniaxial and triaxial compression test of very weakly cemented rock are tested with the rock mechanical system MTS 815 and the complete stress-strain curves are obtained, revealing the post-peak strain softening and expansion deformation characteristics. The experimental results show that the rock sample has obvious volume expansion characteristics before and after the failure. With the increase of the confining pressure, the post-peak volume expansion characteristics are gradually weakened. Based on the crack volumetric strain model, the large dilation deformation failure mechanism of the very weakly cemented rock is analyzed, and the evolution law of rock mass strength and dilatancy parameters with equivalent plastic strain is revealed. At the same time, the pre-peak damage dilatancy and post-peak fracture dilatancy yield criteria are established, and the dilatancy and large-deformation constitutive model of the very weakly cemented rock is constructed.

Key words: very weakly cemented rock, physical and mechanical properties, uniaxial and triaxial compression test, constitutive model, yield criterion