Experimental Study On Mechanical Properties Of Frozen Saturated Medium Sand In Directional Shear Path

The water-rich sand layer seriously restricts the stability of underground engineering excavation,and it is the main stratum for water plugging and reinforcement by the artificial freezing method.Quantitative evolution of mechanical properties of frozen sand and accurate estimation of strength parameters are the key basis and important premise for safety and stability evaluation of frozen engineering.The specimen of the conventional triaxial test was frozen under no pressure,which was used to describe the stress-strain relationship and strength characteristics of frozen soil.This leads to the inconsistency of stress state and stress path between experimental and actual engineering,so it can not reflect the basic characteristics of frozen soil under the complex stress state of current frozen soil engineering.The frozen saturated medium sand is taken as the research object in this paper,and the deformation,strength characteristics and related mechanisms of the compressed frozen saturated sand under directional shear path are systematically studied by experiments.First,the hollow cylinder instrument was renovated by adopting a novel temperature control mode to achieve the fine temperature control accuracy(±0.1?).Under the two typical temperature control modes of direct cooling and cyclic heating and cooling,this equipment can reach the target temperature with high precision at inside,outside and the upper,middle and lower points of the hollow cylindrical sample.During the test,the above temperature can maintain long-term stability,thereby satisfying the temperature control effect of uniform temperature and non-uniform temperature freezing test.Based on the theoretical derivation of the stress-strain component of the hollow cylinder samples,a compression-freezing directional shear stress path loading method under arbitrary shear angle is designed,through which the stress path and the reliability of the data acquisition of the frozen soil hollow cylinder test system are verified.The preparation method of hollow cylindrical samples of frozen saturated sand suitable for the test system and the refinement test process meeting the requirements of test specification and repeatability are developed.Furthermore,a set of experimental research methods on the mechanical properties of frozen saturated sand with complex stress paths are constructed.The surface and internal microstructure of the samples are characterized and visualized by electron microscopy and computed tomography.The geometric and spatial distribution characteristics of grains,pores and ice crystals are further studied.The results show that the frozen saturated standard sand belongs to the typical anisotropic material.The standard sand grains have a non-spherical structure,whose ratio of the major axis to minor axis is about 1.385.The spatial distribution probability of the long axis of the sand grains in the sample prepared by the 'falling sand method' is horizontal > vertical,and the skeleton of sand grains demonstrates anisotropic characteristics.The pore structure is isotropic,but the growth direction of the ice crystal in the pores is horizontal,which leads to the anisotropy of ice crystals.A series of directional shear tests of saturated medium sand frozen under pressure is conducted.The results show that large principal stress direction angle,medium principal stress coefficient,average principal stress and temperature have a significant influence on deformation,peak strength and generalized stress-strain relationship of frozen saturated medium sand.The peak strength decreases gradually with the increase of large principal stress direction.When ?=75°,the strength is only 65% of that when ?=0°,which decreases by 35%.This shows that the traditional method,which ignores the shear direction and directly describes the strength of frozen sand by using the conventional triaxial shear strength,is not in line with the complex stress state of the actual engineering frozen soil and will overestimate the strength of frozen soil.When the medium principal stress coefficient is in the range of 0?b?1,the peak strength decreases with the increase of the medium principal stress coefficient.After the comprehensive analysis of the strength characteristics of saturated medium sand frozen under pressure condition and different directional shear stress paths,it is found that the shape characteristics of the yield surface on the meridian plane are significantly different from those under the non-pressure freezing condition when the average principal stress is 0 ? 10 MPa.On this basis,the mechanism of the change of yield surface caused by the compression and melting of ice crystals under non-pressure freezing condition is revealed.It is found that with the decrease of the middle principal stress coefficient,the strength envelope in the deviatoric plane shows a tendency that is similar to a smooth convex triangle.By improving the SMP criterion and introducing the new relevant parameters to consider the ice-binding force,temperature effect and anisotropy,an extended SMP criterion is finally proposed to obtain the partial plane failure function and the meridian plane failure function,which is more consistent with the actual engineering strength criterion of frozen saturated medium sand in full stress space.