Study On Unloading Mechanical Characteristics And The Damage Constitutive Model Of Sandstone Under Hydro-pressure Cycle

Unloading failure is the most typical form of damage in excavation slope engineering.And the repeated lifting process of reservoir water level has great influence on the stability of rock slope in the Three Gorges reservoir area.Therefore,it is of great significance to study the change of mechanical properties of sandstone during unloading process under cyclic pore water pressure damage.In this paper,the common sandstone in the reservoir area is taken as the research object.The unloading mechanical properties of the sandstone under the hydro-pressure cycle are studied through laboratory tests.,and the following conclusions are obtained:1)In this paper,the unloading test considering the unloading rate under hydro-pressure cycle of saturated sandstone is carried out firstly.And the variation law of stress-strain curve during the test process was obtained,and the relationship between peak confining pressure and confining pressure strain compliance and unloading rate was obtained.It is found that the p eak confining pressure of sandstone will be reduced continuously under unloading failure with the increase of unloading rate,and the brittleness of sandstone is more obvious under high unloading rate.The axial and lateral confining strain compliance decreases with the increase of unloading rate,and the unloading rate has a greater influence on the lateral strain of sandstone.2)The stress-strain curve of unloaded sandstone under different water pressure cycles is obtained through the unloading failure test of sandstone under different hydro-pressure cycle.The variation of peak confining pressure,confining strain compliance and failure characteristics of sandstone samples with the number of hydro-pressure cycle is studied under the action of different hydro-pressure cycle.The results show that with the increase of the number of hydro-pressure cycle,the peak intensity shows an inverse S curve,and the peak confining pressure increases with a positive S shape.According to the hydro-strain curve,the three stages of hydro-cycle influence initial,influence aggravation and saturation are put forward.In the initial stage,the initial confining pressure plays a major role in influencing the sandstone strain.In the aggravation stage,the axial and lateral strain of sandstone increases obviously,and the hydraulic circulation plays a dominant role.During the action saturation stage,the degradation of sandstone by hydro-pressure cycle tends to be stable and both axial and lateral confining pressure strain compliance increased with the increase of hydro-pressure cycle times.But the increase of axial confining pressure strain compliance was significantly greater than that of lateral in the aggravation stage.3)According to the energy calculation principle,the evolut ion law of the total absorbed energy,elastic strain energy and dissipated energy with the unloading rate and the number of hydro-pressure cycle under the action of hydro-pressure cycle is obtained.It is found that elastic strain energy in sandstone unload ing failure process increases with the increase of total absorbed energy and dissipation energy decreases as the increase of unloading rate.The elastic strain energy in the unloading process of sandstone increases in the initial influence and aggravation of the hydraulic cycle,and decreases in the saturation stage of the hydraulic cycle.4)The Hoek-Brown criterion is used as the rock strength criterion and the minimum energy consumption in the rock unloading process is selected as the damage variable.According to the Weibull statistical distribution theory of rock micro-body strength,the comprehensive damage variable in the process of unloading damage of sandstone under hydro-pressure cycle is established based on the equivalence principle and combined with continuous damage theory and the generalized Hooke’s law.Therefore loop saturated sandstone unloading damage constitutive model is deduced under the hydro-pressure cycle.It is found that the constitutive model can be consistent with the experimental curve,which can better reflect the damage evolution and constitutive relation of saturated sandstone under hydro-pressure cycle,and demonstrate the rationality of the method for determining model parameters.