Study On Mechanical Properties And Constitutive Model Of Hard Rock Under Hydro-mechanical Coupling Condition

Because of deep tunnels are appeared constantly in the construction of infrastructure,combine with the peculiar geological structures in west China,the stability of surrounding rock which under the condition of high stress and high underground water pressure has become a hot topic.During the failure of rock,water pressure will stimulate the propagation of cracks and accelerate the destruction of rock,so that the mechanical behavior of rock under the hydro-mechanical coupling condition becomes complicated.In view of this,conducting mechanical test of hard rock which take from Bamiao tunnel,and based on the present studies,analyzing the mechanical properties,acoustic emission characteristics,the mechanical of energy evolution,progressive failure process and brittle feature of sandstone.Based on the analysis results,a new non-linear constitutive models is proposed for hard rocks under the hydro-mechanical coupling condition.The main research contents and results are as follows:(1)Through the rigid MTS815 servo testing machine,the mechanics characteristics of a deep sandstone were studied by the loading and unloading triaxial tests at different confining pressure and water pore pressure.Stress-strain curve has obvious deformation stage,and most of the samples appear brittle characteristics at the post-peak stage.Described the relationship between the maximum principal stress and confining pressure and water pressure by plane equation.It is concluded that the maximum main stress and confining pressure were positively correlated,and negatively correlated with water pressure.The water pressure can weaken rock strength,along with the increase of water pressure,the peak of strain and maximum volume strain trend to decrease,compression limit of rock decreased and failure occurred in advance.Compared with the conventional loading test,the peak strength and peak strain are smaller,the axial strain increment is smaller during the process of stress drop,and the damage process of rock under unloading condition is more severe.(2)Monitoring the acoustic emission phenomenon to analyze the acoustic emission characteristics of rock under the action of hydro-mechanical coupling.The acoustic emission activity is relatively calm at the beginning of the loading.The acoustic emission activity tend to be more active and cumulative ringing counts curve appeared inflection point during the stage of elastic-plastic deformation.When the stress approach the peak intensity,the acoustic emission activity is very active and cumulative ringing counts increase quickly.There are still a lot of acoustic emission activity after peak intensity.Ringing count amplitude decreases with the increase of confining pressure,and the acoustic emission characteristics of sandstone has no obvious regularity along with the change of water pressure.Compared with the conventional test,the acoustic emission rate of rock increases slightly under unloading test.From the perspective of the energy,the energy density increased gradually with the increase of stress,the total energy trend to become a concave curve with the increase of axial strain.Most of the energy stored in the form of elastic strain energy and the dissipation energy is small before the yielding strength.Elastic energy increase slowly and the speed of dissipation energy increase quickly when reaching the yielding strength.Elastic strain energy released and dissipation energy increase instantaneously when the stress reach to peak strength.The energy storage limit of sandstone increases gradually and the proportion of dissipation energy become greater with the increase of confining pressure.The energy storage limit of sandstone decrease gradually and the reduction increase with the increase of water pressure.(3)Progressive failure process and brittle feature of sandstone are studied based on test results.Along with the increase of water pressure,the closure stress of sandstone increase and the damage strength decrease.In the stage of crack closure,water pressure can restrain the close of micro crack.In stable stage of crack growth,pore water pressure will promote the formation of new cracks and stimulate the expansion of the original cracks,which makes the phase become shorter.The ratio of the damage stress to the peak strength decreases with the increase of water pressure in the unstable stage of crack growth.The brittleness of rock decreases with the increase of confining pressure,which indicates the confining pressure can inhibit the deformation and failure of rock.The rate of stress drop increases and the brittleness index increases with the increase of water pressure.Under unloading condition,the brittle characteristics of rock are further improved compared with the conventional loading test.Because of the existence of pore water pressure,the residual bearing capacity of rock is reduced,the brittleness characteristic is enhanced,and the failure process is more sudden and violent.(4)Based on the analysis of the characteristics of rock failure process,a new non-linear constitutive models is proposed for hard rocks under the hydro-mechanical coupling.The piecewise elastic model is used to replace the linear elastic model in the front section of the peak,and the continuous linear softening model is used to describe the stress drop section.The residual segment is regarded as an ideal plastic material.The results of the constitutive model proposed in this paper agree well with the experimental data.It shows that the elastic-brittle-plastic constitutive model can reflect the mechanical behavior of sandstone under the hydro-mechanical coupling condition.