Study On Mechanical Properties And Failure Mechanism Of Red Sandstone Under Principal Stress Rotation

With the increasing of deep rock mass engineering,such as transportation,water conservancy and hydropower,and mining,the rock mass properties encountered in engineering construction become more and more complex,the surrounding rock stress will be redistributed during the excavation of deep rock mass engineering,including changes in the magnitude of the principal stress and rotation of the principal stress direction.At present,conventional rock mechanics testing equipment is mainly used to carry out experimental research on principal stress magnitude,the experimental research of rock mechanics considering principal stress rotation is still in its infancy.Moreover,due to the lack of relevant rock mechanics test equipment,most of the research of principal stress rotation in deep rock engineering is focused on numerical simulation.Therefore,the current research on rock mechanical properties under principal stress rotation is still insufficient,and the failure mechanism of surrounding rock under principal stress rotation is still unclear.Compared with shallow engineering,the principal stress gradient of deep underground rock mass during excavation is larger,and the mechanical properties,failure conditions and forms of surrounding rock mass are more complex.In order to ensure the safety of engineering construction,it is necessary to carry out research on rock mechanical properties and mechanism under complex stress path of deep underground rock mass.This paper adopts the general research idea of "engineering simulation analysis → mechanical phenomenon disclosure → laboratory test study →mechanical property analysis→failure mechanism disclosure".Firstly,through numerical simulation of deep and long tunnel excavation,the temporal and spatial evolution law of principal stress and damage deformation of surrounding rock were studied,and the mechanical phenomenon of principal stress direction rotation during the excavation process was revealed;Secondly,in view of the rock mechanics problem under principal stress rotation,the conventional triaxial compression test and principal stress rotation test of red sandstone hollow cylinder were carried out using the rock hollow cylinder torsion shear test system;On this basis,the mechanical properties under rotation and non-rotation of principal stress are compared and analyzed,and finally,the failure mechanism of red sandstone under rotation of principal stress is revealed,and it provides scientific basis for the safe and efficient construction of actual underground engineering.The main research results and conclusions of this paper are as follows:(1)Numerical simulation of the excavation process of deep and long tunnels is carried out,the evolution law of the surrounding rock stress field during the excavation process is obtained,the changes in the magnitude and direction of the principal stresses of surrounding rocks at different monitoring locations can be divided into three stages:minor adjustment phase,rapid adjustment phase,and steady change phase.The failure proximity(FAI)is used to quantitatively describe the damage change and distribution law of surrounding rock during tunnel excavation,the results show that the damage evolution of surrounding rock affected by excavation is basically consistent with the evolution law of principal stress direction,and the change of principal stress direction has an important effect on the evolution of surrounding rock damage.Based on the evaluation index of stress triaxiality,the stress state change of the surrounding rock during the excavation process is analyzed,it was found that when the tunnel face crossed the monitoring section,the change of the stress state of the surrounding rock shows a transition process from compression→shear→tension→compression,accompanied by the rotational mechanical phenomenon of principal stress.(2)The thin-walled cylinder theory is used to analyze the stress state at the midpoint of the rock hollow cylinder,combined with the rock hollow cylinder torsion shear test system,and the transformation relationship between the stress of the element in the hollow cylinder and four independent test loads is derived.Furthermore,the stress path,test method and technique of conventional rock mechanics test(uniaxial compression test,tensile strength test and triaxial compression test)and principal stress rotation test are analyzed,which lays a theoretical foundation for laboratory rock mechanics test under principal stress rotation and non-rotation condition.(3)Adopt the conventional triaxial stress path with the principal stress not rotating,the hollow cylinder torsional shear test system is used to perform conventional triaxial compression tests on hollow cylindrical specimens of red sandstone.Mohr-Coulomb strength criterion and Hoek-Brown strength criterion were used to calculate the mechanical parameters of red sandstone under the condition of non-rotation of principal stress,and combined with the fracture morphology and fracture angle of red sandstone under different confining pressures,the failure mechanism of red sandstone under the condition of non-rotation principal stress is revealed.The results show that the cohesion of red sandstone increases from the crack initiation point to the peak point,due to the connection and coalescence of the rupture surface after the peak of red sandstone,the cohesion drops sharply until it reaches the minimum value at the residual point,but the internal friction angle is the smallest at the crack initiation point,and gradually increases in the process of stress increasing to the peak,and gradually decreases after the peak.At the same time,with the increase of confining pressure,the rock fracture angle decreases gradually,and the degree of brittleness decreases gradually.(4)Compared with the condition of no rotation of principal stress,carry out the principal stress rotation test under different average stress p,it is found that under the stress path where the principal stress rotates to the specimen failure,the influence of rotation angle of principal stress axis on rock strength is related to the degree of damage inside the rock.on the other hand,the rock strength of the principal stress axis rotation test with a fixed rotation angle and the conventional triaxial compression test are compared and analyzed.It is found that as the average stress increases,under the rotation of principal stress,the strength of red sandstone changes from a strength higher than the conventional triaxial rock to a strength lower than the conventional triaxial rock.Furthermore,establish the functional relationship between the rock strength parameters c,Φ and the average stress p under these two stress paths,it is found that the strength parameters of red sandstone are not sensitive to hydrostatic pressure in conventional triaxial compression tests,however,under the condition of principal stress rotation,the strength parameters change obviously with the hydrostatic pressure,and the principal stress rotation has a very significant effect on the mechanical properties of rock.(5)According to SEM images,the fracture morphology and microscopic failure characteristics of red sandstone fracture surface under different stress paths were qualitatively analyzed.The fractal characteristics of pore structure in the slice image after rock failure were quantitatively studied,under conventional triaxial compression tests,the fractal dimension and porosity of rock pore structure decrease with the increase of confining pressure.Furthermore,the variation of fractal dimension of rock pore structure under different stress paths was compared and analyzed,revealing the failure mechanism of red sandstone under principal stress rotation: Under the lower average stress,the principal stress rotation compacts the cracks inside the rock,and the fractal dimension of the pores is smaller than that under conventional triaxial compression;Under the medium average stress,the damage of red sandstone is aggravated due to the rotation of principal stress,and the fractal dimension of pore structure increases with the increase of average stress;Under the high stress levels,the fractal dimension of pore structure under principal stress rotation decreases with the increase of average stress,indicating that confining pressure at high stress levels inhibits red sandstone damage evolution.