A Micro-Macro Model Of Cyclic Creep Rupture On Brittle Rocks Under High Seepage Pressure

With the rapid development of China’s society and economy,the number,scale,and burial depth of major rock engineering projects have shown an immediate increase,such as the Jinping diversion tunnel,Baihetan hydropower station,Yunnan Central Water Transfer Project,Qinghai-Tibet Railway,Sichuan-Tibet Railway,Bohai Bay Passage,etc.These deep-ground projects are affected by high seepage pressure,high ground stress,and cyclic load disturbance during the construction and operation stages.It is beyond dispute that these effects are important causes of surrounding rock deformation and other disasters.Then it is necessary to fully understand the mechanical properties of rocks under those effects,especially the long-term creep evolution law.Still,there is a lack of research on micro-macro mechanical relationships in this area.This paper establishes a micro-macro mechanical model for creep fracture of brittle rocks,first considering the effect of water content.The model is based on the wing crack extension model,the subcritical crack law,and the crack-strain model,which provides a basis for studying the mechanical properties of rocks under seepage pressure and selecting exemplary mechanical parameters.Second,improving the wing crack extension model draws the strain relationship between the rock stress state and extended crack length under the seepage pressure.The Mohr-Coulomb model is combined to establish the model of brittle rock shear fracture under the influence of seepage pressure.Based on the above micro-macro mechanical model considering water content and seepage pressure,introducing the cyclic axial loading and unloading function and establishing the recovery mechanism of the extended wing crack.The elastic-viscous-plastic creep evolution curve is obtained by combining the Hooke-Kelvin model.Finally,by introducing the evolution path of cyclic seepage pressure,the effect of cyclic seepage pressure on the creep fracture characteristics of brittle rocks under constant stress is studied.Through the above research,some progress has been made in the following areas:(1)The influence of water content on the stress-strain constitutive relationship of brittle rock is studied,and the influence on its characteristic values is analyzed.It is found that the crack initiation stress,peak stress,and elastic modulus all decrease gradually with the increase in water content.By discussing the effect of water content on the creep evolution of rock,it is found that the creep of rock under the action of water should change greatly,and it is more prone to creep failure and failure.(2)According to the macro-micro mechanical model of shear fracture of brittle rock under seepage pressure,it can be found that the cohesion,internal friction angle,shear strength,and shear failure angle all decrease nonlinearly with the increase of seepage pressure.The internal friction angle,shear strength,and shear failure angle increase with the increase of confining pressure,while the cohesion decreases slightly with the growth of confining pressure.(3)Under cyclic axial stress loading and unloading and constant confining pressure,the greater the seepage pressure,the greater the viscoelastic strain,plastic strain,total strain,and rebound value of the rock,while the smaller the cyclic creep failure stress and the shorter the failure time.With the increasing confining pressure,viscoelastic strain,plastic strain,total strain and strain rebound of rock decrease,and cyclic creep failure time is longer.(4)Under the gradual increase of cyclic seepage pressure loading and unloading,the total strain decelerates and increases in both loading and unloading stages while the strain rate decreases gradually.The difference is that the initial value of the strain rate in the loading stage is significantly greater than that in the unloading stage.With the increase of cyclic osmotic pressure loading and unloading value,the viscoelastic strain,plastic strain,total strain,and strain rate all increase.While the viscoelastic rebound value decreases to a certain extent,and the failure time is shorter.