Study On Unloading Deformation Mechanism And Stability Of Excavating Surrounding Rock Mass Of Large Underground Caverns

The geological condition of constructing underground powerhouse site in right band of three gorge project is complex and the excavating scale is great, the length of powerhouse is 311.3m, the width 32.6m and height 87.3m. Excavating a lot of underground rock masses must bring surrounding rock mass stress to change, lead it up to unloading and loosening in the excavated direction, and unloading difference deformation is evident. However, the study method to stability problem of the large project with rock mass excavated usually pertain to the of category loading rock mass mechanics, but on the condition of unloading, mechanical character, constitutive model and mode of cracks extending differ from loading case, so, evaluation to stability of surrounding rock mass with the method of loading rock mass mechanics is faulty. Based on investigation to underground powerhouse excavated of three gorge and a lot of rock unloading test, the paper researches mechanical character, constitutive model, deformation failure mechanism and stability of surrounding rock mass when rock mass unloading by way of statistics, elasto-plastic rock mass mechanics, fracture mechanics, analysis of engineering geology and simulation. The major research findings are described as follows:(1) Based on unloading test of rock, the character of rock deformation on the condition unloading, parameters of unloading rock mass becoming to poor in course of unloading and stress brittle falling off after peak value are revealed. Strong dilatation becourse of unloading makes rock deformation and failure, the character of brittle failure after peak value is evident, and the coefficient of the stress brittle falling takes on fine correlativity with initial confining pressure. Deformation modulus 'E' gradually decreases but Poisson's ratio 'μ' increases in course of unloading, the range of E decreasing is about 5% to 25%,μincreasing about 50% to 300%, the changing extent is enhanced along with initial confining pressure and unloading strength increasing, the change better correlates with volumetric strain. The friction angle of Unloading rock mass (φ) a few increases but the cohesion (c) consumedly decreases relative to general compression tests: a) when unloading confining pressure and loading axial compression on the same time, the peak value of c decreases about 33.2% and the remnants value about 65.3%, the peak value ofφincreases about 14.7% and the remnants value about 33.2%; b) when unloading confining pressure and axial compression on the same time, the peak value of c decreases about 47.8% and the remnants value about 77.6%, the peak value ofφincreases about 9.4% and the remnants value about 5.9%.(2) The rock mass failure on the condition of unloading accords with Griffith yield criteria about yield dot, but accord with Mohr-Coulomb yield criteria after arriving peak value. On the assumption that the rock mass yield accords linear transition from Griffith to Mohr-Coulomb criteria along with volumetric strain on the condition of unloading, the yield criteria and corresponding constitutive equations of unloading rock mass can be established. The stress-strain curve can be divided into four phase: elasticity, unloading yield, brittle after peak value and ideal plastic in remains phase, complete constitutive model can be established by four phase constitutive equations are solved.(3) Based on crack rock mass similar model test, the strength, deformation, failure shape and the crack expanding evolutionary process of crack rock mass on the condition of unloading are revealed. Unloading failure strength of rock mass decrease along with included angle of crack and unloading direction in single crack model. Rock bridge strength of combined model high with low dip angle is minimum and high with high dip angle is relative higher. Crack expanding on the condition of unloading is phase and paroxysmal, the deformation must appear multilevel kick, a great extent phase step when cracks should prong, and the times of displacement kick positively correlate with amount of fresh cracks. Unloading expanding crack commonly bear definite degree tensile, their expanding modes of unloading expansion can be divided into four kinds: shear , tensile, tensile-shear and wing cracks expansion. The bridge failure can be classified into three modes: shear , tensile-shear and tensile failure, especially tensile-shear failure.(4) In course of underground excavation and unloading, the state of stress on cracks surface gradually changes from compressive-shear to tensile-shear, based on rupture mechanics theory, by Contrastive analysis of stress intensity factors when crack expanding under compressive-shear and tensile-shear stress state, we find that the cracks more easily expand on the condition of unloading. The stress intensity factors of embranchment cracks tip are researched in course of unloading, especially deduced under tensile-shear, and the length of crack expansion is made certain.(5) Based on construction monitoring and geological condition of powerhouse, unloading deformation mechanism and stability of excavating surrounding rock mass are analyzed. The systemic numerical simulation is carried out in order to studying on stability of surrounding rock mass including faults on the condition of excavation and unloading, and in order to evaluate the influence of distribution position of faults, initial field stress in site and shear strength of faults on the stability, some quantified index are present, such as the coefficient of maximum stress concentrating, the area of stress loosening, maximum deformation, the deformation of characteristic position and the area of plastic zone.(6) Based on the theory of blocks and analysis of engineering geology, the failure modes of block in powerhouse are summarized. 3-D numerical simulation of typical blocks shows that the secondary stress field, deformation and distribution of plastic zone are different in different position. The quantified index of part stability and evolutionary process of deformation failure of blocks are present. the method of strength reduction is applied in order to evaluate the whole stability of blocks in powerhouse,. The influence of stress on the stability of blocks and the limitation of limit equilibrium theory are explained.(7) the unloading effect when rock mass excavated isn't considered in common elasto-plastical numerical analysis, in the paper, the program of numerical calculation considering parameter becoming to poor in course of unloading compiled by using fish language in FLAC software, and a 3-D numerical simulation of excavation stability of powerhouse is carried out, the calculation results accord with monitoring datum and more agree with actual engineering excavation than common elasto-plastical analysis.