大型地下洞室开挖围岩卸荷变形机理

正在施工的三峡工程右岸地下电站区地质条件复杂，开挖规模大，主厂房长311.3m×宽32.6m×高87.3m，大量地下岩体开挖必将引起围岩应力场的变化，致使围岩应力在开挖方向卸荷松驰，卸荷差异变形明显。而这种大型岩体开挖工程稳定性问题，在过去一般是运用加载岩体力学的方法来研究，但卸荷条件下，岩体的力学特征、本构方程、裂隙扩展方式与加载时是不同的，因此用加载岩体力学的方法来评价围岩的稳定性是不完善的。本文以三峡地下电站主厂房施工开挖为工程背景，以大量岩石(体)卸荷试验和主厂房施工监测成果为基础，以数理统计、弹塑性岩体力学、断裂力学、工程地质分析及数值分析为研究手段，对卸荷条件下岩体的力学特性、本构模型、变形破坏机理及围岩稳定性等方面从理论到工程应用进行了较为系统的研究，取得以下主要研究成果：(1)基于岩石卸荷试验，揭示了岩石卸荷变形、卸荷过程中岩石参数的弱化及峰后应力脆性跌落特征。卸荷引起的强烈扩容是岩石变形破坏的根本原因，具有较强的峰后应力脆性跌落特征，应力脆性跌落系数与初始围压具有较好的相关性。卸荷过程中岩体的变形模量E逐渐减小而泊松比μ逐渐增大，E减小约5％～25％，μ增大约50％～300％，这种变化幅度随初始围压的增大和卸荷程度的增强而变大，其变化与体积应变有较好的相关性。相对常规压缩试验，卸荷岩体的摩擦角φ有所增大而粘聚力c却大大减小：卸载围压且同时升轴压试验的峰值c减小约33.2％，残余c减小约65.3％，峰值φ增大约14.7％，残余φ增大约33.2％；同时卸载围压和轴压试验的峰值c减小约47.8％，残余c减小约77.6％，峰值φ增大约9.4％，残余φ增大约5.9％。(2)卸荷岩体在屈服点附近符合Griffith屈服准则，而到达峰值后符合Mohr-Coulomb屈服准则，假设卸荷岩体屈服是随体积应变从Griffith屈服至Mohr-Coulomb屈服线性过渡，可构造出卸荷岩体的屈服准则，进而求出其相应的本构方程。将岩体卸荷应力～应变曲线分为弹性、卸荷屈服、峰后脆性及残余理想塑性四个阶段，求出其相应段的本构方程，得出卸荷岩体全过程的本构模型。(3)基于裂隙岩体相似模型试验，揭示了卸荷条件下裂隙岩体的强度、变形特征、破坏形态及裂隙扩展演化过程。在单裂隙岩体中，岩体的卸荷破坏强度随裂隙与卸荷方向夹角的增大而减小的，而在双裂隙模型中，陡～缓倾角组合岩桥强度最低，陡倾角组合岩桥的强度相对较高；裂隙卸荷扩展具有阶段性和突发性，变形会出现多级突跳现象，当发展到裂隙贯通时，位移会出现大幅度的阶跃，位移突跳的次数与新生裂纹的数量正相关。卸荷扩展裂隙一般带有一定程度的张性特征，其卸荷扩展方式有剪切扩展、张拉扩展、拉剪复合扩展及冀裂纹扩展四种类型，岩桥卸荷破坏可归纳为剪切破坏、张剪复合破坏和张拉破坏，以张剪复合破坏为主。(4)地下洞室开挖卸荷过程中，裂隙面的应力状态经历了从压剪应力状态逐渐向拉剪应力状态的变化过程，基于断裂力学理论，对压剪及拉剪切应力状态下裂隙扩展时的应力强度因子进行对比分析，说明卸荷条件下裂隙更容易扩展。同时也探究了裂隙扩展过程中分支裂隙尖端的应力强度因子，进而确定裂隙扩展的长度，首次推导了拉剪应力状态下裂隙扩展过程中分支裂隙尖端的应力强度因子。(5)基于主厂房施工监测，结合工程地质条件分析围岩的开挖卸荷变形机理及其稳定性。对含断层围岩的开挖卸荷稳定性进行较系统的数值模拟，提出了以最大应力集中系数、应力松驰区面积、最大变形量、特征部位变形大小和塑性区面积等量化指标评价断层分布部位、初始地应力场和断层抗剪强度参数对围岩稳定性影响的方法，阐述了含断层围岩开挖卸荷变形机理及其稳定特征。(6)根据块体理论及工程地质分析，总结了主厂房开挖块体的失稳模式。对不同部位典型块体数值模拟分析表明不同部位的块体二次应力场、变形及塑性区分布特点不同，提出了块体变形失稳演化机理和局部稳定性评价的量化指标，同时将强度折减法应用到主厂房块体整体稳定性评价中并与极限平衡稳定性计算结果进行比较，阐述了地应力场对块体稳定性的影响和极限平衡计算中存在的缺陷。(7)一般弹塑性数值分析并没有考虑开挖岩体的卸荷效应，利用FLAC软件内置fish语言，编写了考虑卸荷过程中岩体参数弱化的数值计算程序，并对主厂房开挖稳定性进行了模拟计算，计算结果与监测相符，较一般弹塑性计算结果更符合工程开挖实际。
电加热器
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.