Modeling time-dependent mechanical behavior of hard rock considering excavation-induced damage and complex 3D stress states

To investigate the long-term stability of deep rocks,a three-dimensional(3D)time-dependent model that accounts for excavation-induced damage and complex stress state is developed.This model comprises three main components:a 3D viscoplastic isotropic constitutive relation that considers excavation damage and complex stress state,a quantitative relationship between critical irreversible deformation and complex stress state,and evolution characteristics of strength parameters.The proposed model is implemented in a self-developed numerical code,i.e.CASRock.The reliability of the model is validated through experiments.It is indicated that the time-dependent fracturing potential index(xTFPI)at a given time during the attenuation creep stage shows a negative correlation with the extent of excavationinduced damage.The time-dependent fracturing process of rock demonstrates a distinct interval effect of the intermediate principal stress,thereby highlighting the 3D stress-dependent characteristic of the model.Finally,the influence of excavation-induced damage and intermediate principal stress on the time-dependent fracturing characteristics of the surrounding rocks around the tunnel is discussed.

Stress distribution and its influencing factors of bottom-hole rock in underbalanced drilling

The underbalanced drilling has been widely used due to its advantages of high drilling efficiency and low cost etc., especially for hard formation drilling. These advantages, however, are closely related to the stress state of the bottom-hole rock; therefore, it is significant to research the stress distribution of bottom-hole rock for the correct understanding of the mechanism of rock fragmentation and high penetration rate. The stress condition of bottom-hole rock is very complicated while under the co-action of overburden pressure, horizontal in-situ stresses, drilling mud pressure, pore pressure and temperature etc. In this paper, the fully coupled simulation model is established and the effects of overburden pressure, horizontal in-situ stresses, drilling mud pressure, pore pressure and temperature on stress distribution of bottom-hole rock are studied. The research shows that： in air drilling, as the well depth increases, the more easily the bottom-hole rock is broken; the mud pressure has a great effect on the bottom hole rock. The bigger the mud pressure is, the more difficult to break the bottom-hole rock; the max principle stress of the bottom-hole increased with the increasing of mud pressure, well depth and temperature difference. The bottom-hole rock can be divided into 3 regions respectively according to the stress state, 3 direction stretch zone, 2 direction compression area and 3 direction compression zone; the corresponding fragmentation degree of difficulty is easily, normally and hardly.

Experimental study of seepage characteristics of single rock fracture based on stress states and stress history

Through seepage tests under different loading and unloading confining pressures and different hydraulic gradients,the authors studied the effects of stress states and stress history on fracture permeability evolution for single granite fracture and sandstone fracture. The results show that there exists a linear relationship between the seepage discharge and osmotic pressure in sandstone fissure under each level of confining pressure. With the increasing in the confining pressure,the permeability of the fracture decreases,but the decreasing rate is changeing. During the unloading process,the fracture seepage velocity cannot be fully recovered to the size of the loading process. Therefore,in the unloading process of the confining pressure,the recovery of fracture permeability shows obvious hysteresis effects. The flow rate of the fracture remains unchanged during five cycles of loading and unloading processes of the confining pressure. In each cycle,the evolution character of the flow rate with the confining pressure remains unchanged. These experiments show that the seepage characteristics of sandstone and granite fractures are not the same under the same stress state.

Effect of intermediate principal stress on strength of soft rock under complex stress states

A series of numerical simulations of conventional and true triaxial tests for soft rock materials using the three-dimensional finite difference code FLAC3D were presented. A hexahedral element and a strain hardening/softening constitutive model based on the unified strength theory(UST) were used to simulate both the consolidated-undrained(CU) triaxial and the consolidated-drained(CD) true triaxial tests. Based on the results of the true triaxial tests simulation, the effect of the intermediate principal stress on the strength of soft rock was investigated. Finally, an example of an axial compression test for a hard rock pillar with a soft rock interlayer was analyzed using the two-dimensional finite difference code FLAC. The CD true triaxial test simulations for diatomaceous soft rock suggest the peak and residual strengths increase by 30% when the effect of the intermediate principal stress is taken into account. The axial compression for a rock pillar indicated the peak and residual strengths increase six-fold when the soft rock interlayer approached the vertical and the effect of the intermediate principal stress is taken into account.

Experimental study on the slip evolution of planar fractures subjected to cyclic normal stress

The frictional rupture mechanisms of rock discontinuities considering the dynamic load disturbance still remain unclear.This paper investigates the transitional behaviors of slip events happened on a planar granite fracture under cyclic normal stress with diferent oscillation amplitudes.The experimental results show that the activations of fast slips always correlate with unloading of normal stress.Besides,the intensive normal stress oscillation can weaken the shear strength which is recoverable when the normal stress return to constant.The rupture patterns are quantifed by stress drop,slip length and slip velocity.With the efect of small oscillation amplitudes,the slip events show chaotic shapes,compared to the regular and predictable style under constant normal stress.When the amplitude is large enough,the big and small slip events emerge alternately,showing a compound slip style.Large amplitude of the cyclic normal stress also widens the interval diferences of the slip events.This work provides experimental supports for a convincible link between the dynamic stress disturbance and the slip behavior of rock fractures.

Study of the Stress-Strain State in the Mined Potassium Massif with Inclined Bedding

This paper presents results which concern regularities of propagation of discontinuous zones from mined-out space in rock massif with inclined bedding structure. The dependencies of maximal height of these zones from the inclination angle of productive beds were found. The mechanical model of rock massif was chosen as viscoelastic and transversely isotropic. Finite element method was used for calculation performing. Coulumb-Mohr and maximal elongation criteria were used for determination of discontinuous zones.

Shear strength criteria for rock,rock joints,rockfill and rock masses:Problems and some solutions

Although many intact rock types can be very strong,a critical confining pressure can eventually be reached in triaxial testing,such that the Mohr shear strength envelope becomes horizontal.This critical state has recently been better defined,and correct curvature or correct deviation from linear Mohr-Coulomb（MC） has finally been found.Standard shear testing procedures for rock joints,using multiple testing of the same sample,in case of insufficient samples,can be shown to exaggerate apparent cohesion.Even rough joints do not have any cohesion,but instead have very high friction angles at low stress,due to strong dilation.Rock masses,implying problems of large-scale interaction with engineering structures,may have both cohesive and frictional strength components.However,it is not correct to add these,following linear M-C or nonlinear Hoek-Brown（H-B） standard routines.Cohesion is broken at small strain,while friction is mobilized at larger strain and remains to the end of the shear deformation.The criterion ＇c then σn tan φ＇ should replace ＇c plus σn tan φ＇ for improved fit to reality.Transformation of principal stresses to a shear plane seems to ignore mobilized dilation,and caused great experimental difficulties until understood.There seems to be plenty of room for continued research,so that errors of judgement of the last 50 years can be corrected.

某红层深挖路堑基底上拱离心模型试验研究

本文以成自铁路球溪站深路堑工点为研究对象,开展了外业调查、资料整理和模型选取,通过离心机模型试验分析开挖深路堑前后应力场和位移变化规律,探究路堑坡体开挖后基底不同深度水平应力与竖向应力变化的特征、相对大小关系及变形情况,结果表明:(1)深路堑坡体开挖后,基底埋深一定深度范围内,水平应力调整为大于竖向应力,基底中部上拱变形量最大;(2)水平应力与竖向应力的大小关系是导致深路堑基底产生上拱变形的关键因素。

A novel elastoplastic model for Yunnan sandstone under poly-axial loading

The lack of understanding of plastic hardening(softening)laws,especially under anisotropic stress conditions,results in inappropriate geotechnical management.Most of the yielding envelopes do not consider the effect of intermediate principal stress and the influence of Lode's angle.In addition,the application of plastic flow rules regarding yielding surfaces compromises the softening of rock internal friction as well as the influence of Lode's angle on the plastic potential.Moreover,the ductility to brittleness transition in the intermediate principal stress direction still requires a theoretical foundation.In this study,based on poly-axial testing results of Yunnan sandstone,we adopted a failure criterion with the intermediate principal stress proposed by Menétrey and Willam.The proposed new failure envelope was applied to capture the plastic evolution of rock samples.A plastic hardening-softening model is constructed,based on the framework of the plastic theory.The softening envelope is modified to better present the stress drop and considers the deterioration of rock internal friction in the post-peak stage of poly-axial loading.The differential of plastic potential according to the principal stresses is also modified,considering the rotation of Lode's angle in the poly-axial loading tests.The model results were compared with laboratory testing results,which showed great consistency across 9 different loading tests(5 under triaxial stress and 4 under poly-axial stress with 22 stress-strain curves in total).The induced brittleness by the intermediate principal stress is also well captured by the proposed model.

某超高层深基坑开挖对既有隧道变形和内力的影响分析

某超高层深基坑支护结构边线距隧道结构边线最近净距离仅6.7 m,采用有限元分析基坑支护设计和施工对紧邻已建成隧道变形和内力的不利影响。经有限元计算分析可知,通过施加预作用力的方式,使隧道围岩处于安全可控的应力状态,隧道变形和内力变化远小于控制标准,目前基坑已全部施工完成,隧道正常运营未受影响,相关施工经验值得借鉴。

庙堂沥青混凝土心墙堆石坝心墙基座嵌岩深度对基座应力的影响分析

沥青混凝土心墙堆石坝基座作为连接心墙与坝基的特殊结构,其受力状态对大坝的稳定至关重要。以在建的重庆庙堂沥青混凝土心墙堆石坝为例,选取其最大断面,建立二维模型进行有限元计算,分析探讨了基座在未嵌入基岩、半嵌入基岩、全嵌入基岩三种不同布置形式下的应力分布情况。结果表明,基座在基岩中的嵌入深度与基座内部的拉应力之间关联性较强。在蓄水期,不同布置形式之间拉应力相差最大可达334%,同时,由于廊道在蓄水期时受基座内部较大拉应力的影响,容易造成拉裂破坏,通过改变廊道在基座中的位置,降低廊道周围的拉应力,避免廊道产生破坏。廊道位置改变后,周围拉应力相比于原来下降幅度最大可达87%。

红一煤矿井田岩体应力分析及对首采工作面矿压显现影响

为了分析红一煤矿4煤层岩体应力空间分布特征及其与矿压显现的关系,利用地质动力区划方法对井田范围内的活动断裂进行识别和划分,以所划分的活动断块为构架,通过岩体应力状态分析系统计算确定了井田岩体应力状态,划定了不同的4个应力分区,并且明确了不同应力分区的微震及对矿压显现的影响。根据现场采集的各类矿压监测数据,与岩体应力分析结果进行对比分析,确定了矿压显现与岩体应力的耦合关系,确定井田矿压显现主要受到由活动断裂产生的高应力控制。研究结果可为红一煤矿矿压显现预测及其控制提供指导。

武汉周大福金融中心后压浆大直径嵌岩桩承载力计算

对现行行业标准和地方标准中后压浆大直径嵌岩桩承载力计算公式进行比较,指出了适用于武汉周大福金融中心单桩竖向承载力计算的公式,并在考虑上部土层尺寸效应、后压浆提高效应和桩端岩石三向应力状态后形成了三种计算方法,以计算结果与试桩成果进行对比验证,得出了哪种计算方法最优的结论。

不同岩体含水率对隧道围岩安全影响及防控措施的研究

以北京地铁隧道工程为案例,探索在持续降雨的过程中隧道上覆岩体的含水量不断提高,对隧道衬砌顶板、拱肩及拱腰关键部位的应力影响特点以及隧道衬砌最大主应力分布特点,分别设计上覆岩体含水量为10%、15%、20%、25%、30%五种工况,利用FLAC3D有限差分软件进行数值模拟分析计算,得出了含水率对垂直应力和水平应力的影响情况。在不同降雨工况下,对研究区域设计了不同的下凹式绿地雨水渗蓄优化方案,计算出了下凹式绿地在暴雨期间的雨水蓄渗能力,得出了不同工况下凹式绿地雨水渗蓄方案。

岩爆应力状态研究

通过地下洞室围岩的应力状态分析和真三轴压缩试验，研究了岩爆应力状态及其在围岩中的极限深度问题。研究结果表明，片状劈裂岩爆是在双轴压缩应力状态作用下在洞壁面上产生；剪切错动型岩爆是在真三轴应力状态下在围岩内部产生；围岩内部存在着产生岩爆的极限深度区，极限深度的深浅取决于地应力场中的垂直和水平主应力的比值大小。

考虑剪胀及软化的洞室围岩弹塑性分析的统一解

基于俞茂宏统一强度理论,考虑某些材料屈服后的强度衰减(塑性软化)和体积膨胀的特点,用弹性-塑性软化-塑性残余三线性应力-应变模型推导出了洞室围岩塑性残余区半径、塑性软化区半径、洞周边位移、围岩内任一点应力及围岩压力的解析表达式,给出了洞室围岩所处应力状态的判别式。得出的解答具有广泛意义,著名的Kastner公式、Airey公式和现有的一些结果为其特例。通过实例分析了剪胀、软化及不同强度模型对结果的影响。该结果在洞室围岩稳定性分析和加固等方面具有一定的工程实用价值。

岩石应力状态改变引起岩石热状态改变的研究

岩石的应力状态发生改变是否能引起岩石的热状态发生改变，是一个值得研究的重要问题，我们在实验室对岩石试件进行了实验，实验得出岩石的温度随岩石受到的压力增加而显著增加，在试件破裂前出现明显的破裂温度前兆；实验还发现岩石内部温度随压力变化的不均匀性，不同测温点的温度变化不同，其变化量可相差几度。不同岩性和不同颗粒粒径结构的岩石的温度随压力变化量有很大的差别。

深部巷道围岩的分区破裂机制及“深部”界定探讨

通过分析巷道围岩的应力状态和变形破坏多阶段、多水平的性状,揭示巷道围岩最大支撑压力区的体积变形状态及其后果,得出深部围岩区域破裂现象的发生条件、岩体的初始压力和围岩全过程变形状态,尤其是围岩峰值后状态下材料的残留强度关系。根据分区破裂现象的出现条件,提出界定浅部及深部工程活动的标准:浅部工程——坑道最大支撑压力区不破坏的深度;深部工程——坑道最大支撑压力区发生破坏的深度。

真三轴软岩非线性力学试验系统研制

根据深部软岩非线性力学行为研究的需要,针对目前的试验系统不能进行三向拉压、拉剪复合应力试验及加卸载过程模拟的不足,研制了一套能进行三轴拉压、拉剪等多种组合试验和对不同加卸载过程进行模拟的试验系统。介绍这一试验系统的主要结构、技术标准、功能和为取得满意的试验效果所研制和采用的减摩、传力和应力集中消除等技术,所述的试验系统为深入研究深部工程岩体的力学行为提供新手段。

拉剪应力状态下岩体裂隙扩展的断裂力学机制及物理模型试验

开挖卸荷岩体裂隙面通常处于拉剪应力状态。在裂隙应力和变形状态分析的基础上,采用线弹性断裂力学理论和物理模型试验研究了拉剪应力状态下裂隙扩展的力学机制。岩体裂隙在拉剪应力状态下沿裂隙面间的滑动抗剪摩擦力消失,裂隙起裂沿Ⅰ型张拉裂隙断裂韧度KⅠ最小的方向起裂,并最终发展与卸荷拉应力方向垂直;拉剪应力状态下岩体的总位移方向平行于拉应力方向,通过合理的位移假设,基于能量及线弹性断裂力学理论,求解了拉剪应力状态下分支裂隙扩展过程中尖端的动态应力强度因子和扩展长度判据;通过拉剪应力状态下单裂隙扩展物理模型试验验证了理论推导的正确性。