Study of the initiation and propagation of excavation damaged zones around openings in argillaceous rock

The study of the creation and evolution of the excavation disturbed zone (EDZ) in argillaceous rocks is a major issue for the safety of nuclear wastes underground repositories. In this context, the argillaceous Tournemire site has provided a unique opportunity to study the evolution of the EDZ with time thanks to the existence of three openings of different ages. A thorough characterization of the EDZ has been conducted by different means such as visual observation, analysis of samples extracted from drilled boreholes, EDZ permeability measurements, etc. On the basis of these measurements, a conceptual model of the EDZ initiation and propagation at the Tournemire site has been proposed. In order to validate this model, numerical simulations of increasing complexity have been carried out. In a first attempt, the response of the rock mass to the excavation phase, followed by seasonal cyclic variations of temperature and relative humidity inside the opening, has been simulated by means of a purely mechanical analysis, using a simple elastic material model. The EDZ has been estimated by post-processing the calculated stress states, using a Mohr–Coulomb failure criterion. The results obtained show that no EDZ could be predicted unless adopting a low cohesion value for the rock mass. Moreover, the deferred nature of the EDZ formation in Tournemire could not be reproduced. These limitations have then been suppressed by using a coupled viscoplastic-damaging mechanical model, the parameters of which have been identified from different laboratory experiments. With this model, a time evolution of the EDZ could be predicted, but the EDZ pattern could not match the one observed in situ. Finally, in view of the importance of the hydraulic couplings, unsaturated hydro-mechanical calculations have been carried out to investigate the effect of the numerous seasonal variations cycles and the resulting shrinkage.     

多尺度节理岩体力学特性的颗粒流分析

以白云鄂博露天铁矿东矿岩质高边坡为工程背景,结合现场地质调查、室内岩石和节理力学试验等数据,采用等效岩体技术,构建能充分反映节理分布特征的实验室、现场原位试验和工程尺度等多尺度等效岩体模型。通过对各类等效岩体模型进行单轴压缩试验,研究岩体单轴抗压强度、弹性模量等力学特性的尺寸效应和各向异性。研究表明：节理的存在使岩体表现出尺寸效应和各向异性,且随着尺寸的增大,这种特性基本呈逐渐减弱的趋势;研究区域岩体的表征单元体积、单轴抗压强度和弹性模量分别为20 m×10 m×10 m、1.46 MPa和3.91 GPa;岩体单轴抗压强度、弹性模量与轴向尺寸的关系,近似符合渐进式指数函数关系,且该函数能直观地给出工程尺度岩体的力学特性。     

Modeling the Effects of Heterogeneity and Anisotropy on the Excavation Damaged/Disturbed Zone (EDZ)

Summary  When modeling the mechanical behavior of underground excavations, it is necessary to include the influence of the rock mass characteristics on the Excavation Damaged/Disturbed Zone (EDZ). In this paper, the Realistic Failure Process Analysis code, RFPA, is used to model the extent of the EDZ. The inhomogeneous characteristics of rock at the mesoscopic level are included by assuming that the material properties of the constituent elements conform to a Weibull distribution; the anisotropy is incorporated as a transversely isotropic medium; the non-elastic characteristic is simulated via an elastic damage-based constitutive law. A finite element program is adopted as the basic stress analysis tool. In this study, a notable feature is that no a priori assumptions need to be made about where and how fracture and failure will occur – cracking can take place spontaneously and can exhibit a variety of mechanisms when certain local stress conditions are met. The deformation and failure process of anisotropic rock around excavations of different geometries is analyzed, and compared to experimental tests, showing similar fracture patterns. Additionally, the effect of confining stress and of different material layers is modeled and discussed. It is found that the model clearly illustrates that fracturing, both initiation and propagation, occurs as a combination of the stress concentrations and weakness planes introduced via the transverse anisotropy – which could represent either foliations or ubiquitous joint sets. Correspondence: Dr. Shuhong Wang, Box 265, School of Resource and Civil Engineering, Northeastern University, Shenyang 110004, P.R. China     

Analysis of Fracture Mechanics Tests on Opalinus Clay

Many studies have recently been conducted to evaluate various mechanical characteristics of the Opalinus Clay (OPA) formation in view of its potential use as the hosting rock for the Swiss nuclear waste repositories. Its sedimentary bedding makes OPA a transversely isotropic rock and its directional mechanical properties need to be measured. This paper reports on an experimental and computational approach that was adopted to define the parallel-to-bedding fracture mechanics (FM) parameters of OPA in Mode-I. OPA cores from Mont Terri Underground Research Laboratory (URL) were submitted to laboratory tests on notched semi-circular specimens under three-point bending (SCB). In these tests, crack propagation is forced along the notch direction. However, the 45° bedding inclination of the specimen axis frequently deviated the crack from the expected direction. An analysis of the SCB tests was performed by means of non-linear FM techniques and the pertinent Mode-I parameters along the bedding were estimated.     

Assessment of the Excavation-Damaged Zone in a Tall Rock Slope Using Acoustic Testing Method

During slope excavation, high stresses can become concentrated in the rock mass because of stress redistribution. Failure of the rock mass creates an excavation-damaged zone (EDZ) in the slope. The damage reduces the acoustic wave velocity in the rock mass. Results of field tests measuring acoustic wave velocity at the Jinping I Hydropower Station are used here to study the EDZ in a tall rock slope. Two acoustic testing methods were adopted in the field tests: single-hole acoustic testing (SAT) and cross-hole acoustic testing (CAT). The acoustic wave velocity was lower in the EDZ, and the depth of the EDZ increased with decreasing slope elevation. Statistical analysis shows that the acoustic wave velocity obtained by the SAT method is larger than that obtained by the CAT method, and the relative difference between the SAT- and CAT-derived velocities is lower for a high quality rock mass than for a low quality rock mass. The integrity ratio and severity of damage can also be determined by acoustic wave velocity test results, revealing that the integrity ratio and elastic modulus of a rock mass are reduced in the EDZ.     

Investigation into water retention and gas permeability of Opalinus clay

Opalinus clay (OPA) is currently being investigated as a potential host rock for radioactive waste repository. The construction of this repository will lead to an excavation-damaged zone (EDZ) in the surrounding. Its sealing ability is crucial for the safety assessment of the geological repository. The sealing ability of OPA has a close relationship with its water retention and gas permeability behaviours. For this purpose, the water retention and gas permeability of OPA and its comparison with the artificial barrier (i.e. bentonite) were investigated in this study. The results show that OPA absorbed less water than granular bentonite material with equal suction. Compared with the other two similar materials (Boom clay and COx argillite), which were selected as natural engineering barriers for nuclear waste storage in Belgium and France, the suction behaviours of OPA are similar to those of COx argillite but notably different from those of Boom clay. The gas permeability tests show that OPA sample is quite sensitive to the changes in confining pressures but is less sensitive to applied gas pressures. Further, it is found that the OPA is more permeable than bentonite barrier after 10.5 years of hydration. Then, the OPA samples were artificially water-saturated. We find that the sealing ability can be recovered after long-time water saturation. However, when comparing with bentonite barrier, it is still a weak zone. This indicates that the EDZ should be paid more attention for the assessment the performance of the nuclear repository.     

高地应力硬脆性围岩开挖损伤区时效性扩展案例分析——特征与机制

硬脆性围岩在开挖完成后，其强度在高应力的影响下具有明显的时间效应，这导致围岩开挖损伤区的发展也呈现出与时间相关的特征。在岩石强度时效性演化模型的基础上，以锦屏二级水电站试验洞钻孔摄像、声波、变形监测等开挖损伤区实测结果为目标函数，采用正交设计方法、最小二乘支持向量机模型、粒子群优化算法等方法，建立了考虑时间效应的LSSVM-PSO智能反演分析方法，并以锦屏二级水电站试验洞为例，研究了开挖完成后的25 d里，围岩强度在高地应力条件下的时效性演化特征，进而获得这一时段内开挖损伤区扩展过程。研究结果表明：（1）高应力地区，隧洞开挖后，围岩损伤区的主要扩展方向受地应力控制，且最大扩展方向为最小主应力方向，且破坏区（破坏接近度FAI≥2）也集中于该方向； （2）开挖损伤区面积随时间近似呈S形曲线变化，表明开挖损伤区初始发展较为缓慢，随着时间推移呈现线性增加的趋势，最后又逐渐趋于稳定；（3）开挖后第3～10 d为开挖损伤区快速增长阶段。该研究成果对高应力地区硬脆性围岩开挖损伤区时效性演化研究具有指导意义。     

Evolution of anisotropy with saturation and its implications for the elastoplastic responses of clay rocks

Many clay rocks have distinct bedding planes. Experimental studies have shown that their mechanical properties evolve with the degree of saturation (DOS), often with higher stiffness and strength after drying. For transversely isotropic rocks, the effects of saturation can differ between the bed-normal (BN) and bed-parallel (BP) directions, which gives rise to saturation-dependent stiffness and strength anisotropy. Accurate prediction of the mechanical behavior of clay rocks under partially saturated conditions requires numerical models that can capture the evolving elastic and plastic anisotropy with DOS. In this study, we present an anisotropy framework for coupled solid deformation-fluid flow in unsaturated elastoplastic media. We incorporate saturation-dependent strength anisotropy into an anisotropic modified Cam-Clay (MCC) model and consider the evolving anisotropy in both the elastic and plastic responses. The model was calibrated using experimental data from triaxial tests to demonstrate its capability in capturing strength anisotropy at various levels of saturation. Through numerical simulations, we demonstrate the role of evolving stiffness and strength anisotropy in the mechanical behavior of clay rocks. Plane strain simulations of triaxial compression tests were also conducted to demonstrate the impacts of material anisotropy and DOS on the mechanical and fluid flow responses.     

A particle mechanics approach for the dynamic strength model of the jointed rock mass considering the joint orientation

In nature, there exist several forms of anisotropy in rock masses due to the presence of bedding planes, joints, and weak layers. It is well understood that the anisotropic properties of jointed rock masses significantly affect the stability of surface and underground excavations. However, these critical anisotropic characteristics are often ignored in existing uniaxial dynamic failure criteria. This study investigates the effect of a pre-existing persistent joint on the rate-dependent mechanical behaviours of a rock mass using a particle mechanics approach, namely, bonded particle model (BPM), to realistically replicate the mechanical response of the rock mass. Firstly, in order to capture the rate-dependent response of the jointed rock mass, the BPM model is validated using published experimental data. Then, a dynamic strength model is proposed based on the Jaeger criterion and simulation results. To further investigate the dynamic behaviours, the dynamic uniaxial compressive strength (UCS) for anisotropic rock masses with various joint orientations is investigated by subjecting the BPM models to uniaxial compression numerical tests with various strain rate. The proposed dynamic strength model is validated based on numerical simulation results. Finally, the fragmentation characteristics of the jointed rock masses are analysed, which demonstrate that the failure mode affects the dynamic UCS. This is further confirmed by the analysis of the orientations of microscopic cracks generated by the compression loading.     

Coupled THM processes in EDZ of crystalline rocks using an elasto-plastic cellular automaton

This paper aims at a numerical study of coupled thermal, hydrological and mechanical processes in the excavation disturbed zones (EDZ) around nuclear waste emplacement drifts in fractured crystalline rocks. The study was conducted for two model domains close to an emplacement tunnel; (1) a near-field domain and (2) a smaller wall-block domain. Goodman element and weak element were used to represent the fractures in the rock mass and the rock matrix was represented as elasto-visco-plastic material. Mohr–Coulomb criterion and a non-associated plastic flow rule were adopted to consider the viscoplastic deformation in the EDZ. A relation between volumetric strain and permeability was established. Using a self-developed EPCA^2D code, the elastic, elasto-plastic and creep analyses to study the evolution of stress and deformations, as well as failure and permeability evolution in the EDZ were conducted. Results indicate a strong impact of fractures, plastic deformation and time effects on the behavior of EDZ especially the evolution of permeability around the drift.     

雪峰山隧道砂板岩各向异性力学特性的试验研究

利用MTS815 Flex GT岩石力学试验系统,对雪峰山隧道围岩中的砂板岩开展单轴和三轴试验,研究这种砂板岩中的细微层理对岩石变形特性、强度特性及其参数的影响,结果表明：岩石力学特性的各向异性特征显著。层理面与轴向力夹角0°时应力-应变曲线呈不稳定破裂特征,破坏面沿层理面方向发育;夹角90°时曲线呈峰后迅速软化特征,破坏面为对角贯通性剪切破坏。单轴试验中夹角0°的抗压强度比夹角90°高出约20%,弹性模量和变形模量比夹角90°分别约大50%和80%。三轴试验中2种夹角情况破坏时主应力差 相近,夹角0°的弹性模量和变形模量分别比夹角90°时约大6%和20%,围压对砂板岩的各向异性特征有弱化效应。这些结论揭示了该砂板岩各向异性的力学特性,对解决工程实际问题有重要的参考价值     

考虑空间效应的地下洞室爆破开挖松动区参数场位移反分析

将地下洞室爆破开挖松动区视为一个随开挖过程演变的非均匀、非稳定三维扰动场,松动区内岩体力学参数则是一个具有时空演化特性的参数场。考虑爆破开挖扰动空间效应和岩体真实变形响应,提出了真实工作状态下开挖松动区岩体参数场辨识的位移反分析方法。基于局部监测变形空间插补得到的空间位移场,通过分析洞室爆破开挖围岩变形扰动机制,建立了开挖松动区岩体变形模量参数场数值演化模型,并进行了模型适用性和参数敏感性分析。在此基础上,以变形模量参数为例,结合围岩实测位移信息,提出了开挖松动区参数场位移反分析的动态实现过程。将该方法应用于溪洛渡地下洞室群施工期参数场反演和围岩稳定动态反馈评价及预测,结果表明,该方法合理有效,在大型地下洞群施工开挖与快速监测反馈方面具有显著的工程适用性及实用性。     

统计岩体力学(SMRM)——岩体工程地质力学的传承与发展

本文是中国地质学会工程地质专业委员会和中国科学院页岩气与地质工程重点实验室第五届谷德振讲座主讲报告。本文简要回顾了谷德振先生岩体工程地质力学的核心价值和历史贡献,系统介绍了30年来统计岩体力学理论和应用技术探索对岩体工程地质力学的传承和发展。统计岩体力学借鉴经典统计物理学的思想方法,提出了岩体结构几何概率模型、断续介质连续等效的断裂力学能量原理、岩体结构-应力协同控制原理、岩体强度的弱环控制原理、断续裂隙网络渗流力学原理;建立了裂隙岩体本构模型、岩体全过程变形分析方法、高储能岩体特性与岩爆机理模型、圆形硐室围岩弹性变形解析解;发展了岩体数据现场采集技术与装备、全空间方向岩体结构参数、力学参数、渗透系数计算方法,以及各向异性岩体质量分级、工程岩体主动加固方法;开发了岩体工程参数计算系统和数值分析工具JointModel。统计岩体力学的建立对岩体工程地质力学和岩体力学理论与技术进步起到了重要的推进作用。     

The influence of an excavation damaged zone on the thermal-mechanical and hydro-mechanical behaviors of an underground excavation

In Korea, a reference disposal system, KRS, was proposed in 2006 after 10 years of research and development. In the KRS, the high-level radioactive waste repository is considered to be located in a crystalline rock likes granite. For a validation of the feasibility, safety, and stability of the KRS, an underground research tunnel, KURT was constructed in Nov. 2006. During the construction of KURT by a controlled blasting, the size and characteristics of an excavation damaged zone(EDZ) were investigated by in situ as well as laboratory tests. The possible influences of an EDZ around a tunnel on the thermal, hydraulic and mechanical behaviors of the near field were investigated by using hydro-mechanical and thermo-mechanical coupling analyses. From this study, it was found that the existence of an EDZ can influence the thermal, hydraulic, and mechanical behaviors of the near field and it was recommended that an EDZ should be considered as an important parameter during the design of underground repositories.     

Assessments of Strength Anisotropy and Deformation Behavior of Banded Amphibolite Rocks

Assessment of strength anisotropy in transversely isotropic rocks has been one of the most challenging subjects in rock engineering. However, far too little attention has been paid to banded amphibolite rocks. This study aim to evaluate strength and deformation anisotropy behavior of banded amphibolite rocks. The dynamic mechanical tests including ultrasonic pulse test, uniaxial compressive strength, Brazilian test and deformability test were performed on drilled rock samples as a function of foliation plane angle (β = 0°, 30°, 60° and 90°). The results obtained have shown that the dynamic mechanical properties of amphibolite rocks have different values concerning banding plane. Compression and shear waves taken parallel to the foliation plane show highest values than those obtained in the other directions. Under uniaxial test, the banded amphibolite has a U-shaped anisotropy with maximum strength at β = 90° and minimum strength is obtained when β = 30°. Strength anisotropic index ranges between 0.96 and 1.47. It seems that the high range value of anisotropic index is mainly due to slight undulation of foliation planes, that being not perfectly straight. The results of elastic deformation test show that there is no clear dependence on microstructures characteristics of subtype-amphibolite rocks that controlling modulus “shape-anisotropy”. However, in this study, Young modulus values of amphibolite rocks with β follow both types of shape-anisotropy, “U-shape” and “decreased order-shaped”. Thus, this study recommended that further research be undertaken regarding the role of modulus “shape-anisotropy” within the same lithotype.     

矿山采动卸荷岩体力学参数劣化规律研究

矿山开采对采动影响区域内的围岩形成扰动,弱化岩体力学性质,其参数劣化规律是数值模拟与分析计算的主要问题之一。以铜坑矿92#矿体连续采矿顶板诱导崩落试验采场为对象,运用卸荷岩体力学理论,确立了开挖卸荷岩体力学参数计算模拟的计算流程。结合有限元数值模拟方法,建立了岩体卸荷等效模型,进行了6步连续卸荷的计算,得到了地下连续采矿顶板卸荷岩体力学参数的变化规律,并运用多项式实现了卸荷岩体力学参数与卸荷量变化值的函数拟合,获得了采动卸荷岩体力学参数的劣化规律。结果显示,卸荷第6步时,计算不收敛,表明此时顶板已经完全破坏而发生崩落;在卸荷过程中,岩体力学参数均呈逐渐弱化的趋势,其中内摩擦角、凝聚力和弹性模量随开挖卸荷的推进呈逐渐减小的趋势,最终的参数分别相当于初始值的55%、50%和50%,但泊松比呈逐渐增大的趋势,最终值相当于初始值的1.15倍。其结果说明了采动卸荷导致了岩体质量的劣化,卸荷岩体力学参数与卸荷量的函数为采动卸荷力学响应的动态分析提供了理论基础。     

Influences of Mineralogy and Water Content on the Mechanical Properties of Argillite

Callovo-Oxfordian (Cox) argillite was investigated in the context of feasibility studies for an underground repository for nuclear waste. In this study, the influence of mineralogical composition and water content on the mechanical properties of the Cox argillite was investigated. The samples were drilled from three representative depths at which the rock formations have different mineralogical compositions. Micro-indentation and mini-compression tests were performed on samples with different water contents. The elastic and failure properties of argillite were determined and were found to be related to the mineralogical composition and water content. Both the elastic modulus and strength decrease with increasing clay and water contents.     

不同应变率单轴加载条件下页岩的各向异性力学行为

基于静载条件下的常规力学试验可以证明,页岩层理面的存在是导致其力学行为各向异性的主要原因,而应变率的变化势必也影响着页岩各向异性特征的变化。为探究应变率对页岩各向异性力学行为的影响,分别在10-4 s-1、5×10-4 s-1、10-3 s-1和10-2 s-1 4种应变率条件下对不同层理角度的页岩进行单轴压缩力学试验。研究表明:（1）页岩弹性模量的各向异性程度随应变率升高而减弱,泊松比的各向异性程度受应变率的影响不明显;（2）页岩峰值强度随应变率升高而增加,其中45°~60°试样应变率敏感性最强,页岩的强度各向异性度随应变率升高而降低;（3）随着应变率升高,层理面对页岩破坏模式的控制作用得到削弱,破坏模式的各向异性程度减弱。总体而言,页岩的各向异性力学特征随应变率升高而减弱,研究结论有助于页岩力学行为的深入理解。