Evaluation of excavation damaged zones(EDZs)in Horonobe Underground Research Laboratory(URL)

Excavation of underground caverns,such as mountain tunnels and energy-storage caverns,may cause the damages to the surrounding rock as a result of the stress redistribution.In this influenced zone,new cracks and discontinuities are created or propagate in the rock mass.Therefore,it is effective to measure and evaluate the acoustic emission(AE)events generated by the rocks,which is a small elastic vibration,and permeability change.The authors have developed a long-term measurement device that incorporates an optical AE(O-AE)sensor,an optical pore pressure sensor,and an optical temperature sensor in a single multi-optical measurement probe(MOP).Japan Atomic Energy Agency has been conducting R&D activities to enhance the reliability of high-level radioactive waste(HLW)deep geological disposal technology.In a high-level radioactive disposal project,one of the challenges is the development of methods for long-term monitoring of rock mass behavior.Therefore,in January 2014,the long-term measurements of the hydro-mechanical behavior of the rock mass were launched using the developed MOP in the vicinity of 350 m below the surface at the Horonobe Underground Research Center.The measurement results show that AEs occur frequently up to 1.5 m from the wall during excavation.In addition,hydraulic conductivity increased by 2e4 orders of magnitude.Elastoplastic analysis revealed that the hydraulic behavior of the rock mass affected the pore pressure fluctuations and caused micro-fractures.Based on this,a conceptual model is developed to represent the excavation damaged zone(EDZ),which contributes to the safe geological disposal of radioactive waste.

Predicting the Thickness of an Excavation Damaged Zone around the Roadway Using the DA-RF Hybrid Model

After the excavation of the roadway,the original stress balance is destroyed,resulting in the redistribution of stress and the formation of an excavation damaged zone(EDZ)around the roadway.The thickness of EDZ is the key basis for roadway stability discrimination and support structure design,and it is of great engineering significance to accurately predict the thickness of EDZ.Considering the advantages of machine learning(ML)in dealing with high-dimensional,nonlinear problems,a hybrid prediction model based on the random forest(RF)algorithm is developed in this paper.The model used the dragonfly algorithm(DA)to optimize two hyperparameters in RF,namely mtry and ntree,and used mean absolute error(MAE),rootmean square error(RMSE),determination coefficient(R^(2)),and variance accounted for(VAF)to evaluatemodel prediction performance.A database containing 217 sets of data was collected,with embedding depth(ED),drift span(DS),surrounding rock mass strength(RMS),joint index(JI)as input variables,and the excavation damaged zone thickness(EDZT)as output variable.In addition,four classic models,back propagation neural network(BPNN),extreme learning machine(ELM),radial basis function network(RBF),and RF were compared with the DA-RF model.The results showed that the DARF mold had the best prediction performance(training set:MAE=0.1036,RMSE=0.1514,R^(2)=0.9577,VAF=94.2645;test set:MAE=0.1115,RMSE=0.1417,R^(2)=0.9423,VAF=94.0836).The results of the sensitivity analysis showed that the relative importance of each input variable was DS,ED,RMS,and JI from low to high.

Predicting excavation damage zone depths in brittle rocks

During the construction of an underground excavation, damage occurs in the surrounding rock mass due in large part to stress changes. While the predicted damage extent impacts profile selection and support design, the depth of damage is a critical aspect for the design of permeability sensitive excavations, such as a deep geological repository(DGR) for nuclear waste. Review of literature regarding the depth of excavation damage zones(EDZs) indicates three zones are common and typically related to stress induced damage. Based on past developments related to brittle damage prediction using continuum modelling, the depth of the EDZs has been examined numerically. One method to capture stress induced damage in conventional engineering software is the damage initiation and spalling limit(DISL) approach. The variability of depths predicted using the DISL approach has been evaluated and guidelines are suggested for determining the depth of the EDZs around circular excavations in brittle rock masses. Of the inputs evaluated, it was found that the tensile strength produces the greatest variation in the depth of the EDZs. The results were evaluated statistically to determine the best fit relation between the model inputs and the depth of the EDZs. The best correlation and least variation were found for the outer EDZ and the highly damaged zone(HDZ) showed the greatest variation. Predictive equations for different EDZs have been suggested and the maximum numerical EDZ depths, represented by the 68% prediction interval, agreed well with the empirical evidence. This suggests that the numerical limits can be used for preliminary depth prediction of the EDZs in brittle rock for circular excavations.

Assessing fracturing mechanisms and evolution of excavation damaged zone of tunnels in interlocked rock masses at high stresses using a finitediscrete element approach

Deep underground excavations within hard rocks can result in damage to the surrounding rock mass mostly due to redistribution of stresses.Especially within rock masses with non-persistent joints,the role of the pre-existing joints in the damage evolution around the underground opening is of critical importance as they govern the fracturing mechanisms and influence the brittle responses of these hard rock masses under highly anisotropic in situ stresses.In this study,the main focus is the impact of joint network geometry,joint strength and applied field stresses on the rock mass behaviours and the evolution of excavation induced damage due to the loss of confinement as a tunnel face advances.Analysis of such a phenomenon was conducted using the finite-discrete element method (FDEM).The numerical model is initially calibrated in order to match the behaviour of the fracture-free,massive Lac du Bonnet granite during the excavation of the Underground Research Laboratory (URL) Test Tunnel,Canada.The influence of the pre-existing joints on the rock mass response during excavation is investigated by integrating discrete fracture networks (DFNs) of various characteristics into the numerical models under varying in situ stresses.The numerical results obtained highlight the significance of the pre-existing joints on the reduction of in situ rock mass strength and its capacity for extension with both factors controlling the brittle response of the material.Furthermore,the impact of spatial distribution of natural joints on the stability of an underground excavation is discussed,as well as the potentially minor influence of joint strength on the stress induced damage within joint systems of a non-persistent nature under specific conditions.Additionally,the in situ stress-joint network interaction is examined,revealing the complex fracturing mechanisms that may lead to uncontrolled fracture propagation that compromises the overall stability of an underground excavation.

Calibration of coupled hydro-mechanical properties of grain-based model for simulating fracture process and associated pore pressure evolution in excavation damage zone around deep tunnels

The objective of this paper is to develop a methodology for calibration of a discrete element grain-based model(GBM)to replicate the hydro-mechanical properties of a brittle rock measured in the laboratory,and to apply the calibrated model to simulating the formation of excavation damage zone(EDZ)around underground excavations.Firstly,a new cohesive crack model is implemented into the universal distinct element code(UDEC)to control the fracturing behaviour of materials under various loading modes.Next,a methodology for calibration of the components of the UDEC-Voronoi model is discussed.The role of connectivity of induced microcracks on increasing the permeability of laboratory-scale samples is investigated.The calibrated samples are used to investigate the influence of pore fluid pressure on weakening the drained strength of the laboratory-scale rock.The validity of the Terzaghi’s effective stress law for the drained peak strength of low-porosity rock is tested by performing a series of biaxial compression test simulations.Finally,the evolution of damage and pore pressure around two unsupported circular tunnels in crystalline granitic rock is studied.

Permeability and pressure distribution characteristics of the roadway surrounding rock in the damaged zone of an excavation

Research on the permeability and pressure distribution characteristics of the roadway surrounding rock in the excavation damaged zone(EDZ) is beneficial for the development of gas control technology. In this study, analytical solutions of stress and strain of the roadway surrounding rock were obtained, in which the creep deformation and strain softening were considered. Using the MTS815 rock mechanics testing system and a gas permeability testing system, permeability tests were conducted in the complete stress-strain process, and the evolution characteristics of permeability and strain were studied over the whole loading process. Based on the analytical solutions of stress and strain and the governing equation of gas seepage flow, this paper proposes a hydro-mechanical(HM) model, which considers three different zones around the roadway. Then the gas flow process in the roadway surrounding rock in three different zones was simulated according to the engineering geological conditions, thus obtaining the permeability and pressure distribution characteristics of the roadway surrounding rock in three different zones. These results show that the surrounding rock around the roadway can be divided into four regions-the full flow zone(FFZ), flow-shielding zone(FSZ), transitive flow zone(TFZ), and in-situ rock flow zone(IRFZ). These results could provide theoretical guidance for the improvement of gas extraction and gas control technology.

Predicting the excavation damaged zone within brittle surrounding rock masses of deep underground caverns using a comprehensive approach integrating in situ measurements and numerical analysis

Excavation Damaged Zone(EDZ)scope is important for optimizing excavation and support schemes in deep underground caverns.However,accurately predicting the full EDZ scope within the surrounding rock masses of deep underground caverns during excavation remains a pressing problem.This study presents a comprehensive EDZ scope prediction approach(CESPA)for the brittle surrounding rock masses of deep underground caverns by coupling numerical simulation with quantitative analysis of borehole wall images and ultrasonic test results.First,the changes in both P-velocity(V_(p))and joint distribution of the surrounding rock masses before and after excavation damage are captured using ultrasonic tests and borehole digital cameras.Second,the quality Q-parameters of the surrounding rock mass before and after excavation damage are preliminarily rated with the rock mass descriptions provided by borehole wall images,and the rock mass V_(p)-parameter values are determined according to the V_(p)-borehole depth curves.Third,the Q-parameter ratings are further finely adjusted by updating the related Q-values to be similar with the Q-values estimated by V_(p)-parameter values.Fourth,the initial and residual mechanical parameters for the rock mass deterioration model(RDM)are estimated by the adjusted Q-parameter ratings based on the modified Q-based relations,and the elastic modulus deterioration index(EDI)threshold to describe the EDZ boundary is determined with the V_(p)-parameter values.Finally,EDZ scope is predicted using the elastoplastic numerical simulation with RDM and EDI based on the mechanical parameter estimates and EDI threshold.Analyses of applications in Sub-lab D1 in Jinping II project show that CESPA can provide a reliable and operable solution for predicting full EDZ scopes within the brittle surrounding rock masses of deep underground caverns.

Parametric modeling on spatial effect of excavation-damaged zone of underground cavern

Regarding excavation-damaged zone (EDZ) around underground opening as non-homogeneous rockmass with spatial deterioration effect on stuffiness and strength, a parametric model of EDZ using radius-displacement-dependent deformation modulus (RDDM) was proposed. Considering the nonlinearity characteristic of deformation and locality otherness of surrounding rock, deterioration parameter field of deformation modulus of rockmass around opening was quantitatively calculated through a given function. Applicability for multi-cavern condition and parameter sensibility of the model was analyzed by numerical experiments using synthetic data. Furthermore, the model was applied to identify EDZ of underground caverns of Pubugou hydropower station by calculating deterioration parameter field. Based on the parametric analysis of spatial effect and geological investigation, it is recognized that large radial deformation of deep fractured rock at the spandrel position and insufficient supporting bolts mainly result in great deformation pressure to act on the shotcrete and cause partial crack and spalling. It is shown that deterioration parameter field along the longitudinal axis of main powerhouse is evidently non-homogeneous in space and distributes exponentially along the radius from the opening. The model provides a simple and convenient way to identify the EDZ in the working state for rapid construction feedback analysis and support optimization of underground cavern from quantitative point of view and also aids in interpreting monitoring displacements and estimating support requirements.

Distribution characteristics and the evolution law of excavation damage zone in the large-span transition section of high-speed railway tunnel based on microseismic monitoring

Purpose–The microseismic monitoring technique has great advantages on identifying the location,extent and the mechanism of damage process occurring in rock mass.This study aims to analyze distribution characteristics and the evolution law of excavation damage zone of surrounding rock based on microseismic monitoring data.Design/methodology/approach–In situ test using microseismic monitoring technique is carried out in the large-span transition tunnel of Badaling Great Wall Station of Beijing-Zhangjiakou high-speed railway.An intelligent microseismic monitoring system is built with symmetry monitoring point layout both on the mountain surface and inside the tunnel to achieve three-dimensional and all-round monitoring results.Findings–Microseismic events can be divided into high density area,medium density area and low density area according to the density distribution of microseismic events.The positions where the cumulative distribution frequencies of microseismic events are 60 and 80%are identified as the boundaries between high and medium density areas and between medium and low density areas,respectively.The high density area of microseismic events is regarded as the high excavation damage zone of surrounding rock,which is affected by the grade of surrounding rock and the span of tunnel.The prediction formulas for the depth of high excavation damage zone of surrounding rock at different tunnel positions are given considering these two parameters.The scale of the average moment magnitude parameters of microseismic events is adopted to describe the damage degree of surrounding rock.The strong positive correlation and multistage characteristics between the depth of excavation damage zone and deformation of surrounding rock are revealed.Based on the depth of high excavation damage zone of surrounding rock,the prestressed anchor cable(rod)is designed,and the safety of anchor cable(rod)design parameters is verified by the deformation results of surrounding rock.Originality/value–The research provides a new method to predict the surrounding rock damage zone of large-span tunnel and also provides a reference basis for design parameters of prestressed anchor cable(rod).

基于改进XGBoost算法的深部巷道松动圈智能预测研究

深部巷道爆破开挖后由于爆炸冲击和原位应力动态卸载耦合作用,围岩内不可避免地产生松动圈,进而影响结构的稳定性,因此对松动圈厚度进行超前预测显得非常重要。依托多座地下矿山松动圈测试作为研究对象,共获取300组有效数据样本。采用4种主流的超参数优化算法,即遗传算法(GA)、灰狼优化算法(GWO)、粒子群优化算法(PSO)和樽海鞘算法(SSA)对XGBoost算法进行优化,并以此构建4种松动圈预测混合模型。采用R2、RMSE、MAE和MAPE指标对预测模型的性能进行对比分析,并开展松动圈厚度参数的敏感性分析。最后,将最优的PSO-XGBoost模型应用于地下矿山运输巷道进行工程验证。结果表明:在群体规模分别为90、70、60和100时,GA-XGBoost、GWO-XGBoost、PSO-XGBoost和SSA-XGBoost模型取得了最佳的预测表现。其中,PSO-XGBoost模型在训练集和测试集中的相关系数分别为0.9244和0.8787,具有最佳的预测性能。相比基准模型(XGBoost、RF、SVM和LightGBM),优化后模型松动圈的预测精度和性能均得到显著提升。巷道当量直径(TD)和围岩地质强度指标(GSI)对松动圈厚度的影响最为显著,垂直主应力也具有明显的影响。优化后的XGBoost模型在实际工程中的应用结果显示实测值与预测值误差在10%以内,PO-XGBoost具有工程应用价值。

Comprehensive assessment on dynamic roof instability under fractured rock mass conditions in the excavation disturbed zone

The damage process of fractured rock mass showed that the fracture in rocks induced roof collapse in Yangchangwan Coal Mine, China. The rock mass was particularly weak and fractured. There occurred 6 large-scale dynamical roof falls in the excavation disturbed zone （EDZ） with the collapsing volume of 216 m^3. First, the field detailed geological environment, regional seismic dynamics, and dynamic instability of roadways were generally investigated. Second, the field multiple-index monitoring measurements for detecting the deep delamination of the roof, convergence deformation, bolt-cable load, acoustic emission （AE） characteristic parameters, total AE events, AE energy-releasing rate, rock mass fracture, and damage were arranged. Finally, according to the time-space-strength relations, a quantitative assessment of the influence of rock-mass damage on the dynamic roof instability was accomplished.

Fracture development around deep underground excavations: Insights from FDEM modelling

Over the past twenty years, there has been a growing interest in the development of numerical modelsthat can realistically capture the progressive failure of rock masses. In particular, the investigation ofdamage development around underground excavations represents a key issue in several rock engineeringapplications, including tunnelling, mining, drilling, hydroelectric power generation, and the deepgeological disposal of nuclear waste. The goal of this paper is to show the effectiveness of a hybrid finitediscreteelement method （FDEM） code to simulate the fracturing mechanisms associated with theexcavation of underground openings in brittle rock formations. A brief review of the current state-of-theartmodelling approaches is initially provided, including the description of selecting continuum- anddiscontinuum-based techniques. Then, the influence of a number of factors, including mechanical and insitu stress anisotropy, as well as excavation geometry, on the simulated damage is analysed for threedifferent geomechanical scenarios. Firstly, the fracture nucleation and growth process under isotropicrock mass conditions is simulated for a circular shaft. Secondly, the influence of mechanical anisotropy onthe development of an excavation damaged zone （EDZ） around a tunnel excavated in a layered rockformation is considered. Finally, the interaction mechanisms between two large caverns of an undergroundhydroelectric power station are investigated, with particular emphasis on the rock mass responsesensitivity to the pillar width and excavation sequence. Overall, the numerical results indicate that FDEMsimulations can provide unique geomechanical insights in cases where an explicit consideration offracture and fragmentation processes is of paramount importance. 2014 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting byElsevier B.V. All rights reserved.

Formation mechanism of extension fractures induced by excavation of a gallery in soft sedimentary rock,Horonobe area,Northern Japan

这份报纸集中于一个画廊的挖掘在日本的 Horonobe 区域在软沉积岩石中导致的破裂的形成机制。画廊印射的详细骨折显示破裂由两个都先存在组成砍破裂和挖掘损坏了地区(EDZ ) 破裂。EDZ 破裂对应于与床上用品飞机或 transgranular 裂缝联系的弱飞机。EDZ 破裂对先存在终止砍破裂。甚至为在软沉积岩石中的挖掘，因此， EDZ 破裂的形成被先存在控制破裂和更早弱的飞机。

深部硬岩开挖损伤区时效行为的中间主应力效应研究

为探究中间主应力对深部硬岩开挖损伤区时效行为的影响,基于Perzyna弹粘塑性理论和应变能理论及考虑中间主应力的三维屈服准则,建立了硬岩蠕变全过程的粘塑性损伤耦合本构模型,并将其嵌入到工程岩体破裂过程分析软件(CASRock)中,通过比较模拟结果与试验结果,验证了模型和程序的有效性。进一步模拟了锦屏地下实验室二期工程#1实验室的时效行为,并探究了不同中间主应力作用下硬岩开挖损伤区的时效破裂过程,发现硬岩的时效破裂行为表现出明显的中间主应力区间效应;通过总粘塑性应变与中间主应力的等时曲线簇发现中间主应力的区间效应受时间影响;开挖损伤区的存在能促进其内部围岩时效破裂发展,从而扩大塑性区的范围。

基于围岩应力监测和振动监测的采场稳定性研究

采用自主设计的多功能监测杆对某金属矿采场开挖过程中的围岩稳定性进行实时监测,记录采场回采过程中监测杆长度范围内岩层垂直方向应力变化和围岩振动事件。结果表明,开挖面积增大会导致围岩所受应力和峰值速度呈现上升趋势,反映出围岩内部发生破裂次数增多,围岩稳定性下降,松动圈深度增加。对于采用集中端头锚固式锚杆进行支护的矿山,锚杆长度大于松动圈深度时,能更有效地控制松动圈演化和岩体冒落。该采场完成为期一个月的开采后,岩层沉降使监测杆长度范围内岩层垂直方向应力增加约3.9 MPa,振动事件强度满足我国爆破振动安全许可标准,综合评价围岩稳定性较好。

砂质泥岩峰后破裂承载特征与块体分布规律研究

岩石峰后阶段的变形承载规律对巷道破碎区围岩的稳定性控制具有重要意义。为研究岩石峰后阶段的破裂演化规律与承载特征,以砂质泥岩为试验对象,开展了控制应变量为峰值应变不同倍数的单轴压缩试验,并应用非线性分形理论阐明了峰后破裂块体的分布规律。基于试验结果,构建了峰后含贯通破裂面岩样的力学分析模型。结果表明:(1)砂质泥岩全应力-应变曲线呈现出单峰状与多峰状两种形态的峰后应力跌落方式;(2)峰后阶段,试样破坏模式由低倍峰值应变下的张性破坏逐渐转变为高倍峰值应变下的剪切滑移破坏;(3)同应变量下试样破裂块体的分形维数内部大于外部,且二者与试样的峰后应变倍数值均具有显著的正相关性;(4)岩石在峰后阶段的持续破坏方式与块体间接触破裂面特性有关,破裂块体仅会在一定的倾角范围内沿破裂面滑动,依据所建立的分析模型可准确解释砂质泥岩峰后阶段破裂承载特性与块体分布规律。

Numerical simulations for describing generation of excavation damaged zone: Important case study at Horonobe underground research laboratory

The aim of the present research was to establish a case study for the prediction of the unknown EDZ(Excavation Damaged Zone)distribution using a numerical analysis calibrated by replicating the trends in the EDZ observed from one of the representative underground research fields in Japan(Horonobe URL).In this study,a 2D numerical analysis using a damage model,which can determine rock deformation and fracturing simultaneously,is presented.It was calibrated to reproduce the excavation of the gallery at the Horonobe URL at a depth of 350 m.Simulated results show an excellent agreement with the extent of the measured EDZ and capture the failure modes of EDZ fractures suggested by the in-situ observations.Finally,the calibrated numerical analysis was used to realistically estimate the EDZ formation for the geological disposal of high-level radioactive waste(HLW)under the same environment as that of the above-mentioned galley at the Horonobe URL.Consequently,it was shown that the tensile/shear hybrid fractures dominantly constituted the EDZ and propagated to a maximum extent of about 0.3 m from the cavity wall during the cavity excavation for the HLW disposal.Overall,the calibrated numerical analysis and resulting estimations,targeted for the environment at the depth of 350 m at the Horonobe URL,where mudstone is located,should be useful for predicting the trends in the EDZ distribution expected in the implementation of HLW disposal projects under deep geological conditions,such as those that exist in Japan,which are dominated by sedimentary rocks,including mudstone。

First-arrival traveltime tomography for monitoring the excavation damaged zone in the Horonobe Underground Research Laboratory

Subsurface excavation results in the formation of a zone called excavation damaged zone(EDZ)around the tunnel wall.An EDZ is a major concern in the field of high-level radioactive waste disposal because it may act as a flow path after the closure of a repository.In this study,first-arrival traveltime tomography was repeatedly conducted on the EDZ at a depth of 350 m in the Horonobe Underground Research Laboratory.However,the acquired data was highly affected by the support structure on the drift wall.For proper visualization of the EDZ,information about the structure was incorporated into the inversion by modifying the model constraint.The synthetic study showed that the approach reproduced the EDZ in the model without the artifacts.The method was applied to field data,and the EDZ around the drift was detected.The inversion was extended to a time-lapse inversion to trace the changes in P-wave velocity in the EDZ.The synthetic study demonstrated that temporal changes in the P-wave velocity distribution could be detected.Data obtained from 12 surveys under open-drift conditions were analyzed by time-lapse inversion.The results indicated that the EDZ did not undergo sealing or evolution at the site for approximately seven years.

地下洞室开挖爆破围岩松动圈的数值分析计算

根据数值计算提出了在地下洞室开挖爆破中,确定围岩松动圈的方法。给出了地下洞室开挖爆破的三维弹塑性损伤有限元计算方法。该方法论述了爆破荷载、重力荷载和开挖荷载的计算、施加以及迭代的方法,可以合理反应爆破对围岩稳定的影响和锚固支护施加的效果;结合岩石松动圈的实测方法,推导了判定围岩开始松动的损伤系数阈值公式,可以结合数值计算得到的洞周围岩损伤系数分布,确定在开挖爆破作用下围岩松动圈的范围。对实际工程的分析计算表明,根据数值计算判定的围岩松动范围与实测松动范围的规律基本一致,量值吻合较好,其结果为地下洞室开挖爆破的围岩松动圈的确定提供了有效的方法。

锦屏二级水电站深埋隧洞开挖损伤区特征分析

在高应力条件下,岩体强度和应力之间的尖锐矛盾将导致围岩出现损伤,损伤是不同应力条件下围岩状态的直接体现。利用声波检测和钻孔电视对锦屏二级深埋引水隧洞的一典型断面进行全断面测试,声波测试结果显示,断面上低波速带断面形态呈不对称状,与断面应力分布也并不完全对应。在每个声波钻孔中补充钻孔电视,对破裂发育深度和围岩内部实际构造特征有了更直观的认识。为对损伤区特性进行更加准确的描述,利用FLAC3D计算在洞周不同位置处关键点的应力路径,对关键点的应力状态进行分析。在UDEC泰森多边形离散的基础上增加对于节理的描述,分析节理对损伤区分布的影响,模拟结果表明,节理的存在改变了隧洞开挖后洞周的应力分布,从而导致围岩破损和破坏区域的差异。最后,借助于颗粒流程序PFC对隧洞开挖后围岩的损伤区进行模拟,所揭示出的损伤局部化特征和损伤区、破裂区分布特征与现场实际具有很好的一致性。