Strength of massive to moderately jointed hard rock masses

The Hoek-Brown(HB) failure criterion and the geological strength index(GSI) were developed for the estimation of rock mass strength in jointed and blocky ground where rock mass failure is dominated by sliding along open joints and rotation of rock blocks. In massive, veined and moderately jointed rock in which rock blocks cannot form without failure of intact rock, the approach to obtain HB parameters must be modified. Typical situations when these modifications are required include the design of pillars,excavation and cavern stability, strainburst potential assessment, and tunnel support in deep underground conditions(around s1/s ci 0.15, where s1 is the major principal compressive stress and s ciis the unconfined compressive strength of the homogeneous rock) in hard brittle rocks with GSI ! 65. In this article, the strength of massive to moderately jointed hard rock masses is investigated, and an approach is presented to estimate the rock mass strength envelope using laboratory data from uniaxial and triaxial compressive strength tests without reliance on the HB-GSI equations. The data from tests on specimens obtained from massive to moderately jointed heterogeneous(veined) rock masses are used to obtain the rock and rock mass strengths at confining stress ranges that are relevant for deep tunnelling and mining;and a methodology is presented for this purpose from laboratory data alone. By directly obtaining the equivalent HB rock mass strength envelope for massive to moderately jointed rock from laboratory tests,the HB-GSI rock mass strength estimation approach is complemented for conditions where the GSIequations are not applicable. Guidance is also provided on how to apply the proposed approach when laboratory test data are not or not yet available.     

A comparative review in regards to estimating bearing capacity in jointed rock masses in northeast Jordan

There are a number of different methods used for estimating the bearing capacity in jointed rock masses. In this paper, the geological strength index (GSI) introduced by Hoek et al. (1995) was used to estimate the bearing capacity of the rock mass via rock mass rating (RMR). An empirical relationship is proposed to estimate the bearing capacity of the rock mass using the GSI-dependent toughness factor (TF). The proposed formula was correlated with bearing capacity equations used in the literature. The regression analyses showed exponential relationships with a high correlation coefficient.     

Influence of degree of interlock on confined strength of jointed hard rock masses

The strength of jointed rock mass is strongly controlled by the degree of interlock between its constituent rock blocks. The degree of interlock constrains the kinematic freedom of individual rock blocks to rotate and slide along the block forming joints. The Hoek–Brown (HB) failure criterion and the geological strength index (GSI) were developed based on experiences from mine slopes and tunneling projects in moderately to poorly interlocked jointed rock masses. It has since then been demonstrated that the approach to estimate the HB strength parameters based on the GSI strength scaling equations (called the ‘GSI strength equations’) tends to underestimate the confined peak strength of highly interlocked jointed rock masses (i.e. GSI > 65), where the rock mass is often non-persistently jointed, and the intact rock blocks are strong and brittle. The estimation of the confined strength of such rock masses is relevant when designing mine pillars and abutments at great depths, where the confining pressure is high enough to prevent block rotation and free sliding on block boundaries. In this article, a grain-based distinct element modeling approach is used to simulate jointed rock masses of various degrees of interlock and to investigate the influences of block shape, joint persistence and joint surface condition on the confined peak strengths. The focus is on non-persistently jointed and blocky (persistently jointed) rock masses, consisting of hard and homogeneous rock blocks devoid of any strength degrading defects such as veins. The results from this investigation confirm that the GSI strength equations underestimate the confined strength of highly interlocked and non-persistently jointed rock masses. Moreover, the GSI strength equations are found to be valid to estimate the confined strength of persistently jointed rock masses with smooth and non-dilatant joint surfaces.     

Effects of confinement on rock mass modulus: A synthetic rock mass modelling (SRM) study

The main objective of this paper is to examine the influence of the applied confining stress on the rock mass modulus of moderately jointed rocks (well interlocked undisturbed rock mass with blocks formed by three or less intersecting joints). A synthetic rock mass modelling (SRM) approach is employed to determine the mechanical properties of the rock mass. In this approach, the intact body of rock is represented by the discrete element method (DEM)-Voronoi grains with the ability of simulating the initiation and propagation of microcracks within the intact part of the model. The geometry of the pre-existing joints is generated by employing discrete fracture network (DFN) modelling based on field joint data collected from the Brockville Tunnel using LiDAR scanning. The geometrical characteristics of the simulated joints at a representative sample size are first validated against the field data, and then used to measure the rock quality designation (RQD), joint spacing, areal fracture intensity (P₂₁), and block volumes. These geometrical quantities are used to quantitatively determine a representative range of the geological strength index (GSI). The results show that estimating the GSI using the RQD tends to make a closer estimate of the degree of blockiness that leads to GSI values corresponding to those obtained from direct visual observations of the rock mass conditions in the field. The use of joint spacing and block volume in order to quantify the GSI value range for the studied rock mass suggests a lower range compared to that evaluated in situ. Based on numerical modelling results and laboratory data of rock testing reported in the literature, a semi-empirical equation is proposed that relates the rock mass modulus to confinement as a function of the areal fracture intensity and joint stiffness.     

A quantitative comparison of strength criteria for hard rock masses

Knowledge of the rock mass strength is important for the design of all types of underground excavations. A frequently applied approach for estimation of the rock mass strength is through an empirical failure criterion, often in conjunction with rock mass classification/characterisation systems. This paper presents a review of existing methods to estimate the rock mass strength using empirical failure criteria and classification/characterisation systems—in this study, commonly denoted as estimation methods. A literature review of existing methods is presented, after which a set of methods were selected for further studies. The selected methods were used in three case studies, to investigate their robustness and quantitatively compare the advantages and disadvantages of each method. A Round Robin test was used in two of the cases. The case studies revealed that the N, Yudhbir-RMR₇₆, RMi, Q-, and Hoek–Brown-GSI methods, appeared to yield a reasonable agreement with the measured strengths. These methods are thus considered the best candidates for realistic strength estimation, provided that care is taken when choosing values for each of the included parameters in each method. This study has also clearly shown the limits of presently available strength estimation methods for rock masses and further work is required to develop more precise, practical, and easy-to-use methods for determining the rock mass strength. This should be based on the mechanical behaviour and characteristics of the rock mass, which implies that parameters that consider the strength of intact rock, block size and shape, joint strength, and physical scale, are required.     

Linearization of the Hoek and Brown rock failure criterion for tunnelling in elasto-plastic rock masses

We present a novel methodology for estimation of equivalent Mohr–Coulomb strength parameters that can be used for design of supported tunnels in elasto-plastic rock masses satisfying the non-linear empirical Hoek–Brown failure criterion. We work with a general adimensional formulation of the Hoek–Brown failure criterion in the space of normalized Lambe's variables for plane stress, and we perform linearization considering the stress field in the plastic region around the tunnel. The procedure is validated using analytical solutions to a series of benchmark test cases. Numerical solutions are also employed to validate the procedure in cases for which analytical solutions are not available. Results indicate that the stress field in the plastic region around the tunnel, as well as the linearization method employed and the quality of the rock mass, has a significant impact on computed estimates of equivalent Mohr–Coulomb strength parameters. Results of numerical analyses also show that our proposed linearization method can be employed to estimate loads and moments on the tunnel support system. We recommend the equating model responses (EMR) method to compute equivalent Mohr–Coulomb strength parameters when the tunnel support pressure is accurately known, and we further show that our newly introduced linearization method can be employed as an alternative to the best fitting in the existing stress range (BFe) and best fitting in an artificial stress range (BFa) methods, providing performance estimates that are generally better than estimates of the BFe and BFa methods when differences with the response of the Hoek–Brown rock mass are of engineering significance (say more than 10%).     

煤层底板岩体阻水能力原位测试研究

介绍了煤层底板岩体阻水能力的测试原理、测试方法和测试过程 ,分析了测试成果的可靠性 ,并对杨庄煤矿六煤底板岩体的阻水能力进行了评价 ,为煤矿底板突水防治提供了一条新途径。     

Directional rock mass rating (DRMR) for anisotropic rock mass characterization

Bulletin of Engineering Geology and the Environment - Rock mass classification systems are used to categorize and estimate the role of the most significant parameters influencing rock mass behavior...     

Modification of rock mass strength assessment methods and their application in geotechnical engineering

Due to complicated structures and discontinuities in surrounding rock mass, existing empirical failure criteria cannot meet the requirements of engineering practice such as tunnels. To improve estimation accuracy on the strength of rock mass with joints, a modified chart of the Geological Strength Index using Hoek–Brown criteria was further tested to estimate rock mass strength [Lin et al. (2014) Bull Eng Geol Environ 4(73):1245–1258], and, in this paper, new strength estimation equations for jointed rock mass were then modified based on a large dataset obtained from Chinese projects. Here, standard drilling time is first introduced and described in this study, and then used as a parameter to estimate rock strength. Different empirical formulas based on joint density, rock mass classification, Hoek–Brown criteria, and elastic wave velocity are thus used to estimate rock mass strength by using data from the Jiubao tunnel. The results estimated based on different empirical formulas were similar, indicating that the modified assessment method presented in this paper can be used to estimate rock mass strength under certain circumstances. Cross-correlation of different empirical methods provides significant confidence in predicted rock mass strength calculations.     

深部矿区煤岩体强度测试与分析

 基于钻孔触探法原理,开发出小孔径井下煤岩体强度测定装置。在实验室对34个煤岩样品进行试验：在煤岩块上钻取标准试件,测量单轴抗压强度;在留下的钻孔中,用煤岩体强度测定装置测定探针临界载荷,分析探针破坏钻孔壁煤岩的形态;然后确定煤岩块单轴抗压强度与探针临界载荷的关系。试验表明,探针破坏钻孔壁煤岩的形状、深度及范围与煤岩性质密切相关。煤岩体强度越高,破坏范围、侵入深度越小,破坏形状越规则。结合井下实测数据,回归得出描述探针临界载荷与煤岩体单轴抗压强度关系的公式。同时,分析临界载荷的离散性及控制措施,讨论结构面对煤岩体强度的影响及测试分析方法,并在典型的深部矿区——新汶矿区进行井下原位测试。新汶矿区巷道顶板不同岩性的岩层强度相差很大,不同矿井的岩层强度也存在明显差别。煤层强度由于煤帮出现破碎区、煤层性质不均匀、煤层结构面分布不均匀等原因变化较大,出现明显的波动。基于井下煤岩体强度实测数据的巷道支护设计,符合井下环境中的煤岩体条件,设计的合理性与可靠性显著提高,巷道围岩稳定性与支护状况得到明显改善。最后分析钻孔触探法存在的问题,并提出改进建议。     

渗流对岩体强度影响的机理研究

渗流作用对岩体强度的影响主要表现在渗水对岩体的物理化学作用和力学作用,并结合材料力学理论推导出了存在空隙水压力的岩体中任一面上的有效应力及岩体抗剪强度的降低量,其结果对评价渗流作用下工程岩体的稳定与破坏具有十分重要的意义.     

Compression–shear strength criterion of coal–rock combination model considering interface effect

Surrounding rocks around coal tunnel in western mining area of China are typical composite structures composed of weakly cemented soft rock and hard coal, and the tunnel stability is closely related to the overall mechanical behavior of the combination body. The equivalent homogeneous model of coal–rock combination body and its stress state expressions were firstly established based on the strain energy equivalency principle. Then, the general compression–shear failure criterion of the equivalent model which takes into account the cohesive strength of the interface between coal and soft rock was derived by assuming that the yielded mediums all met Mohr–Coulomb criterion. Furthermore, accuracy of the proposed analytical model was verified by carrying out laboratory test for coal-mudstone specimen, and it found that the theoretical results were in good agreement with the test values. Strength of the combination body lies between the strong body and weak body. Finally, the effects of interface cohesion strength, rock thickness and stress level on the failure behavior of combination model were analyzed based on the analytical model. Results show that the proposed model not only contains the classical sliding failure theory for two-dimensional weak plane presented by Jaeger, but also reflect strength behavior of a more complex composite model composed of different rock mediums and structural plane. Thus, the analytical model provides theoretical basis for further studying the mechanical behavior of coal–rock combination model.     

煤层顶底板岩石点荷载强度与拉压强度对比试验研究

 利用自主研制的点荷载仪,通过室内对煤矿顶底板常见岩性岩石进行点荷载试验、单轴抗压试验、抗拉试验对比分析,研究由点荷载强度快速准确确定单轴抗压强度及抗拉强度的方法。试验结果表明：点载荷强度较低的粗砂岩极易受到加载速率和加载方向影响,其预测值与实际值的误差相对较大;而对于点荷载强度较大的岩石,3种强度的线性相关性较高。点荷载试验的应用,为煤矿顶底板岩石强度确定及岩体分类提供了重要依据。     

Influence of data analysis when exploiting DFN model representation in the application of rock mass classification systems

Discrete fracture network (DFN) models have been proved to be effective tools for the characterisation of rock masses by using statistical distributions to generate realistic three-dimensional (3D) representations of a natural fracture network. The quality of DFN modelling relies on the quality of the field data and their interpretation. In this context, advancements in remote data acquisition have now made it possible to acquire high-quality data potentially not accessible by conventional scanline and window mapping. This paper presents a comparison between aggregate and disaggregate approaches to define fracture sets, and their role with respect to the definition of key input parameters required to generate DFN models. The focal point of the discussion is the characterisation of in situ block size distribution (IBSD) using DFN methods. An application of IBSD is the assessment of rock mass quality through rock mass classification systems such as geological strength index (GSI). As DFN models are becoming an almost integral part of many geotechnical and mining engineering problems, the authors present a method whereby realistic representation of 3D fracture networks and block size analysis are used to estimate GSI ratings, with emphasis on the limitations that exist in rock engineering design when assigning a unique GSI value to spatially variable rock masses.     

大光包滑坡滑带碎裂岩体原位钻孔剪切试验研究

安县大光包滑坡是汶川地震触发的最大规模滑坡,滑坡南侧暴露长约1.8 km顺层滑带,其岩体高度碎裂化,引起广泛关注。为准确评价滑带碎裂岩体的强度参数,笔者在前人研究的基础上开展了细致的野外调查工作,采用法国Phicometre岩土两用原位钻孔剪切试验仪对大光包滑坡滑带碎裂岩体进行了原位剪切试验。将试验结果与Hoek–Brown岩体强度准则估值和基于工程地质类比法的力学参数建议值进行了对比分析。基于以上研究,提出了大光包滑坡南侧顺层滑带碎裂岩体力学参数建议值:内聚力为245~480 k Pa,内摩擦角为25.0°~26.5°。     

钻屑温度法预测冲击地压的试验研究

钻屑温度法是通过测试钻孔过程中钻屑温度来判断煤矿冲击地压危险性的方法。钻屑温度法考虑了煤体应力、煤体物理力学性质等因素,具备许多传统方法不可替代的优点,是一种很有发展的预测预报方法。钻屑温度包括钻头温度、钻孔温度和煤屑温度。通过理论分析和试验研究相结合,总结了钻孔过程中钻屑温度的变化规律：钻屑温度随着煤体应力的增大而增高,随着煤体强度增加而增高,但随着推进速度增大而降低,钻屑温度变化规律与煤体应力及钻屑量具有较好的一致性,研究结果为煤矿动力灾害进行预测预报提供一定的理论与试验基础。     

A generic method for rock mass classification

Rock mass classification (RMC) is of critical importance in support design and applications to mining, tunneling and other underground excavations. Although a number of techniques are available, there exists an uncertainty in application to complex underground works. In the present work, a generic rock mass rating (GRMR) system is developed. The proposed GRMR system refers to as most commonly used techniques, and two rock load equations are suggested in terms of GRMR, which are based on the fact that whether all the rock parameters considered by the system have an influence or only few of them are influencing. The GRMR method has been validated with the data obtained from three underground coal mines in India. Then, a semi-empirical model is developed for the GRMR method using artificial neural network (ANN), and it is validated by a comparative analysis of ANN model results with that by analytical GRMR method.     

富水破碎岩体注浆加固模拟试验及应用研究

设计了一套适用于富水破碎岩体的注浆加固模拟试验装置。选用PO42.5水泥、SAC42.5水泥和自主研发材料对富水破碎灰岩体进行了注浆加固试验。在分析加固结石体的单轴压缩试验结果的基础上,研究了注浆压力、注浆材料和破碎岩体间泥质充填物对注浆加固效果的影响和作用规律。采用极差分析法确定了影响注浆加固效果的主控因素及各因素间相互作用关系。验证了自主研发材料用于富水破碎岩体注浆加固治理的效果显著优于传统水泥材料。将上述试验结论成功用于指导工程实践,取得了良好的注浆治理效果。     

An analytical model for estimating rock strength parameters from small-scale drilling data

The small-scale drilling technique can be a fast and reliable method to estimate rock strength parameters. It needs to link the operational drilling parameters and strength properties of rock. The parameters such as bit geometry, bit movement, contact frictions and crushed zone affect the estimated parameters. An analytical model considering operational drilling data and effective parameters can be used for these purposes. In this research, an analytical model was developed based on limit equilibrium of forces in a T-shaped drag bit considering the effective parameters such as bit geometry, crushed zone and contact frictions in drilling process. Based on the model, a method was used to estimate rock strength parameters such as cohesion, internal friction angle and uniaxial compressive strength of different rock types from operational drilling data. Some drilling tests were conducted by a portable and powerful drilling machine which was developed for this work. The obtained results for strength properties of different rock types from the drilling experiments based on the proposed model are in good agreement with the results of standard tests. Experimental results show that the contact friction between the cutting face and rock is close to that between bit end wearing face and rock due to the same bit material. In this case, the strength parameters, especially internal friction angle and cohesion, are estimated only by using a blunt bit drilling data and the bit bluntness does not affect the estimated results.