Assessment of deformation modulus of weak rock masses from pressuremeter tests and seismic surveys

The deformation modulus of intact rock can be determined through standardized laboratory tests for heavily jointed rock masses but this is very difficult, while in situ tests are time-consuming and expensive. In this study, the deformation modulus of selected heavily jointed, sheared and/or blocky, weathered, weak greywacke, andesite and claystone were assessed, based on pressuremeter tests, geo-engineering characterization and seismic surveys. Empirical equations based on GSI and RMR values are proposed to indirectly estimate the deformation modulus of the greywackes. For the andesites, the spacing of the discontinuities is greater than the length of the pressuremeter probe hence the intact rather than rock mass deformation modulus is obtained. The pressuremeter test results from the claystones could not be correlated with the field data; the relationship between the ratio of rock mass modulus to intact rock modulus and RQD appears to give a better estimation of the deformation modulus.      

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.     

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.     

Measurements of and correlations between block size and rock quality designation (RQD)

Various measurements of the block size or degree of jointing (i.e. density of joints, RQD, block volume, joint spacing) are described. It is concluded that the RQD measurements are encumbered with several limitations and that this parameter should be applied with care. These limitations influence the engineering results where RQD is applied in classification systems, numerical modelling and other engineering assessments.The three-dimensional block volume (V_b) and the volumetric joint count (J_v) measurements give much better characterizations of the block size. As the block size forms an important input to most rock engineering calculations and estimates, it is important to select the most appropriate method to measure this parameter.Correlations between various measurements of block size have been presented. It turned out difficult to find any reliable correlation between RQD and other block size measurements. An adjusted, better equation between RQD and J_v than the existing is presented, though still with several limitations.More efforts should be made to improve the understanding on how to best measure the block size in the various types of exposures and patterns of jointing.     

Determination and applications of rock quality designation(RQD)

Characterization of rock masses and evaluation of their mechanical properties are important and challenging tasks in rock mechanics and rock engineering.Since in many cases rock quality designation(RQD)is the only rock mass classification index available,this paper outlines the key aspects on determination of RQD and evaluates the empirical methods based on RQD for determining the deformation modulus and unconfined compressive strength of rock masses.First,various methods for determining RQD are presented and the effects of different factors on determination of RQD are highlighted.Then,the empirical methods based on RQD for determining the deformation modulus and unconfined compressive strength of rock masses are briefly reviewed.Finally,the empirical methods based on RQD are used to determine the deformation modulus and unconfined compressive strength of rock masses at five different sites including 13 cases,and the results are compared with those obtained by other empirical methods based on rock mass classification indices such as rock mass rating(RMR),Q-system(Q) and geological strength index(GSI).It is shown that the empirical methods based on RQD tend to give deformation modulus values close to the lower bound(conservative) and unconfined compressive strength values in the middle of the corresponding values from different empirical methods based on RMR,Q and GSI.The empirical methods based on RQD provide a convenient way for estimating the mechanical properties of rock masses but,whenever possible,they should be used together with other empirical methods based on RMR,Q and GSI.     

Critical strain and squeezing of rock mass in tunnels

The squeezing of tunnels is a common phenomenon in poor rock masses under high in situ stress conditions. The critical strain parameter is an indicator that allows the degree of squeezing potential to be quantified. It is defined as the strain level on the tunnel periphery beyond which instability and squeezing problems are likely to occur. Presently, in the literature, the value of critical strain is generally taken as 1%. It is shown in this study that the critical strain is an anisotropic property and that it depends on the properties of the intact rock and the joints in the rock mass. A correlation of critical strain with the uniaxial compressive strength, tangent modulus of intact rock and the field modulus of the jointed mass is suggested in this paper. It is also suggested that the modulus of deformation being anisotropic in nature should be obtained from field tests. In absence of field tests, use of a classification approach is recommended, and, expressions are suggested for critical strain in terms of rock mass quality Q. A rational classification based on squeezing index (SI) is proposed to identify and quantify the squeezing potential in tunnels. Applicability of the approach is demonstrated through application to 30 case histories from the field.     

A 3D synthetic rock mass numerical method for characterizations of rock mass and excavation damage zone near tunnels

In practice, a damage zone is generally formed after tunnel excavation in jointed rock mass. This damage zone is closely related to rock mass properties and requires careful examination in order for cost effective supporting designs. In this research, a synthetic rock mass (SRM) numerical method is applied for characterizations of the jointed rock mass and excavation damage zone (EDZ) near underground tunnels in 3D. The SRM model consists of bonded particles and simulates deformation and crack propagation of the rock mass through interactions between these particles. The effects of joint stiffness and distribution on the rock mass properties are systematically examined by comparing the numerical data with an empirical geological strength index (GSI) system and an associated Hoek-Brown strength criterion. The numerical results suggest that rock mass properties are comparable to the empirical GSI/Hoek-Brown system only when inclined joints are simulated in the rock mass subjected to axial loading. The rock mass is strengthened and the empirical GSI/Hoek-Brown characterization becomes inappropriate when the joints are less favorable to shear sliding. The SRM method is then applied for characterizations of tunnel EDZ. It appears that the depth and location of the EDZ are a function of the tunnel orientation, joints, and in situ stresses. The EDZ depth is expected to be higher when inclined joints are simulated. The EDZ area is reduced when the joints in the rock mass are horizontally and vertically distributed.

     

压剪应力条件下岩体裂纹扩展概率模型研究

经典Ｗｅｉｂｕｌ统计断裂理论从统计的角度建立了拉剪应力条件下裂纹扩展的统计断裂模型。而岩体工程中的断续节理岩体的裂纹有相当一部分是处于压剪应力作用下,应用经典Ｗｅｉｂｕｌ统计断裂理论建立的裂纹扩展概率模型是不适宜的。本文对断续节理岩体中压剪应力条件下裂纹扩展的随机性从断裂力学角度进行了研究,建立了压剪应力条件下裂纹的断裂力学概率模型,并讨论了节理岩体的失效概率与裂纹几何、力学各参数之间的关系。     

A new estimation method and an anisotropy index for the deformation modulus of jointed rock masses

The deformation modulus of a rock mass is an important parameter to describe its mechanical behavior.In this study,an analytical method is developed to determine the deformation modulus of jointed rock masses,which considers the mechanical properties of intact rocks and joints based on the superposition principle.Due to incorporating the variations in the orientations and sizes of joint sets,the proposed method is applicable to the rock mass with persistent and parallel joints as well as that with nonpersistent and nonparallel joints.In addition,an anisotropy index AIdmfor the deformation modulus is defined to quantitatively describe the anisotropy of rock masses.The range of AIdmis from 0 to 1,and the more anisotropic the rock mass is,the larger the value of AIdmwill be.To evaluate the proposed method,20 groups of numerical experiments are conducted with the universal distinct element code(UDEC).For each experimental group,the deformation modulus in 24 directions are obtained by UDEC(numerical value)and the proposed method(predicted value),and then the mean error rates are calculated.Note that the mean error rate is the mean value of the error rates of the deformation modulus in 24 directions,where for each direction,the error rate is equal to the ratio of numerical value minus predicted value to the numerical value.The results show that(i)for different experimental groups,the mean error rates vary between 5.06%and 22.03%;(ii)the error rates for the discrete fracture networks(DFNs)with two sets of joints are at the same level as those with one set of joints;and(iii)therefore,the proposed method for estimating the deformation modulus of jointed rock masses is valid.     

Classification and assessment of rock mass parameters in Choghart iron mine using P-wave velocity

Engineering rock mass classification,based on empirical relations between rock mass parameters and engineering applications,is commonly used in rock engineering and forms the basis for designing rock structures.The basic data required may be obtained from visual observation and laboratory or field tests.However,owing to the discontinuous and variable nature of rock masses,it is difficult for rock engineers to directly obtain the specific design parameters needed.As an alternative,the use of geophysical methods in geomechanics such as seismography may largely address this problem.In this study,25 seismic profiles with the total length of 543 m have been scanned to determine the geomechanical properties of the rock mass in blocks Ⅰ,Ⅲ and Ⅳ-2 of the Choghart iron mine.Moreover,rock joint measurements and sampling for laboratory tests were conducted.The results show that the rock mass rating(RMR) and Q values have a close relation with P-wave velocity parameters,including P-wave velocity in field(V_(PF)).P-wave velocity in the laboratory(V_(PL)) and the ratio of V_(PF) V_(PL)(i.e.K_p = V_(PF)/V_(PL).However,Q value,totally,has greater correlation coefficient and less error than the RMR,In addition,rock mass parameters including rock quality designation(RQD),uniaxial compressive strength(UCS),joint roughness coefficient(JRC) and Schmidt number(RN) show close relationship with P-wave velocity.An equation based on these parameters was obtained to estimate the P-wave velocity in the rock mass with a correlation coefficient of 91%.The velocities in two orthogonal directions and the results of joint study show that the wave velocity anisotropy in rock mass may be used as an efficient tool to assess the strong and weak directions in rock mass.     

Responses of jointed rock masses subjected to impact loading

Impact-induced damage to jointed rock masses has important consequences in various mining and civil engineering applications. This paper reports a numerical investigation to address the responses of jointed rock masses subjected to impact loading. It also focuses on the static and dynamic properties of an intact rock derived from a series of laboratory tests on meta-sandstone samples from a quarry in Nova Scotia, Canada. A distinct element code (PFC2D) was used to generate a bonded particle model (BPM) to simulate both the static and dynamic properties of the intact rock. The calibrated BPM was then used to construct large-scale jointed rock mass samples by incorporating discrete joint networks of multiple joint intensities into the intact rock matrix represented by the BPM. Finally, the impact-induced damage inflicted by a rigid projectile particle on the jointed rock mass samples was determined through the use of the numerical model. The simulation results show that joints play an important role in the impact-induced rock mass damage where higher joint intensity results in more damage to the rock mass. This is mainly attributed to variations of stress wave propagation in jointed rock masses as compared to intact rock devoid of joints.     

Control of rock joint parameters on deformation of tunnel opening

Tunneling in complex rock mass conditions is a challenging task, especially in the Himalayan terrain, where a number of unpredicted conditions are reported. Rock joint parameters such as persistence, spacing and shear strength are the factors which significantly modify the working environments in the vicinity of the openings. Therefore, a detailed tunnel stability assessment is critically important based on the field data collection on the excavated tunnel’s face. In this context, intact as well as rock mass strength and deformation modulus is obtained from laboratory tests for each rock type encountered in the study area. Finite element method(FEM) is used for stability analysis purpose by parametrically varying rock joint persistence, spacing and shear strength parameters, until the condition of overbreak is reached. Another case of marginally stable condition is also obtained based on the same parameters. The results show that stability of tunnels is highly influenced by these parameters and the size of overbreak is controlled by joint persistence and spacing. Garnetiferous schist and slate characterized using high persistence show the development of large plastic zones but small block size, depending upon joint spacing; whereas low persistence, low spacing and low shear strength in marble and quartzite create rock block fall condition.     

基于三维裂隙网络的RQD尺寸效应与空间效应的研究

 岩石质量指标(RQD)是岩土工程与地质工程中的重要概念,在岩体性质分析中起到了重要作用,但RQD存在着明显的尺寸效应与空间效应,这在以后岩体工程设计计算中并没有引起充分的重视。为研究RQD的空间效应,采用三维裂隙网络模拟方法模拟现场真实岩体,并在模型中设立大量测线获取岩体中不同位置的RQD值。结果表明,岩体中不同位置的RQD值并不相同,存在明显的空间效应。为获取可整体代表岩体好坏程度的RQD值,需在大量RQD样本的基础上进行分析。另外,为更好地体现岩体的非均质性,研究不同阈值下的RQD范围,最终确定可充分反映待分析岩体的最佳阈值,为4 m。尺寸效应是RQD参数的重要性质,通过改变测线长度的方法探讨RQD在不同尺度下的变化情况。总结、提出RQD的计算模型与公式,即：A-A模型、T-T模型、A-A-S模型、Priest-Hudson公式与Senz-Kazi公式。分别研究与对比不同模型下RQD的尺寸效应,结果表明,采用A-A-S模型可充分减少尺寸效应带来的RQD的计算误差;当阈值较大时,Priest-Hudson公式与Senz-Kazi公式存在一定的误差,基于三维裂隙网络模拟的方法进行RQD的计算将会更为准确。     

Anisotropic characteristics of jointed rock mass: A case study at Shirengou iron ore mine in China

Rock mass is characterized by the existence of distributed joints whose properties and geometry strongly affect the mechanical behavior of jointed rock masses. A finite element model considering the anisotropic characteristics of fractured rock mass was proposed which could deal with a wide variety of joint distribution in rock mass and then applied in Shirengou iron ore mine in Tangshan, China. First, the scale effects and anisotropy were investigated by using multi-scale discrete fracture network models under uniaxial compression tests. Then, the principal direction of elasticity was found and used in the constitutive law of the equivalent continuum model. Finally, the deformation and failure behavior were studied and verified through site-specific microseismic data. It is found that the stress and damage zone are influenced by joint orientation. This proposed model can efficiently study the effects of rock joints on rock mass behavior and thus contribute to a more reasonable explanation on the dominant effect of the joint sets on deformation and failure of rock mass.     

基于纵波波速的块状岩体GSI系统

 为减少目前GSI系统对现场地质观察的依赖程度,降低其应用难度,且使其能更加准确地反映一定深度范围内的岩体特性,根据现有研究成果与GSI系统输入参数的定性、定量对应关系,建立基于纵波波速的GSI系统,据此获得大岗山坝区岩体的GSI,通过对比分析此结果与经验公式的结果及基于GSI的岩体变形模量的预测值与实测值的分布规律及其相关性,探讨了将岩体完整系数和岩石风化程度系数作为GSI系统输入参数的可行性。结果表明：基于岩体完整系数和岩石风化程度系数的GSI系统基本可行;风化程度输入参数采用风化岩石与未风化岩石的波速比平方较为合理;岩体完整系数和岩石风化程度系数丰富了GSI系统的输入参数。     

DDA子块体开裂模拟算法的优化与验证

非连续变形分析(DDA)是计算离散可变形块体系统力学响应的数值计算方法,其在连续岩体开裂破坏模拟中的应用也已得到研究。在以往的预离散子块体DDA开裂模拟方法中,通过采用虚拟节理对子块体进行粘结以模拟连续体的变形,进而根据虚拟节理面上的块间接触力判断沿预设节理面的拉伸或剪切破坏。该算法能够较好的模拟岩体的开裂路径和破坏形态,但受预设虚拟节理面方向对块间接触力大小的影响,由此得到的岩石开裂强度与实际之间可能产生较大差异。本研究不再根据块间接触力进行开裂判断,而改进为根据邻近子块体的应力状态进行开裂判断,并仍假定裂纹沿虚拟节理面产生。用新的DDA程序对压缩载荷作用下圆盘试件的拉伸开裂破坏和方形试件的剪切开裂破坏进行了模拟。算例表明,改进后的开裂算法具有高的开裂计算准确性,并大大减小了预设虚拟节理分布对开裂破坏强度及破坏路径模拟结果的影响。     

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.     

A geo-engineering classification for rocks and rock masses

In this article, an attempt is made to assess the reliability of predicting the uniaxial compressive strength and the corresponding modulus of a rock mass by current approaches. These two basic engineering properties, when estimated from rock mass rating (RMR), Q and geological strength index (GSI), indicate hardly any change in the modulus ratio with the change in the quality of the rock mass from very good to very poor. However, the modulus ratio obtained from the relations involving the joint factor, J_f, indicate a definite decrease in the modulus ratio with a decrease in the quality of the rock mass. The strength and modulus in the unconfined and confined states, the modulus ratio and failure strain in the unconfined case were linked to J_f in earlier publications based on a large experimental database. Some of these relations were adopted to verify the response of jointed test specimens, the response of the rock mass during excavations for mining and civil underground chambers, in establishing ground reaction curves including the extent of the broken zone, and the bearing capacity of shallow foundations.The joint factor is now linked to RMR, Q and GSI. The prediction of compressive strength and modulus of the rock mass appears to be more suitable. For classifying the rock, based on these properties, the Deere and Miller engineering classification, applicable to intact rocks, has been suitably modified and adopted. The results of different modes of failure of jointed specimens establish definite trends of changes in the modulus ratio originating from the intact rock value on the modified Deere and Miller plot. A geo-engineering classification is evolved by considering strength, modulus, quantifiable weathering index and lithological aspects of the rock.     

非贯通节理岩体单轴压缩动态损伤本构模型

非贯通节理岩体是同时含有节理、裂隙等宏观缺陷及微裂隙、微孔洞等细观缺陷的复合损伤地质材料,基于此提出了在非贯通节理岩体动态损伤本构模型中应同时考虑宏、细观缺陷的观点。首先对基于细观动态断裂机理的经典动态损伤本构模型——TCK模型进行了阐述,其次针对目前节理岩体损伤变量定义中仅考虑节理几何参数而未考虑其强度参数的不足,基于能量原理和断裂力学理论推导得出了同时考虑节理几何及强度参数的宏观损伤变量(张量)的计算公式;第三,基于Lemaitre等效应变假设推导了综合考虑宏、细观缺陷的复合损伤变量(张量);第四,借鉴前人基于复合材料力学的观点,考虑了节理法向及切向刚度等变形参数对岩体动态力学特性的影响,进而建立了基于TCK模型的非贯通节理岩体单轴压缩动态损伤本构模型。并利用该模型讨论了载荷应变率、节理内摩擦角、节理厚度、节理法向及切向刚度和节理倾角等对岩体动态力学特性的影响规律。计算结果与目前的理论及试验研究结果比较吻合,从而说明了该模型的合理性。