Characterisation and engineering properties of tectonically undisturbed but lithologically varied sedimentary rock masses

Tectonically undisturbed sedimentary rocks deposited in a quiescent shallow marine environment often include a sequence of strata that may present significant lithological variety at the scale of an engineering structure. Such rock masses exhibit engineering properties that are significantly different from tectonically disturbed rock masses of similar composition. For example, molasse consists of a series of tectonically undisturbed sediments of sandstones, conglomerates, siltstones and marls, produced by the erosion of mountain ranges after the final phase of an orogeny. They behave quite differently from flysch which has the same composition but which was tectonically disturbed during the orogeny. The molasses behave as continuous rock masses when they are confined at depth and the bedding planes do not appear as clearly defined discontinuity surfaces. Close to the surface the layering of the formations is discernible and only then similarities may exist with the structure of some types of flysch. Therefore extreme care is necessary in the use of geotechnical classification systems for the selection of design parameters, in order to avoid penalizing the rock mass unnecessarily. A discussion on the use of the geological strength index (GSI) for the characterisation of such rock masses is presented. Two GSI charts are proposed for estimating the mechanical properties of these masses: one mainly for tunnels; and the second for surface excavations. An example is given to illustrate the process of tunnel design in molassic rocks.     

节理岩体力学参数的选取与应用

在岩块的变形和强度参数及岩体结构面的野外统计窗资料的基础上，利用损伤力学和Hook-Brown准则进行节理岩体力学参数的选取，从而为岩体工程数值模拟提供较可靠的参数。最后，给出了该方法应用于浙江三门核电厂人工边坡数值计算的实例。     

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.     

富溪双连拱隧道围岩强度及稳定性评价

围岩强度指标是各种隧道工程设计中不可缺少的重要参数,准确预测围岩强度对隧道支护设计和稳定计算十分重要。针对富溪隧道地质条件复杂、围岩稳定性差等问题以现场岩石点荷载试验推测岩石强度,以隧道围岩的Q指标统计为基础,利用基于Q指标的经验公式推测围岩抗压强度,给出了富溪隧道各个断面围岩的抗压强度,据此评价各段围岩的稳定性。研究表明,采用的3个经验公式中,无论平均值,还是分散度,Yudhbir公式预测结果都最接近规范标准值,说明本文所用方法用于隧道工程是可行的。该方法简单、实用,预测结果能够反映围岩工程特性,符合实际,可以作为隧道工程中定量评价围岩强度的有效方法,为隧道设计、施工提供可靠依据。     

一种确定节理岩体残余强度参数方法的探讨

 岩体的地质强度指标(GSI)集中考虑岩体结构和结构面表面特征2个方面的因素,已被广泛地应用于岩体强度参数与变形参数的计算中。在GSI系统的基础上,采用残余GSI量化评价方法,通过对峰值地质强度指标GSI进行折减,以得到残余地质强度指标 ,据此来计算节理岩体的残余强度参数。首先,结合岩体分类指标法(RMi),在 和 的基础上探讨残余岩块体积 和残余节理条件系数 取值的确定方法;然后,由 和 计算出 ,据广义Hoek-Brown准则计算出节理岩体的残余强度参数,重点对4种典型岩体的残余强度取值进行分析,讨论残余岩体体积对岩体残余强度参数的影响;最后,通过对岩体原位剪切实验数据和一桥墩承台开挖边坡的稳定性反分析,证实基于 指标的节理岩体残余强度参数确定方法的合理性和可靠性,为节理岩体残余强度参数的确定提供一条新的思路。     

基于有限元方法的裂隙岩体等效强度参数研究

 利用等效连续介质力学方法分析裂隙岩体中工程稳定性时,选择合理的裂隙岩体等效强度参数是取得可靠分析结论的前提。依据建立的裂隙岩体网格,结合岩石统计损伤模型和结构面损伤模型,通过有限元数值方法研究裂隙岩体加载中的渐进破坏过程,以及裂隙岩体等效抗压强度的尺寸效应和各向异性。分析结果表明,从岩块到岩体,岩体等效抗压强度很快降低至稳定幅度,岩体抗压强度的各向异性也不显著。最后,对10 m尺寸的岩体进行不同围压下抗压强度的数值分析,得到裂隙岩体的强度参数,并和Hoek-Brown经验准则进行对比。文中的分析方法对于工程裂隙岩体的宏观参数取值具有参考价值。     

对Hoek-Brown公式的改进及其工程应用

阐述了岩体力学参数研究的工程意义，介绍了国内外常用的几种岩体力学参数的确定方法，指出了Hoek-Brown强度准则和有关改进公式中存在的不足，建立了可以考虑岩体受扰动程度的修正系数Km,Ks，给出了Hoek-Brown强度准则的改进公式。修正系数Km,Ks由完整性系数Kv来表征，既能反映岩体受扰动程度，又方便获取，克服了Hoek-Brown公式和前人改进公式的不足。提出的改进公式，能够更好地确定介于扰动和未扰动之间的岩体力学参数。以厂坝铅锌矿岩体力学参数研究为例，分别用提出的改进公式和其他几种方法进行计算，结果表明，所提出的改进公式在确定介于扰动和未扰动之间的岩体力学参数时与实际情况更为接近。在对比研究的基础上，进一步说明了提出的改进公式的合理性。     

Computing in-situ strength of rock masses based upon RQD and modified joint factor: Using pressure and damage sensitive constitutive relationship

In this study, a new model was presented for computing strength of rock masses based upon in-situ observations of RQD popularly known as rock quality designation. This model links up the rock mass parameters from in-situ investigations with the strength parameters of jointed rocks obtained from laboratory scale experimental observations. Using the constitutive relation, the author derived a pressure and damage sensitive plastic parameter to determine strength of rock masses for varied extents of discontinuity and pressure induced damage. The test results show that plasticity characterized by hardening and softening inclusive of damage invariably depends upon mean pressure and extent of deformations already experienced by rock masses. The present work explores the test data that reveal the dependence of in-situ strength on incremental joint parameters obtained from the joint number,joint orientation, joint roughness, gouge parameters and water pressure. Substituting the relationship between the RQD and modified joint factor with that between modulus ratio and strength ratio, the model shows successfully that using damage inclusive plastic parameter and RQD provides a relationship for estimating the strength of rock masses. One of the main objectives of this work is to illustrate that the present model is sensitive to plasticity and damage together in estimating in-situ strength of rock masses in foundations, underground excavation and tunnels.     

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.     

Heterogeneous rock mass classification by means of the geological strength index: the <Emphasis Type="Italic">San Mauro</Emphasis> formation (Cilento,Italy)

This paper describes an application of the geological strength index (GSI) method to the San Mauro formation, which is characterized by sandstones alternating with argillaceous marls. The Sandstone/Pelite (S/P) ratio and structural complexity were determined. Geo-structural and geo-mechanical surveys were undertaken in situ and rock samples were tested in the laboratory. A map of the S/P ratio was produced showing the bedrock divided in four classes. Three ranges of GSI values were identified. The values of the intact UCS and of the constant m _i were appropriately reduced to reflect the variable presence of sandstone compared with the pelitic fraction. A “weighted average” of the intact strength properties of the hard and weak layers was adopted. The values for the intact materials were reduced from 20 to 60% depending on the GSI categories of the heterogeneous rock mass. In this way, seven classes of rock masses characterized by different values of GSI, reduced UCS and m _i values were identified.     

岩石试验抗压、抗拉区间强度及代表值可信度研究

研究岩石区间非定值强度问题以期适应实验室试验有限性、随机性的特点,以此反映岩石强度参数取值的区间性特征及可能性分布。首先,提出了岩石的抗压区间强度、抗拉区间强度的概念;其次,基于区间分析、未确知数学理论,结合实验室实际,给出了区间强度的求解方法,并提出了区间内不同强度代表值的可信度确定方法,有利于工程计算时强度参数的选取。最后,通过小样本和大样本的分析比较,对本文方法的合理性进行了验证。小样本研究表明,取可信度为95%时,岩石抗压、抗拉强度代表值小于常规均值计算得到的值,且常规均值计算求取的岩石抗压、抗拉强度代表值可信度在65%左右,可见通过常规均值计算的结果是偏危险的;将某矿山砂岩的320个试样（大样本）随机分成64组,每组5个试样（小样本）,研究发现,320个岩样强度呈正态分布,把平均值238.17 MPa作为该区域砂岩的名义值,64个传统均值有32个的取值的大于名义值,证明小样本下（5个样本）,用传统均值代表岩石强度其保证率为50％;64个区间强度有55个包含名义值,准确率为86％,且另外6个区间强度上限值均小于名义值,可见用区间强度代表岩石强度的保证率为95％。区间强度较传统均值更充分地表达了岩石强度的信息,完善了岩石强度参数确定方法,为实际工程设计施工、强度校核及可靠性研究中岩石强度参数的取值提供一条新途径。     

Nonlinear shear behavior of rock joints using a linearized implementation of the Barton-Bandis model

Experiments on rock joint behaviors have shown that joint surface roughness is mobilized under shearing,inducing dilation and resulting in nonlinear joint shear strength and shear stress vs.shear displacement behaviors.The Barton-Bandis(B-B) joint model provides the most realistic prediction for the nonlinear shear behavior of rock joints.The B-B model accounts for asperity roughness and strength through the joint roughness coefficient(JRC) and joint wall compressive strength(JCS) parameters.Nevertheless,many computer codes for rock engineering analysis still use the constant shear strength parameters from the linear Mohr-Coulomb(M-C) model,which is only appropriate for smooth and non-dilatant joints.This limitation prevents fractured rock models from capturing the nonlinearity of joint shear behavior.To bridge the B-B and the M C models,this paper aims to provide a linearized implementation of the B-B model using a tangential technique to obtain the equivalent M-C parameters that can satisfy the nonlinear shear behavior of rock joints.These equivalent parameters,namely the equivalent peak cohesion,friction angle,and dilation angle,are then converted into their mobilized forms to account for the mobilization and degradation of JRC under shearing.The conversion is done by expressing JRC in the equivalent peak parameters as functions of joint shear displacement using proposed hyperbolic and logarithmic functions at the pre-and post-peak regions of shear displacement,respectively.Likewise,the pre-and post-peak joint shear stiffnesses are derived so that a complete shear stress-shear displacement relationship can be established.Verifications of the linearized implementation of the B-B model show that the shear stress-shear displacement curves,the dilation behavior,and the shear strength envelopes of rock joints are consistent with available experimental and numerical results.     

用Hoek-Brown强度准则计算隧道围岩的cm和φm值

把Mohr-Coulomb强度包络线与Hoek-Brown经验强度曲线联系起来,推导出用Hoek-Brown强度曲线的斜率计算围岩岩体抗剪强度参数的公式,指出这样得到的抗剪强度参数将与围岩岩体所处的应力状态有关,对不同埋深,开挖卸荷的围岩岩体的数值模拟尤为有效。     

指数型岩石真三轴强度准则

岩石强度是工程岩体稳定性评价和结构优化设计的前提和基础,通过岩石真三轴试验分析了岩石强度演变特征:(1)随着最小主应力的增加,岩石强度逐渐增大,但增幅逐渐减小并趋于零;(2)岩石广义压拉强度比随静水压力的增大呈先增大再减小的规律,并最终趋于1,即在π平面上呈三角形向圆形转变的过程;(3)岩石强度随中间主应力增大表现为先增大再减小的过程。三参数型指数函数完全符合岩石强度在子午面上的基本特点,而L_(WW),L_(MN)和L_(YMH) 3个罗德函数准确反映了中间主应力对岩石强度的影响,且无条件满足拉压子午面区间光滑、连续、外凸性要求。利用π平面广义拉压强度比和罗德函数将指数型拉压子午面结合,构建了指数型真三轴强度准则,分析了强度参数对岩石强度的影响及其空间包络特征。最后采用14种岩石的真三轴试验数据对指数型真三轴强度准则进行了最优拟合误差分析,结果表明指数型真三轴强度准则均具有良好的拟合精度,可正确描述软–硬不同性质岩石的强度特征。     

露天矿山岩体力学参数估算法的应用研究

露天矿山边坡稳定性分析中岩体力学参数的取值至关重要,Hoek-Brown强度准则可以充分反映岩石的非线性破坏的特点,广泛应用于工程实践;结构面作为岩体的一部分,其对岩体的影响不可忽略;结构面的稳定性由抗剪强度决定,对结构面力学参数的估计同样是评价岩体强度的重要组成部分。利用轮廓曲线法和回弹法量取结构面粗糙度和壁岩强度量化地质强度指标GSI,并对潜在滑移结构面抗剪强度精准取值。结合广义Hoek-Brown强度准则估算和兴矿山II-B边坡岩体力学参数,通过极限平衡法分析其整体稳定性系数;利用Barton-Bandis模型线性拟合,计算结构面抗剪强度,对滑体落岩的稳定性进行分析。计算结果表明,整体边坡安全系数F_s=1.285,稳定性较好;含有潜在滑移结构面的岩体安全系数F_s=0.911,局部存在单平面滑移可能。计算结果符合现场实际情况,经反算法验证后,证明通过该方法估算得到的岩体强度参数及结构面抗剪强度是合理的。     

Back-analysis of rock mass strength parameters using AE monitoring data

Most back-analyses in geotechnical engineering are based on methods that utilize field displacement monitoring data. In the present study, a novel method is developed to back-calculate rock mass strength parameters from AE (acoustic emission) monitoring data in combination with FEM stress analysis. The method is based on the important concept of generalized AE initiation threshold of rock masses, established from comprehensive data analysis of laboratory test and underground monitoring programs using AE and microseismic (MS) techniques. An easy-to-use Wizard is developed in Microsoft Excel™ to assist site engineers to perform the back-analysis. The efficient solver in Excel is utilized to reach the optimization solution of an objective function with constraints. The Wizard allows the user to complete the analysis process in an interactive fashion. One example is given to demonstrate the back-analysis process using AE monitoring data recorded from a cavern site. The rock mass strength parameters identified from this approach compare well with field test data, suggesting that the tool can be used effectively to back-calculate rock mass strength parameters from AE monitoring data.     

Hoek-Brown强度准则研究进展与应用综述

1980年E. Hoek和E. T. Brown提出了Hoek-Brown(H-B)强度准则,已充分得到岩石力学与工程研究者的认同,并进行研究和应用。首先系统地阐述H-B强度准则研究进展：E. Hoek和E. T. Brown对H-B强度准则的研究成果、三维H-B强度准则、H-B强度准则岩石和岩体参数研究、考虑层状节理的H-B强度准则及其参数的各向异性研究。再对过去30 a国内外基于H-B强度准则工程应用的成果进行总结。最后对笔者所开展的H-B强度准则最新研究工作进行介绍：提出一种真正意义上的广义三维H-B强度准则,并采用3种Lode角函数进行屈服面修正,使其可以直接应用于后续本构模型建立和数值软件嵌入;采用三维颗粒流模型进行微观数值建模,对H-B强度准则岩石和岩体参数开展微观研究并建立多尺度联系,为参数的确定提供更加可靠的依据。     

基于三维形态空间分析和仿真试验的岩体结构面剪切强度参数研究

 获取结构面可靠的强度指标是研究岩体强度的重要基础。因不同结构面样本的巨大差异,以及剪切测试难以重复进行的实际情况,直接通过试验获取剪切参数通常难度较大且结果不理想。以具有定向变形特征的岩体剪切结构面为研究对象,探索性提出采用结构面剪切测试–结构面表面形态三维空间分析–数值仿真试验相结合方法,获取真实可靠的结构面强度参数。该方法优势表现为：岩体结构面摩擦角的2个组成部分即基本摩擦角和爬坡角完全分离,且能考虑结构面形态三维特性及剪切变形方向特征;同时,力学测试和数值试验结合,克服了单纯依靠剪切测试无法重复的重大难题。     

Influence of grain size on strength of polymineralic crystalline rock: New insights from DEM grain-based modeling

Grain size effect on rock strength is a topic of great interest in geotechnical engineering.A consensus obtained from earlier laboratory tests is that rock strength generally decreases with the increase of grain size for both silicate and carbonate rocks;however,some recent numerical results conflict with such laboratory test results.To address this intriguing issue,the effect of grain size on strength of polymineralic crystalline rock with low porosity is investigated numerically using the grain-based modeling(GBM) approach in discrete element method(DEM) by interpreting micro-cracking process in response to loading.In agreement with some previous DEM simulation results,the simulated rock strength is found to increase with increasing grain size for both homogeneous and heterogeneous models,even when the number of assembled disks in one mineral grain changes.The mechanism of strength increase with increasing grain size is mainly associated with the number of assembled smooth-joint contacts along grain interfaces and the generation of grain boundary cracks in response to loading.The grain interfaces significantly weaken the integrity of the rock model,which is similar to effects of inherent defects in real rock.As the grain size increases,fewer grain interfaces are built in the model and the rock strength becomes much higher.Hence,by solely changing the mineral grain size in a model,the mechanism of grain size effect as observed in laboratory tests cannot be replicated.To address this issue,a method of degradation of grain boundary strength parameters is used to mimic the possible mechanism of grain size effect.The simulated strength using the method becomes comparable with those obtained from laboratory tests when the heterogeneity in the rock is considered.Degradation of grain boundary parameters with increasing grain size provides a plausible explanation for the grain size effect on rock strength.