Prediction of strength and deformability of an interlocked blocky rock mass using UDEC

The accurate prediction of strength and deformability characteristics of rock mass is a challenging issue.In practice,properties of a rock mass are often estimated from available empirical relationships based on the uniaxial compressive strength(UCS).However,UCS does not always give a good indication of in situ rock mass strength and deformability.The aim of this paper is to present a methodology to predict the strength and deformability of a jointed rock mass using UDEC(universal distinct element code).In the study,the rock mass is modelled as an assemblage of deformable blocks that can yield as an intact material and/or slide along predefined joints within the rock mass.A range of numerical simulations of uniaxial and triaxial tests was conducted on rock mass samples in order to predict the equivalent mechanical properties for the rock mass under different loading directions.Finally,results are compared with the deformability parameters obtained by analytical methods.     

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.     

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.

     

Properties of the rock mass in the region of the pumped-storage plant mloty,Poland

The paper presents, with due regard to geological characteristics, the properties of the metamorphic and sedimentary rock mass in the region of the pumped-storage power station which is under construction at Mloty. The strength properties and the deformability of the rock mass have been described on the basis of in situ and laboratory investigations, depending on the direction of the operation of the shear forces with respect to the foliation planes. The results of investigations of the anisotropy of the compression strength of the rock have been presented for those rocks in the region of the pressure inlet tunnels. In the region of the upper reservoir the properties of the rock eluvium, which will constitute the foundation for a high embankment were examined. The filtration properties of the rock mass have been determined for the whole area. The results of complex geophysical investigations have been utilized in order to obtain details of the geological features and to establish the properties of the mass, especially along sections between the particular points of the in situ investigations.     

Water-induced variations in mechanical properties of clay-bearing rocks

This study presents the results obtained from laboratory tests carried out on different types of clay-bearing rock collected from various parts of Turkey, to quantify the effects of water content on mechanical properties of the rocks, and to develop a method for estimating the rock strength and deformability at any water content based on physical properties. For this purpose, in addition to physical properties of the rock types collected, needle penetration resistance, uniaxial compressive strength, tensile strength and modulus of elasticity were determined on a number of specimens with different water contents. The results suggest that with increasing the water content the reductions in the uniaxial compressive strength, modulus of elasticity and tensile strength are up to 90%, 93% and 90%, respectively, from oven-dried to saturated conditions. Based on a series of empirical models developed and the comparisons between the experimentally determined mechanical properties and those predicted from the models, it is concluded that the suggested models seem to be very practical tools to estimate the mechanical properties of the clay-bearing rocks at any water content using the coefficients related to some physical properties of the rock material such as dry unit weight, water absorption by weight and porosity. In addition, needle penetration test is particularly suitable for such rocks to indirectly estimate their uniaxial compressive strength.     

The geological strength index: applications and limitations

The geological strength index (GSI) is a system of rock-mass characterization that has been developed in engineering rock mechanics to meet the need for reliable input data, particularly those related to rock-mass properties required as inputs into numerical analysis or closed form solutions for designing tunnels, slopes or foundations in rocks. The geological character of rock material, together with the visual assessment of the mass it forms, is used as a direct input to the selection of parameters relevant for the prediction of rock-mass strength and deformability. This approach enables a rock mass to be considered as a mechanical continuum without losing the influence geology has on its mechanical properties. It also provides a field method for characterizing difficult-to-describe rock masses. After a decade of application of the GSI and its variations in quantitative characterization of rock mass, this paper attempts to answer questions that have been raised by the users about the appropriate selection of the index for a range of rock masses under various conditions. Recommendations on the use of GSI are given and, in addition, cases where the GSI is not applicable are discussed. More particularly, a discussion and suggestions are presented on issues such as the size of the rock mass to be considered, its anisotropy, the influence of great depth, the presence of ground water, the aperture and the infilling of discontinuities and the properties of weathered rock masses and soft rocks.     

Correlation of the Hoek–Brown failure criterion for a sparsely jointed rock mass with an extended plane of weakness theory

An appropriate estimate of rock mass strength is necessary for the design of civil and mining structures built in or on rock. Rock mass is an inhomogeneous and anisotropic material with complex behaviour, which contains random planes of discontinuities that tend to reduce its strength. The direct estimation of this strength is practically unfeasible, due to difficulties in sampling and testing. This has led to the development of empirical failure criteria. These, express the strength of the rock mass in terms of properties of the intact rock and the discontinuities. The Hoek–Brown criterion is the most widely accepted one. However, albeit its use for many years, no experimental in situ validation with the actual rock mass strength has been demonstrated. Therefore, the Hoek–Brown criterion is investigated analytically through an extended plane of weakness theory, already validated with experimental evidence on physical specimens. Various intact rock qualities with blocky and very blocky structure are examined. The results indicate deviations in the rock mass strength predicted by the two approaches, especially when the intact rock strength is low.     

General report,session I: Engineering properties of carbonate rocks

After reviewing the geological classification of carbonate rocks, a method of describing carbonate rocks for engineering purposes is developed following the recommendations set out by the I.A.E.G. Mapping Commission. Particular attention is paid to the details of the weathering profile developed on carbonate rocks. The engineering classification of carbonate rocks is based on both the engineering behaviour of the rock as a material and in the mass. Physical properties, including strength and deformation characteristics, are used to determine a modulus ratio for a wide range of limestones including the English chalk. In addition to rock material properties, and engineering classification of the rock mass has to take account of structure, discontinuities, and details of the weathering profile. Assessment of rock mass properties requiresin situ testing to determine deformation characteristics. If the usually limited amount of data on such quantitativein situ properties is related to other measures of rock quality, then engineering geological mapping can be used to extend the applicability of the limitedin situ data to a whole site. RQD has been related to the compressibility of the rock mass, and a rock mass factor has been proposed that links the deformability of the rock material to that of the rock mass. Rock mass classifications for large underground openings may be used with limestones. Engineering problems associated with carbonate rocks are briefly reviewed, as are the properties of limestones as a building stone and as aggregate.     

A New Approach to Estimate Side Resistance of Rock Socketed Drilled Shafts

The uniaxial compressive strength of intact rock is widely used as the sole factor to estimate the side resistance of drilled shafts socketed in rock. However, the side resistance should also depend on in situ rock mass characteristics such as rock type, strength, joint frequency, and weathering conditions. In this study, a new approach is proposed to estimate the side resistance of rock socketed drilled shafts. The approach is based on the GSI rock mass classification, and the Hoek and Brown failure criterion. Inclusion of all these factors into the proposed method explains, to some extent, the differences that are obtained when various empirical methods are used. Comparisons between observed side resistances from field case studies and estimated side resistances with the proposed method show a good correlation. The method appears to be robust since the results are relatively insensitive to estimation from the different rock mass classification and properties and are comparable with predictions from previous empirical correlations.     

A specific upscaling theory of rock mass parameters exhibiting spatial variability: Analytical relations and computational scheme

In this paper the representation of geological conditions in a numerical simulation model is considered. By the expression “geological conditions” we mean the 3D volume geometry of the geological formations, the spatial variability exhibited by the rock parameters inside each of these geological volumes and the necessary upscaling of the rock deformability and strength parameters that are determined in the laboratory from cores collected in the field. A specific theory is outlined of how to go from laboratory tests, geological information and field measurements and observations to the full-scale numerical or “ground model” that includes, apart from initial and boundary conditions and ground water, the rock constitutive laws and associated material parameters for use in simulation models. The term “specific” used in the title of this paper stems from the fact that other possible approaches for the same problem may be applied; i.e. empirical rock mass classification systems, explicit modeling of joints in the rock by the distinct element or finite element methods, homogenization techniques, etc. The method used to take into account the spatial variability of rock mass properties by virtue of Geostatistical Theory and 3D modeling tools is also outlined, with an example case study.     

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.     

粘土矿物与斜坡失稳

作为微米级材料，粘土矿物的单晶尺寸及特殊晶体结构使其集合体-粘土呈现低渗透性、分散-凝絮性及粘滞性等重要工程特性。除沉积岩斜坡含粘土矿物外，76％以上的斜坡岩石造岩矿物还可形成次生粘土矿物。粘土矿物广泛分布于原生沉积岩、三大岩类风化带及第四系松散堆积物构成的斜坡中。泥屑岩因含粘土矿物而易于失稳，且水稳定性差。次生粘土矿物的形成将引起源矿物、矿物集合体及岩块的强度与变形特性的显著变化，诱发岩石向松散介质转化。宏观结构面是粘土矿物最主要的形成和聚集场所，结构面及其内侧一定范围内不同成因粘土矿物的淀积将导致结构面及岩体强度的衰减。粘土矿物是滑坡滑带的常见矿物组分；粘土滑带可以在滑体与滑床之间起到润滑作用。滑带中粘土矿物的含量越多，其润滑效应越显著。粘土型滑带还具有隔水边界之功效，可将滑体与周围介质隔离，使之成为独立水文地质单元，提高坡体拦截、吸收渗入水的能力及斜坡稳定对降雨过程的敏感度。粘土矿物可以促进斜坡时效变形，甚至成为斜坡破坏的关键因素，对斜坡演化及失稳的贡献是显著的。     

Strength and failure modes of rock mass models with non-persistent joints

Most problems faced by the practicing rock engineer involve the evaluation of rock mass strength and deformability. The theoretical evaluation of the mechanical properties of fractured rock masses has no satisfactory answer because of the great number of variables involved. One of these variables, the influence of which over rock mass behavior is poorly documented, is the degree of fracture persistence. This paper presents the results of biaxial tests performed on physical models of rock with non-persistent joints. The failure modes and maximum strengths developed were found to depend on, among other variables, the geometry of the joint systems, the orientation of the principal stresses, and the ratio between intermediate stress and intact material compressive strength (σ₂/σ_c). Tests showed three basic failure modes: failure through a planar surface, stepped failure, and failure by rotation of new blocks. Planar failure and stepped failure are associated with high strength behavior, and small failure strains, whereas rotational failure is associated with a very low strength, ductile behavior, and large deformation.     

破碎岩体注浆加固体强度估计

在注浆压力和时间满足规范要求时,注浆前岩体强度、注浆后加固体强度、浆液结石体强度是影响注浆加固效果的主要因素。在分析了已有纯水泥浆结石体强度试验的基础上,利用非线性拟合分析,提出了P.O. 42.5水泥结石体28 d强度与水灰比之间关系的经验公式。对已有岩体注浆前后强度试验值进行了无量纲分析,建立了量纲为1的参量——强度增长率和原岩体与浆液结石强度比,进而通过非线性拟合分析,提出了破碎岩体注浆加固体强度增长率的经验公式。根据莫尔库仑强度理论,推导出岩体注浆前后剪切强度参数增长率的计算公式。在现场进行了破碎岩体注浆试验,结果表明,强度增长率试验值与经验公式计算值基本吻合,满足岩体工程注浆设计的需要。     

岩体变形模量随深度的变化关系

针对花岗岩，使用岩石力学参数管理系统ＲＯＭＥＤＡ（ＲｏｃｋＭｅｃｈａｎｉｃｓＤａｔａ）中的数据，推导了岩体变形模量随深度的变化关系，并把这种关系应用到三峡永久船闸边坡岩体变形模量的预测中。     

Utilizing rock mass properties for predicting TBM performance in hard rock condition

The key parameters on the estimation of tunnel-boring machine (TBM) performance are rock strength, toughness, discontinuity in rock mass, type of TBM and its specifications. The aim of this study is to both assess the influence of rock mass properties on TBM performance and construct a new empirical equation for estimation of the TBM performance. To achieve this aim, the database composed of actual measured TBM penetration rate and rock properties (i.e., uniaxial compressive strength, Brazilian tensile strength, rock brittleness/toughness, distance between planes of weakness, and orientation of discontinuities in rock mass) were established using the data collected from one hard rock TBM tunnel (the Queens Water Tunnel # 3, Stage 2) about 7.5 km long, New York City, USA. Intact rock properties were obtained from laboratory studies conducted at the Earth Mechanics Institute (EMI) in the Colorado School of Mines, CO, USA. Based on generated database, the statistical analyses were performed between available rock properties and measured TBM data in the field. The result revealed that rock mass properties have strong affect on TBM performance. It is concluded that TBM performance could be estimated as a function of rock properties utilizing new equation (r = 0.82).     

Characteristics of the composition,structure, and physico-mechanical properties of permafrost rock soil in massifs

Investigations of the composition, structure, and physico-mechanical properties of permafrost rock soil in massifs are codified. Tables are presented for characteristics of frozen and thawed rock soils, and are compared with the deformability, strength, and other characteristics of frozen rock soils on the basis of investigations on specific projects. It is demonstrated that after thawing, strong permafrost rocks are frequently transformed into weak saturated soils.     

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。     

Anisotropy of strength and deformability of fractured rocks

Anisotropy of the strength and deformation behaviors of fractured rock masses is a crucial issue for design and stability assessments of rock engineering structures, due mainly to the non-uniform and non- regular geometries of the fracture systems. However, no adequate efforts have been made to study this issue due to the current practical impossibility of laboratory tests with samples of large volumes con- taining many fractures, and the difficulty for controlling reliable initial and boundary conditions for large-scale in situ tests. Therefore, a reliable numerical predicting approach for evaluating anisotropy of fractured rock masses is needed. The objective of this study is to systematically investigate anisotropy of strength and deformability of fractured rocks, which has not been conducted in the past, using a nu- merical modeling method. A series of realistic two-dimensional （2D） discrete fracture network （DFN） models were established based on site investigation data, which were then loaded in different directions, using the code UDEC of discrete element method （DEM）, with changing confining pressures. Numerical results show that strength envelopes and elastic deformability parameters of tested numerical models are significantly anisotropic, and vary with changing axial loading and confining pressures. The results indicate that for design and safety assessments of rock engineering projects, the directional variations of strength and deformability of the fractured rock mass concerned must be treated properly with respect to the directions of in situ stresses. Traditional practice for simply positioning axial orientation of tunnels in association with principal stress directions only may not be adequate for safety requirements. Outstanding issues of the present study and su~zestions for future study are also oresented.     

基于BQ的破碎岩体注浆加固强度增长理论

正确评估破碎岩体注浆加固后的强度对于岩石工程设计十分重要。基于莫尔库仑强度准则,建立了注浆前后破碎岩体强度增长理论,推导出单轴抗压强度增长率、单轴抗拉强度增长率、内摩擦系数增长率和内聚力增长率之间的关系方程。根据已有的基于BQ的岩体内聚力和内摩擦角的经验公式,推导出注浆后破碎岩体内摩擦系数增长率和内聚力增长率随岩体质量指标增长量（ΔBQ）变化的表达式。再根据已有的注浆后破碎岩体单轴抗压强度增长率的经验公式和各强度增长率之间的关系方程,得到隐含ΔBQ的非线性方程,可通过数值解法求出了ΔBQ的值,进而很容易求得各强度增长率的值。分析表明,随着岩体质量指标由小到大变化,注浆后强度增长率开始较大,并很快减小,而后趋于平缓;一般情况下,内聚力增长率约为摩擦系数增长率的2～5倍,单轴抗压强度增长率约为单轴抗拉强度增长率的2～3倍。