Rock Mass Classification Schemes Research Articles

Upscaling the Mechanical Properties of a Fractured Rock Mass Using the Lattice-Spring-Based Synthetic Rock Mass (LS-SRM) Modeling Approach—Comparison of Discontinuum, Continuum and Empirical Approaches

A numerical characterization of a fractured rock mass and its mechanical behavior using a discontinuum approach was carried out utilizing lattice-spring-based synthetic rock mass (LS-SRM) models. First, LS-SRM models on a laboratory scale were created to reproduce standard rock mechanical tests on Triassic sandstone samples from a quarry in Germany. Subsequently, the intact rock properties were upscaled to an element volume representative for geotechnical applications, recalibrated and combined with a Discrete Fracture Network (DFN) model. The resulting fractured rock mass properties are compared to predictions from empirical relationships based on rock mass classification schemes and the DFN-Oda-Geomechanics approach. Modeling results reveal a significant reduction in the strength of the fractured rock mass compared to the intact rock, showing a high agreement with empirically calculated values. Results for the deformation modulus reveal a significant reduction induced by the fracture network and a good agreement compared to the results obtained by other approaches. It is shown that the LS-SRM allows analyzing the complex mechanical behavior during failure of rock masses, including crack initiation, propagation and coalescence. The resulting rock mass properties are key parameters for a wide range of geotechnical applications and can be used for large-scale numerical modeling as well.

A new perspective for support design of shallow tunnels in mudstone formation: basic philosophy and engineering practice

The traditional support design of rock tunnels mainly relies on the comparison of the geological condition and tunnel characteristics with those prescribed in rock mass classification schemes. The supporting patterns and supporting parameters can then be empirically determined in accordance with the classified grades of rock masses. However, a variety of rock mass classification schemes are used worldwide, which require different input indices. Different classification schemes may lead to varied design suggestions for the same project. Thus, determining the most reliable suggestion presents a challenge. In addition, most of the traditional rock mass classification schemes neglect the intrinsic and distinct properties of rock masses and often either underestimate or overestimate the necessary support requirements. In this study, a new design philosophy is proposed by emphasizing the importance of considering the intrinsic properties of rock mass. Based on a tunneling project in Pakistan, we first determined the basic mechanical properties of mudstone, which is the major formation in the tunneling region. Then, a series of model tests was conducted to analyze the tunnel stability under different scenarios, from which a reduced supporting system that was weaker and more flexible than those determined from the classic rock mass classification schemes was derived. Practical application showed that the proposed supporting system was adequate and reliable. Apart from referring to the rock mass classification scheme, the current study highlights the importance of considering the mechanical properties of rock masses for tunnel support design.

Estimating the rock mass deformation modulus: A comparative study of empirical methods based on 48 rock mass scenarios

Abstract The rock mass deformation modulus, Erm, is an input parameter for most numerical modeling to verify the deformation behavior of rocks due to rock engineering activities within/on it. Among the most common methodologies used for estimating this parameter, empirical correlations based on rock mass classification schemes (e.g., RQD, RMR, GSI, and Q) stand out the most, principally because of their low cost when compared to the other methods. Herein, the main correlations used in practice are evaluated and comparted for 48 different rock quality scenarios, previously characterized and classified according to rock mass classification systems. The results obtained by each of the empirical correlations demonstrated that normalized correlations, that is, based on the ratio of the rock mass and intact rock modulus, Erm/Ei, underestimate the Erm values when compared to those results obtained from not-normalized correlations in the scenarios of better quality rock masses. For poor quality rock mass scenarios, both non-normalized and normalized correlation presented similar results. The correlations proposed by Hoek and Diederichs (2006) and Galera et al. (2007) estimated more central Erm values when compared to the other correlations, for all quality scenarios, while the Mitri et al. (1994) and Sonmez et al. (2006) methods estimated most high and low values of Erm, respectively.

APPLICABILITY OF THE GEOLOGICAL STRENGTH INDEX (GSI) CLASSIFICATION FOR THE TRUSMADI FORMATION AT SABAH, MALAYSIA

During the feasibility and preliminary design stages of a project, when very little detailed information on the rock mass and its geomechanic characteristics is not available, the use of a Rock Mass Classification Scheme (RMCS) can be of considerable benefit. Various parameters were used in order to identify the RMCS. The parameter comprised of Rock Quality Designation (RQD), Rock Mass Rating (RMR), Rock Structure Rating (RSR), Geological Strength Index (GSI), Slope Mass Rating (SMR), etc. In this paper, we present the results of the applicability of the Geological Strength Index (GSI) classification for the Trusmadi Formation in Sabah, Malaysia. The GSI classification system is based on the assumption that the rock mass contains a sufficient number of “randomly” oriented discontinuities such that it behaves as a homogeneous isotropic mass. In this study, the GSI relates the properties of the intact rock elements/blocks to those of the overall rock mass. It is based on an assessment of the lithology, structure and condition of discontinuity surfaces in the rock mass and is estimated from visual examination of the rock mass exposed in outcrops or surface excavations. A total of ten (10) locations were selected on the basis of exposures of the lithology and slope condition of the Trusmadi Formation. The Trusmadi Formation regionally experienced of two major structural orientations NW-SE and NE-SW. It consists mostly of dark grey shale with thin bedded sandstones, typical of a turbidite deposit. This unit has been subjected to low grade of metamorphism, producing slates, phyllites and meta-sediments and intense tectonic deformation producing disrupted or brecciated beds. Quartz vein are quite widespread within the joints on sandstone beds. The shale is dark grey when fresh but changes light grey to brownish when weathered. The results are classified as “Poor Rock” to “Fair Rock” in term of GSI. The poor categories (TR2 and TR7) represent slickensided, highly weathered surfaces with compact coatings or fillings or angular fragments. It is also characterized as blocky/ disturbed/seamy, which folded with angular blocks formed by many intersecting discontinuity sets. The fair categories can be divided into two (2) types; type 1 (TR1, TR6 and TR8) which represent as smooth, moderately weathered and have altered surfaces. It is also characterised as very blocky rock, which indicates interlocked, partially disturbed ass with multi-faceted angular blocks formed by 4 or more joint sets. Type 2 (TR3, TR4, TR5, TR9 and TR10) which represent as smooth, moderately weathered and have altered surfaces but characterized as blocky/disturbed/seamy, which folded with angular blocks formed by many intersecting discontinuity sets. It also has persistence of bedding planes or schistosity.

A Preliminary Assessment of Rock Slope Stability in Tropical Climates: A Case Study at Lafarge Quarry, Perak, Malaysia

The stability analysis of rock slopes is a complex task for the geotechnical engineers due to the complex nature of the rock mass in a tropical climate that often has discontinuities in several forms, and consequently, in several types of slope failures. In this work, a rock mass classification scheme is followed in a tropical environment using the Rock Mass Rating (RMR) and Geological Strength Index (GSI) combined with the kinematic investigation using the Rocscience Software Dips 6.0. The Lafarge quarry is divided into ten windows. In the RMR system, the five parameters uniaxial compressive strength (UCS), rock quality designation (RQD), discontinuity spacing, discontinuity condition, and groundwater conditions are investigated. The RMR values range from 51 to 70 (fair to good rock mass), and the GSI values range from 62 to 65 (good to fair rock mass). There is a good and positive correlation between RMR and GSI. The kinematic analysis reveals that window A is prone to critical toppling, window H to critical wedge-planar failure, and window G to critical wedge failure. From the results obtained, it can be concluded that the kinematic analysis combined with the rock mass classification system provides a better understanding to analyze the rock slope stability in a tropical climate compared with considering the rock mass classification system individually.

Numerical modeling of influence parameters in cavabililty of rock mass in block caving mines

The prediction of rock mass cavability is needed in the design of block caves to estimate the undercut size required to achieve continuous caving. The cavability of the rock mass will control mine design and economic issues for a given geological environment and is a fundamental issue in establishing a successful block caving mine. With recent interest in applying caving techniques to stronger rock masses, the reliable prediction of rock mass cavability is becoming increasingly important. The two most universally applied methods for cavability prediction are the empirical stability graph approach developed by Laubscher and numerical methods. Empirical approaches developed to predict rock mass cavability have been based on the assessment of rock mass conditions using recognized rock mass classification schemes. Rock mass classification is the first stage in empirical approaches to predicting rock mass cavability. The numerical approaches also commonly utilize rock mass classification schemes to assign rock mass properties and apply failure criteria within the models. A reliable prediction of cavability of rock mass depends on the accurate identification of the influence parameters in rock mass cavability. The cavability of a rock mass is based on many factors such as natural and induced factors, affect the caving performance. In this paper the effect of seven parameters, the compressive strength of intact rock (UCS), joint orientation, joint persistence, joint density (P32), joint friction, confined stress and hydraulic radius (HR), is investigated using numerical technique named the Synthetic Rock Mass (SRM). The SRM is a numerical modeling technique which combines the Bonded Particle Model (PFC) for intact rock and Discrete Fracture Network (DFN) simulation. To assess the influence of each parameter in numerical modeling, the value of one parameter is changing while the values of other six parameters are fixed. It is found that in-situ stress and hydraulic radius are the most effective parameters in cavability of rock mass in block caving mines.

Automatic extraction of blocks from 3D point clouds of fractured rock

This paper presents a new method for extracting blocks and calculating block size automatically from rock surface 3D point clouds. Block size is an important rock mass characteristic and forms the basis for several rock mass classification schemes. The proposed method consists of four steps: 1) the automatic extraction of discontinuities using an improved Ransac Shape Detection method, 2) the calculation of discontinuity intersections based on plane geometry, 3) the extraction of block candidates based on three discontinuities intersecting one another to form corners, and 4) the identification of “true” blocks using an improved Floodfill algorithm. The calculated block sizes were compared with manual measurements in two case studies, one with fabricated cardboard blocks and the other from an actual rock mass outcrop. The results demonstrate that the proposed method is accurate and overcomes the inaccuracies, safety hazards, and biases of traditional techniques.

Tensile Strength of Geological Discontinuities Including Incipient Bedding, Rock Joints and Mineral Veins

Geological discontinuities have a controlling influence for many rock-engineering projects in terms of strength, deformability and permeability, but their characterisation is often very difficult. Whilst discontinuities are often modelled as lacking any strength, in many rock masses visible rock discontinuities are only incipient and have tensile strength that may approach and can even exceed that of the parent rock. This fact is of high importance for realistic rock mass characterisation but is generally ignored. It is argued that current ISRM and other standards for rock mass characterisation, as well as rock mass classification schemes such as RMR and Q, do not allow adequately for the incipient nature of many rock fractures or their geological variability and need to be revised, at least conceptually. This paper addresses the issue of the tensile strength of incipient discontinuities in rock and presents results from a laboratory test programme to quantify this parameter. Rock samples containing visible, natural incipient discontinuities including joints, bedding, and mineral veins have been tested in direct tension. It has been confirmed that such discontinuities can have high tensile strength, approaching that of the parent rock. Others are, of course, far weaker. The tested geological discontinuities all exhibited brittle failure at axial strain less than 0.5 % under direct tension conditions. Three factors contributing to the tensile strength of incipient rock discontinuities have been investigated and characterised. A distinction is made between sections of discontinuity that are only partially developed, sections of discontinuity that have been locally weathered leaving localised residual rock bridges and sections that have been ‘healed’ through secondary cementation. Tests on bedding surfaces within sandstone showed that tensile strength of adjacent incipient bedding can vary considerably. In this particular series of tests, values of tensile strength for bedding planes ranged from 32 to 88 % of the parent rock strength (intact without visible discontinuities), and this variability could be attributed to geological factors. Tests on incipient mineral veins also showed considerable scatter, the strength depending upon the geological nature of vein development as well as the presence of rock bridges. As might be anticipated, tensile strength of incipient rock joints decreases with degree of weathering as expressed in colour changes adjacent to rock bridges. Tensile strengths of rock bridges (lacking marked discolouration) were found to be similar to that of the parent rock. It is concluded that the degree of incipiency of rock discontinuities needs to be differentiated in the process of rock mass classification and engineering design and that this can best be done with reference to the tensile strength relative to that of the parent rock. It is argued that the science of rock mass characterisation may be advanced through better appreciation of geological history at a site thereby improving the process of prediction and extrapolating properties.

Classification of cut slopes in weathered meta-sedimentary bedrocks

In order of abundance, the meta-sedimentary rocks along Pos Selim Highway in Perak state Malaysia comprise quartz mica schist, graphitic schist, and quartzite layers. Field investigations revealed that these meta-sedimentary rocks have gradational weathering profile based on differences particularly in textures, hardness, lateral changes in colour, and consistency of material extension. The results from uniaxial compressive strength tests confirmed field observations whereby failure occurred mostly on outcrops having joints almost perpendicular to foliation. From the kinematic analyses, the investigated cut slopes are unstable with possibilities of wedge and planar failures. Application of rock mass classification schemes including Rock Quality Designation (RQD) and Rock Mass Rating (RMR) yielded almost similar poor to good quality ranges for each investigated rock mass. While Slope Mass Rating (SMR) classified the cut slopes from stable to unstable slopes, this study categorized them into one actively unstable, four marginally stable and five stable slopes. ResumenEn orden de abundancia, las rocas metasedimentarias a lo largo de la carretera Pos Selim, en el estado Perak de Malasia, se componen de esquistos de cuarzo mica, esquistos de grafito y capas de cuarzo. Las investigaciones de campo revelan que estas rocas metasedimentarias tienen perfiles de meteorización progresiva basados en diferencias particulares como textura, dureza, cambios laterales de color y consistencia del material de extensión. Los resultados de los ensayos uniaxiales de esfuerzo de compresión confirmaron las observaciones de campo por las cuales se estableció que las fallas ocurrieron mayormente en los afloramientos con coyunturas perpendiculares hacia la foliación. De los análisis cinemáticos se desprende que los taludes de corte investigados son inestables con posibilidades de fallas planas y de cuña. La utilización de esquemas de clasificación rocosa como el índice RQD (del inglés Rock Quality Designation) y la clasificación geomecánica de Bienawski o RMR (del inglés Rock Mass Rating) evidencia rangos similares de baja y buena calidad para cada masa rocosa estudiada. Mientras que el índice de taludes SMR (del inglés Slope Mass Rating) clasificó los taludes de corte de estables a inestables, este estudio los categorizó de uno activamente inestable, cuatro marginalmente estables y cinco estables.

Discrete Fracture Network approach to characterise rock mass fragmentation and implications for geomechanical upscaling

Natural fragmentation is a function of the fracture length and connectivity of naturally occurring rock discontinuities. This study reviews the use of a Discrete Fracture Network (DFN) method as an effective tool to assist with fragmentation assessment, primarily by providing a better description of the natural fragmentation distribution. This approach has at its core the development of a full-scale DFN model description of fracture orientation, size and intensity built up from all available geotechnical data. The model fully accounts for a spatially variable description of the fracture intensity distribution. The results suggest that DFN models could effectively be used to define equivalent rock mass parameters to improve the predictability achieved by current geomechanical simulations and empirical rock mass classification schemes. As shown in this study, a mine-scale DFN model could be converted to equivalent directional rock mass properties using a rapid analytical approach, allowing the creation of a rock mass model that incorporates the influence of a local variable structure with continuous spatial variability. When coupled with more detailed numerical synthetic rock mass simulations for calibration and validation, a balanced and representative approach could be established that puts more equal emphasis on data collection, local- and large-scale characterisation, conceptualisation and geomechanical simulation.

Rock Mass Classification and Slope Stability Assessment of Road Cut Slopes in Garhwal Himalaya, India

There are many rock mass classification schemes which are frequently used for different purposes such as estimation of strength and deformability of rock masses, stability assessment of rock slopes, tunneling and underground mining operations etc. The rock mass classification includes some inputs obtained from intact rock and discontinuity properties which have major influence on assessment of engineering behaviour of rock mass. In the present study, detail measurements were employed on road cuts slope faces in Garhwal Himalayas to collect required data to be used for rock mass classification of Rock Mass Rating (RMR) and Geological Strength Index (GSI). The stability assessment of rock slopes were also done by using Slope Mass Rating. In addition the relation between RMR and GSI were also evaluated using 50 data pairs.

A method for the design of longwall gateroad roof support

A longwall gateroad roof support design method for roadway development and panel extraction is demonstrated. It is a hybrid numerical and empirical method called gateroad roof support model (GRSM), where specification of roof support comes from charts or equations. GRSM defines suggested roof support densities by linking a rock-mass classification with an index of mining-induced stress, using a large empirical database of Bowen Basin mining experience. Inherent in the development of GRSM is a rock-mass classification scheme applicable to coal measure strata. Coal mine roof rating (CMRR) is an established and robust coal industry standard, while the geological strength index (GSI) may also be used to determine rock-mass geomechanical properties. An elastic three-dimensional numerical model was established to calculate an index of mining induced stress, for both roadway development and longwall retreat. Equations to calculate stress index derived from the numerical modelling have been developed. An industry standard method of quantifying roof support is adopted as a base template (GRSUP). The statistical analyses indicated that an improved quantification of installed support can be gained by simple modifications to the standard formulation of GRSUP. The position of the mathematically determined stable/failed boundary in the design charts can be changed depending on design criteria and specified risk.

Application of rock mass classification techniques to weak rock masses: A case study from the Ruahine Range, North Island, New Zealand

Rock mass classification systems were first generated for use in engineering applications, but their potential for utilization in geomorphic studies has since been recognized. These techniques were mostly developed for hard rock environments, and questions remain about their applicability to weak rocks. Here, the applicability of three rock mass classification techniques (rock mass strength (RMS), rock mass rating (RMR), and slope mass rating (SMR)) to weak rock masses was analyzed. Techniques incorporated parameters such as uniaxial compressive rock strength, discontinuity condition and orientation, and groundwater ratings. Rock mass classification values were determined from 14 profiles sited on recently excavated road cuttings on the Saddle Road, in the Ruahine Range, North Island, New Zealand. This is an important transport route across the North Island’s axial ranges, with the road excavated into weak late Pliocene – Early Pleistocene mudstone. Mean slope and minimum slope angle were measured at each profile in concert with the rock mass classification schemes. The three classification techniques all appear to have limited usefulness given the subaerial conditions prevalent at the study site. It would appear that the relative weightings of the different parameters within the RMR, RMS, and SMR classification schemes would need modifying for weak rock masses, but the precise details of this are difficult to determine.

Quantitative geophysical log analysis in coal measure sequences

Geophysical logging is routinely undertaken as part of coal mine exploration programs. With minimal analysis, coal seam depth can be determined and estimates made of coal quality, lithology and rock strength. Quantitative log interpretation will add to this information. We discuss log responses in terms of the mineralogy of the clastic sedimentary rocks frequently found in the black coal mining areas of the Sydney and Bowen Basins. We find that the log responses can be tied to the mineralogy with reasonable confidence. If a full suite of logs is run, ambiguities in the interpretation may be resolved. A key driver in this work is geotechnical characterisation. With this additional geophysical input, it should be possible to develop improved rock mass classification schemes.

Rock mass properties for engineering design

The determination of rock mass properties for engineering design is considered from twoperspectives. These are in-situ measurement, including classification-based methods, and the limitations of the classification approach. Several measurement methods are available which will give useful results, if used appropriately. The choice of different methods will depend on the nature of therock mass. The required accuracy should be considered realistically and, in many cases, high levels of accuracy are not, in fact, necessary. Examples are included, which show how the mass strengths of mine pillars were determined with acceptable accuracy using well known rock mass classification schemes, modified as necessary to accommodate local conditions. Rock mass classification is a widely used, economical and extremely useful basis for determining properties, but there are dangers in uncritical application. The classification methodology is critically examined, and the use of multivariate statistics applied in a multi-dimensional space is considered to optimize the usefulness of the measured data.

Stability Analysis of Jointed/Weathered Rock Slopes Using the Hoek-Brown Failure Criterion

ABSTRACT The failure of rock slopes usually occurs along the discontinuities. But for weathered or highly jointed rock mass, failure surface is often curved as in soil slopes. To assess the stability of jointed/weathered rock slopes, shear strength reduction technique is adapted for use with the non-linear Hoek-Brown failure criterion, an empirical approach to estimating rock mass strength. Hoek-Brown strength parameters can be estimated using rock mass classification schemes such as RMR or GSI. Numerical results obtained by strength reduction are compared with limit analysis upper bound solutions derived using a series of linear failure surfaces tangent to the actual non-linear failure surface. Results are presented in graphs of normalized slope height versus RMR that could be used as stability charts.

A coupled hydromechanical system defined through rock mass classification schemes

Changes in the hydraulic conductivity field, resulting from the redistribution of stresses in fractured rock masses, are difficult to characterize due to complex nature of the coupled hydromechanical processes. A methodology is developed to predict the distributed hydraulic conductivity field based on the original undisturbed parameters of hydraulic conductivity, Rock Mass Rating (RMR), Rock Quality Designation (RQD), and additionally the induced strains. The most obvious advantage of the methodology is that these required parameters are minimal and are readily available in practice. The incorporation of RMR and RQD, both of which have been applied to design in rock engineering for decades, enables the stress-dependent hydraulic conductivity field to be represented for a whole spectrum of rock masses. Knowledge of the RQD, together with the original hydraulic conductivity, is applied to determine the effective porosity for the fractured media. When RQD approaches zero, the rock mass is highly fractured, and fracture permeability will be relatively high. When RQD approaches 100, the degree of fracturing is minimal, and secondary porosity and secondary permeability will be low. These values bound the possible ranges in hydraulic behaviour of the secondary porosity within the system. RMR may also be applied to determine the scale effect of elastic modulus. As RMR approaches 100, the ‘softening’ effect of fractures is a minimum and results in the smallest strain-induced change in the hydraulic conductivity because the induced strain is uniformly distributed between fractures and matrix. When RMR approaches zero, the laboratory modulus must be reduced significantly in order to represent the rock mass. This results in the largest possible change in the hydraulic conductivity because the induced strain is applied entirely to the fracture system. These values of RMR bound the possible ranges in mechanical behaviour of the system. The mechanical system is coupled with the hydraulic system by two empirical parameters, RQD and RMR. The methodology has been applied to a circular underground excavation and to qualitatively explain the in situ experimental results of the macropermeability test in the drift at Stripa. Copyright © 1999 John Wiley & Sons, Ltd.

Development of a preliminary rock mass classification scheme for near-surface excavation : Inyang, H I Int J Surf MinV5, N2, 1991, P65–74

Development of a preliminary rock mass classification scheme for near-surface excavation : Inyang, H I Int J Surf MinV5, N2, 1991, P65–74

Development of a preliminary rock mass classification scheme for near-surface excavation

ABSTRACT In rock excavation work, the resistance of a rock mass depends partially on the mode of loading, shape of the loading instrument, rock moisture conditions and the orientations of the significant flaws relative to that of the applied force. Flaws or discontinuities in rocks may be cracks, bedding planes, weak grain boundaries and faults. Laboratory tests on rocks are often conducted on small cores of reasonably intact material. Generally, rocks decrease in strength with increase in sample size because the probability of the presence of large strength-controlling flaws increases with sample size. This is a plausible explanation for the disparity between laboratory and field specific energies often observed in rock excavation. Since the synergistic effects of discontinuity properties, scale of excavation and moisture condition can not be effectively evaluated using laboratory test results solely, it is necessary to use judgement along with such results in rating rock masses. Such a rating may involv...

Post-test assessment of simulations for in situ heater tests in basalt—Part I. heater test description and rock mass properties

Thermomechanical simulations were undertaken both a priori and a posteriori for two full-scale heater tests in shallow tunnels at the Near Surface Test Facility (NSTF) at Hanford, Washington. The Basaltic rock mass was investigated by an extensive laboratory and field programme involving geological characterization, core drilling and borehole testing, a field thermal test and a large-scale in situ flat jack test isolating 8 m 3 of columnar basalt. Rock mass thermal and mechanical properties were determined from these characterization data for input into thermomechanical simulations. The heater test systems are described along with the instrumentation, which consisted of numerous thermocouples, borehole extensometers, borehole deformation gauges and vibrating wire stressmeters. The determination of the rock mass thermal and mechanical properties from laboratory test data, geological characterization data and back-analysis of field property tests are detailed. The idealization of the basaltic rock mass was determined from interpretation of extensive geological characterization data; involving detailed field geologic mapping, borehole and core logging, rock mass classification schemes, impression packer tests, borehole geophysical tests, borehole jack deformation tests and laboratory and field experiments.