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.     

Estimation of bearing capacity of basalts at the Atasu dam site,Turkey

This paper describes the results of the engineering geological investigations and bearing capacity studies carried out at the proposed site of the rock fill Atasu Dam, to be constructed on basalts and pyroclastics. Rock mass strength and modulus of elasticity of the rock mass were determined using the Hoek–Brown empirical strength criterion. Rock mass classifications for the dam rock foundation were undertaken following the RMR, Q and GSI systems and the stress distributions using the finite element technique. To estimate the bearing capacity of the basalts, different empirical equations were used and compared.      

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.      

Engineering geological evaluation and preliminary support design for the metro extension tunnel,Ankara, Turkey

The paper reports an assessment of the engineering geological characteristics of the rock mass to be encountered between Mecidiye and Gazino stations on the new extension of the Ankara metro and the determination of appropriate support and excavation methods. The rock mass quality was estimated using the rock mass rating (RMR), geological strength index (GSI) and rock mass quality (Q) systems and the tunnel divided into sections. The RMR, Q and NATM systems were used to determine the support and excavation methods in these areas. The deformations and stress concentrations around each tunnel section were investigated and the interaction of the support systems with the rock mass was analyzed using finite element software. It is concluded that rock mass classification systems should be used in tandem with numerical tools, although it is emphasized that the estimation of rock mass properties is not an exact science and both rock properties and numerical models should be refined based on observations and the results of instrumentation installed during the construction of a tunnel.      

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

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

The correlation between RMR and Q systems in parts of Iran

One of the approaches for characterising rock masses with discontinuities due to the presence of engineered designs is to use rock mass classification. Thus far, many classification systems, including RMR, Q, and GSI, have been proposed in the literature. Their parameters are based on site investigations, such as surface/subsurface fracture studies and well coring, as well as laboratory experiments. When sufficient information is not available, the utilisation of several rock mass classification systems is useful to compile a more complete understanding of the composition and characteristics of a rock mass. Thus, many correlations have been drawn to relate different systems, especially between RMR and Q systems. In this study, the best correlation coefficient between RMR and Q systems was determined with the aim of suggesting a new potential correlation for various geotechnical activities in parts of Iran. To accomplish this aim, rock mass parameters for the RMR and Q systems were assessed by considering their values separately for more than 800 stations at 14 different sites and applying statistical procedures to the data. Finally, a new correlation was determined.     

Post-failure behavior of two mine pillars confined with backfill

Researchers from the National Institute for Occupational Safety and Health used a series of instruments (borehole extensometers, earth pressure cells, and embedment strain gauges) to study the post-failure behavior of two pillars confined by backfill in a test section at the Buick Mine near Boss, MO, USA. Evaluation of these pillars was part of a research project to assess the safety of the test section when high-grade support pillars were mined.Data from borehole extensometers installed in several backfill-confined pillars and numerical modeling indicated that these pillars failed during extraction of the support pillars. Failure was corroborated by the post-yield pillar strain response in which the immediate elastic strain was negligible compared to the time-dependent strain component measured between blasting rounds.A three-dimensional, finite-element program with an elastic perfectly plastic material model was calibrated using extensometer data to estimate rock mass modulus and unconfined compressive strength. The resulting rock mass modulus was 45–60% of the average deformation modulus obtained from laboratory tests, and the calibrated compressive strength was 40% of average laboratory values. A rock mass modulus equal to 52% of the average laboratory deformation modulus was calculated using the rock mass rating (RMR) system. Rock mass strength was calculated with the generalized Hoek–Brown failure criterion for jointed rock and indicated that in situ strength was 33% of laboratory strength. Post-failure stresses calculated by the finite-element model were larger for confined pillars than post-failure stresses in unconfined pillars calculated using empirical plots. Data from the calibrated model provided a strain-hardening stress-versus-strain relationship. This knowledge is critical for the design of mines that use partially failed pillars to carry overburden load.     

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.     

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.     

Simulation of excavations in jointed rock masses using a practical equivalent continuum approach

A simple practical equivalent continuum numerical model previously presented by Sitharam et al. (Int. J. Rock Mech. Min. Sci. 38 (2001) 437) for simulating the behaviour of jointed rock mass has been incorporated in the commercial finite difference programme fast Lagrangian analysis of continua (FLAC). This model estimates the properties of jointed rock mass from the properties of intact rock and a joint factor (J_f), which is the integration of the properties of joints to take care of the effects of frequency, orientation and strength of joint. A FISH function has been written in FLAC specially for modelling jointed rocks. This paper verifies the validity of this model for three different field case studies, namely two large power station caverns, one in Japan and the other in Himalayas and Kiirunavara mine in Sweden. Sequential excavation was simulated in the analysis by assigning null model available in FLAC to the excavated rock mass in each stage. The settlement and failure observations reported from field studies for these different cases were compared with the predicted observations from the numerical analysis in this study. The results of numerical modelling applied to these different cases are systematically analysed to investigate the efficiency of the numerical model in estimating the deformations and stress distribution around the excavations. Results indicated that the model is capable of predicting the settlements and failure observations made in field fairly well. Results from this study confirmed the effectiveness of the practical equivalent continuum approach and the joint factor model used together for solving various problems involving excavations in jointed rocks.     

A practical procedure for the back analysis of slope failures in closely jointed rock masses

Where closely jointed rock masses are encountered in slopes, failure can occur both through the rock mass, as a result of combination of macro and micro jointing, and through the rock substance. Determination of the strength of this category of rock mass is extraordinarily difficult since the size of representative specimens is too large for laboratory testing. This difficulty can be overcome by using a non-linear rock mass failure criterion or by back analysis of such slopes to estimate the rock mass strength. In this paper, a practical procedure and a computer program are presented for the back determination of shear strength parameters mobilized in slopes cut in closely jointed rock masses which obey a non-linear failure criterion rather than a linear one. The procedure shows that the constants to derive normal stress dependent shear strength parameters of the failed rock masses can be determined by utilizing a main cross-section and without a pre-determined value of rock mass rating (RMR). Trials are made for different RMR_m and RMR_s values corresponding to various possible combinations of the constant m and s, which are used in the Hoek–Brown failure criterion, satisfying the limit equilibrium condition. It is also noted that the procedure provides a quick check for the rock mass rating obtained from the site investigations. The method is used in conjunction with the Bishop's method of analysis based on circular slip surfaces. The procedure outlined in this paper has also been satisfactorily applied to documented slope failure case histories in three open pit mines in Turkey.     

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

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

Some new Q-value correlations to assist in site characterisation and tunnel design

The rock mass quality Q-value was originally developed to assist in the empirical design of tunnel and cavern reinforcement and support, but it has been used for several other tasks in rock engineering in recent years. This paper explores the application of Q and its six component parameters, for prediction, correlation and extrapolation of site investigation data, and for obtaining first estimates of some input data for both jointed distinct element and continuum-approximation modelling. Parameters explored here include P-wave velocity, static modulus of deformation, support pressure, tunnel deformation, Lugeon-value, and the possible cohesive and frictional strength of rock masses, undisturbed, or as affected by underground excavation. The effect of depth or stress level, and anisotropic strength, structure and stress are each addressed, and practical solutions suggested. The paper concludes with an evaluation of the potential improvements in rock mass properties and reduced support needs that can be expected from state-of-the-art pre-injection with fine, cementicious multi-grouts, based on measurements of permeability tensor principal value rotations and reductions, caused by grout penetration of the least favourable joint sets. Several slightly improved Q-parameter ratings form the basis of the predicted improvements in general rock mass properties that can be achieved by pre-grouting.     

Engineering geological appraisal of the Sulakyurt dam site,Turkey

This paper discusses the engineering geological investigations, diversion tunnel support design and slope stability assessment studies carried out at the Sulakyurt dam site, northeast of Ankara, Turkey. The Sulakyurt dam will be used for flood flow control and water storage for irrigation. Engineering geological mapping, discontinuity surveys, core drilling, water absorption and laboratory tests were undertaken. The RMR, Q and GSI approaches were used to estimate the rock mass quality, site characteristics, rock mass parameters and appropriate tunnel support elements. The results of kinematic and limit equilibrium analyses for the slopes on the right and left banks are reported.      

Bearing ratio of reinforced fly ash overlying soft soil and deformation modulus of fly ash

This paper presents the laboratory study on the bearing ratio of unreinforced and reinforced fly ash overlying soft soil beds of a total of 11 fly ash samples collected from different thermal power plants located in the Eastern part of India. The thickness of the bottom clay layer (H_c) was maintained as 100 mm in the bearing ratio mould. The upper layer thickness of compacted fly ash (H_f) was varied. The values of the ratio H_f/H_c used were 0.75, 1.00 and 1.25 in this study. The fly ash layer was reinforced with single layer and double layers of geotextiles. The effects of (i) position and number of layers of geotextiles, (ii) thickness of the compacted fly ash layer overlying soft soil layer, and (iii) moulding water content of the soft soil, on the bearing ratio of fly ash are highlighted. The inclusion of geotextile into the compacted fly ash bed enhances the bearing ratio. An increase in the thickness of compacted fly ash layer over the soft soil layer also increases the bearing ratio of the compacted fly ash bed. The values of unconfined compressive strength and deformation modulus of all the fly ash samples are also presented. Empirical relationships to estimate deformation modulus of fly ash from unconfined compressive strength and relationships between initial tangent modulus and secant modulus of fly ash are presented. It may be concluded from this research study that reinforced compacted fly ash overlying soft soil with a geotextile layer at the interface can find potential application in the construction of roads over soft soil.     

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.     

单轴压缩条件下柱状节理岩体变形和强度各向异性模型试验研究

 柱状节理岩体作为一种典型的结构岩体,由于柱状节理构造的存在,其变形和强度表现出显著的各向异性特性。为研究柱状节理岩体的力学各向异性,采用模型试验方法,以石膏、水泥和水的混合物为模型材料,制作具有不同柱体倾角(? = 0°～90°)的圆柱形柱状节理岩体试件,通过单轴压缩试验得到柱状节理岩体在不同柱体倾角?下的变形模量和单轴抗压强度。在此基础上绘制出变形和强度随柱体倾角变化的各向异性曲线,分析柱状节理岩体变形和强度的各向异性特性：柱状节理岩体变形和强度各向异性曲线都呈现近似“U”型,单轴抗压强度在? = 30°时取得最小值,在? = 90°时取得最大值,强度各向异性比达到1.5,表现出较显著的各向异性;变形模量在? = 30°～60°范围内取得较小值,侧向应变比大于0.5。同时,根据试验结果,总结柱状节理岩体在单轴压缩应力条件下的4种典型破坏模式,并对其破坏机制进行分析。     

Modifications to the GSI for granite in drilling

The Geological Strength Index (GSI) is widely used to estimate mechanical parameters of rock mass, but estimation of GSI is relatively subjective because of the lack of quantitative parameters. Hence, the existing method for estimating GSI is not suitable for rock samples from drilled cores. Inspired by the GSI estimation method of flysch (Marinos and Hoke, Proceedings of the GeoEng2000 at the international conference on geotechnical and geological engineering, Melbourne, Technomic publishers, Lancaster, pp 1422–1446, 2000), this paper proposes two parameters, the rock mineral condition and rock core length, for use in a modified, more quantitative method for estimating GSI. This modified method can be used for granite specimens from drilled cores. It uses images of typical core samples instead of the sketches given by Hoek’s GSI estimation method. A modified chart of GSI for granite, which is suitable for estimating mechanical parameters of rock mass from drilled core samples, is then developed. The E (elastic modulus) and Q _ult (ultimate bearing capacity) values obtained using our modified method were compared with those from experimental data; the results indicate that the modified method can produce reasonable GSI values and can be used to estimate the mechanical parameters of rock mass from drilled cores.     

Determination of geological strength index of jointed rock mass based on image processing

The geological strength index(GSI) system,widely used for the design and practice of mining process,is a unique rock mass classification system related to the rock mass strength and deformation parameters based on the generalized Hoek-Brown and Mohr-Coulomb failure criteria.The GSI can be estimated using standard chart and field observations of rock mass blockiness and discontinuity surface conditions.The GSI value gives a numerical representation of the overall geotechnical quality of the rock mass.In this study,we propose a method to determine the GSI quantitatively using photographic images of in situ jointed rock mass with image processing technology,fractal theory and artificial neural network(ANN).We employ the GSI system to characterize the jointed rock mass around the working in a coal mine.The relative error between the proposed value and the given value in the GSI chart is less than 3.6%.