Geological Strength Index System Research Articles

Coal Structure Prediction Based on Type-2 Fuzzy Inference System for Multi-Attribute Fusion: A Case Study in South Hengling Block, Qinshui Basin, China

The accurate prediction of coal structure is important to guide the exploration and development of coal reservoirs. Most prediction models are interpreted for a single sensitive coal seam, and the selection of sensitive parameters is correlated with the coal structure, but they ignore the interactions between different attributes. Part of it introduces the concept of the geological strength index (GSI) of coal rocks in order to achieve a multi-element macroscopic description and quantitative characterization of coal structure; however, the determination of coal structure involves some uncertainties among the properties of coal, such as lithology, gas content and tectonic fracture, due to their complex nature. Fuzzy inference systems provide a knowledge discovery process to handle uncertainty. The study shows that a type-2 fuzzy inference system (T2-FIS) with multi-attribute fusion is used to effectively fuse pre-stack and post-stack seismic inversion reservoir parameters and azimuthal seismic attribute parameters in order to produce more accurate prediction results for the Hengling block in the Shanxi area. The fuzzy set rules generated in this paper can provide a more reliable prediction of coal structure in the GSI system. The proposed system has been tested on various datasets and the results show that it is capable of providing reliable and high-quality coal structure predictions.

Mechanical Parameters of Deep-Buried Coal Goaf Rock Mass Based on Optimized GSI Quantitative Analysis

In order to obtain the accurate mechanical parameters of deep-buried coal goaf rock mass, the limitation of geological strength index (GSI) in concealed rock mass is analyzed. Based on the test result and analysis of the current normative standards, the classification indexes of rock mass structural are optimized based on discontinuity distance d and rock mass integrity index K v . The ratio of rock mass saturated strength to dry strength, η, is introduced, quantization formula of structural surface conditions is proposed, and the influence of groundwater and rock types is included in structural surface condition classification. The GSI system is improved to better suit all types of deep-buried and water-rich rock masses. Furthermore, the rock mass disturbance factor D’s quantitative formula is listed according to the Hoek–Brown (HB) criterion. Taking the goaf roof under railway as an example, the parameters of deep-buried rock mass are obtained based on the improved quantitative GSI system and HB criterion. This research provides a scientific reference for achieving geological parameters and engineering designing in goaf areas.

Stochastic assessment of hard rock pillar stability based on the geological strength index system

Stochastic assessment of hard rock pillar stability based on the geological strength index system

A fully automatic-image-based approach to quantifying the geological strength index of underground rock mass

The conventional discontinuity survey process in the mining industry is known to be a time-consuming one and it is technically challenging due to the limited accessibility of fresh rock exposures. A rapid and robust rock mass property quantification system is desirable for rock structure design during mining operations. In this work we develop an image-based and fully automatic rock mass Geological Strength Index (GSI) rating system. The proposed method involves a series of novel algorithms to quantify the GSI rating based on data recovered from digital images. The proposed GSI system includes both Structure Rating (SR) and Joint Condition Digital Imaging (JCDI) to represent the bulk rock and discontinuity surface conditions of the rock mass. Local histogram equalization and self-adaptive gamma correction were introduced into the pre-processing of the rock face images. Compared to conventional histogram equalization, local histogram equalization can effectively restrain the uneven distribution of brightness often present. Self-adaptive gamma correction based on the image properties automatically enlarges the difference between discontinuity areas and intact rock. A novel algorithm combining region growing and the Hough transform is proposed for the automatic extraction of discontinuity areas. Laboratory and field tests demonstrated that the algorithm possesses an advantage over existing methods with regard to better noise suppression and that it can yield reasonable results for images taken in poor photography conditions. Discontinuities were characterized using a novel algorithm comprising area thinning, skeleton linking, spurs removal, and sampling. Using this algorithm, four parameters of the discontinuity can be quantified: length, orientation, separation width, and JRC value. The proposed approach was validated by two field-based case studies.

SLOPE STABILITY ASSESSMENT OF THE SOUTHWEST OVERALL SLOPE OF THE QAJARAN OPEN-PIT MINE BASED ON THE GSI ROCK MASS CLASSIFICATION SYSTEM

The application of assessing the fracturing quality of rock masses based on the classification system for determining the boundary parameters of the Qajaran open-cast mine slope is substantiated. In the certain geomechanical environment, the long-term slope stability of the south-west wall of Qajaran open-pit mine based on the Geological Strength Index (GSI) classification system is estimated. A comparative analysis between the results with other well-known classification systems, such as MRMR, Q-system and RQD, and the outputs of the proposed GSI system is made. A good agreement between the results obtained from Barton’s Q-system and Hoek’s GSI classification systems is found. It is highly desirable to quantify the rock mass based on 2 and/or more classification systems (e.g. Hoek’s GSI and Barton’s Q-system) to provide much more reliable and reasonable estimates. The stability analysis of Qajaran open-pit overall slope at different scales based on the confirmed lower and upper bounds of the GSI classification system and compared with the results of the accepted stability criteria is performed. It has been shown that when there are limited input data of the jointed rock mass in the current geomechanical environment, it is expedient to use Hoek’s GSI classification system to determine the geometric parameters at different scales at the first development stage of the rock slope designs and estimate the stability of the overall slopes of deep open-pit mines based on Hoek-Brown failure criterion.

A shear strength model for anisotropic blocky rock masses with persistent joints

The Mohr-Coulomb (MC) shear strength parameters, cohesion c and angle of friction φ, are required in numerical models. Currently, the geological strength index (GSI) system has been widely used for estimating shear strength of blocky rock masses in rock engineering. However, the GSI system does not include the effect of joint orientation β on the mechanical properties of a rock mass, which means that the shear strength model cannot reflect anisotropic rock mass strength behavior caused by joint orientation. In this research, UDEC-based synthetic rock mass models, which are calibrated by experimental data, are adopted to study the effect of joint orientation on the shear strength of blocky rock masses with two perpendicular joint sets. The values of cohesion c and angle of friction φ estimated from the numerical simulation are compared with those calculated from the empirical shear strength models based on the GSI system. Comparison of the results shows that the existing empirical model overestimates the shear strength of rock mass when 10° <β < 45°, which may impose a risk on engineering design. To rectify the problem, based on the analysis of shear strength parameters of rock masses with various joint orientations, anisotropic weighting factors are proposed to modify the existing model. The proposed modified model is capable of providing conservative but more accurate estimation of the shear strength of rock mass, which is important for safe engineering designs.

Numerical characterization for rock mass integrating GSI/Hoek-Brown system and synthetic rock mass method

Rock mass characterization is a key issue to derive mechanical properties of jointed rock mass. In this research, the strength, modulus and post-peak behaviour of rock masses with varied joint orientations and confining stresses are characterized by integrating an empirical geological strength index (GSI)/Hoek-Brown system and a synthetic rock mass (SRM) numerical method. The SRM model is a 3D distinct element model supporting explicit simulation of discontinuous joints and crack propagation in the rock mass. The rock mass structure as denoted by a discrete fracture network is correlated to the GSI system through a quantitative factor of rock block size. The strength and modulus are then derived from the GSI/Hoek-Brown system and used as references for a series of synthetic rock mass models. A comparative study shows that when the predominant joints are steeply inclined in the rock mass subjected to axial loading, the empirical GSI/Hoek-Brown system provides reliable mechanical properties. When the joints are less favorable to shear sliding, the GSI/Hoek-Brown system underestimates the rock mass strength. Cracking characteristics are also investigated in selected SRM models, where few cracks are simulated when inclined joints are inserted in the rock mass and low confining stresses are applied. This research finally shows the method to construct continuum rock mass models using the numerical parameters of the SRM models. The numerical results suggest that the global failure zone is thin when the post-peak is elasto-brittle, and the failure zone is much wider when the post-peak is elasto-plastic. The involvement of the additional failed zones is responsible for the post-peak ductility. The paper is meant to explore new methods to derive high quality geo-data for jointed rock mass.

An Empirical UCS Model for Anisotropic Blocky Rock Masses

The Hoek–Brown (HB) failure criterion is one of the most widely used failure criteria in rock engineering. Based on the Geological Strength Index (GSI) system, a number of empirical models have been proposed in parallel with this criterion to estimate the strength and deformation properties of rock masses such as uniaxial compressive strength (UCS) and deformation modulus. However, the GSI system does not incorporate the effects of joint orientation β on the quality of a rock mass. This means that these empirical models cannot capture anisotropic rock mass strength caused by joint orientations. In this research, UDEC rock mass models, which are calibrated by laboratory data, are used to investigate the effects of joint orientation on rock mass strength in an unconfined state. The values of UCS obtained from the numerical simulation are then compared with those calculated from traditional empirical UCS models based on the GSI system. The comparison study shows that the value of UCS is significantly overestimated by the traditional empirical model when 10° < β < 45°, which will have serious safety implications for engineering designs. To rectify the problem, based on the analysis of numerical simulation results, an anisotropic weighting factor fβ is proposed to be used to refine the empirical UCS model. The modified UCS model is demonstrated to be capable of giving conservative but more accurate prediction of the rock mass strength for various joint orientations, which will result in more optimal and safer engineering designs.

The Hoek–Brown failure criterion and GSI – 2018 edition

The Hoek–Brown criterion was introduced in 1980 to provide input for the design of underground excavations in rock. The criterion now incorporates both intact rock and discontinuities, such as joints, characterized by the geological strength index (GSI), into a system designed to estimate the mechanical behaviour of typical rock masses encountered in tunnels, slopes and foundations. The strength and deformation properties of intact rock, derived from laboratory tests, are reduced based on the properties of discontinuities in the rock mass. The nonlinear Hoek–Brown criterion for rock masses is widely accepted and has been applied in many projects around the world. While, in general, it has been found to provide satisfactory estimates, there are several questions on the limits of its applicability and on the inaccuracies related to the quality of the input data. This paper introduces relatively few fundamental changes, but it does discuss many of the issues of utilization and presents case histories to demonstrate practical applications of the criterion and the GSI system.

An Improved Rock Mass Characterization Method Using a Quantified Geological Strength Index and Synthetic Rock Mass Model

In this paper, an improved rock mass characterization method is presented. This method used a quantified geological strength index (GSI) and synthetic rock mass (SRM) model based on a discrete fracture network (DFN) and bonded particle model. A quantitative method is initially introduced to assist in the use of the conventional GSI chart, employing the rock block volume as the quantitative factor. The block volume for a given DFN model is calculated by a volumetric joint count, which is defined as the number of fractures intersecting a cube with unit length. The peak strength and deformation modulus of the rock mass are estimated based on the derived GSI, and then selected as references to constrain mechanical parameters of joints in the SRM model. A parametric study shows a positive correlation between the joint stiffness/friction coefficient and both the peak strength and deformation modulus. The joint friction coefficients of 0.5, 1.0, and 1.5 are suggested to correspond to the surface quality of ‘FAIR’, ‘GOOD’, and ‘VERY GOOD’ in the GSI chart. An advantage of the SRM over the GSI lies in the characterization of the post-peak strain softening behavior. The numerical results show that lower joint stiffness and friction coefficient result in a more ductile post-peak behavior. Additional SRM models with varied joint distributions show the complexities of rock mass characterization, where the rock mass strength, modulus, and post-peak strain softening rate may vary significantly with only one aspect of distributions of the joints, such as intensity or diameter. Triaxial testing is also conducted, and the SRM peak strengths are compared with the Hoek–Brown strength criterion based on the GSI system. The strengths of the regular triaxial models show general convergence with the Hoek–Brown criterion, but the strengths of the true triaxial models are less than those of the regular triaxial models due to deviatoric confining stresses. Meanwhile, ductile post-peak behavior is generally simulated in the triaxial testing. Cracking models are finally investigated and presented, behaving as additional information to explain the changes in the rock mass strength and deformability with different confining stresses. The method presented in this paper is meant to help geologists and engineers to quantify the GSI for jointed rock mass, and characterize the rock mass modulus and strength. Meanwhile, the synthetic rock mass method provides an alternative for rock mass characterization in addition to the traditional GSI method.

Maintaining geological reality in application of GSI for design of engineering structures in rock

In rock engineering design, significant advances have occurred in recent years in numerical modelling capability with increasing trends to ever greater use of synthetic rock mass models and discrete fracture network (DFN) modelling, but all too often with little comparative improvement in geologic base data. As a consequence, there is even more need that reliable estimates be available of strength and deformation characteristics of the rock masses on which or within which engineering structures are to be created, be it a tunnel, a foundation or a slope. Geological Strength Index (GSI) characterization, linked with Hoek-Brown strength determination as a basis for modelling has been widely adopted by engineers and geologists involved in design and construction of engineering structures. The need for geological definition of rock mass properties required as inputs into numerical analysis, constitutes one of the greatest reasons for application of the GSI chart, allowing characterization of even difficult-to-describe rock masses, including tackling even the most problematic of weak and complex rock masses. Back-analyses of tunnels, slopes and foundation behaviour using GSI and its reliable application in rock engineering designs attest to its reliability. With continuing use worldwide, the GSI system has continued to evolve, but greater understanding is needed in the definition of input constants, for establishing both GSI and intact rock properties. This need for improved evaluation, particularly from a geological perspective, is addressed in this paper. Geological processes of tectonism, weathering and alteration all affect GSI. Evaluation of these factors, which are each critical to proper GSI definition, are analyzed based on real rock mass cases. Suggested ranges in variability of intact rock parameters uniaxial compressive strength σci and material constant mi for common rock masses are presented in the context of a composite new GSI chart. This chart allows selection of appropriate GSI ranges for any specific rock suite. Specific key engineering geological characteristics that differentiate igneous, metamorphic and sedimentary one from each other are highlighted through discussion of various example rock units (including gneisses, granites, ophiolites, limestones, schists, siltstones/mudstones/shales, and molassic and flysch formations). Illustrations are given of how geological differentiation dictates variability in geotechnical properties of most common rock masses.

PREDICTION OF ROCK MASS PROPERTIES, TUNNEL STABILITY AND SUPPORT PRESSURE BY GEOLOGICAL STRENGTH INDEX (GSI) IN CROCKER FORMATION: A CASE STUDY

This study was conducted along a tunnel in Crocker Formation. The objectives of this study are to determine the value of Geological Strength Index (GSI) and to predict rock mass properties, very unfavourable discontinuities combination and tunnel support pressure for rock bolts or shotcrete for the tunnel. Engineering geological mapping, rock sampling and estimation of GSI values and the disturbance factor were conducted along the tunnel faces. Laboratory analysis includes Point load and dry density test and data analysis consists of kinematic analysis and limit equilibrium analysis. The rock mass was characterised by 94.88 MPa UCS, 0.024 MN/m3 unit weight, widely space and high persistency of discontinuities. The GSI value is 50 with 0.8 disturbance factor. The cohesion, tensile strength and friction angle are 3.671 MPa, 0.056 MPa and 25.20°, respectively. There are eight possibilities of discontinuities combinations on tunnel roof that have factor of safety (F.O.S) lower than 2 and combination of joints 2, 4 and 6 has the highest maximum wedge volume of 28.37 m3. The maximum support pressure of rock bolts or shotcrete for F.O.S of 2 is 0.04 MN. The individual discontinuity plane has been identified to overestimate friction angle and cohesion values, then the GSI system should be applied in homogeneous or isotropic and not in structurally controlled rock masses.

A revised, geotechnical classification GSI system for tectonically disturbed heterogeneous rock masses, such as flysch

Use of the geological strength index (GSI) rock mass classification system and the associated m, s and a parameter relationships linking GSI with the Hoek–Brown failure criterion provides a demonstrated, effective and reliable approach for prediction of rock mass strength for surface and underground excavation design and for rock support selection for most “normal” rock masses. One of the key advantages of the index is that it allows characterization of rock masses difficult to describe, such as flysch, and the geological reasoning it embodies, allowing adjustments to be made to its ratings to cover a wide range of rock masses and conditions compared to a typical engineering approach. Flysch, having high heterogeneity in its petrographic nature and a tectonically disturbed structure, forms very weak rock masses in many cases and needs a particular geotechnical classification according to the engineering geological characteristics it presents. After a decade of application of the GSI for the classification of heterogeneous rock masses (Marinos and Hoek 2001), this paper attempts to re-evaluate or verify the original values and to contribute to the appropriate selection of the index for various conditions. A revised GSI diagram for heterogeneous rock masses, such as flysch, is presented, where a certain range of GSI values for every rock mass type is proposed according to the siltstone-sandstone participation and their tectonic disturbance. Data from the design and construction of a large number of tunnels in a variety of geological conditions were assessed for this purpose. In addition to the GSI values, the selection of the appropriate “intact” rock properties for this type of heterogeneous rock mass is also discussed, where characteristic σci, Ei and modulus ratio (MR) values are proposed.

Weathering characterization for landslides modeling in granitoid rock masses of the Capo Vaticano promontory (Calabria, Italy)

This work focuses on developing multidisciplinary researches concerning weathering profiles related to landscape evolution of the Capo Vaticano promontory on the Calabria Tyrrhenian side (southern Italy). In this area, the tectonic uplift, occurred at least since Pleistocene, together with the Mediterranean climatic conditions, is the main cause of deep weathering and denudation processes. The latter occurred on the outcropping rocks of the crystalline-metamorphic basement, made up of weathered granitoids, in turn belonging to the Monte Poro granitoid complex (intermediate to felsic plutonic rocks covered by Cenozoic sedimentary successions). Field observations coupled to borehole explorations, geophysical surveys, and minero-petrographical analyses allowed the characterization of the granitoid outcrops typical of the studied area in terms of kind and degree of slope instability. This characterization was based on suitable correlations verified between several factors as weathering degree, elastic properties of rocks, and discontinuity features. Weathering profiles are mainly composed by rock masses varying from completely weathered rock with corestones of highly weathered rock (classes IV–V) to slightly weathered rocks (class II). The weathered rocks are involved in several landslide typologies such as debris flow (frequency 48.5%), translational slide (frequency 33.3%), and minor rock fall and rotational slide (frequency 9%). The achieved data allowed the establishment of a general correlation between weathering degree and type of slope instability. Debris flow-type instabilities are predominant on the steeper slopes, involving very poor rock masses ascribed to the shallowest portions of the weathering class IV. Translational slides are less widespread than the previous ones and often involve a mixture of soil and highly weathered rocks. Rotational slides are more frequently close to the top of the slopes, where the thicknesses of more weathered rocks increase, and involve mainly rock masses belonging to the weathering classes IV and V. Rock falls mostly occur on the vertical escarpments of the road cuts and are controlled by the characteristics of the main discontinuities. The assessment of rock mass rating and slope mass rating, based on the application of the discontinuity data, allowed respectively an evaluation of the quality of rock masses and of the susceptibility of rock slopes to failure. The comparison between the last one and the real stability conditions along the cut slopes shows a good correspondence. Finally, the geological strength index system was also applied for the estimation of rock mass properties. The achieved results give a worthy support for a better understanding of the relationship between the distribution of landslides and the geological features related to different weathering degrees. Therefore, they can provide a reliable tool to evaluate the potential stability conditions of the rock slopes in the studied area and a general reference framework for the study of weathering processes in other regions with similar geological features.

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%.

Determination of the mechanical parameters of rock mass based on a GSI system and displacement back analysis

It is very important to obtain the mechanical paramerters of rock mass for excavation design, support design, slope design and stability analysis of the underground structure. In order to estimate the mechanical parameters of rock mass exactly, a new method of combining a geological strength index (GSI) system with intelligent displacment back analysis is proposed in this paper. Firstly, average spacing of joints (d) and rock mass block rating (RBR, a new quantitative factor), surface condition rating (SCR) and joint condition factor (J c) are obtained on in situ rock masses using the scanline method, and the GSI values of rock masses are obtained from a new quantitative GSI chart. A correction method of GSI value is newly introduced by considering the influence of joint orientation and groundwater on rock mass mechanical properties, and then value ranges of rock mass mechanical parameters are chosen by the Hoek–Brown failure criterion. Secondly, on the basis of the measurement result of vault settlements and horizontal convergence displacements of an in situ tunnel, optimal parameters are estimated by combination of genetic algorithm (GA) and numerical simulation analysis using FLAC3D. This method has been applied in a lead–zinc mine. By utilizing the improved GSI quantization, correction method and displacement back analysis, the mechanical parameters of the ore body, hanging wall and footwall rock mass were determined, so that reliable foundations were provided for mining design and stability analysis.

Estimation of rock block size distribution for determination of Geological Strength Index (GSI) using discrete fracture networks (DFNs)

Rock mass classification provides fundamental data for a numerical stability analysis of rock structures. Among rock mass classification systems, the RMR and Q systems often are used for rock support system selection and the Geological Strength Index (GSI) system for estimating rock mass strength and deformation parameters. Moreover, the GSI system is the only rock mass classification that is directly linked to engineering design parameters such as the Mohr–Coulomb or Hoek–Brown strength parameters or the rock mass modulus. However, the original application of the GSI system requires long-term experience and a careful approach because of the fact that its use is a subjective decision. A quantitative approach to assist a less experienced engineer in assigning representative GSI values was presented. It employed the rock block volume and joint conditions as quantitative characterisation factors. Their approach is founded on the linkage between descriptive geological terms and measurable field parameters, such as joint spacing and joint roughness. In this study, a discrete fracture network (DFN) model incorporated with stochastic simulation is applied to characterise rock block size distribution for determination of the GSI. The fracture frequency obtained from the core logging data is analysed and provided to the DFN model as input data. Realisation of the DFN and its verification are conducted to establish the joint systems corresponding to the original fracture frequency. As a result, the stochastic simulation can successfully provide the information on the rock block size distribution to the procedure of the GSI determination.

A Photogrammetric Approach for Stability Analysis of Weathered Rock Slopes

The geological strength index (GSI) system is dependent on the rock block volumes and the joint surface conditions. The weathering degree of rock slopes and their strength properties also depend on these characteristics. This study thus focuses on the use of the GSI system and the Hoek–Brown strength criterion to estimate the engineering parameters for weathered rock masses. Photogrammetric methods based on 3D surface models are used to obtain reliable data on the joint sets in rock slopes, instead of general site investigation using labor-intensive techniques. Photogrammetric surveys were conducted on weathered rock slopes in Gold Coast, Australia to obtain the joint spacing, orientation and roughness. The 3D models are then used to estimate the block sizes the joint roughness coefficients (JRC). The block volumes and JRC values were then used to estimate GSI values. Then parametric studies using the finite element method is conducted to investigate the stability of the slope using the GSI values.

Sensitivity analysis of GSI based mechanical parameters of the rock mass

Recently, the rock mechanical and rock engineering designs and calculations are frequently based on Geological Strength Index (GSI) method, because it is the only system that provides a complete set of mechanical properties for design purpose. Both the failure criteria and the deformation moduli of the rock mass can be calculated with GSI based equations, which consists of the disturbance factor, as well. The aim of this paper is the sensitivity analysis of GSI and disturbance factor dependent equations that characterize the mechanical properties of rock masses. The survey of the GSI system is not our purpose. The results show that the rock mass strength calculated by the Hoek-Brown failure criteria and both the Hoek-Diederichs and modified Hoek-Diederichs deformation moduli are highly sensitive to changes of both the GSI and the D factor, hence their exact determination is important for the rock engineering design.

APPLICATION OF THE ROCK MASS CLASSIFICATION SYSTEMS TO PILLAR DESIGN IN LONGWALL MINING FOR ABU-TARTUR LONGWALL PHOSPHATE MINING CONDITIONS

Pillars are designed to ensure regional stability or local support in stopes and along drifts, or to yield under a measure of control. In all cases, the strength of the material and the variations in strength must be known both for the pillar and for the roof and floor. The stability in longwall faces depends mainly on the interaction between the roof strata, face support, roadway support and dimensions of pillars. The main aim of this paper is to apply rock mass classification systems to longwall pillar design at Abu-Tartur mining area. The pillar load is estimated taking into account the physical and mechanical properties of phosphate deposit and roof rock, panel width, mining height, depth below surface. Two methods from classification systems are used in calculation pillars stress and strength to pillars design namely Geological Strength Index (GSI) and Rock Mass Rating (RMR) systems. GSI values for immediate, main roof rocks and phosphate ores are determined from geological conditions, as lithology, structure of the interlocking of rock blocks and the conditions of the surfaces between these blocks. RMR value can be determined by correlation it with GSI system. The pillar widths calculated by applying rock mass classifications (GSI& RMR) are 49m and 64m at a factor of safety 2 and panel width 100m with extraction ratios of 70 and 64 % respectively. The data used in calculations are collected from geological reports of the company and from laboratory tests of phosphate ores and shale rocks in the roof.