Geological Strength Index Values Research Articles

A new indicator for estimating the degree of mining-induced land subsidence: the overburden’s average GSI value

Underground coal mining leads to land subsidence, which, in turn, results in damage to buildings and infrastructure, disturbs the original ecological environment, and hinders the sustainable development of coal mining cities. A reasonable estimation of land subsidence, on the other hand, is the foundation for building protection, land reclamation, and ecological environment reconstruction. However, when we applied the existing land subsidence estimation theory to the deep mining areas of the Ordos coalfield in western China, there was a significant deviation between the estimations and the measurements. To explain such unusual case, we propose using the overburden’s average GSI (Geological Strength Index) value instead of the compressive strength (UCS) of rock specimens for a better representation of the overburden’s overall properties. By using on-site subsidence monitoring results and historical data, we provided evidence which supports that the overburden’s average GSI value has a much greater impact on subsidence rates than the UCS. Subsequently, we investigated the relationship between three typical overburden’s GSI values and the subsidence rates via a calibrated numerical model, revealing the variation patterns of maximum surface subsidence when the overburden’s average GSI value is set at 30, 50, and 75, respectively. Finally, on the basis of the measured and simulated results, we discussed a non-conventional strip mining method for mining subsidence control in the deep mining areas of the Ordos coalfield in western China, and explained why it is possible and what are the significant advantages behind. The proposed methods, findings, and suggestions in this paper are therefore quite helpful for researchers and engineers who wish to estimate and control the mining-induced land subsidence, as well as for those who are particularly interested in the study of environment science related to land subsidence.

Determination of Physical and Geomechanical Properties of Black Shale Hosting Lead and Zinc Ore in Eyingba, Ebonyi

This study was conducted to investigate the physical and geo-mechanical properties such as specific gravity, porosity, uniaxial compressive strength (UCS) etc. of shale, hosting lead and zinc ore in Enyigba, Ebonyi State, Nigeria. The strength properties of the black shale were analyzed because the variability in the strength of rock masses must be properly accounted for, to determine their utility in engineering design. Several tests were carried out on the intact shale samples, and the results were converted to bulk mass strength values using the Hoek–Brown criterion. The results showed that the UCS of the black shale ranged from 38.3 to 43.7 MPa for a geological strength index (GSI) value of 75; and from 54.2 to 60.4 MPa for a GSI value of 80. The specific gravity was 2.89 while the bulk density and porosity values were 290.3 kg/m3 and 15.56% respectively. Also, it was determined that there is a strong correlation between the UCS and specific gravity of the rock, with negligible levels of correlation, both positive and negative, between the UCS and the other mechanical properties examined. In conclusion, the black shale sample from Enyigba is characterized as a medium to hard rock that is suitable for engineering and mining projects.

Evaluation of Engineering Geological Conditions for Slope Stability Analysis of Diversion Tunnel Portal of Jlantah Dam, Karanganyar, Central Java

This paper presents the research results carried out to analyze the slope stability of the diversion tunnel portal of Jlantah Dam based on the quality of the rock mass. The classification of the rock mass quality at the tunnel location refers to the Geological Strength Index (GSI) method while the analysis of portal slope stability is modelled numerically with the element method using Phase2 software. The modelling of the tunnel portal slope design with and without earthquake load was carried out to obtain the safety factor (SF) value. The results showed that the study area consists of residual soil, andesite breccia and lapilli tuff rocks with rock mass quality based on the GSI value ranging from poor to fair. The inlet portal slope is composed of rocks that have poor and fair mass quality while the inlet section is composed of rocks that have poor mass quality. The SF value>1.3 for conditions without earthquake effect was achieved on slope design 1V:1.5H and SF value>1.1 for conditions with earthquake effect was achieved on slope design 1V:2H. The SRF value at the portal outlet location is greater than the SRF value at the portal inlet location for the same slope design conditions indicating that the rock mass quality at the portal outlet location is better than the rock mass quality at the portal inlet location.

EXCASS system and Rock Mass Rating (RMR) classification for determining excavation method and tunnel support at Pelosika Dam Spillway Tunnel

Pelosika dam spillway tunnel was designed to drainage flood discharge and prevent overtopping. In recent investigations, the rock mass classification has not been done specifically in the spillway tunnel. This research aims to obtain technical geological data and information to define excavation methods and tunnel support. For more comprehensive research, engineering geological investigations were carried out regarding the Geological Strength Index (GSI), selecting excavation methods using EXCASS system and Rock Mass Rating (RMR) for tunnel support. The geological mapping showed that the study site was composed of 2 lithological units: quartz muscovite phyllite and pebblely sand deposits. The GSI value in the tunnel is between 14-25, indicating poor and very poor quality. Adopting the EXCASS system, the optimum method of excavation (EPIopt) in the Pelosika dam spillway tunnel are ripper easy and digger. While RMR values of 9 – 25 are classified as poor quality. Consequently, the appropriate method for the excavation sequence is using the top heading and bench and multiple drifts. And the recommended support system is a systematic rock bolt with wire mesh and shotcrete in the crown and walls with medium to light steel rib.

Engineering Geology of Diversion Tunnel Area at the Meninting Dam Construction, West Lombok, Province of West Nusa Tenggara, Indonesia

The construction of Meninting Dam is undertaken to resolve the water needs in Meninting Watershed, West Lombok, Province of West Nusa Tenggara, Indonesia. Therefore, creating a diversion tunnel is imperative to avert the river flow as the dam project commences. Also, engineering geology work on the soil and rocks, including the classification of physical and mechanical properties were conducted in the intended tunnel site. These considerations were necessary because of the unavailability of rock identification data using the GSI (Geological Strength Index) method which used to design the portal slopes as a significant factor in tunnel safety. The results show the proposed area for diversion tunnel construction to be in the lithology of the polymict breccia and the lapilli tuff units, and the soil conditions were included in the SM category (silty sand). The level of surface rock weathering was divided into 3 units, including: highly weathered residual soil lapilli tuff, and highly, as well as moderately weathered polymict breccias. Meanwhile, 4 units were identified on the rock cores (subsurface) comprising highly weathered residual soil of lapilli tuff, and highly, moderately, and slightly weathered polymict breccias. Based on GSI (Geological Strength Index) of rock and surface data from the area of study, the rock quality was grouped as poor (GSI values: 21-40) to very poor (GSI values: 0-20). However, drilling data classified the mass quality subsurface rocks in fair (GSI values 41-55), poor (GSI values: 21-40), particularly in tunnels, and very poor (GSI values 0-20). Therefore, the rock mass quality is possibly used to design the slope of the tunnel portal as 45⁰-55⁰, in order to ensure safety.

Estimating the Geological Strength Index (GSI) in Regional Seismic-Landslide Zonation Using the Empirical Regression Model

The assessment of the strength parameters of geological formations in regional scale which encounters thousands of slopes is a complicated approach and time consuming and needs huge field work. This issue is an important research topic concerning the regional seismic-landslide susceptibility analysis or hazard zonation. An empirical regression model was presented to estimate the Geological Strength Index (GSI) with an implication on geological quadrangle of Gorgan region at Alborz mountains range (north of Iran). Two main sets of data were applied in this study: (1) geomorphological data including the slope height, aspect, and distance from faults and distance from thrusts and (2) the physical and mechanical properties of rocks including the unit weight, uniaxial compressive strength (σci), and the petrographic constant (mί) of intact rock. The first group was extracted from a 1 : 100,000 digital geologic map and 10 m digital elevation model (DEM) and the second group was obtained from the Hoek–Brown failure criterion recommended tables. Linear regression equations were generated applying data collected from 294 studied stations using SPSS software. The regression equation predicted GSI in terms of (1) the distance from faults, (2) the distance from thrusts, and (3) the uniaxial compressive strength (σci). The equation had an R2 value of 0.739 and thus fit well to the data. The new method in its present state was recommended for the estimation of the GSI values in regional scale conditions for the assessment of landslide susceptibility and hazard mapping or post events landslide occurrence prediction in the case of probable big earthquakes in Alborz area that is required for emergency responses. The results indicated that the estimation error was about ±30 while the average error was within +5 and −5 and average error percentage was about 3%.

COMPARATIVE ANALYSIS OF DIFFERENT CALCULATION METHODS OF THE GEOLOGICAL STRENGTH INDEX (GSI) BASED ON QUALITATIVE AND QUANTITATIVE APPROACHES

In this research, the dispersion of the Geological Strength Index (GSI) values obtained with quantitative and qualitative approaches has been evaluated in four rock outcrops of different quality. The subjective component associated with qualitative or visual methods has been studied by conducting a virtual survey in a group of forty participants constituted by civil engineers, geological engineers, and mining engineers from Peru, Spain, and Chile, who were given a data sheet with a photograph and a basic description of each rock mass. The results showed that the GSI values fit a normal distribution characterized by a mean value and a standard deviation, which in some cases could present moderate to high coefficients of variation (COVs). This paper also includes the study of the dispersion of the GSI values obtained with quantitative formulations that have been evaluated and incorporated into regional databases to assess trends, mainly in the GSI-RMR’ relationships. The results indicate that the average GSI values reported with both approaches are similar; however, with the quantitative methodologies, COV values were classified as low to moderate, which is better adjusted to the suggested COV values for the GSI. Despite this, quantitative methodologies must be used with caution, taking into account the characteristics of the rock masses on which the relationships have been defined.

EQUIVALENT GEOLOGICAL STRENGTH INDEX (GSI) APPROACH WITH APPLICATION TO ROCK MASS SLOPE STABILITY

A considerable amount of slope stability analysis has been observed in jointed rock masses in which the GSI (Geological Strength Index) estimated at the outcropping level is considered input data to define the rock mass strength. However, this procedure is unsuitable when the rock outcrop scale and the slope scale are significantly different (e.g. open-pit slopes), resulting in an overestimated rock mass strength. For this reason, and in the absence of criteria to modify the GSI based on the scale effects, in this research, a new GSI version is proposed, called GSIe or “equivalent GSI”. To define an expression for obtaining the GSIe in terms of the rock mass properties, comparative stability analyses were conducted in a series of hypothetical slopes using two approaches: the first considers the rock mass as a discontinuous medium of rock blocks separated by discontinuities; the second considers the rock mass as an equivalent continuous medium characterized by an equivalent GSI. For the adequate equivalent GSI value, evaluated in each analyzed slope, the safety factor and the failure surface are similar in both approaches. In conformity with the results, a GSIe formulation in terms of the slope height, the spacing, the intact rock strength, the persistence, and the joint conditions has been proposed. Finally, the formulation was validated by applying it in five cases of mining slopes where the failure occurred.

Freeze-Thaw Effects on Stability of Open Pit Slope in High-Altitude and Cold Regions

The cycle of the freeze-thaw action must be taken into account in the stability analysis of an open pit slope in the high-altitude and cold regions, because the natural process of freeze-thaw poses a significant effect on mechanical properties of the rock mass. To achieve this purpose, a linear relationship between the geological strength index (GSI) and the Tianshan slope rock mass rating (TSMR) system is established considering the effect of the freeze-thaw action by introducing a freeze-thaw correction coefficient δ. The GSI value is modified for rock mass in high-altitude and cold regions. The improved Hoek-Brown criterion considers the influences of the freeze-thaw action and steep and gentle slopes. The research outcome is applied in the No. 4 minefield open pit coal mine in the Muli mining area. Numerical calculations are performed by inputting rock mass mechanical parameters obtained in traditional and modified criterions, to discuss the influences of the freeze-thaw action on the stabilities of both the present mining slope and the final slope at the end of the designed mining. The results show that the safety factors of the original slope are 2.33 and 1.67, respectively, while after the modification, they are 2.14 and 1.61, respectively. In terms of the No. 4 minefield open pit coal mine, the slope stability meets the design requirement, although taking the freeze-thaw cycle into account.

Bearing capacity of ring foundations over a rock mass using numerical analysis

ABSTRACT Ring foundations are used for circular industrial structures and often placed over a rock mass. However, the bearing capacity factors for ring foundation are not available in the literature. Hence upper and lower bound finite element limit analysis (FELA) was performed to evaluate the bearing capacity factors for the ring foundation resting over a rock mass. The Hoek-Brown constitutive model was adopted to study variations in the geometry of ring foundation, Geological Strength Index (GSI), intact rock constant (mi), and unit weight of rock mass (γ) on the bearing capacity of the ring foundation. The bearing capacity factors Nσ are presented for the different values of GSI, mi, and γ. The failure patterns under varying ring geometry and the rock mass unit weight are presented and discussed. The results show an increase in the bearing capacity with an increase in Hoek-Brown parameters for a rock mass. The bearing capacity is observed to be decreasing with an increase in the ratio (Ri/Ro) of the inner radius (Ri) to the outer radius (Ro) of the ring foundation. Design tables are provided as a ready reference for practitioners to evaluate the bearing capacity of the ring foundations resting over a rock mass.

Ultimate bearing capacity of strip footing resting on rock mass using adaptive finite element method

An essential requirement during the design of structures is a rational assessment of the ultimate bearing capacity of the footing resting on rock mass (qu). The qu is influenced by several parameters, and the simultaneous consideration of all such factors during the assessment of ultimate bearing capacity is a cumbersome process. Considering that the available literature lacks in providing the influence of various parameters of the rock mass on qu, an attempt has been made in this study to evaluate qu of a strip footing resting on a rock mass, which was presumed to follow the latest form of Hoek-Brown failure criterion using finite element modeling. The results obtained in this study were validated with the previous findings of the bearing capacity factors (Nσ0) for a footing resting on weightless rock mass. A comprehensive study has been performed to get an insight into the factors affecting the qu of a footing resting over the rock mass by varying the Geological Strength Index (GSI) ranging from 10 to 100, as well as the disturbance factor (D) at 0 and 1, together with the embedment depth of the footing (Df). The qu is determined by the load-settlement response obtained from numerical simulations and the observed potential failure planes from the simulations are analyzed and discussed. The results showed that the qu was greatly influenced by the GSI of the rock mass and the effect of D reduces with higher values of GSI. The study recommends that the qu of a poor mass can be increased by placing the footing at a deeper depth (Df).

A GSI-softening model for characterizing strength behavior of thermally-damaged rock

Strength behavior of thermally-damaged rock is quite different from that of undamaged or intact rock. In this study, a concept of degradation of geological strength index (GSI) is used and a GSI-softening model in the framework of generalized Hoek-Brown (HB) failure criterion is proposed to characterize the strength behavior of thermally-damaged rock. The proposed GSI-softening model is then verified using several sets of test data collected from previous publications. It is found that the fast increase of strength with increasing confining pressure for thermally-damaged rock can be well captured using the proposed GSI-softening model. The results also show that the GSI value degrades from 100 of undamaged rock to a value ranging from 72 to 88 for the thermally-damaged rocks examined in this study. The material parameter for the thermally-damaged rock is about 1.5 to 3 times higher than the original value of undamaged rock, which indicates that the frictional strength of thermally-damaged rock is much more prominent than that of undamaged rock. The degraded GSI value generally decreases linearly as the temperature in the treatment increases for the examined rocks in this study, while the material parameter linearly increases with the increase in the treatment temperature.

Investigation of Failed Slopes Based on a New Method of Back Analysis in Rock Masses: A Case Study

Small variation in shear strength parameters results in remarkable changes in the safety factor (SF) of a rock slope. In this regard, rock mass strength of a failed slope can be estimated with reasonable accuracy using back analysis approach. The conventional back analysis (BA) procedure is based on trial and error approach on the mb and s parameters of Hoek–Brown failure criterion in conjunction with the Geological Strength Index (GSI) classification system. A new BA procedure presented in this paper, a functional relationship, that indicted all possible combinations of mb and s constants, was derived using stability analysis of a slope assuming that the SF of the slope is equal to unity. Then, another relationship is established between mb and s based on the magnitude of GSI. Intersection of the two relationships when plotting on a graph, defines a unique value for mb and s. Finally, effect of the constant a in the generalized Hoek–Brown failure criterion on the GSI value obtained from the method BA is discussed and its effect is considered in the BA approach. The most advantage of the proposed method is that a reliable GSI is obtained in a relatively short time. For better explanation of the new BA procedure, this method is used for the back analysis of a failed slope along the national railway of Iran. Comparing results obtained from the new BA approach and the traditional back analysis method well-specified which the new proposed BA approach increases the accuracy of parameter estimation.

Study of Rock Mass Rating (RMR) and Geological Strength Index (GSI) Correlations in Granite, Siltstone, Sandstone and Quartzite Rock Masses

A comprehensive understanding of geological, structural geological, hydrogeological and geotechnical features of the host rock are essential for the design and performance evaluation of surface and underground excavations. The Hungarian National Radioactive Waste Repository (NRWR) at Bátaapáti is constructed in a fractured granitic formation, and Telfer Gold Mine in Australia is excavated in stratified siltstones, sandstones and quartzites. This study highlights relationships between GSI chart ratings and calculated GSI values based on RMR rock mass classification data. The paper presents linear equations for estimating GSI from measured RMR89 values. Correlations between a and b constants were analyzed for different rock types, at surface and subsurface settings.

Bearing capacity of interfering strip footings on rock mass

ABSTRACT The present study computes the ultimate bearing capacity of closely spaced strip footings resting on a rock mass. The investigation is executed numerically by using the lower bound finite element limit analysis in association with the semidefinite programming (SDP). Rock mass is assumed to be a perfectly plastic material and follows the modified Hoek-Brown failure criterion. The magnitude of efficiency factor (ξσ ), which is the ratio of the ultimate bearing capacity of interfering footings to isolated footing, is assessed for different clear spacing distances between strip footings. The influence of various rock strength parameters, such as normalised uniaxial compressive strength (σci /γ B), geological strength index (GSI) and Hoek-Brown material parameter (mi ) on the footing interference, are also studied extensively. The results of the present study indicate that the interference effect increases continuously with an increase in mi value; however, it reduces with an increase in GSI value. Detailed variations of the bearing capacity with mentioned parameters are shown and compared with the available literature. It is found that the present study generally provides a lower estimate of ξσ value.

Slope Stability Analysis Based on Rock Mass Rating, Geological Strength Index and Kinematic Analysis in Vindhyan Rock Formation

The Rock Mass Rating (RMR), Geological Strength Index (GSI) and kinematic analysis are widely employed in geotechnical engineering practice. The RMR and GSI both are used for rock mass characterization while the kinematic analysis is applied to identify the mode of failure of slope based on rock discontinuities and slope face characteristics of rock. An attempt has been made to analyze the RMR, GSI and Kinematic Analysis using in-situ data corresponding to Vindhyan sandstone. The data has been collected from outcrops at seven different locations in and around Markundi hill along SH-5, Chopan, Sonbhadra. The relationship between RMR and GSI for Vindhyan sandstone in the study area has been attempted using generalize equation proposed by Hoek (2013) while the kinematic analysis has been applied to decipher the weak zone of failure using RocScience Dips software. The lowest RMR value estimated is 35 for location S-4 and the highest RMR value is 58 for location S-1. Other locations viz, S-2, S-3, S-5, S-6 and S-7 have the RMR value from 42 to 49 which infers that the Vindhyan sandstone existing at Markundi is characterized by both poor as well as fair rock mass. The GSI value of the location S-1 is estimated as 34 (minimum) and 46 (maximum) of location S-4. GSI value of the same rock formation also corroborates that the rock mass belongs to poor as well as fair rock mass. The wedge failure is inferred due to intersection of two sets of joints oriented in north-west and north-east direction. The line of intersections of these two discontinuities are observed in wedge shaped block. It is concluded that the locations under investigations have tendency of wedge failure.

Recommendations of excavation and support systems of Pamukkulu Dam diversion tunnel based on correlation of rock mass classification between RMR and GSI

This paper presents the results of geological engineering research conducted to determine the character of rock masses, recommendations of tunnel excavation method and support system based on standup time estimates in unsuported conditions. The investigation was conducted by observing rock mass quality based on the newest bore log sample test results in 2019 using Rock Mass Rating (RMR) and Geological Strength Index (GSI) rock mass classification. The results showed that area consist of lithology in the form of porphyryc lava basalt and pyroclastic volcanic breccia. Rock mass has a slightly weathering alteration rates. Intact rocks have Uniaxial Compressive Strength (UCS) values ranging from 100-250 Mpa to >250 Mpa and are a category of strong rocks. Rock mass has fair to good rock quality class III-II based on RMR values between 53-69, GSI values between 48-64. The roof span required is obtained from the tunnel planning roof span of 10 meter, with a stand-up time of 70 hours without support system and immediate collapse for 5 days. The recommended excavation methods are excavation by drill and blast on top heading and bench: 1,5-3 meter advance in top heading tunnel face, and then can be recommended support system using rock bolts (20 mm diameter, fullly bonded): systematic bolts 4 meter long, spaced 1,5-2 meter in crown and bench with wiremesh in crown then shotcrete: 50-100 mm in crown, and 30 mm in sides, without steel ribs support.

Analysis of engineering geology conditions based on the quality of rock mass in the Tiga Dihaji Dam’s diversion tunnel, South Sumatera, Indonesia

This research has examined the rock mass quality case study in the Tiga Dihaji Dam’s diversion tunnel. Observations of geological conditions were carried out on the surface and subsurface of the study site and show that the study area consists of tuffaceous sandstone and carbonate interbeds. The method of this study is based on the classification of the Geological Strength Index (GSI), Rock Mass Rating (RMR), and the Q-system. The results indicate that tuffaceous sandstone has a GSI value of 15 - 87.5 (very poor - very good), RMR 48 - 82 (fair - very good), and Q-system 0.01 – 60.0 (exceptionally poor - very good). Meanwhile, carbonate interbeds have a low value, with a GSI value of 10.5 - 77.5 (very poor to very good), RMR 17.0 – 56.0 (very - poor fair), and Q-system 0 - 35.2 (exceptionally poor - good). Moreover, a correlation was made between rock mass quality for conditions in the study area. The correlation between GSI and RMR was obtained by the equation GSI = 2.2885RMR 82.567 (R2 = 0.6653), RMR and Q-system RMR = 2.0175ln(Q) + 63.061 (R2 = 0.4987), and GSI and Q-system GSI = 7.2119ln(Q) 54.578 (R2 = 0.8095).

Geomechanical characterisation of discontinuous greywacke from the Wellington region based on laboratory testing

ABSTRACT Greywacke rock masses are the most prominent (∼20%) bedrock type across New Zealand. This paper reviews the publicly available laboratory testing data on the geomechanical properties of greywacke from the Wellington region and compares the results with two recent studies. This includes compilations of density, unconfined compressive strengths (UCS), tensile strengths, Poisson’s ratio, Young’s Modulus, and mi factor. This paper also presents the first published geomechanical data from a cataclasite zone in the greywacke of the Wellington region. The overall results presented in this paper highlight the influence of both weathering and discontinuities on the geomechanical properties derived from the testing of core samples. The results are discussed in the context of sampling bias, scale effects and recent developments in the geomechanical characterisation of discontinuous rock. Two different approaches for characterising the rock mass geomechanical properties of greywacke at the project-scale are presented. The first approach uses the geological strength index (GSI) as the main parameter to capture the change in rock mass character with depth while the UCS remains constant. The second approach uses the UCS as the main parameter to capture the change in rock mass character with depth, while the GSI value remains constant.

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.