Design Of Underground Excavations Research Articles

Veined Rock Performance under Uniaxial and Triaxial Compression Using Calibrated Finite Element Numerical Models

Geotechnical rockmass characterization is a key task for design of underground and open pit excavations. Hydrothermal veins influence excavation performance by contributing to stress-driven rockmass failure. This study investigates the effects of vein orientation and thickness on stiffness and peak strength of laboratory scale specimens under uniaxial and triaxial compression using finite element numerical experiments of sulfide veined mafic igneous complex (CMET) rocks from El Teniente mine, Chile. The initial numerical models are calibrated to and validated against physical laboratory test data using a multi-step calibration procedure, first of the unveined Lac du Bonnet granite to define the model configuration, and second of unveined and veined CMET. Once calibrated, the numerical experiment involves varying the vein geometry in the veined CMET models by orientation (5 to 85°) and thickness (1, 4, 8 mm). This approach enables systematic investigation of any vein geometry without limitations of physical specimen availability or complexity of physical materials. This methodology greatly improves the value of physical laboratory test data with a limited scope of vein characteristics by using calibrated numerical models to investigate the effects of any other vein geometry. In this study, vein orientation and thickness were both found to have a significant impact on the specimen stiffness and peak strength.

Implementation of the Displacement Discontinuity Method in Geotechnical Case Studies

This paper uses the displacement discontinuity method, one of the boundary element methods, to solve two major engineering problems. The first one addresses the safe design of underground excavations in fractured rock masses. The implemented method was used to control the slip of discontinuities passing through a circular opening at 45°. Special contact elements were used to simulate a possible slip on the cracks. At the same time, stress intensity factors (SIFs) were calculated using the gradient elasticity theory (special tip elements where numerical integrations are needed were excluded). The crack propagation due to shear slip occurrence was defined using the criterion of maximum tangential stress at an angle of 71° from the initial crack direction. The second one involved in the crack’s propagation was solved by applying pressure to the circular opening, while a part of it was transferred to the cracks, approximating the mechanism of hydraulic fracture. Finally, the implementation of higher elasticity elements in the cracks provided an accurate estimation of SIFs, showing an error of about 4%, as confirmed by comparisons with existing type I loading solutions.

Geomechanical characterization of rock mass rating and numerical modeling for underground mining excavation design


 
 
 The objective of the study is the geomechanical characterization of the rock mass rating RMR system and numerical modeling for mining underground excavation design of the Djebel El Ouahch tunnel, in Constantine (Algeria).The geological and geotechnical character- ization of the rock mass is important for the design of underground mining excavations. In this article, we present the results of the RMR characterization of the rock mass and the numerical modeling by the finite element method (FEM), under the conditions of the Djebel El Ouahch tunnel, Constantine (Algeria).The RMR system is a useful tool for characterization of the rock mass quality and establishing the appropriate support system. For poor rock (Class IV), the excavation should be top heading and bench 1.0 m – 1.5 m advance in top heading. Support should be installed concurrently with excavation, 10 m from face. Rock bolting should be systematic with 4 m – 5 m long, spaced 1.5 m – 1.5 m in the crown and walls with wire mesh, Shotcrete of 100 m -150 mm in the crown and 100 mm in sides. The steel sets should be light to medium ribs spaced 1.5 m only when required. The rock mass consists of generally poor rocks with average stand- up time of 10 hours for 2.5m span with mass cohesion ranges between 100 kPa – 200 kPa and rock mass friction angle ranges from 15° to 35°. The FEM project due to its precision calculates the safety factor and evaluates the principal deformations and displacements of the rocks mass .The originality of this work lies in the use of two different approaches , the RMR system and numerical method (FEM) for analyzing the quality and evaluation of the deformations and displace- ments of rock mass .This method has become a very common practice in underground mining excavation design.This study illustrates that the results obtained by RMR of the argillite rock mass in the case is 28.00 ,ranging from 21.0 to 40.0 classified as Class IV (Poor Rock), while the results of FEM reveal that in accordance with the poor quality of the rocks, large deformations and displacements were observed around the underground mining excavation, which can be at the origin of the ruptures. The value of the safety factor of the order of 0.95 to 1.24 shows the instability of the excavation, and the appearance of very considerable hazard zones in the argillite layer.
 
 

Estimation of Modulus of Deformation Using Rock Mass Rating—A Review and Validation Using 3D Numerical Modelling

The Himalayan region has enormous potential for hydropower development. However, variations in geological and geotechnical conditions pose challenging tasks for the designers. If these variations are not tackled in a timely manner during underground excavations, especially for caverns, instabilities may occur, resulting in time and cost over-runs. For sustainable hydropower development, minimizing these over-runs is necessary. The modulus of deformation (Ed) of a rock mass is an essential input parameter required in the design of underground excavations. This study involves collecting the results of extensive in situ tested values for various hydroelectric projects in the Himalayan regions, along with the rock mass rating (RMR) values at 35 test sites. Ed is estimated empirically based on statistical analysis. Comparisons were made with the empirical equations already available in the literature, using RMR and the proposed equation for estimating Ed. Although different researchers have proposed many equations for estimating the value of Ed using RMR, a gap exists in validating such equations. In this regard, the proposed equation for Ed was verified by carrying out 3D numerical-modelling studies using FLAC3D, an explicit finite-difference software for an underground powerhouse cavern and comparing the displacement values with the field instrumentation data.

Predicting Angle of Internal Friction and Cohesion of Rocks Based on Machine Learning Algorithms

The safe and sustainable design of rock slopes, open-pit mines, tunnels, foundations, and underground excavations requires appropriate and reliable estimation of rock strength and deformation characteristics. Cohesion (𝑐) and angle of internal friction (𝜑) are the two key parameters widely used to characterize the shear strength of materials. Thus, the prediction of these parameters is essential to evaluate the deformation and stability of any rock formation. In this study, four advanced machine learning (ML)-based intelligent prediction models, namely Lasso regression (LR), ridge regression (RR), decision tree (DT), and support vector machine (SVM), were developed to predict 𝑐 in (MPa) and 𝜑 in (°), with P-wave velocity in (m/s), density in (gm/cc), UCS in (MPa), and tensile strength in (MPa) as input parameters. The actual dataset having 199 data points with no missing data was allocated identically for each model with 70% for training and 30% for testing purposes. To enhance the performance of the developed models, an iterative 5-fold cross-validation method was used. The coefficient of determination (R2), mean absolute error (MAE), mean square error (MSE), root mean square error (RMSE), and a10-index were used as performance metrics to evaluate the optimal prediction model. The results revealed the SVM to be a more efficient model in predicting 𝑐 (R2 = 0.977) and 𝜑 (R2 = 0.916) than LR (𝑐: R2 = 0.928 and 𝜑: R2 = 0.606), RR (𝑐: R2 = 0.961 and 𝜑: R2 = 0.822), and DT (𝑐: R2 = 0.934 and 𝜑: R2 = 0.607) on the testing data. Furthermore, to check the level of accuracy of the SVM model, a sensitivity analysis was performed on the testing data. The results showed that UCS and tensile strength were the most influential parameters in predicting 𝑐 and 𝜑. The findings of this study contribute to long-term stability and deformation evaluation of rock masses in surface and subsurface rock excavations.

Mechanical properties and fracturing of rock-backfill composite specimens under triaxial compression

In the mining industry, backfill is used to stabilize mined-out underground excavations, and backfill made from mine tailings has the added advantage of being sustainable and eliminating the hazards of storing tailings on the surface. Triaxial compressive tests were conducted on laboratory specimens containing both rock and backfill, in order to better understand the effects of rock/backfill volume fraction and confining pressure on the strength and fracturing in rock-backfill systems. The importance of the interface between rock and backfill was also studied by conducting before and after CT scanning. The results are very interesting and show that shear and tensile fracturing in the rock as loading progresses occurs concurrently with slip and failure of the rock-backfill interface, resulting in unique stress-strain behavior, crack dilatancy, and failure types. The backfill shows very little fracturing but has a major effect on the effective strength and elastic properties of the combined specimens. The results provide important details on the failure process of rock-backfill systems and provide guidelines for the safe design of underground excavations in rock.

Evaluation of Weathered Rock Mass Strength and Deformation Using Weathering Indices

Rock weathering and the resulting effects on geotechnical properties of weathered rock mass has long been a concern for geotechnical engineering practicing since the strength and deformational properties of weathered rock masses is needed for analysis and design of underground openings, opens pits and excavations for foundations of dams, bridges and high-rise buildings in and on weathered rock mass. It is a very challenging task to obtain required core samples to determine the strength and deformation behavior for the assessment of weathered rock mass performance. This is because of weathering which causes change in fabric of rock by creating voids and micro cracks after dissolving grain boundaries particularly, at higher stage of weathering. This paper attempt to present a method of sampling and testing of weathered rocks and proposed a weathered rock mass indices chart for estimating rock mass strength and deformation properties. To investigate the applicability of the proposed weathered rock mass indices chart for other regions, available data from literature for same rock has been used along with the extensive testing data generated in the present investigation Deccan plateau basaltic rocks.

Modelling of sensitivity of underground space stability to the in situ stress uncertainties: case study at the Bukov underground research facility phase II (Rozna mine, Czechia)

Numerical modelling has been widely used in the underground excavation design, where the in situ stress state plays a crucial role in the stability analysis. However, determination of an exact stress state for a specific geological region remains uncertain due to the complicated tectonic nature and measurement limitations. The stability is thus better analysed by considering the in situ stress as a finite spectrum and pinpointing the possible worst-case scenario. The most probable scenarios of in situ stress states in the Rožná mine area were analysed based on the varying trends in principal stress ratio and mean stress values obtained from four different measurement/analysis campaigns. The influence of different in situ stress judgement on the deformation and failure characteristics of the Bukov Underground Research Facility (URF) (Phase II, Czech) were investigated by the finite volume program FLAC3D. Results show that the increased horizontal stress anisotropy and the mean stress level jointly increase the overall deformation and lower the URF stability. Such influences on the roadway horizontal convergence are more considerable than the vertical ones. A mathematical model considering mean stress and horizontal stress ratio was proposed to quantitatively describe the overall stability, especially useful for excavations possessing complicated configuration.

Evaluation of the Influence of In Situ Stress on the Stability of Mine Pit Walls: A Case Study of Songwe Mine

The investigation of the influence of in situ stress in Open Pit Mine (OPM) projects has not been accorded a deserved attention despite being a fundamental concern in the design of underground excavations. Hence, its long-term potential adverse impacts on pit slope performance are overly undermined. Nevertheless, in mines located in tectonically active settings with a potential high horizontal stress regime like the Songwe mine, the impact could be considerable. Thus, Using FLAC3D 5.0 software, based on Finite Difference Method (FDM) code, we assessed the role of stress regimes as a potential triggering factor for slope instability in Songwe mine. The results of the evaluated shearing contours and quantified strain rate and displacement values reveal that high horizontal stress can reduce the stability performance of the pit-wall in spite of the minimal change in Factor of Safety (FoS). Since mining projects have a long life span, it would be recommendable to consider “in situ stress-stability analyses” for OPM operations that would be planned to extend to greater depths and those located in tectonically active regions.

A comprehensive underground excavation design (CUED) methodology for geotechnical engineering design of deep underground mining and tunneling

Underground excavation designs in mining and civil engineering projects are associated with a different geometric, complex ground structure, field–stresses, and groundwater conditions. Due to diverse influence factors and various requirements for each project, the design procedure should be customised based on the site-specific factors. The design procedure is only a direct and triggering process, while in a complex ground condition, mixed failure mechanisms may occur. The objective of this research is to present a new methodology "Comprehensive Underground Excavation Design" called CUED method with emphasis on diagnosis of ground behaviour and failure mechanism(s) in deep and hard rock conditions for long-term life expectations. The CUED method proposed in six steps including ground characterisation, diagnosis of ground behaviour, identifying failure mechanism, design analysis to manage ground behaviour, construction, field measurements/monitoring and design update. A procedure has been defined for each step by determination of input data, processing data and output data, so-called IPO approach. IPO is applied to determine parameters of the CUED method in each step. Based on the proposed method, rock mass composition provides sufficient information to the diagnosis of ground behaviour. Then, different types of failure modes and related mechanisms are evaluated following three main factors: rock mass structures, stress concentration and construction conditions/key features of the projects. In some cases, only one failure mechanism is dominant, but in many cases, may failure starts with one mechanism and then followed by other mechanism or combination of mechanisms. It makes the design more complicated and should be adjusted during operational stages. Design analysis to manage ground behaviour is carried out to determine appropriate strategy in an unstable ground such as selecting suitable excavation method, excavation sequences, and ground stabilisation methods regarding time-cost model of the projects. Meanwhile, the design parameters are implemented in the construction stage by excavation, loosening, depressurisation/stress management, quality control of materials and installing ground support systems. Optimisation of design parameters is performed through new acquired data in field measurements to reduce the risk of rock failures. The proposed design procedure was verified through several case studies from mining and underground excavation projects, and some typical cases were presented. The purpose of the proposed design method is to attempt to increase productivity, cost optimisation, safety improvement, and consequently diminish instability in underground excavations. The research results indicated that the proposed method efficiently increase safety and optimise the project's cost and time. The presented CUED method could be used as engineering tools for underground excavation design.

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.

Physical investigation on the behaviours of voussoir beams

Abstract It is important to understand the behaviour of voussoir beams in the design of underground excavations. There are a large number of discrepancies between the results obtained by different researchers so far. To resolve these discrepancies, physically testing the stress distribution at the midspan and abutment of voussoir beam is the key. In this paper, a simple two-block voussoir beam was built with loading system. The displacement and strain of the voussoir beam were measured by a digital image correlation system. The strain distribution along the joints of voussoir beams was firstly revealed. Then, the relationship between transverse load and midspan displacement of voussoir beam was presented. It is found that the stress distributions at the midspan and abutment are different both in shape and depth. The stress distribution is changing as the loading changes as well.

A New Method for Forecasting Uniaxial Compressive Strength of Weak Rocks

The uniaxial compressive strength of weak rocks (UCSWR) is among the essential parameters involved for the design of underground excavations, surface and underground mines, foundations in/on rock masses, and oil wells as an input factor of some analytical and empirical methods such as RMR and RMI. The direct standard approaches are difficult, expensive, and time-consuming, especially with highly fractured, highly porous, weak, and homogeneous rocks. Numerous endeavors have been made to develop indirect approaches of predicting UCSWR. In this research work, a new intelligence method, namely relevance vector regression (RVR), improved by the cuckoo search (CS) and harmony search (HS) algorithms is introduced to forecast UCSWR. The HS and CS algorithms are combined with RVR to determine the optimal values for the RVR controlling factors. The optimized models (RVR-HS and RVR-CS) are employed to the available data given in the open-source literature. In these models, the bulk density, Brazilian tensile strength test, point load index test, and ultrasonic test are used as the inputs, while UCSWR is the output parameter. The performances of the suggested predictive models are tested according to two performance indices, i.e. mean square error and determination coefficient. The results obtained show that RVR optimized by the HS model can be successfully utilized for estimation of UCSWR with R2 = 0.9903 and MSE = 0.0031203.

Incorporating scale effect into a failure criterion for predicting stress-induced overbreak around excavations

The evaluation of the depth of brittle failure around excavations is of major importance in order to optimize the design of underground excavations and ensure the safety of workers and equipment. The current proposed approaches to evaluate it are related to a single scale of study (intact rock or rock mass scale). Therefore, they are scale-dependent, and cannot be applied for all excavation diameter. In this paper, a generalized failure criterion including the scale effect for predicting stress-induced overbreak around excavations is developed. This failure criterion is based on the damage initiation relation (σ1= Aσ3+ Bσc). The scale effect is included into it through a relation proposed to evaluate the B parameter and depending on the scale of study. The fit parameters of the relation proposed have been defined considering a database at both rock mass and intact rock scales arising from a literature review. For intact rock scale, the B parameter is defined as a function of the diameter of the excavation, expressed following a potential form. For rock mass scale, the B parameter is defined equal to 0.35, regardless the diameter of the excavation. Based on the proposed B parameter relation, the depth and extension of the brittle failure around excavations can be evaluated for any scale of study.

Statistical assessment of intact rock properties for two underground mining projects at Raglan Mine, Quebec, Canada

ABSTRACT The design of underground mining excavations relies on geotechnical characterization of intact rock through laboratory testing. As mining project development progresses through prefeasibility to production stages, the reliability of estimates of rock mechanics properties needs to increase. However, it is challenging to determine the number of tests needed to adequately estimate intact rock properties at different development stages. This paper looks at the early stages of an underground mining project in the Canadian Arctic where two field and laboratory testing campaigns were conducted to evaluate intact rock tensile and uniaxial compressive strength. Results were statistically compared to target confidence levels associated with different stages of a mining project. The study provides insights for planning future field and laboratory testing campaigns. The methodology also provides a quantitative means to assess whether additional laboratory testing is needed to improve tensile and uniaxial compressive strength estimates.

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.

Selecting the optimal drilling and blasting design for underground excavations in underground copper mine 'Jama' Bor

Drilling and blasting operations are very important part of underground excavation design. They should be performed in accordance with some criteria, such as fragmentation, excavation stability and cost. In the paper are considered three drilling and blasting designs in order to reach the optimal one. To do so, all the criteria should be simultaneously considered in the analysis. For analysis is used Analytic Hierarchy Process – AHP method. Using AHP method, drilling and blasting patterns in underground copper mine “Jama” Bor are investigated and analysed. According to the obtained results, alternative 2 with contour drilling and blasting pattern was selected to be the best decision. Application of this alternative comparatively satisfies both fragmentation and excavation stability.

Measurement of elastic properties in Brazilian disc test: solution derivation and numerical verification

Mechanical properties of rocks/soils are key inputs in the design and analysis of surface structures (e.g., slope, foundation) and underground excavations (e.g., tunnel, cavern, borehole, and fracture) in earth engineering practice. Brazilian disc test has been extensively used to measure tensile strength for various rocks. In this study, we propose to measure Young’s modulus, Poisson’s ratio and tensile strength in one Brazilian test through making two additional measurements, i.e., two relative displacements using LVDT, or normal strains by pasting two strain gauges, which are perpendicular to each other, at the center of the disc’s side faces. The analytical solutions of the equivalent strains calculated from LVDT measurements or measured directly by strain gauges are derived from elasticity theory. The analytical solutions are verified with measurements in a numerical model. New formulae for calculating Young’s modulus and Poisson’s ratio from two measured strains are provided and their insensitivity to gauge size is illustrated.

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.

The support characteristic curve for blocked steel sets in the convergence-confinement method of tunnel support design

Construction of support characteristic curves is an important aspect of the implementation of the convergence-confinement method of tunnel support design. This paper presents a detailed analysis of the equations required to construct the support characteristic curve of a combined support system consisting of closed circular steel sets linked to the periphery of the tunnel by equally spaced prismatic wood blocks. This particular problem has been treated in the classical literature of underground excavation design in rock, although without providing details of the origin or validity of the equations. This paper fills this gap, providing the derivation and validation of all the equations required to construct the support characteristic curve for the combined support system. The paper shows that both the stiffness and maximum load that the combined support system can take increases as the angle between equally spaced wood blocks is decreased (or the number of blocks for the steel section is increased). In the limit, when the angle between wood blocks is assumed to be zero, the paper shows that the classical equations for construction of support characteristic curve for a steel set subjected to uniform radial load are recovered. The paper presents also a practical example illustrating the application of all equations needed to construct the support characteristic curve for the combined support system.