Underground Openings Research Articles

Stability Assessment of Old Underground Coal Mine Openings for the Safety of Overlying Railway Tracks

This paper presents the stability assessment of a century old coalmine situated under a railway track. The mine openings may lead to the surface subsidence which can endanger the track safety. To assess the stability, this paper presents a unified approach of observational, empirical, and analytical methods. Observational approach monitored the immediate roofs with geotechnical instruments during a period. In empirical approach, the coal pillar safety factors were assessed using rock mass classification. Finally, the analytical study was performed using numerical simulation. All these studies found that the openings and the coal pillars are stable for the track safety.

A Closed-Form Analytical Solution for Circular Opening in Rocks Using Drucker–Prager Criterion

Wellbore and tunnel problems are of true triaxial stress state, even if the ground is under axisymmetric loading condition. A closed-form analytical solution is proposed using Drucker–Prager failure criterion. The solutions are obtained for rock mass exhibiting elastic–perfectly plastic or elastic–brittle–plastic behaviour. The proposed solution is then compared with the finite element analysis (FE-analysis) results. Parametric studies are also carried out. The results of the proposed analytical solution are found to be in good agreement with the FE-analysis results. The proposed analytical solution can thus be used for predicting the stresses and deformation of underground circular openings considering true triaxial stress state.

Effects of water saturation on the strength and loading-rate dependence of andesite

Water saturation conditions in the rock masses around underground openings vary with place and time. For the long-term stability assessment of underground structures, it is essential to understand the effect of water on the time-dependent behavior of rocks. In this study, the effects of water saturation on the loading-rate dependence of strength, which is one of the time-dependent characteristics of rocks, were investigated both theoretically and via experiments. First, the effects of loading rate and water saturation on rock strength and its variability were clarified by means of a theory that is based on both stochastic and rate processes. Next, alternating loading-rate tests were conducted on Sanjome andesite using uniaxial compression testing under various water saturation conditions. The test results showed that the rock strength increases with a decrease in water saturation and that an increase in strength accompanied by a tenfold increase in loading rate is almost constant under various water saturation conditions. All of the test results were consistently explained by our theory of the dependence of rock strength and its variability under different loading rates and water saturation conditions. It was also found that the dependence of strength on the loading rate is equivalent to that on the water saturation, and that the strength obtained from a test could be converted to one under an arbitrary loading rate and water saturation condition. In addition, we proposed a constitutive equation based on these theoretical and experimental results that could reproduce the loading-rate dependence of stress-strain curves under various water saturation conditions using a single set of constants.

Influence of Shock Losses on Air Distribution in Underground Mines

Air resistances are classified depending on their locality in underground mines. Three groups of local resistances are distinguished: shaft and mining horizon intersections; shaft and air channel intersections; stope and ventilation duct intersections in blind drifts. Influence of each group of the local resistances on the mine depression is evaluated as function of geometry and aerodynamics of underground openings. The criteria are proposed for estimating percentage influence of each group on the total mine depression are proposed. The calculation methods are determined for each group to be advisably used in quantitative analysis of air distribution in underground mine ventilation networks under the ventilation mode change.

Numerical Analysis of Tunnel Design Effect of Excavation Damaged Zone on Principal Stresses and Total Displacement

The development of Excavation damaged zone (EDZ) in an underground opening can significantly affect the overall performance of the opening. Generally, it is believed that the presence of such zone in any kind of underground opening can pose big problem to seepage flow, stability and subsequently impair the general performance and functionality of the excavation. In order to assess the influence of EDZ on excavation, the behavior of the principal stresses and the total maximum displacement was observed in the present study. Therefore, this study is aimed at numerical design of underground tunnel and determining the effect of EDZ on the principal stresses (sigma 1 and sigma 3) and total maximum displacement surrounding the excavation. The principal stresses (sigma 1 and sigma 3) were observed to decrease with increasing EDZ thickness and corresponding increase in elastic modulus, while the shear stress was observed to increase. The total maximum displacement was also observed to decrease with increasing EDZ thickness and corresponding increase in permeability. The general conclusion drawn from the analysis is that mechanical properties principal stresses and (total maximum displacement) decrease with the development of EDZ around the tunnel.

Experimental study of dynamic bending failure of Laurentian granite: loading rate and pre-load effects

In deep underground rock engineering projects, rocks are under static pre-load and they may further experience dynamic load due to earthquakes or production blasts. It is thus desirable to consider dynamic failure of rocks subjected to static pre-load. Besides, bending load is commonly encountered near underground openings. Therefore, this study considers the effect of the pre-load on the dynamic bending strength of Laurentian granite (LG). Using a modified split Hopkinson pressure bar system, the semi-circular bend (SCB) method is applied to carry out the bending tests. Five groups of SCB specimens are tested under different pre-loads and loading rates. The results show that under a given pre-load, the flexural tensile strength of LG increases with the loading rate, and decreases with the static pre-load at a given loading rate. The total flexural tensile strength is roughly independent of the pre-load. An empirical equation is used to represent the effects of the loading rate and the pre-load force on the dynamic flexural tensile strength. Furthermore, the flexural tensile strengths measured from SCB tests have higher values than the tensile strengths measured using the Brazilian disc method for the same rock. A nonlocal failure theory is utilized to quantitatively interpret this discrepancy.

Effect of tunnel overburden stress on the rock brittle failure depth

Tunneling under high overburden stresses results in many tunnel instability problems due to the rock overstressing. Understanding and simulating the rock failure process is the major issue of a deep excavation to achieve an appropriate rock support system that provides possible cost-effective and stable construction. The excavation of the Pahang Selangor Raw Water Transfer Tunnel is considered in this paper. Three critical cases of the project are analyzed. A possible rock brittle failure was predictable at the tunnel sidewalls under a depth of more than 500 m. The rock overstressing is analyzed based on the in situ stress conditions, intact rock strength, and actual failure depth observed at the site. Failure zones are simulated using the cohesion softening–friction hardening model and compared with the site observed failures. A review of underground openings excavated in different rock mass conditions showed that the ratio of the maximum boundary stress to the uniaxial compressive strength (σθmax/σci) is suggested as the key parameter to determine the tunnel instability problems. In this study, an attempt is made to investigate the influence of the maximum tangential boundary stress to the uniaxial compressive strength ratio(σθmax/σci) on the rock brittle failure depth, stress distribution, and displacement of the rock mass around the tunnel. A parametric study is implemented using different tunnel depths including the actual tunnel depths. The results show that with increasing tunnel depth or (σθmax/σci) ratio, the risk of spalling, rock burst, and other tunnel instabilities are increasing.

The Dosage of Superplasticizer in Cemented Coal Waste Backfill Material Based on Response Surface Methodology

Cemented coal waste backfill material (CCWBM) is developed for backfilling the goaf in coal mines. As fresh CCWBM slurry is generally transported into underground openings through a pipeline, the fluidity of fresh slurry becomes one of the most important properties. Adding superplasticizer is considered to improve the flow performance of the slurry without alerting the mechanical performance of filling body. The dosage of superplasticizer (SP) is related to filling cost, thus response surface methodology (RSM) is adopted to study the influence of material composition on SP when target slump is 250 mm. The effects of fly ash content, fine gangue ratio, and mass concentration on SP are analyzed using the software of Design-Expert and central composite design (CCD), and models are established for SP. Results show that the SP model coincides greatly with the test results and can be applied to analyze and predict SP in CCWBM. Mass concentration, fly ash content, and fine gangue ratio influence SP from high to low. The interaction of fine gangue ratio and mass concentration between SP is the most significant. The fact that the improved aggregate space model can be applied to analyze the fluidity of CCWBM is proved too. The research results provide guidance for the design and preparation of CCWBM with favourable performance and low cost in practical production.

Stress redistribution and formation of the pressure arch above underground excavation in rock mass

Pressure arch utilises the self-stabilizing capacity of surrounding rock and is benefit to the roof stability of underground excavations. The analysis of it provides a new stability assessment tool for underground engineering. In this paper, a series of numerical simulations were carried out to investigate the formation and properties of pressure arch and the self-stabilisation mechanism of surrounding rock. It is found that stress redistribution results in the formation of the pressure arch. The inner and outer boundary of arching area are determined as the principal stress trajectories through the inflection points of the principle stress-depth curves on the centre line of caverns. Furthermore, parametric analysis illustrates that in situ stress ratio and excavation size have more significant effect on the inner and outer boundaries and the arch thickness than roof shape and overburden depth. Finally, it can be observed that the various trends of the inner boundary height with different influence factors are same with the displacement of the crown, and it could be used as an indicator for stability assessment. Based on the observed phenomena, a design scheme is proposed to determine the maximum size of the underground openings cross-section without support in agreement with the geomechanical conditions.

The failure mechanism and construction practice of large underground caverns in steeply dipping layered rock masses

The stability of large underground caverns in steeply dipping layered rock masses is not only affected by the engineering geological characteristics of surrounding rock masses, but also by the excavation sequences and reinforcement measures. Certain places around the periphery of underground openings, such as the high sidewalls, would exhibit significant displacement increases and various failure patterns if the excavation and support were not properly designed. These issues can, according to the basic principles of structurally-controlled concept, be largely attributed to the mechanical properties of bedding planes and other discontinuities. In order to clarify the possible mechanisms behind these failures and to realize the dynamic adjustment of construction, a thorough investigation was conducted to the underground caverns of the Wudongde hydropower station throughout the excavation process. The geological conditions and mechanical properties of the rock masses in the study area were first introduced in detail. Four examples were then presented following the overall logic from geological background to failure mechanism and finally to reinforcement practice. The first example focused on the local wedge-type instability analyses, including the collection of necessary information, the identification of potential blocks, as well as the determination strategies for the factors of safety. The second example illustrated typical tensile failure mode of steeply inclined strata induced by large-scale excavation. The sudden normal unloading of bedding planes led to observable fractures along these weaknesses and considerable increases in sidewall displacements. Large quantities of extra reinforcements had to be installed to control further development of fractured range of the upstream sidewall. The third example highlighted the long-range influence of excavations on the stability of downstream roof, where multistage displacement increase and shotcrete cracking took place as a result of stress concentration and strata buckling, possibly also of the local unfavorable discontinuities. The last example was related to the failure mode of combined discontinuities on the surfaces of piers between two pits. Lessons learned from these examples mainly involve what the mechanical behaviors of the steeply dipping layered rock masses in different places after excavation will be like, and to what extent these behaviors could be predicted or anticipated, which then put emphasis on the importance of geological investigations, and how the optimization of excavation sequence and bench height as well as the timely installation of regular supports could effectively reduce the risks of instability.

Experimental Study on Shear Behavior of a Rock Discontinuity Under Various Thermal, Hydraulic and Mechanical Conditions

Discontinuities in rock mass behave as weak planes, and it is crucial to understand their behavior when assessing the stability of underground structures. Shear characteristics of discontinuities are usually affected by the interaction among overburden stress, tectonic stresses, water pressure by groundwater level and temperature at depth. Thus, it is necessary to evaluate the variation of the frictional properties of rock discontinuities at different conditions. In this study, a series of shear tests were carried out for three types of rocks (Daejeon granite, Goheung diorite, and Linyi sandstone) having a saw-cut surface to investigate the shear characteristics of a rock discontinuity under various thermal-hydro-mechanical conditions in a triaxial compression chamber. In addition to the tests on saw-cut specimens, the effect of surface roughness on shear characteristics was examined. Cement mortar was used to reproduce identical rough discontinuities having JRC values of 2 and 12. The testing conditions were determined considering in situ conditions at the vicinity of underground opening such as radioactive waste disposal facilities, enhanced geothermal systems, and oil reservoirs. The experimental results were analyzed based on Coulomb’s and Patton’s failure criterion. It was observed that the shear characteristics of rock discontinuities were sensitive to confining and water pressure but not to a temperature below 80 °C. XRD analysis and SEM observation were performed to identify the mechanism responsible for the change of friction angle. Clay minerals having a layer lattice structure that results in weakening of the bonding of minerals may have reduced the friction angle.

Geodynamic Hazard Assessment for Tectonic Structures in Underground Mining of North Ore Bodies in the Oktyabrsky Deposit

The tectonic structures of different rank are distinguished in the Norilsk Region. The first-order block structure is formed by intersection of rank I geodynamically active faults: Khatanga, Imangda-Kystykhtakh, Norislk, Fokin-Tangaralakh megafaults and other. Rank II faults are the Norilsk-Kharaelakh fault, which intersects the mine field, and other 7 structures. Using geological sections and boring records obtained within the mining lease, slip faults and oblique-slip faults of rank III, represented by ridges in the terrain, are plotted in the satellite image. In the rockburst-hazardous high-grade ore sites in Glubokaya mine, it is recommended to arrange safe zones by advance boring of destressing holes in underground openings in zones of rank III faults.

NUMERICAL ANALYSES OF UNDERGROUND OPENINGS IN COMPETENT ROCK MASSES: CONTINUOUS VS DISCONTINUOUS

The paper is aimed at comparing the results of numerical analyses of underground openings in competent rock masses like the Carrara Marble (Italy) by considering a real and well documented case study. More specifically, 3D FEM and DEM analyses were carried out on a rock-mass model interested by two faults and three sets of discontinuities. The geometrical model is representative of deep underground openings where spalling - cracks and rock bursts can occur. PLAXIS 3D and 3DEC were used for the analyses. Intact rock and rock mass characterization of Carrara Marble was inferred from available technical literature. The analysis results were compared in terms of principal stresses and displacements in a number of monitoring points around the opening. The main practical interest is to find out a reliable approach for evaluating the stability of very large openings in a competent rock mass like Carrara marble. For such a purpose, a number of available in-situ stress measurements were used.

Experimental study of rockburst under true-triaxial gradient loading conditions

Due to the underground openings, the tangentially concentrated stress of the tunnel remains larger at excavation boundary and decreases toward the interior of the surrounding rock with a certain gradient. In order to study the effect of different gradient stress on rockburst, the true-triaxial gradient and hydraulic-pneumatic combined test apparatus were carried out to simulate the rockburst processes. Under the different gradient stress conditions, the rock-like specimen (gypsum) was tested independently through three principal stress directions loading--fast unloading of single surface--top gradient and hydraulic-pneumatic combined loading, which systematically analyzed the macro-mesoscopic damage phenomena, force characteristics and acoustic emission (AE) signals of the specimen during rockburst. The experimental results indicated that the rockburst test under the gradient and hydraulic-pneumatic combined loading conditions could perfectly reflect the rockburst processes and their stress characteristics; Relatively high stress loading could cause specimen failure, but could not determine its mode. The rockburst under the action of gradient stress suggested that the failure mode of specimen mainly depended on the stress gradient. When the stress gradient was lower, progressive and static spalling failure occured and the rockburst grades were relatively slight. On the other hand, shear fractures occurred in rockbursts accounted for increasingly large proportion as the stress gradient increased and the rockburst occurred more intensely and suddenly, the progressive failure process became unconspicuous, and the rockburst grades were moderate or even stronger.

Numerical Study on Zonal Disintegration of Deep Rock Mass Using Three‐Dimensional Bonded Block Model

Zonal disintegration, a phenomenon of fractured zones and intact zones distributed alternately in deep rock mass, is different from the excavation‐damaged zone of shallow rock mass. In this study, bonded block model of 3DEC was employed to study the fracture mode and origination condition of zonal disintegration. Initiation, propagation, and coalescence progress of fracture around the roadway boundary under different triaxial stress conditions are elaborated. Numerical simulation demonstrated that zonal disintegration may occur when the direction of maximum principal stress is parallel to the roadway axis. It is interesting to find that the fracture around the roadway boundary traced the line of a spiral line, while slip‐line fractures distributed apart from the roadway boundary. The extent of the alternate fracture zone decreased as the confining pressure increased, and alternate fracture zone was no longer in existence when the confining pressure reaches a certain value. Effects of roadway shape on zonal disintegration were also studied, and the results indicated that the curvature of the fracture track line tends to be equal to the roadway boundary in shallow surrounding rock of the roadway, while the fractures in deep surrounding rock seems unaffected by the roadway shape. Those findings are of great significance to support design of deep underground openings.

Large deformation of tunnels in longwall coal mines

Large deformation of tunnels in longwall coal mines creates massive cracks in the surrounding rocks, which may cause groundwater loss and threaten the coal mine safety. Rock mechanical characterization is crucial for understanding the large deformation of underground openings. Based on the theoretical analysis and laboratory triaxial tests, this study aims to propose a strength degradation model of the rocks for a deep understanding on the large deformation of underground openings. The proposed model captures the non-linear post-peak rock strength degradation which is dependent on the plastic strains accumulated under the decreasing confining pressures. The stress-dependent elastic properties were not considered in this study. The proposed model was incorporated in a finite difference package (FLAC3D) for engineering application. The simulated surface convergence of the underground opening matches the in-mine measurements, which supports the proposed model. More importantly, the simulated crack zone after tunnel excavation agrees with the observations from borehole images. Therefore, the proposed model provides a helpful tool to evaluate the safe margin of an underground tunnel if it is close to a subsurface water-bearing formation. It also sheds lights on the underground tunnel design to prevent the adjacent groundwater from inrush.

Mechanical analysis and interpretation of excavation damage zone formation around deep tunnels within massive rock masses using hybrid finite–discrete element approach: case of Atomic Energy of Canada Limited (AECL) Underground Research Laboratory (URL) test tunnel

The construction of an underground opening leads to changes in the in situ stress regime surrounding the excavation. The opening influences the rock mass owing to the redistribution of the stresses and results in the disturbance of the surrounding ground. At great depths, massive to slightly or moderately fractured rock masses are usually encountered, and under high stresses, they are more likely to behave in a brittle manner during an excavation. While constitutive models have been developed and proposed for the numerical simulation of such excavations using continuum mechanics, this brittle response cannot be simulated accurately enough, since the material behaviour is governed by fracture initiation and propagation. On the contrary, discontinuum approaches are more suitable in such cases. For the purposes of this paper, the brittle behaviour of hard, massive rock masses and the associated spalling failure mechanisms were simulated by employing a finite–discrete element method (FDEM) approach using Irazu software. The generated numerical model was utilized to replicate field conditions based on the observations at the Atomic Energy of Canada Limited (AECL) Underground Research Laboratory (URL) test tunnel located in Pinawa, Manitoba, Canada. The model results are compared with field observation data to explicitly demonstrate the suitability of the method.

An equivalent anchoring method for anisotropic rock masses in underground tunnelling

Rock bolting has been widely utilized for reinforcing underground openings, such as tunnels and mining. To ease the complexity of numerically simulating of the bolted rock mass, a novel method for equivalent anchoring simulates the reinforcement effect of the bolts by considering the anisotropic characteristics of the layered rock mass. The jointed material model in ABAQUS is improved by incorporating the anisotropy through the programming language FORTRAN. Based on the improved jointed material model, the equivalent anchoring parameters of the bolted layered rock masses are studied through a series of numerical tests. The numerical outcomes reveal that the improved jointed material model can be used to simulate the anisotropic features of the layered rock mass by comparing the classic and analytic solutions. The degree to which the supporting parameters affect the equivalent reinforcement performance are different in anisotropic rock masses, and the sensitivity of their influence is bolt length > bolt spacing > bolt diameter. Furthermore, an equivalent anchoring model for bolted rock masses (the formula of the improved parameter between the ratio of bolt length to tunnel diameter and the ratio of bolt diameter to bolt spacing) is proposed through numerical results analysis by the multiple linear regression method. An application case shows the field monitoring agreed well with the predicted vertical displacement of the tunnel demonstrating the feasibility of the proposed simulation method for the bolted rock mass. The results show the proposed method in this study can be conveniently utilized for efficient simulation of the mechanical behavior of bolted rock masses.

Determination of temporary support for squeezing zone underground mine PT. Cibaliung Sumberdaya

Underground Mining activity will always face deformation and instability of the rock mass around the underground openings. A large deformation was reported to occur on decline tunnel of PT. Cibaliung Sumberdaya (PT. CSD). Prior to this event, the same tunnel, which the tunnel wall had been refurbished into the proposed original shape. This implies that the squeezing zone had developed around the tunnel and the same support system couldn’t resist the increasing load. Therefore, an analysis of squeezing zone around the excavation should be carried out. Conducted analysis started by the modelling the excavation should be carried out. Conducted analysis started by modelling the excavation the tunnel to get causing the cross cut failure. Then, back analysis was carried out to estimate the decreasing strength of the rock mass inside the squeezing. After that, squeezing zone can be determine the support systems to prevent the failure of rocks. The strain from back analysis results showing that similarity compared to the monitoring result when the modulus deformation E and the strength parameter mb and s of Hoek-Brown Failure criteria of the materials in the squeezing zone is 40 % peak strength. Analysis for temporary support shows that need 250 mm thick shotcrete, 1 m of spacing rock bolting, and 0.5 spacing reinforcement beam. Although, the floor of tunnel should be given extra consideration because could given high load, so must be installing rock bolt to helping stabilization.

Failure Mechanism and Acoustic Emission Characteristics of Rock Specimen with Edge Crack Under Uniaxial Compression

There are many cracks in rock mass which have a significant influence on the stability of rock engineering. Most of current researches on the failure mechanism of cracked rock are mainly focused on the internal cracks. However, edge cracks around underground openings are common in many engineering fields. Understanding the failure mechanism of rock with edge cracks is essential to assess the stability of underground structures. Rock failure is closely related to the cracking processes and acoustic emission characteristics is deemed as an effective means of monitoring the cracking processes of rock. In this study, a series of numerical simulations utilizing particle flow software PFC2D were performed to investigate the influence of the length and position of edge crack on the failure mechanism and acoustic emission characteristics of rock specimens with edge cracks under uniaxial compression. The results show that: the strength of specimens is sensitive to the change of crack length and position; the directions of crack propagation in each specimen is in good agreement, the different crack tip position and the specimen boundary lead to different failure modes; the influence of crack length on acoustic emission is greater than that of crack position.