Opening In Rock Mass Research Articles

A New Numerical Procedure for the Excavation Response in Mohr–Coulomb Rock Mass Exhibiting Strain-Softening Behavior

A new numerical procedure for calculating the excavation response of the Mohr–Coulomb rock mass considering strain-softening behavior is proposed in this article. In this method, the plastic zone of the strain-softening rock mass is divided into the sufficient small plastic concentric annulus with constant radial stress increment, where the stress and strain distributions are characterized based on the existing analytical solutions of the brittle–plastic rock in the plastic zone. According to the equilibrium equation, geometric equation, and Mohr–Coulomb yield criterion, the stresses of each annulus can be calculated, and the explicit form of the displacement can also be determined by invoking the non-associated flow law and Hooke’s law. On this basis, the excavation disturbance-induced response and the ground reaction curve (GRC) in the strain-softening rock mass can be calculated by iterative computation. The proposed method is verified by comparing both the numerical simulation results and the existing theoretical solutions. Extensive computations are then carried out to clarify some practical questions, including the effect of ground condition, the computation efficiency, and the engineering applicability. It is found that the proposed numerical procedure behaves more efficiently and accurately than the previous one for the strain-softening rock mass. This might, therefore, provide convenience and benefits from a computation standpoint for the preliminary design of underground openings in rock masses with slight deformation.

Investigating the mechanical and acoustic emission characteristics of brittle failure around a circular opening under uniaxial loading

The size of underground openings in rock masses in metal mines is critical to the performance of the openings. In this study, the mechanical and acoustic emission (AE) characteristics of brittle rock-like specimens containing a circular opening with different ratios of opening diameter to sample size λ (λ = 0.1, 0.13, 0.17, 0.2, and 0.23) were investigated under uniaxial compression with AE monitoring. The results indicate that the opening size strongly affected the peak strength and the elastic modulus. Crack initiation first started from the upper surface of the specimens, not from the periphery of the openings. Tensile and shear cracks coexisted on the roof and floor of the specimens, whereas tensile cracks were dominant on the two sides. The fracture mode of samples with openings was partially affected by the relative size of the pillars and openings. The AE response curves (in terms of counts, cumulative energy, cumulative counts, and b-value) show that brittle failure was mainly a progressive process. Moreover, the AE information corresponded well with microcrack evolution in the samples and thus can be used to predict sample failure.

Maximum Strength of Opening in Crack-Weakened Rock Mass

Based on the uniform strength and minimized stress intensity factor, the maximum strength shape of an opening in rock mass is theoretically analyzed. The criterion and solution method are proposed for the problem on prevention of failure in rock mass with opening under the action of tectonic forces and gravity. The constructed closed system of algebraic equations enables minimization of stress intensity factors depending on mechanical and geometrical parameters of rock mass.

Influences of seepage force and out-of-plane stress on cavity contracting and tunnel opening

The effects of seepage force and out-of-plane stress on cavity contracting and tunnel opening was investigated in this study. The generalized Hoek-Brown (H-B) failure criterion and non-associated flow rule were adopted. Because of the complex solution of pore pressure in an arbitrary direction, only the pore pressure through the radial direction was assumed in this paper. In order to investigate the effect of out-of-plane stress and seepage force on the cavity contraction and circular tunnel opening, three cases of the out-of-plane stress being the minor, intermediate, or major principal stress are assumed separately. A method of plane strain problem is adopted to obtain the stress and strain for cavity contracting and circular tunnel opening for three cases, respectively, that incorporated the effects of seepage force. The proposed solutions were validated by the published results and the correction is verified. Several cases were analyzed, and parameter studies were conducted to highlight the effects of seepage force, H-B constants, and out-of-plane stress on stress, displacement, and plastic radius with the numerical method. The proposed method may be used to address the complex problems of cavity contraction and tunnel opening in rock mass.

An iterative method for the linearization of nonlinear failure criteria

Linearization of a nonlinear failure criterion may be required if a software incorporating the nonlinear failure criterion is not readily available or if an additional computational difficulty is encountered due to the nonlinearity of the failure criterion. In this paper, a novel method is proposed for the linearization of nonlinear failure criteria. The proposed method is based on an iterative procedure and the least-squares regression is used for the linearization. The material is assumed to be elastic-perfectly plastic. By conducting analytical and finite element analyses of stresses and displacements around underground openings in rock mass governed by the generalized Hoek–Brown failure criterion, it is shown that the proposed method is convergent, efficient and effective.

Rock Engineering Design in Frozen and Thawing Rock: Current Approaches and Future Directions

The purpose of this review is to present the state-of-the-art methods, problems and potential future design techniques used for rock engineering in frozen ground, with particular regard to the design of mine openings in rock masses that may be subject to thawing. The impetus for this work is the recognition that civil and mining infrastructure is extending into such frozen material, and the design procedures currently available may not be optimal. Additionally, civilian infrastructure in frozen ground in alpine regions (e.g. central Europe) is also becoming subject to thawing conditions. In any such scenario, increased heat exchange with an exposed rock mass surface will cause thawing to occur. In general, a rock mass is stronger in frozen conditions than in dry conditions (i.e. without ice or water), but a rock mass will be least strong at the time ice-filled discontinuities are thawing, resulting in a significant shear strength reduction. In this review, we will discuss the various existing design methods in the context of the phenomena involved in the formation of frozen rock and the behaviour of thawing rock. We will show the deficiencies of these methods and highlight potential developments that will lead to future robust design protocols.

Elasto-plastic analysis of a circular opening in rock mass with confining stress-dependent strain-softening behaviour

In strain-softening rock mass, it is widely accepted that the confining stress in the plastic zone around a circular opening varies with the radial distance. The variable confining stress gives rise to a variable critical plastic softening parameter (η∗). This paper aims to take the variable η∗ into account in the development of elasto-plastic solutions for stress and strain states around a circular opening. First, a variable confining stress model (VCSM) is recommended to account for the effect of the variable η∗; a criterion is presented for judging whether or not the rock mass transfers from the plastic softening state to the residual state. Then, a new numerical procedure for the implementation of VCSM is proposed on the basis of Hoek–Brown failure criterion and the non-associated flow rule. In the proposed procedure, the plastic softening and residual zones are divided into a set of concentric rings by an assigned radial stress increment. The increments of stress and strain for each ring can be calculated in a successive manner by the finite difference method. Finally, by using the proposed procedure, a series of parametric studies is conducted to compare VCSM and existing constant confining stress models (CCSM), and also to investigate the variation of the stress components, strain-softening parameter, and strength parameters in the plastic softening zone. The results indicate that CCSM tends to underestimate the deformation of surrounding rock mass with poor quality. The effect of the critical plastic softening parameter on the stability of a circular opening in strain-softening rock mass includes two different aspects: one is to govern the strength parameters of the plastic zone, the other is to control the radii of the plastic softening and residual zones.

Physical model simulation of shallow openings in jointed rock mass under static and cyclic loadings

Physical model simulations have been performed to determine the effects of depth, joint spacing and orientation on the maximum unsupported span of shallow underground openings under static and cyclic loads. Cubical and rectangular blocks of Phu Phan sandstone are arranged in a vertical test frame to simulate a two-dimensional representation of single rectangular openings in rock mass with two mutually perpendicular joint sets. Results indicate that the normalized maximum span (W/S V) rapidly increases with the normalized depth (D/S H), and tends to approach a certain limit for each joint spacing ratio, S V:S H. The maximum span increases with decreasing S V:S H ratio. Under S V = S H condition, increasing the joint angles from 0° to 45° reduces the maximum span by about 20%. At shallow depths the acceleration of 0.225 g can reduce the maximum span by up to 50%. The impact of the cyclic loads however reduces as the depth increases. The test results under both static and cyclic loadings compare reasonably well with those calculated from discrete element analyses using the UDEC code.

Thermally Induced Behavior of the Openings in Rock Mass Affected by High Temperatures

The temporary storage of hot water in openings excavated in rock may be worthwhile for multiple land-use applications, environmental safeguards, and energy conservation. When used for hot water storage, the rock mass around the openings will respond by coupled thermomechanical effects induced by the hot water. In this study, a submergence test of granite and sandstone at 20 and 95°C was used to examine the thermal behavior of the rock mass around the openings to obtain the mechanical properties of uniaxial compressive strength and elastic modulus. A uniaxial compression creep test of the rock at high temperatures was also performed and calculations with various constants for the creep were examined. With the results obtained in this work, the temperature and stress distribution around the openings when hot water is stored was analyzed by considering the creep properties. The thermal behavior and stability of the openings were also examined. The displacement of the openings was predicted to be approximatel...

The Effect of the Intermediate Principal Stress on the Ground Response of Circular Openings in Rock Mass

b strength criterion coefficient in the unified strength criterion c cohesive strength G shear modulus m compressive-tensile strength ratio pi, p cr i , po internal pressure, critical internal pressure and external pressure pc radial stress at the elasto-plastic interface ri, ro internal radius and external radius rc extent of the plastic zone u, ui radial displacement and its value at internal wall of the opening material parameter in the unified strength criterion "r , " radial and tangential strains ’ friction angle Poisson’s ratio 1, 2, 3 principal stresses 13, 12, 23 normal stresses defined as 13 1⁄4 ð 1 þ 3Þ=2, 12 1⁄4 ð 1 þ 2Þ=2, 23 1⁄4 ð 2 þ 3Þ=2

The plastic zones and displacements around underground openings in rock masses containing a fault

Faults are the commonly encountered large geological discontinuities in hard rock masses, most severe underground structure instability is found to be closely associated with the faults presence nearby. The parametric study carried out in this paper using numerical method (UDEC) has identified some fault parameters to be really critical for the underground structure stability. These fault parameters are fault dips, fault shear strength and fault locations relative to the underground structure. This numerical investigation revealed that faults affect the stability of underground structure by the tendency of increasing the plastic zones, displacements and causing both asymmetrically distributed in the rock masses adjacent to the excavation. The relationship of the induced plastic zones, maximum displacements varying with these fault parameters was established. The distribution of plastic zone and displacement was graphically presented and the mechanisms such effects were discussed. These results offer a guideline in support design.

In situ strength criteria for tunnel design in highly-stressed rock masses

Abstract The selection of rock mass strength parameters and the approach to assessing the mechanical and thermal-mechanical rock mass behaviour around tunnels are important aspects of designing underground openings for a spent nuclear fuel waste repository. This paper demonstrates how strength criteria based on in situ observations can be used to design stable underground openings in rock masses subjected to adverse stress conditions. The findings are based upon work conducted at AECL’s Underground Research Laboratory (URL) regarding rock mass strength around tunnels in sparsely fractured granite.