Excavations In Jointed Rock Masses Research Articles

Study on size effect of jointed rock mass and influencing factors of the REV size based on the SRM method

Based on the Yaojiayu tunnel of Binlai Expressway, the size effect of jointed rock mass and influencing factors of the representative elementary volume (REV) were studied. The point cloud image of jointed rock mass near the tunnel face was collected by 3D laser scanning technology. Through the Geology Plane software independently developed by Shandong University, the structural plane data were processed and obtained. According to the structural plane data, the discrete fracture network (DFN) was established. Based on the laboratory test results of rock samples in the tunnel site, the particles were embedded into DFN to construct the synthetic rock mass (SRM). The size effect and REV of the jointed rock mass were studied based on the uniaxial compressive strength (UCS) and Young's modulus obtained by uniaxial compression test simulations. It is concluded that the REV size of the jointed rock mass is 7 m × 7 m × 14 m, which provides the criterion and calculation parameters for simulating tunnel excavation in jointed rock mass by the continuous deformation calculation method. In order to study the influence of joint geometric parameters (trace length, spacing, and dip angle) on REV size, different multi-scale SRM models were established by changing the parameters of structural planes. It is observed that within a certain size range, the REV size is negatively correlated with the joint trace length and dip angle, while positively correlated with the joint spacing. And the size of REV is gradually stable with the increase of geometric parameters of joints. Based on the influence law of joint geometry parameters on REV, the REV size range of different engineering can be estimated approximately.

A 3D synthetic rock mass numerical method for characterizations of rock mass and excavation damage zone near tunnels

In practice, a damage zone is generally formed after tunnel excavation in jointed rock mass. This damage zone is closely related to rock mass properties and requires careful examination in order for cost effective supporting designs. In this research, a synthetic rock mass (SRM) numerical method is applied for characterizations of the jointed rock mass and excavation damage zone (EDZ) near underground tunnels in 3D. The SRM model consists of bonded particles and simulates deformation and crack propagation of the rock mass through interactions between these particles. The effects of joint stiffness and distribution on the rock mass properties are systematically examined by comparing the numerical data with an empirical geological strength index (GSI) system and an associated Hoek-Brown strength criterion. The numerical results suggest that rock mass properties are comparable to the empirical GSI/Hoek-Brown system only when inclined joints are simulated in the rock mass subjected to axial loading. The rock mass is strengthened and the empirical GSI/Hoek-Brown characterization becomes inappropriate when the joints are less favorable to shear sliding. The SRM method is then applied for characterizations of tunnel EDZ. It appears that the depth and location of the EDZ are a function of the tunnel orientation, joints, and in situ stresses. The EDZ depth is expected to be higher when inclined joints are simulated. The EDZ area is reduced when the joints in the rock mass are horizontally and vertically distributed.

Effect of Contributing Parameters on the Behaviour of a Bolted Rock Joint Subjected to Combined Pull-and-Shear Loading: A DEM Approach

The mechanical performance of fully grouted rock bolts is essential in the stability of underground excavations in jointed rock masses. This research implements a new cohesive contact model in distinct element codes (PFC2D) to investigate the fracturing response of rock-like and grout material, as well as the bolt–grout interface. The results are compared in detail with experimental observations. The proposed modelling approach, used in conjunction with the distinct element method (DEM), successfully predicted the behaviour of grout failure and the bolt–grout interface’s shear response. We then developed a novel numerical, stepwise pull-and-shear test (SPST) scheme to further analyse the mechanical behaviour of bolted rock joints subjected to simultaneous pull–shear loading. The cohesive DEM framework proposed in this paper was used to carry out the SPST scheme numerically. The mechanism involved in enhancing the shear strength of bolted rock joint was determined by monitoring the $$ \sigma_{\text{n}}^{\text{i}} $$ and its corresponding contact chain force network during the pull-out test. The influence of pretension stress, the rig angle of the bolt profile, and the constant normal stiffness (CNS) condition are assessed systematically. In particular, the pretension stress magnitude at which the synthetic rock bolting system exhibits the highest shear resistance is identified. The findings from this research highlight the sensitivity of bolted rock joints to the simultaneous pull-and-shear loading, boundary conditions, and bolt–grout interface configurations.

Probability distributions of shotcrete parameters for reliability-based analyses of rock tunnel support

A common support measure for underground excavations in jointed rock masses to support loose blocks is to apply a thin shotcrete layer to the periphery of the excavation and systematically install rockbolts into the surrounding rock mass. In this support system, large blocks are carried by the rockbolts and small blocks are carried by the thin shotcrete layer. To verify the shotcrete layer’s load-bearing capacity and to stringently account for the large uncertainties incorporated in the variables involved in determining its capacity, analytical calculations in combination with reliability-based methods can be used. However, a lack of knowledge exists regarding the magnitude and uncertainty of shotcrete characteristics (thickness, adhesion, flexural tensile strength, residual flexural tensile strength, and compressive strength), making it difficult to apply reliability-based methods. A statistical quantification of these characteristics is therefore important to facilitate reliability-based methods in design and verification of shotcrete support. In this paper, we illustrate how shotcrete support against small loose blocks can be viewed as a correlated conditional structural system and how this system can be analyzed using reliability-based methods. In addition, we present a unique amount of data for the aforementioned variables, which are all incorporated in the design and verification of a shotcrete layer’s ability to sustain loads from small loose blocks. Based on the presented data, we statistically quantify and propose suitable probability distributions for each variable. Lastly, we illustrate how the proposed probability distributions can be used in the design process to calculate the probability of exceeding the shotcrete’s load-bearing capacity. Both the probabilistic quantification and the defined correlated conditional structural system along with the illustrative calculation example are followed by a discussion of their implications.

Stability analysis of support systems using a coupled FEM-DFN model (Case study: a diversion tunnel at Lorestan dam site, Iran)

Various structural discontinuities, which form a discrete fracture network, play a significant role in the failure conditions and stability of the rock masses around underground excavations. Several continuum numerical methods have been used to study the stability of underground excavations in jointed rock masses but only few of them can take into account the influence of the pre-existing natural fractures. In this work, the pre-existing fractures are explicitly modeled as a Discrete Fracture Network (DFN) model, which is fully coupled with the FEM modeling for stability analysis of support systems in a diversion tunnel at the Rudbar Lorestan dam site. Hence, at first, using the surveyed data in the diversion tunnel and an estimation of the suitable probability distribution function on geometric characteristics of the existing joint sets in this region, the 3D DFN model was simulated using the stochastic discrete fracture networks generator program, DFN-FRAC3D. In the second step, a coupled 2D Finite Element Method and the prepared stochastic model were used for analysis of existent (based on technical reports) recommended support systems. The objective here is to grasp the role of the fracture networks on the results of the tunnel stability analysis using FEM modeling and also to compare the results with those obtained through stability analysis without considering the effect of fractures.

Optimization of structural contact stiffness and strength for discrete simulation of progressive failure of healed structure

AbstractGeotechnical analysis for underground excavation design in complex tectonic environments requires an increased understanding and more rigorous consideration of the impact of healed or "intrablock" structure, such as veins, on rockmass behaviour. Intrablock structure occurs between blocks of rock defined and bounded by "interblock structure", the network of joints and other fractures conventionally considered in classic rockmass characterization, classification or rockmass property estimation. Discrete simulation of fractures has become a more commonplace model analysis technique for excavations in jointed rockmasses. Here too, however, special attention is required to simulate intrablock structure within the model. In particular, the selection and evolution of stiffness and strength values for the model discontinuity elements must follow a different logic than that adopted for fractures and true joints. A new concept to better represent the behaviour of intrablock structure in explicit numerical models is proposed and tested in this paper by means of finite element method (FEM) analysis and case study data from a 1, 200 m deep drift. This approach changes the stiffness and strength values of failed intrablock structural elements between pre‐peak ("primary"), post‐peak ("secondary"), and ultimate ("tertiary") states. The FEM models in the tertiary state match 96 % of overbreak patterns along the case drift, versus 80 % in primary state models. These findings suggest that the proposed method is a good option to more accurately model the influence of intrablock structure on rockmass behaviour.

Numerical investigation of arching mechanism to underground excavation in jointed rock mass

In this paper, the discontinuous deformation analysis (DDA) is applied to numerically investigate the formation of pressure arch and the stabilization mechanism of rock bolts for underground excavation in laminated rock mass. A global pressure arch with elevated stress is observed sustaining the weight of the overburden and providing the natural shield over the opening. Below the pressure arch is a loosened zone, where a group of local voussoir beam arches are formed to transfer the weight of each layer to the abutments. Detailed parametric studies regarding the in-situ stress ratio, joint friction angle, overburden depth and dip angle of strata are conducted and their influences on the formation of global pressure arch and local voussoir beam arches are identified and discussed. Based on the observed phenomena, the corresponding reinforcement scheme is proposed and the effectiveness and stabilization mechanism are examined.

터널 굴착하중 조건에서의 절리암반의 탄성계수 예측

절리를 포함한 암반에서 터널굴착시 발생하는 내공변위는 터널의 안정성과 필요 확보공간 및 시공성을 위해서 매우 중요한 인자이다. 터널굴착시 발생할 수 있는 내공변위의 크기는 탄성계수가 큰 영향을 미치는 인자이며 특히 절리면이 터널거동을 지배하는 암반에서는 신뢰성 있는 절리암반의 탄성계수를 산정하는 것은 매우 중요하다. 절리암반의 탄성계수는 암석종류, 절리조건, 하중조건 등과 같은 많은 인자에 의해서 영향을 받는다. 그럼에도 불구하고 기존의 대부분의 연구는 암석 및 절리, 터널 굴착하중 조건 등을 체계적으로 고려하지 않고 압축하중 조건에 근거한 대략적인 경험식에 초점을 두고 있다. 그러므로 본 연구에서는 터널 굴착하중 조건에서의 절리암반의 탄성계수를 보다 합리적으로 추정하기 위하여 암석 및 절리조건을 체계적으로 고려하였다. 본 연구에서는 암석종류, 절리전단강도, 절리경사각, 절리군의 수 및 절리간격을 해석인자로서 고려하였다. 다양한 암석 및 절리조건을 고려하여 수치해석적 매개변수 연구를 수행하였고, 그 결과를 기존의 경험적인 방법들과 비교분석하였으며, 다양한 암석 및 절리조건에서의 탄성계수에 대한 변화도표를 제시하였다. 본 연구를 통해 얻어진 결과는 절리암반에서 터널굴착으로 인해 발생되는 터널 내공변위를 파악하는데 실무적으로 활용될 수 있을 것으로 기대된다. Tunneling-induced displacement in a jointed rock mass is an important factor to control tunnel stability and to secure a demanded space and construction quality. The magnitude of the inducible displacements is significantly affected by an elastic modulus and therefore, in a rock mass where a joint controls tunnel behavior, it is very important to estimate an elastic modulus of jointed rock mass reliably. Elastic modulus of jointed rock mass is affected by many factors such as rock type, joint condition, and loading condition. Nevertheless, most existing studies were focused on rough empirical relationships based on compressive loading conditions, which are different from tunnel excavation loading conditions, without a systematic approach of rock, joint, and loading conditions together. Therefore, this study considered rock and joint conditions systematically to estimate an elastic modulus of jointed rock mass under tunnel excavation loading. The controlled factors considered in this study are rock types and joint conditions (joint shear strength, joint inclination angle, number of joint sets, and joint spacing). Numerical parametric studies have been carried out with a consideration of different rock and joint conditions; the results have been compared with existing empirical relationships; and charts of elastic modulus change of different rock and joint conditions have been provided. The results are expected to have a great practical use for estimating the convergence induced by tunnel excavation in jointed rockmass.

A softening block approach to simulate excavation in jointed rocks

The instability of rock masses as a result of excavation is one of the most important rock mechanics problems. This paper proposes a new “softening block” approach for the simulation of stage-wise sequential excavations in jointed rock masses. The concept of a softening block is presented and the corresponding algorithm developed and implemented into the original Discontinuous Deformation Analysis program proposed by Shi. The proposed softening block approach obviates the need to remove the excavated blocks from the calculation model, which further simplifies the work of pre-processing for sequential excavation simulations. Finally, two numerical examples are presented to demonstrate the validity and capability of the proposed approach. The results indicate that it can be used to simulate the excavation in jointed rock mass efficiently and accurately.

Optimized mesh generation for two-dimensional finite element analysis of underground excavations in rocks masses traversed by joints

Deformation and distribution of stresses around underground excavations in jointed rock masses are influenced by yield and slip along joints. Since stresses are often computed using non-linear finite element analysis, and considering the excavation sequence, a number of what-if scenarios are performed, requiring considerable time. Often an accurate solution is only required in a limited part of a larger model. While mesh optimization can reduce the number of elements and analysis effort, the previously published optimization method by the author only considered the effect of excavations at a region of interest. The method was based on a cost function to prioritize the automatic removal of geometric detail from the boundary representation of the domain and the ensuing mesh. The inclusion of the effects of joints and excavation interaction in a stress analysis model, as part of the mesh optimization process, is developed in this paper with an application to a representative stress analysis case. The resulting optimized meshes can represent the displacement and stresses at the region of interest with considerable accuracy while capturing the yield along joints in the rock mass.

Simulation of excavations in jointed rock masses using a practical equivalent continuum approach

A simple practical equivalent continuum numerical model previously presented by Sitharam et al. (Int. J. Rock Mech. Min. Sci. 38 (2001) 437) for simulating the behaviour of jointed rock mass has been incorporated in the commercial finite difference programme fast Lagrangian analysis of continua (FLAC). This model estimates the properties of jointed rock mass from the properties of intact rock and a joint factor (Jf), which is the integration of the properties of joints to take care of the effects of frequency, orientation and strength of joint. A FISH function has been written in FLAC specially for modelling jointed rocks. This paper verifies the validity of this model for three different field case studies, namely two large power station caverns, one in Japan and the other in Himalayas and Kiirunavara mine in Sweden. Sequential excavation was simulated in the analysis by assigning null model available in FLAC to the excavated rock mass in each stage. The settlement and failure observations reported from field studies for these different cases were compared with the predicted observations from the numerical analysis in this study. The results of numerical modelling applied to these different cases are systematically analysed to investigate the efficiency of the numerical model in estimating the deformations and stress distribution around the excavations. Results indicated that the model is capable of predicting the settlements and failure observations made in field fairly well. Results from this study confirmed the effectiveness of the practical equivalent continuum approach and the joint factor model used together for solving various problems involving excavations in jointed rocks.

Modelling the large deformations in stratified media—the Cosserat continuum approach

Methods employing continuum approximation in describing the deformation of layered materials possess a clear advantage over explicit models, However, the conventional implicit models based on the theory of anisotropic continua suffers from certain difficulties associated with interface slip and internal instabilities. These difficulties can be remedied by considering the bending stiffness of the layers. This implies the introduction of moment (couple) stresses and internal rotations, which leads to a Cosserat-type theory. In the present model, the behaviour of the layered material is assumed to be linearly elastic; the interfaces are assumed to be elastic perfectly plastic. Conditions of slip or no slip at the interfaces are detected by a Coulomb criterion with tension cut off at zero normal stress. The theory is valid for large deformation analysis. The model is incorporated into the finite element program AFENA and validated against analytical solutions of elementary buckling problems in layered medium. A problem associated with buckling of the roof and the floor of a rectangular excavation in jointed rock mass under high horizontal in situ stresses is considered as the main application of the theory. Copyright (C) 1999 John Wiley & Sons, Ltd.

Analysis of open excavations in jointed rock masses using finite and distinct element methods : Carter, J P; Alehossein, H; Luo, S Q; Choi, S K; Muhlhaus, H B Proc 7th ISRM International Congress on Rock Mechanics, Aachen, 16–10 September 1991V1, P683–687. Publ Rotterdam: A A Balkema, 1991

Analysis of open excavations in jointed rock masses using finite and distinct element methods : Carter, J P; Alehossein, H; Luo, S Q; Choi, S K; Muhlhaus, H B Proc 7th ISRM International Congress on Rock Mechanics, Aachen, 16–10 September 1991V1, P683–687. Publ Rotterdam: A A Balkema, 1991

Analytical model for estimating keyblock sizes in excavations in jointed rock masses : Kuszmaul, J S; Goodman, R E Proc Conference on Fractured and Jointed Rock Masses, Lake Tahoe, 3–5 June 1992P26–33. Publ California: Lawrence Berkeley Laboratory, 1992

Analytical model for estimating keyblock sizes in excavations in jointed rock masses : Kuszmaul, J S; Goodman, R E Proc Conference on Fractured and Jointed Rock Masses, Lake Tahoe, 3–5 June 1992P26–33. Publ California: Lawrence Berkeley Laboratory, 1992

A numerical method for modelling large displacements of jointed rocks. I. Fundamentals

A numerical method called the block-spring model for analyzing heavily jointed rocks is presented in this paper. This model permits the assessment of stability of both underground and surface excavation in jointed rock mass and is capable of simulating large displacements. The model simulates the jointed rock mass by an assemblage of rigid blocks interacting through contacts. By relating the contact forces to the relative displacements between the blocks and directly applying the equilibrium equations, the displacements of the blocks and subsequently the contact forces can be determined. An iteration procedure is applied to describe the progressive failure along the joints. The proposed model can identify unstable blocks by considering the rearrangement of the blocks. A computer program has been written in FORTRAN based on the procedures of the proposed model. Some simple examples are presented in the present paper to demonstrate the capability of the proposed model. Advanced features of the model including simulation of rock bolts and groundwater are presented in a companion paper, in which two case histories have been analyzed in detail with the proposed model. Key words : jointed rocks, stress, deformation, analysis, numerical model, excavation, anchors, groundwater.

Damage mechanics analysis for underground excavation in jointed rock mass : Kyoya, T; Kusabuka, M; Ichikawa, Y; Kawamoto, T Proc International Symposium on Engineering in Complex Rock Formations, Beijing, 3–7 November, 1986P506–513. Publ Beijing: Science Press, 1986

Damage mechanics analysis for underground excavation in jointed rock mass : Kyoya, T; Kusabuka, M; Ichikawa, Y; Kawamoto, T Proc International Symposium on Engineering in Complex Rock Formations, Beijing, 3–7 November, 1986P506–513. Publ Beijing: Science Press, 1986

Damage mechanics analysis for underground excavation in jointed rock mass : Kyoya, T; Kusabuka, M; Ichikawa, Y; Kawamoto, T Proc International Symposium on Engineering in Complex Rock Formations, Beijing, 3–7 November, 1986P506–513. Publ Beijing: Science Press, 1986

Damage mechanics analysis for underground excavation in jointed rock mass : Kyoya, T; Kusabuka, M; Ichikawa, Y; Kawamoto, T Proc International Symposium on Engineering in Complex Rock Formations, Beijing, 3–7 November, 1986P506–513. Publ Beijing: Science Press, 1986

Numerical study of excavation support loads in jointed rock masses : Voegele, M D; Fairhurst, C Proc 23rd Symposium on Rock Mechanics, Berkeley, 25–27 August 1982, P673–683. Publ New York: AIME, 1982

A computer study of excavation support loads in tunnels and other excavations in discontinuous rock masses was undertaken with the distinct element method introduced by Cundall (1971). In the distinct element method the rock mass is modelled as a composition of individual blocks of rock defined by the joint planes. Normal and shear deformation laws are ascribed to the joint surfaces; the blocks of rock themselves are considered to be rigid. The particular version of the program utilized in this study ran a mini computer and used interactive techniques for the input of geometrical data such as joint orientation and spacing and subsequent display of rock mass deformation and other data. Two major aspects of the behaviour of excavations in jointed rock masses are examined in this paper: 1) the behaviour of unsupported excavations in rock masses with varying joint configuration, joint properties and in situ stress fields; and 2) the modification of the behaviour of similar excavations when support is provided. In the latter case the excavation behaviour was examined through the use of ground reaction curves. For the covering abstract of the symposium see IRRD 284392. (Author/TRRL)