Modelling the deformation of underground excavations in layered rock masses (original) (raw)
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Simulation of excavations in jointed rock masses using a practical equivalent continuum approach
International Journal of Rock Mechanics and Mining Sciences, 2002
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 (FLA C). This model estimates the properties of jointed rock mass from the properties of intact rock and a joint factor (Jr), which is the integration of the properties ofjoints to take care of the effects of frequency, orientation and strength of joint. A FISH function has been written in FLA C 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 FLACto 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 sol',ing various problems involving excavations in jointed rocks. (t)
International Journal of Rock Mechanics and Mining Sciences, 1997
The paper discusses the results of a numerical study which examines the influence of joint constitutive models on the response of a specific jointed rock mass made up of an anisotropic rock (slate). It presents a comparison between predicted convergences and displacements of rock mass surrounding a pilot gallery, and those measured during field investigations. To perform this analysis, the two-dimensional Distinct Element Method code, UDEC, is used and three joint laws are compared. Based on field investigations for site characterisation, two models are constructed according to fracture density (MODEL A and MODEL B). For each of them, the influence of joint constitutive law on the stability of the gallery is examined, and comparisons between the investigations and predictions are made. Examination of the results shows that there is no noticeable change in stress magnitudes between laws. The displacement magnitudes depend on (1) the constitutive law, (2) the input model parameters and (3) the fracture density. A parametric study in the case of MODEL A indicates that a relatively good match between predicted and measured displacements around the gallery can be observed in certain areas.
Numerical Analysis of Roof Failure Mechanisms of Cavities in a Soft Rock
At the University of Napoli Federico II (Italy), a research program was recently started on the behavior of underground openings in soft rocks, with the aim of finding simple tools for evaluating roof safety conditions. As a first step, assuming a simplified geometry of the cavity, parametric numerical analyses were carried out in plane strain with a FD code and a FE code, by varying the depth and the width of the cavity as well as the thickness of the roof beam. The soft rock (tuff) in which cavities are dug underneath Napoli was modeled as a linear elastic-perfectly plastic material. Two critical roof failure mechanisms were considered: one for which cracks progressively open until it is assumed that a block falls from the roof (local collapse), and one for which the whole roof collapses (general collapse). The results allow collapse loci for the two mechanisms to be plotted, which can be used also for different soft rocks. The results show that the local collapse locus basically ...
Numerical Simulation of Stress Arching Effect in Horizontally Layered Jointed Rock Mass
Symmetry, 2021
Stress arching effect during the excavation of broken surrounding rock in underground engineering has an important influence on the stability of surrounding rock after underground excavation. To determine the stress arching effect in horizontally layered jointed rock mass, the stress arching characteristics of surrounding rock mass after excavation is analyzed in this study by using a series of numerical tests. The formation mechanism of stress arch is revealed through a comparison of the stress characteristics of a voussoir beam structure and theoretical analysis of multi-block mechanical relationship of jointed rock mass. The method for determining the boundaries of a stress arching zone is proposed, and the influence of various factors on a stress arch is further discussed. Results show that after the excavation of horizontally layered jointed rock mass, the stress arch bunch (SAB) is formed in the lower strata above the cavern, and the global stress arch (GSA) is formed in the h...
Impact of constitutive models on the numerical analysis of underground constructions
Acta Geotechnica, 2008
The constitutive model frequently used in numerical calculations of tunnel excavation is linear-elastic perfectly plastic with a Mohr-Coulomb (MC) failure criterion. Generally, this leads to shallower and wider surface settlement troughs than those observed experimentally. It is therefore necessary to use adapted constitutive models for the design of underground works. In this paper, three constitutive models are implemented in a two-dimensional simulation of an underground excavation in plane strain: a linear-elastic perfectly plastic model (the MC model), an elastoplastic model with isotropic hardening [the hardening soil (HS) model, Schanz et al., Beyond 2000 in computational geotechnics, Balkema, Rotterdam, pp. 281-290, 1999 and an extension of this model which implies an evolution of the stiffness modulus in the small-strain range according to the strain level (the HS model with smallstrain stiffness ''HS-Small'', Benz, Small-strain stiffness of soils and its numerical consequences. Ph.D. thesis, Universitat Stuttgart, 189 pp., 2007). The study is based on the results of drained triaxial compression tests representing an overconsolidated clay (Gasparre, Advanced laboratory characterisation of London clay. Ph.D. thesis, Imperial College London, 598 pp., 2005); and is then applied to a shallow tunnel. The impact of the constitutive model is highlighted as well as the limits of the simplest constitutive model.
Application of a transversely isotropic brittle rock mass model in roof support design
International Journal of Mining Science and Technology, 2012
Accurate modelling of the potential failure modes in the rock mass is an essential task towards a robust design of roof support systems in coal mines. The use of generalised rock mass properties based on averaged properties (e.g. Hoek-Brown model) has been found to limit the capability to reproduce the actual rock mass behaviour which may include a wide range of interacting and complex failure mechanisms such as shear and tension fracturing of the intact rock and shear and separation of pre-existing discontinuities, including re-activation. Recent studies have also shown that traditional models, such as the Mohr-Coulomb, may not accurately describe the behaviour of the intact rock, particularly for stress induced failures where spalling and slabbing are observed. This is mainly due to the cohesion and friction components of the shear strength of the intact rock not being mobilised at the same rate with strain-softening of the former component playing an essential role in the post peak behaviour. In addition, coal measure rocks are often transversely isotropic, both by way of the preferred orientation of clay particles within the finer grained lithology and by bedding textures and bedding partings, and this is often ignored in computer simulations. A newly developed transversely isotropic brittle rock mass model is applied in the simulation of a hypothetical and simple roadway development. A Cohesion Weakening-Friction Strengthening (CWFS) approach is adopted to describe the intact rock where the mobilisation and strain-softening of the two shear strength components are linked to plastic deformation. Failure and plastic deformations are also allowed to develop along any number of ubiquitous joint sets using a conventional Mohr-Coulomb failure criterion and applying the stress-strain correction accordingly. The impacts of anisotropy and brittle rock on the development of the excavation disturbed zone or height of softening, as often referred to, are investigated and their implication in the roof support design discussed.
Shortcomings in the standard continuum based Implicit Joint model of layered rocks
2010
Layered rock masses can be described efficiently using a continuum formulation. There are two distinctive continuum based formulations that are found in the published literatures e.g. conventional continuum formulation based models such as Ubiquitous Joint model and non conventional formulation based models such as Cosserat Continuum models. Such equivalent continuum models may provide reasonably accurate predictions when joint slips are minimal i.e. when the shearing is in the direction of layering and rock layer bending can be neglected. However, when joint slips are large and loading direction is not aligned with the direction of layering models based on conventional continuum theories may considerably overestimate the deformation since the bending rigidity of the rock layers are not incorporated in such model formulations. For the case of rock layers with bending stiffness, an Accurate Continuum model can be formulated successfully on the basis of Cosserat Continuum theory. The ...
Verification for numerical modelling of jointed rock mass using thin layer elements
International Journal for …, 1991
Thin layer finite elements are employed to model the response of rock joints in finite element analyses of rock mechanics problems. The case of a circular opening embedded in a rock mass is investigated. Results are obtained for both solid and jointed rock masses, and the effects of the presence of joints on the stress and strain field are investigated. Results from the finite element analysis art verified with respect to measurements from physical modelling. It is shown that the comparison between measured and computed results is satisfactory and that the presence ofjoints has a pronounced effect on the stress and strain field as compared to that realized in the absence of any rock joints. This study provides valuable data which contribute to the verification of thin layer finite elements for modelling the response of rock joints.