Numerical simulation of deep excavations in rock masses

Deep excavations are widely used for various purposes in industries including civil, mining, military, petroleum, energy and environmental engineering. The number and size of excavations have increased steadily in recent years. In engineering practice, designers are particularly interesting in making accurate predictions of the magnitudes of movements in the surrounding rock masses associated with the construction activities, and the likely extent of damage to adjacent structures and facilities. It is therefore of interest to develop reliable and efficient techniques which are able to predict accurately the deformation and stress distributions around excavations in the rock masses. On the basis of both theoretical and practical considerations, this thesis attempted to develop efficient techniques for analysis and to provide a better understanding of the rock mass behaviour during excavation. Rock masses are seldom found in nature without joints or discontinuities which can have a significant effect on the gross mechanical response. In practice, it is almost impossible to explore all of the joint systems or to investigate all their mechanical characteristics. Moreover, when the spacing between the joints is small in comparison with the length scale of interest (such as tunnel width or foundation size), a simulation incorporating the jointing explicitly is very difficult and costly to implement. In these cases, the use of the equivalent continuum model to simulate behaviour of jointed rock masses could be valuable. A concept of "deformation equivalence" has been adopted to derive an equivalent continuum model for simulation of elastic behaviour of jointed rock mass. At higher levels of applied stress either the intact rock or some of the joints or both may yield, and subsequently they will behave plastically. An elastoplastic equivalent continuum model containing any finite number of joint sets in which any of the joint sets and the surrounding intact rock may undergo plastic deformation has been described. A quadratic boundary element method for an anisotropic medium has been developed. It has been demonstrated that the use of quadratic elements can yield accurate results with relatively coarse boundary element meshes, and can not only more adequately represent a curved boundary but it can also give more accurate results than constant elements. A coupled finite element and boundary element method for analysis of boundary value problems in anisotropic rock masses has been developed. This method is characterised by using equivalent continuum models to represent the rock mass as an equivalent, anisotropic, elastic or elastoplastic continuum. In this method. the presence of joints, the sequential excavation or construction, inhomogeneous materials, material non—linearities can be considered in regions of interest which can be represented numerically by a finite element mesh, and the boundary discretisation can be used to represent the response of anisotropic rock masses in the far field. The formulation has been validated by comparing available analytical solutions. In order to provide a detailed understanding of the behaviour of a jointed rock mass around excavations, numerical studies for both circular openings and deep basements excavated in jointed rock masses have been carried out. For circular openings, the predictions illustrate the effects of joint spacing and shear stiffness on the ground response curve, and the zones in the rock mass around the tunnel where intact rock has yielded plastically and where the joints have yielded in shear or have opened. Plots of the dimensionless ground response curves, the vectors of displacement and the contours of the yield ratio for tunnels at various dimensionless depths in jointed rock masses have been presented. For basement excavation problems, the numerical results illustrate the effects of anisotropy of the rock mass and the orientation and spacing of the joint sets around the excavation. I The deformation and stress distributions, the stability of rock masses around the basements and effect of dilatancy in rock joints on the movements of the rock mass have been studied. A case history of a basement excavation adjacent to railway tunnels in rock masses is also presented to demonstrate the capability of the proposed method in solving practical problems. The excavation is located in the central business district of Sydney and is approximately 30m deep with a total of about 500,000m3 of rock having been excavated. Good agreement has been found between the field measurements and numerical predictions