ABSTRACT:

The effects of strength anisotropy of rock masses on bearing capacity of shallow foundations are the focus of this paper. The source of the strength anisotropy can be attributed to many factors and the motivation of this study is to concentrate on the effects of planes of weakness, such as joints and bedding planes. A constitutive model is presented for this class of materials and series of finite element analysis are used for the evaluation of bearing capacity. To validate the applicability of the proposed model, the finite element results obtained in this study are compared to the results obtained by limit analysis, lower and upper bound solutions and also finite element analyses presented by other researchers.

1 INTRODUCTION

The stability of shallow foundations is traditionally evaluated using the bearing capacity equation that is derived from the limit equilibrium approach and incorporates various approximations for the bearing capacity factors. A more rigorous methodology is that of limit analysis, which provides both lower and upper bound assessments. A more accurate approximation involves a numerical assessment based on finite element analysis. The latter can incorporate more advanced constitutive models that reflect the significant features of the mechanical response of the material.

The issue that has not yet been broadly addressed in the assessment of ultimate bearing capacity is that of material anisotropy. Sedimentary rocks, such as shale, limestone and mudstone, are typically formed by deposition and progressive consolidation. Such formations usually have a distinct internal structure, which is characterized by the appearance of multiple sedimentary layers. Besides bedding planes, the geometric layout of networks of joints and other types of discontinuities in a rock mass are significant contributors to the complex mechanical behavior of such geomaterials, e.g. Hoek & Brown (1980), Hoek (1983), Zienkiewicz & Pande (1977). Since the strength characteristics are generally weaker along these fissures and planes, their presence in a rock mass significantly influences the response of geotechnical structures such as slopes, tunnels, excavations, foundations, oil wellbores, etc.

In this paper, a constitutive framework is outlined that describes the mechanical response of transversely isotropic geomaterials. The focus of the framework is mostly on the anisotropy in the strength characteristics. An implicit integration scheme has been developed and implemented in a finite element code RS2 (Rocscience Inc. 2015). Various bearing capacity problems have been analyzed to illustrate the effects of anisotropy. The results obtained in this study are compared to the work of other researchers that have used the finite element method (Alehossein et al. 1992), limit analysis (Davis 1980, Zeng et al. 2000) and lower and upper bound solutions (Sutcliffe et al. 2004).

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