Abstract

One of the major challenges facing rock engineers is that of establishing the bulk properties of the fractured rock mass on which or in which they are working. These are controlled principally by the geometry of the fracture network and the properties of the individual fractures. The network is built up by the superposition of separate fracture sets, each related to a geological event (burial tectonism and exhumation). In structural geology ‘fracture analysis’ is used to determine the order in which the sets are superimposed and knowing this, the 3D geometry of the network can be determined. Examination of the fracture surfaces can also reveal whether they are shear or extensional. Provided with this information the rock engineer can then combine it with site specific tests on the properties of the individual fracture sets and begin to quantify the likely physical behaviour of rock masses on an engineering scale. This paper presents a brief introduction to the concepts of fracture analysis, and goes on to show how these can usefully by integrated with typical rock mechanics analyses to give improved data for rock engineering design.

Introduction

The rock masses in which rock engineering takes place are the result of various geological processes. Such processes and their products are well understood by geologists, and they use this understanding to interpret the genesis of a rock mass. Here, we explore how such understanding can bring benefits to rock engineering.

We begin with a brief discussion of the geological processes that generate fractures in a rock during its cycle within the crust from (for sedimentary rocks) deposition, through burial and diagenesis, possible deformation linked to plate motion (orogenesis, i.e. mountain building) and finally exhumation to its present position at the Earth’s surface. Each of these processes is likely to result in the formation of a fracture set whose orientation and type (shear or extensional) will be determined by the stress field acting at the time. Crucially, the different types are likely to possess different mechanical properties.

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