Geomechanics and the Emergence of Soft Rock TechnologyE.H. Davis Memorial Lecture

Engineering problems involving soft rocks have traditionally been solved by extrapolating from the historically separate technologies of either soil mechanics or rock mechanics. This has generally lead to tenuous, conservative solutions which have contributed little to the overall understan ding of soft rock behaviour.

It is argued that soft rocks are not vague extensions of these two geotechnical sciences but are in fact central to one continuous science extending from soft soils through to hard rocks. All geotechnical materials behave according to the same engineering principles, with obvious differences a function of degree rather than of fundamental nature. Consequently, it is demonstrated that for soft rock technology to emerge as a rational engineering form, the established methods of soil mechanics and rock mechanics should be applied together and not in isolation.

Introduction

For many years, soft rock technology has been considered to fall into a poorly defined area somewhere between soil mechanics and rock mechanics. Solutions to engineering problems involving soft rocks have depended a great deal on the type of problem involved and consequently on the background of the engineers associated with that problem.

For example, when dealing with civil engineering works such as foundation design on a soft rock, an approach based on soil mechanics has been dominant. This technology has used an extrapolation of the concepts and methodologies of materials such as clays and sands to describe the behaviour of soft rocks. Unfortunately, because soils engineers have had comparatively little exposure to soft rocks (or rocks of any kind) in their training and technical literature, these extrapolations have tended to be very tentative and conservative, often leading to very costly design solutions. It would appear that little concession has been given to these harder, more brittle, dilatant and often highly discontinuous materials.

Conversely, when soft rocks have been encountered in a mining or a tunneling environment, it has usually been a specialist in rock mechanics who has sought a solution. Consequently, the technology applied has usually involved the often highly empirical hard rock approach which has been much more concerned with the defects within the rock mass, rather than the rock material itself. It follows, with this approach, that there often has been little consideration given to the characteristics of soft rocks; to their considerably lower strength, and the fact that this could be approaching the strength of the defects, to their less brittle behaviour, to their more compressible skeleton, and to the influence of pore water pressures.

It follows that soft rocks have been traditionally viewed as materials which have existed on the fringes of the two established quantitative engineering geosciences as is shown in Figure 1. Being fringe dwellers, they have never really had the attention that their frequency of occurrence in engineering problems would seem to demand. Consequently, soft rock engineering has consisted of a somewhat random and unconnected collection of largely qualitative data from many different locations. The worth of this data has been further deValued because soil mechanics and rock mechanics each possess their own, and often contrasting, methods of classification, testing, interpretation and design. It is not surprising, therefore, to find that soft rock technology is still wallowing in its primordial slime and that soil mechanics and rock mechanics still exist in almost total isolation.

Just recently, there have been signs that the barriers that seem to have existed between the soil and rock fraternities are beginning to crack, with a marked increase in communications between the two communities, particularly through their common interest in soft rocks. It is the author’s conviction that the sooner this happens, the sooner we will begin to develop a comprehensive understanding of the behaviour of soft rocks. As a corollary to this, since soft rocks share many characteristics with both soils and harder rocks, it is believed that the bridge created by soft rock technology will do much to unite soil and rock mechanics, and therefore, benefit geomechanics in general.

So that this process may be encouraged and perhaps accelerated, this paper will consider the nature of soft rocks and how this relates to geotechnical materials in general. It will be argued that soft rocks are not merely appendages to the two principal geotechnical sciences but are in fact central to one continuous spectrum of geotechnical materials. Soft rocks must therefore display characteristics that are shared by soils and hard rocks, even though these characteristics may not be immediately obvious. For such a perspective of geomechanics, it would seem important that the behaviour of soft rocks should be considered from both directions, to ensure that major misconceptions and errors are not introduced due to the extrapolation from one direction only. A number of examples covering testing procedures, properties, modelling, analysis and design will be offered to support the above views.