1. Introduction

Rock slopes are loaded by three categories of forces:

(a) weight of the rock mass,

(b) thrust and weight of the dam, and

(c) seepage forces of percolating water.

In order to study the stability of an abutment it is necessary to know all these forces. The seepage forces, however, are often ill-known and sometimes not known at all. So far, engineers use artificial means to confine the seepage field within well defined boundaries. The available means are grouting and drainage.

Thanks to recent developments in rock mechanics, it is now possible to evaluate the respective roles of grouting and drainage on the safety of dam abutments, particularly for arch dams. This is vital as a misunderstanding of these roles could lead to designs that are either needlessly costly, or, worse, give a false impression of safety. A number of findings are valid for rock slopes not supporting a dam.

It is proposed here to develop the latest ideas on this difficult subject. Some of the concepts are still of a speculative nature and numerous field measurements are required to confirm their validity. The few observations made so far do, however, justify the approach.

2. Basic concepts

Consider a rock mass containing a great number of fissures, each of them having a constant width, and assume that the seepage flow is:

(a) steady

(b) laminar

The second condition is valid for rocks with fine fissures and moderate hydraulic gradient. Excluding physico-chemical action, seepage of water can have three

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