Stress and strain during river downcutting

In situ stress changes caused by fluviatile erosion are shown to be capable of setting up failure conditions in rock: a) well below the river bed level where plastic deformations are possible; and b) at or just below the river bed where failure is brittle in character. By contrast, gravity loadings at the toe of a valley, even in areas of high relief, can be an order of magnitude less than residual tectonic stresses. While gravity loadings lead to normal slope instability along rock defects, they are shown to be incapable of causing toe buckling or other creep deformations in moderately strong rock.

Introduction

Disruptions to the regional structure of rock fonnations are often evident at the toes of deep valleys; bed separation and bedding fractures are adequately documented. Stress relief phenomena such as open (tension) joints are likewise recorded deep into valley sides.

In recent years descriptions of more elaborate defonnations in valley slopes have appeared in the literature. Folding, warping and toe buckling of moderately strong to strong rocks, Fig. 1, have been attributed to gravity forces by Giraud et al (90), Nemcok (72), Ter-Stepanian (74) and Zischinsky (66). These proposals appear to have been based predominantly on field observations of rock structures on the slopes of valleys, together with some qualitative data on creep phenomena in rocks. However, as far as the author is aware, no quantitative comparisons have been made between rock strengths and valley loadings, to check on the feasibility of these proposals.

The following is an attempt to make a quantitative assessment of the stress changes which could be involved in valley development. A comparison is then made between the role of in situ stress and that of gravity forces in the defonnation of in situ rock.