Excavation-Induced Deterioration of Rockslopes

Dawn T. Nicholson, Alastair C. Lumsden and Steve R. Hencher

Engineered and quarried rockslopes are subject to excavation-induced deterioration over engineering time, leading to unplanned maintenance and constituting a safety hazard. Whilst it is standard practice to evaluate deep-seated instability for excavated slopes, scant attention is given to shallow surface processes at the design stage and subsequently. This is because deterioration is not perceived as a significant risk, it is difficult to quantify, and its mechanisms are poorly understood. Deterioration is defined here as the shallow, progressive alteration, detachment and removal of material from a parent mass by mechanical and chemical processes. It occurs because excavation releases confining pressures leading to expansive recovery of the rock mass, and because the newly-created slope is exposed to the external environment.

The primary aim of ongoing research at the University of Leeds is to develop a systematic approach to the evaluation of rockslope deterioration potential. Such an evaluation should be applicable both to existing and proposed rockslopes, and should lead ultimately to appropriate design modifications or to the selection of remedial treatments. In pursuit of this aim, a selective field investigation of the extent, nature and consequences of deterioration has been undertaken for excavated rockslopes in the UK. Nearly 100 sites have been characterised in terms of rock mass and material properties (eg fracture characteristics, rock mass structure, lithological properties and weathering grade) and external factors. These have included environmental conditions (eg hydrology and climate), stress conditions (eg stress release and quarry blasting), slope and engineering factors (eg geometry, remedial measures and excavation method), and time since exposure. Records have also been made of (i) slope morphology (particularly forms attributable to deterioration), (ii) evidence of weathering activity and (iii) the products of deterioration.

This data has led to the identification of several distinct types, or modes, of deterioration, which, collectively, result characteristically from shallow surface weathering and erosion acting on slopes excavated in rock. Each mode is distinct in its frequency of occurrence, its velocity of movement, the size of constituent material, and event magnitude. In addition, each mode of deterioration is unique in terms of its implications for slope maintenance, safety hazard and appropriate remedial treatments.

The deterioration modes recognised are grouped according to their frequency of occurrence: Isolated modes such as rockfall, debris flow, debris slide and rock slide tend to be large magnitude events which occur infrequently and were rarely in evidence. Conversely, slabfall, toppling, blockfall, scaling, stonefall and grainfall can be described as sporadic, and occur frequently even in otherwise intact rock masses. Evidence for sporadic modes was recorded almost universally on the slopes investigated, commonly being controlled by the removal, reinforcement or support of individual blocks. Deterioration modes such as ravelling (block, stone and grain), flaking, wash, rock creep (flexural toppling) and solution occur on a semi-continuous basis and are best controlled by containment or surface protection. These mechanisms tend to occur in highly fractured, fissile, or weak rock masses and create the greatest maintenance burden.

For each of the slopes investigated, deterioration modes were compared with external factors, and rock mass and material properties of the host rock(s). This has shown that there is no one intrinsic slope characteristic or external factor which controls the mode of deterioration, but that each mode of deterioration responds to the presence of one or more specific conditions. Though the relationships are complex, the nature of the fracture network and rock material strength are commonly of overriding importance in determining deterioration mode. Various distinct types of rock mass structure (eg regular or irregular blocky, massive, weak massive, fissile, composite and layered) which rely heavily upon these two parameters, have also been identified, and correlate well with deterioration modes identified. As such, many excavated rockslopes may have a built-in propensity to deteriorate in a certain fashion, a factor which can be exploited in any evaluation of deterioration potential.

A new, systematic, three stage engineering evaluation of deterioration, called Rockslope Deterioration Assessment (RDA), is currently under development. In the first stage of RDA, a range of potential rock and environmental controls on deterioration are given a rating to determine the deterioration susceptibility class. Stage two of RDA comprises the assessment of deterioration mode using the classification discussed herein. The final stage utilises the results of stages one and two to provide guidelines for slope design modifications and remedial treatments.