Dawn T. Nicholson and Steve R. Hencher
When a new rockslope is being designed, most engineers base their design criteria on the avoidance of deep-seated failure. Potential failure mechanisms are identified during site investigation, and preventive techniques such as modification of slope geometry, support measures and slope drainage, are then incorporated into the design. However, progressive near-surface deterioration can be a major problem during the lifetime of an engineered rockslope. Deterioration can lead to high maintenance costs and may constitute a safety hazard, yet it receives scant attention at the design stage.
This paper considers the fundamental causes of the deterioration of rockslopes in engineering time and illustrates how greater understanding of the mechanisms involved can be translated into better application of preventive techniques and design criteria. The extent and nature of near-surface deterioration is influenced by rock mass and material properties, climatic controls, applied dynamic and static stresses, and by a variety of engineering factors. The agents of deterioration include both physical and chemical processes, including stress release, frost shattering, cycles of wetting and drying, chemical decomposition and root wedging. These mechanisms invariably lead to the dilation of existing open and incipient discontinuities allowing increased ingress of water, rock fragments, organic material and other damaging elements.
The research involves qualitative inspection of engineered rockslopes in the UK in order to identify the principle modes of deterioration and the resulting practical implications. At a more fundamental level, the development of incipient discontinuities into open fractures is being investigated by experimental weathering of rock core. Ultrasonic velocity is being used for non-destructive monitoring of core deterioration, and the results are being correlated with variables including lithological properties, depth of origin and exposure time.
A new classification, the Rockslope Deterioration Assessment (RDA) method, is introduced, and is aimed at determining the susceptibility of new cut slopes to deterioration. The RDA provides guidance on the nature of deterioration to be expected, and advises on appropriate preventive and remedial measures. An outline of the RDA is given in Figure 1. The parameters required to determine a susceptibility class for the rock mass, prior to excavation, are identifiable at the site investigation stage. These parameters should be confirmed during and post excavation, in addition to a further consideration of additional influencing factors.
The paper illustrates the consequences of deterioration with reference to a number of case studies in Hong Kong and the UK (see Figure 2). These examples consider rockslope deterioration in terms of its nature, its relationship to the slope system, and the breakdown and failure of engineering preventive measures.