Deterioration of Excavated Rockslopes:
Mechanisms, Morphology and Assessment

Dawn T. Nicholson

Results are reported of laboratory simulated weathering processes on a range of sedimentary rocks and the investigation of the deterioration of rock masses as observed on engineered and quarried rockslopes.

The simulated weathering processes include freeze-thaw, wetting and drying, salt weathering and slaking. It is found that the rock properties of pore volume, saturation coefficient and microporosity exert greatest influence on susceptibility to breakdown. For stronger rocks it is found that durability correlates well with high strength and elasticity.

A range of rock flaws visible in hand specimen are described and their influence on rock deterioration assessed. Linear flaws such as laminations and stylolites are more likely to be associated with breakdown, and the role of structural weaknesses is most evident in stronger rocks. Rock breakdown mode due to experimental weathering is found to closely resemble material weathering of source slopes in the field. A range of rock breakdown mechanisms are inferred from changes in pore microstructure and rock strength. There are indications of a progression from deterioration which is invisible and involves modification of the existing pore structure, to macro deterioration resulting from generation of new void space and microcracks.

After field investigation of more than two hundred rockslopes deterioration is found to be widespread, and there is little evidence of a systematic approach to its assessment or mitigation. Fracture spacing, rock strength and lithology are found to be the most influential factors in rockslope deterioration and these are used to define a characteristic range of rock mass types. A range of morphological forms attributed to deterioration are defined and described. An engineering classification of deterioration modes is presented, based on constituent material size, velocity of movement and frequency of occurrence. Deterioration modes correlate well with rock mass type.

A new rock mass classification, called Rockslope Deterioration Assessment (RDA) is proposed, dealing specifically with shallow, weathering and erosion-related rockslope processes. RDA is divided into three stages; a ratings assessment of deterioration risk, a qualitative review of the likely deterioration hazard, and guidance on appropriate mitigation. The findings of the experimental work are incorporated into stage one of RDA where appropriate. Notable in this respect is the emphasis in RDA on evaluation of fracture spacing on the basis of all fractures present, whether open or incipient, and whether natural, or induced by blasting, weathering or stress release. RDA is applied to the slopes investigated in the fieldwork and shows that certain types of rock mass are associated with higher risk of failure. There is also an element of predictability in the occurrence of deterioration modes. Correlation between stage one of RDA and Rock Mass Rating is examined and it is shown that, although there are some similarities, a fundamental difference relates to the basis upon which fracture spacing is assessed.