Project:   Investigation of highway Subsidence Client:     Highland Regional Council
	Caledonian Geotech were engaged to undertake an investigation of a newly constructed road which crosses the moor of the Mhoine to ascertain the reasons for the sudden appearance of large subsidence hollows within the carriageway and to estimate the potential for future failures. The complex nature of the geological weaknesses necessitated the progressive employment of a variety of investigative techniques. These included normal and infra-red air photography, engineering geological mapping, topographic survey, seismic refraction and ground probing radar.
	A number of natural depressions, which were observed from air photography to traverse the moor, followed lines directly associated with the underlying geology. Seismic refraction and ground-radar traverses across such lines revealed a number of cavities within the bedrock surface, partially infilled by till. Geological mapping confirmed that joint patterns within the rock would result in joint intersections occurring at the positions of the cavities. Water seepage through the peat resulted in mobilisation of the till, which had drained, at such localities, down into the body of the rock, thereby undermining the peat and creating the topographic depressions. The original investigation had not permitted an understanding of this natural mechanism to be gained, and the construction of the new road, using the classical approach of peat removal had thereby artificially induced a much larger scale version of the subsidence phenomenon.
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