Instability developed early in 1965 during the construction of earthworks on the southern lengh of the Sevenoaks By-pass, Kent. The By-pass was subsequently realigned over this lengh. This report describes some of the investigations carried out along the line of the original route and considers the stability of the earthworks.

The invetigations revealed eight areas where some history of instability was indicated. The material within these areas were found to contain natural slip surfaces running approximately parallel with the ground surface. The main factor governing the feasibility of earthwork construction across these areas was the requirement that the the water table in the unatable areas be depressed to a level below that of the main slip surface and maintained at that position. If this could be achieved, preliminary stability valculations showed that an embankment about 5 m in height could be constructed using light-weight fill and side-slopes of 1 in 4.

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