M40 MOTORWAY BANBURY IV CONTRACT - LIME STABILISATION

This paper describes the problems encountered with the use of lime stabilisation in the M40 Motorway Banbury IV contract in England, and alerts highway engineers to a mechanism of lime stabilisation which has not been generally appreciated. The problems on Banbury IV appeared to occur only where the road ran over lower lias rock; they emerged at the end of the winter after the lime-stabilised element of the capping layer was constructed during Spring and Summer 1989. It was found that the carriageway had lifted by up to 150mm; the lime-stabilised layer had softened, was moister than expected, and its thickness had increased from 250mm to 400mm. The unexpected heave of the lime-stabilised material, found here and during very recent American research, emphasises the need for detailed chemical and laboratory studies of both soil and ground water in any project considering the use of lime stabilisation; conventional test methods are sometimes insufficient. The results show that the heave is caused by ettringite/thaumasite formation. In order for this to occur, the following conditions are necessary: a) high pH conditions; b) a sufficient clay mineral content giving an adequate supply of alumina, silica and carbonates; c) the presence of sulphates; d) the correct temperature conditions; and e) availability of sufficient water. These conditions were present on Banbury IV. The Department of Transport is now considering the implications for specifying lime stabilisation. (TRRL)