A laboratory study of the development of earth pressure behind integral bridge abutments

A conventional bridge abutment utilises bearings to support the bridge deck, and expansion joints to allow it to slide as temperature changes occur. But experience has shown that expansion joints often leak, leading to deterioration of underlying structural elements, and are expensive to maintain and replace. As a result integral abutments, which are fully fixed with respect to the bridge deck, are increasingly being recommended. However, there is uncertainty about the magnitude of the earth pressures that they should be required to support. Available evidence from model tests and from field instrumentation does not provide a basis upon which to predict either the circumstances under which thermal cycling will lead to significant increases in earth pressure, or the levels to which they might rise. This paper reports the result of laboratory tests on natural clay samples, on pluviated sand specimens, and on glass ballotini, all of which have been subjected to the stress paths and levels of cyclic straining that a typical integral bridge abutment might impose on its retained soil. The results show that whereas the natural clay and the glass ballotini showed no lateral stress accumulation, regardless of strain levels and stress excursions, the pluviated sand specimens experienced systematic increases in lateral stress for almost all cyclic strain levels, eventually reaching states of stress at, or close to, both active and passive. The underlying mechanisms of stress increase are explored, and it is concluded that particle shape is an important factor in determining the response of soil to this special type of loading.