Abstract
The lateral pressures induced by the compaction of clay against rigid structures and the subsequent behaviour of the compacted clayey backfill with time (dissipation of the pressures), and when wetting takes place (swelling pressures), have been investigated. The use of compacted clays behind bridge abutments and other retaining structures could lead to significant savings in arease where granular fill is either unavailable or expensive. Preliminary laboratory work was undertaken in order to assess the basic relationships between the main variables involved in the process of compaction (such as energy, soil parameters, and the stresses induced). A subsequent laboratory investigation was carried out to study the change (dissipation) of the compaction pressures with time, as well as the lateral pressures induced when swelling occurs. Compaction and swelling apparatusses were developed for these laboratory tests which incorporated a new small size total earth pressure gauge. The final part of this investigation involved a pilot scale experiment jointly undertaken by the Geotechnical Engineering Division of the Department of Civil Engineering, University of Surrey, with the Ground Engineering section of the Transport and Road Research Laboratory of the Department of Transport. The results from this experiment indicated that large compaction lateral pressures (more than double the earth pressure obtained from an "at rest" classical analysis) can be induced in well compacted stiff clayey backfills. Existing compaction theory developed for granular soils cannot be used to predict either the magnitude or the distribution of these pressures. Following the latest draft for a revised edition of the "Specification for Road and Bridge Works" the air voids content of the fill was used to control the effectiveness of the compaction process instead of the method specification which is current practice. Therefore a rapid and reliable method for determining the dry density in-situ was needed. The performance of the TROXLER 3411-b in direct measurement mode was investigated in order to establish an experimental correspondance between the nuclear density tests and the other standard methods (i.e., sand replacement and core cutter) . For the second stage of this experiment, water was supplied to the fill and after a period of swelling of approximately four months, the lateral swelling pressures have reached magnitudes of around four times the predicted "at rest" values, and are in excess of the theoretical passive pressures. Considering that the mechanisms involved in the compaction and swelling behaviour of clayey backfills have not yet been clearly established and that there is still no theory able to predict with the accuracy and detail required the magnitude and distribution of the earth pressures investigated, it is suggested that very stiff clays of high plasticity should be carefully studied before being chosen as backfill material.