At present, the Department of Transport limits stabilised sub-bases to cement bound materials of strength no lower than Type CBM1 and requires these materials to be built to the same thickness as unbound granular, Type 1 sub-base. Research has therefore been carried out to assess the structural performance of various stabilised materials in order to widen the range of materials permissible as sub-base and to develop economical foundation designs. The research entailed the establishment of mix designs for cement bound crushed rock and sand materials and a lime and cement treated silty clay. These materials were produced and constructed at full scale as sub-bases on a weak clay subgrade in TRL's Pavement Test Facility. The performance of these sub-bases, under wheel loads representative of construction traffic, was compared with that of an unbound granular, Type 1 sub-base. Laboratory tests also determined the dependence of the structural properties of the sub-base materials on their age. The results of the full scale structural tests were used to select equivalent thicknesses of the stabilised sub-bases that produced foundations of similar stiffness to Type 1 sub-base. It was shown that stabilised sub-bases built on subgrades of CBR between 2.5 and 3 per cent could be laid thinner than Type 1 sub-base for equivalent or better performance. Unit costs of the sub-bases were collated and applied to the proposed thickness designs for the sub-bases and it was demonstrated that stabilised sub-bases were economically viable compared to Type 1 sub-base. It was also shown that stabilised sub-bases, which were laid to marginally greater thicknesses than the equivalent thicknesses, produced foundations of markedly greater stiffness than traditional, unbound granular foundations. This behaviour could permit reductions in roadbase thickness. It is recommended that lime and cement treated soil should be considered for inclusion as sub-base in Department of Transport Standards. Trials on the road network should also be carried out on the stabilised materials to further develop material specifications and foundation designs and to assess reductions in roadbase thickness permissible for stiff foundations. (A)
Abstract