RECENT INVESTIGATIONS INTO THE USE OF PLASTIC LATERITES AS BASES FOR BITUMINOUS-SURFACED LOW-VOLUME ROADS

The development of specifications for bases for bituminous-surfaced roads in temperate zones is described in this paper. These specifications were developed in the 1920s and 1930s when compaction plant was light compared with that available today. These specifications have been adopted with limited modifications by developing countries in tropical zones for low-volume roads. The majority of lateritic gravels do not comply with the requirements of these specifications and bases of crushed stone or stabilized materials are normally used. A description is provided of full-scale, trial sections of road in Kenya and Malawi in which plastic lateritic gravel bases were used and the test results of site investigations gathered over a number of years. These trial sections have performed satisfactorily. A 3-year laboratory study of the Kenya laterite has been undertaken at Imperial College London to ascertain the reasons for their satisfactory performance. The construction procedures used and the relative densities found in the subgrade are also described. It is concluded that the satisfactory performance of the plastic laterite bases was a result of their high degree of compaction, the grading that resulted in low permeability and high stiffness, the construction procedures adopted, and the well-drained subgrade.