Abstract Deltaic lateritic soils of the Niger delta region of Nigeria differ markedly fiom the other lateritic soils because of some mode of formation related peculiarities. As the most widely used soil materials for road earthworks in the entire region, their continual exploitation based on properties of the more matured and widely studied lateritic soils of the hinterland has met with mixed degrees of failure and success. In this study, samples of this soil material fiom different locations were stabilised mechanically with or without sand addition and chemically with controlled proportions of cement, cement-sand and cement-geosta. Strength of plain mechanically-stabiied (i.e. merely compacted) soil was found to be directly dependent on the compacted density which itself is dependent on the percentage fines, F while sand stabiiation was also found to be additionally dependent on the optimum sand content (OSC) or the most effective sand content that ., ,, ."I. .* , .I* will produce the densest state of compaction. OSC is itself dependent on F and this was used to develop a graphical model for predicting the various road design paramete&. A graphical model was also developed to harmonise laboratory and field compaction. Cement-related chemical stabilization was found to depend solely on the fabric structure developed through apparent cohesion over time in course of cement hydration. Although pbin cement stabilization significantly improve strength, soakedCBR of 280% required for suitability as a base course material was achieved at rather high cement contents in excess of 12% which is at variance with the economic ceiling of about 7% specified by first volume of FMW Specifications on roadworks. Composite stabiliiation with sand and cement achieved this feat (i.e. 180% CBR) with about 6% and 36% cement and sand contents respectively. Geosta addition to soilcrete was also found to be a considerable improvement over plain cement stabilization and that even at very low geosta content less than or equal to 2% depending on the optimum geosta content, OGC and the percentage fines. Analytical and graphical models were also presented to predict influence of the various stabilkation methods using as indicator parameter the percentage fines, F obtainable fiom simple gradation tests, particularly wet-sieving. As a resuIt of these and other soil-property-related failures of roads in particular, it was also recommended that all road related agencies (governmental and non-governmental) should be more research oriented through proposal and funding.
