Investigation of Sub-Base Material Performance on Rural Roads in Niger State

Sub-base layer quality significantly determines rural road pavement performance, particularly in tropical lateritic environments such as Niger State, North Central Nigeria. This study investigates the engineering performance of sub-base materials currently in use on rural roads in three LGAs of Niger State: Bida, Agaie, and Lapai. In-situ Dynamic Cone Penetrometer testing was conducted at 200-metre intervals on 12 road sections to assess sub-base strength variability. Core samples at critical locations were collected for laboratory analysis including CBR, gradation, Atterberg limits, and compaction characteristics. Performance is assessed against Federal Ministry of Works sub-base specification requirements. Results indicate that in-situ sub-base CBR varies considerably along assessed sections, ranging from 12 to 68 percent. Mean CBR of 28 percent falls within the minimum 30 percent specification in 44 percent of tested locations. Plasticity Index of sub-base materials ranges from 6 to 22 percent, with 35 percent of samples failing the maximum 12 percent specification. Road sections constructed with quarry-crushed granite sub-base show consistently higher performance than those using laterite sub-base. The study concludes that material source selection and field compaction quality control are the primary determinants of sub-base performance in Niger State rural roads, and recommends mandatory laboratory pre-qualification of all sub-base materials before placement.

Keywords: sub-base performance, rural roads, DCP testing, Niger State, North Central Nigeria