Effects Of Locust Bean (Parkia Biglobosa) Pod Extract on The Consistency Limits of Marginal Soils for Road Pavement Layer Construction

Earthworks in road pavement layer construction present huge challenges with severe financial implications especially when suitable material for base course and subbase course are not found along the road corridor or within reasonable haulage distances. In most cases, it may be easy to find marginal materials or materials that need some form of stabilization. Chemical and mechanical stabilization is common but could be expensive. Plant extracts for stabilization hold promise but more research is required to identify locally available plants that can be cultivated or applied at scale if found useful. In this exploratory study, the potential effects of stabilization of marginal lateritic gravel soils using locust bean pod extract (LBPE) were studied for Consistency Limits of natural gravel materials for road pavement layer construction. The extract was prepared by pounding and soaking the locust bean pods in tap water for intervals of up to 28 days. The soil sample was obtained from Agyei-Ano South, Sunyani, and tested in the geotechnical laboratory of Sunyani Technical University (STU). The soil was analyzed for index properties and classified as Silt-Clay material (A-6(5)) and clayey gravels (GC) using the AASHTO and Unified Soil Classification Systems respectively. Consistency limit tests for the control and test specimens were determined using distilled water and LBPE respectively. The soils stabilized with LBPE were found to generally exhibit 3.5% – 7.1% higher Liquid Limit (LL), and 0.9% – 15.0% higher Plastic Limit (PL) than the control. Compared with the control, the Plasticity Index (PI) values were lower by up to 22%. Lower PI is desirable in road construction materials. Improvements of up to 20.2% in the Consistency Index and up to 55 % in the liquidity index of the LBPE samples over the control were observed. This is an indication that LBPE could improve the engineering properties and change the classification of the lateritic soil, in this instance, from G30 grading envelope plasticity index requirement to the G40 grading envelope plasticity index requirement. The optimum soaking duration was found to be 3 days.