Obtaining Suction Distribution Within Vadose Zone of Highway Pavement System in Southwestern Nigeria Using Physico-Empirical Approach

Abstract

The stability of the pavement system on old Ogbomoso-Ilorin road in southwestern Nigeria basement complex was investigated. This was carried out by estimating the suction, degree of saturation and effective stress of the soils within vadose zone from soil–water characteristic curve (SWCC) and suction stress characteristic curves (SSCCs) employing physico-empirical approach. For this purpo se, the grain-size distribution (GSD) curves of forty two disturbed soil samples, scooped from borings at anomalously low resistivity points delineated by 2-D Electrical resistivity imaging were fitted using empirical equation. Permeability and Atterberg limits tests were also carried out for engineering classification of the subsoil. The GSD curves indicated the soils are uniformly-, poorly- and gap-graded. The permeability of the soils ranged within 1.13 × 10–6 − 9.35 × 10–5 m/s suggests very fine sands, silts/clay-silt laminate of poor sub-drainage. The intermediate suction range values obtained from SWCCs were less than 150 kPa indicate matric suction. The soils suction; 1.053–79.529 kPa, varied with air entry values; computed (1.350–3.907 kPa) and estimated (0.406–3.672 kPa), indicating the soils could shrink. The high suction gradient; 0.223–12.403%/kPa, indicates decreased evaporation flux, the ability of the soils to generate flow and suggests the soils are deformable. The SSCCs indicated the degree of saturation at which minimal soils deformation could occur varied between 0.230 and 0.560 with maximum suction stress of 1.9–8.0 kPa. The presumed equilibrium suction of the area within the range 17.365–79.529 kPa was relatively high compared with suctions of all other soils. This indicates decreasing suction and hence significance shrinkage of the soils. The pavement materials properties would therefore have undergone significant volume change and impaired the stability of the pavement.