<b>Index Properties and Shear Strength Characteristics of Sub-Grade Soil Along </b> <b>Mariam-Awa Road, Southwest Nigeria</b>

Abstract

This study evaluates the index properties and shear strength characteristics of subgrade soils along the Mariam–Awa Road, Ago-Iwoye, Southwestern Nigeria. Thirty-one soil samples (18 disturbed, 13 undisturbed) were collected from 18 locations at depths between 0.4m and 1.5m. Laboratory tests conducted include grain size analysis, specific gravity, natural moisture content, Atterberg limits, compaction, California Bearing Ratio (CBR), and shear strength analysis. Grain size curves were plotted for classification and interpretation. Results showed that fines passing the No. 200 sieve ranged from 20% to 61%. Only 7 of the 18 disturbed samples met the Federal Ministry of Works and Housing (FMWH) specification of <35% fines. Specific gravity averaged 2.66, consistent with the recommended standard. Natural moisture content (6.5%–10.7%) was found to be within the permissible limits. However, Atterberg limits exceeded FMWH thresholds, with liquid limits of 41%–47%, plastic limits of 13%–16%, and plasticity indices of 28%–31%, indicating high plasticity. Maximum Dry Density (MDD) ranged from 1.808–1.898 Mg/m³, and Optimum Moisture Content (OMC) ranged from 9.8%–16%. All MDD values were satisfactory, while 14 out of the treated 18 samples had acceptable OMC (≤15%). Unsoaked and soaked CBR values ranged from 46%–86% and 22%–70%, respectively. According to approved and permissible National and International Highway and Transport Standards, only 10 samples met the unsoaked CBR requirement (≥ 80%), and 11 met the soaked requirement (≥ 30%). In conclusion, the majority of soils (68%) along the route are suitable for subgrade use, though localized weak zones require treatment.