Profiles of Problematic Soils and Spatial Distribution: Implication on Foundation Construction in Parts of Kosofe Lagos, Southwestern Nigeria.

Geotechnical data were complemented with geophysical investigation and employed to delineate problematic soils in parts of Kosofe Lagos, Southwestern Nigeria. The study area was chosen because of known issues regarding cracks in buildings and differential settlement of infrastructures founded on soils in the area. The aim is to generate profiles and maps of the spatial distribution of the subsurface soils to aid in foundation planning. Forty eight borehole logs and nine Vertical Electrical Soundings were compiled to delineate the different subsurface lithology which include peat, clay and sand. The results showed that the peat layer has maximum thickness of about 18.25 m but absent in some boreholes. This is underlain by clay unit with thickness ranging between 2.50-28.50 m. Sand unit constitute the third layer delineated with maximum thickness of 14 m. There is a general thickening of peat soils in the northern parts, especially around the streams in the area, which is instructive on the role of stream in the formation of the peat. The clay on the other hand is thickest around the northeastern and southeastern parts. The soil profiles generated reveal that the area is underlain by thick peat and clay having significant lateral, vertical variation and rapidly changing lithological facie over short distances. The extensive occurrence of these poor engineering soils calls for adequate engineering precaution in designs of building foundation

Non-Invasive Geophysical Investigation of Failure Along a Section of Ago-Iwoye Market Road, Ago-Iwoye, Southwestern Nigeria

A detailed geophysical investigation was carried outto determine ifthe subsurface geology is responsible for the failures along this road section. This study aims to imageand identifycompetent, moderately competent,and incompetent zones of the subsurface soiland thus to findout ifgeology is responsiblefor the failure along this section of the road.Electrical Resistivity Imaging (ERI) using the Wenner array 2D Imaging, and Vertical Electrical Sounding (VES) using the Schlumberger array were adopted for this survey. The geoelectric section from the VES revealed the presence of 3 lithological layers; topsoil, weathered layer (clay, sandyclay, clayey sand),and basement,which is also true for the electrical resistivity images revealed by the 2D electrical resistivity imaging. The results showed that the shallow subsurface beneath the road section is mostly underlain with clay thatis geotechnically incompetent andnot suitable for construction. The causes of road failure are believed to be theresult of the incompetent clayey topsoiland theweathered layer seen inthe profiles, which expand whenabsorbing water and shrink when drying, thuscausing instability beneath the pavement due tothe low shear strength and high compressibility,and also due to fractured basement rocks at shallow depths

Integrated Resistivity, Index, and Strength Characteristics of Subgrade Soils: Implication for Highway Pavement Failure Studies in North-Central Nigeria

Integrated geophysical and geotechnical studies have been carried out to determine the geological cause(s) of the failure of sections of Ajaokuta – Anyigba Highway, North-central Nigeria. Forty-eight (48) Vertical Electrical Soundings (VES) were conducted on failed and stable sections of the highway. Also, twenty-one (21) subgrade soil samples close to VES stations from the unstable and stable sections of the highway were subjected to laboratory geotechnical analyses which include grain size distribution, Atterberg limits, compaction (Optimum Moisture Content, OMC, and Maximum Dry Density, MDD) and California bearing ratio (CBR) at soaked and unsoaked states following American Society for Testing and Material (ASTM) standards as appropriate. The geophysical results show that low resistivity (10–100 Ohms-m) inferred as clay/silt of low competence characterizes the subgrade soils of the unstable segment. While higher resistivity (148–272 Ohms-m) interpreted as sandy-clay/silt with moderate competence was obtained for the subgrade soils of the stable segment. Results of Geotechnical tests show that the subgrade soils of the unstable segment have geotechnical properties that generally fall below required standard specifications. Strong correlations of R = 0.86, 0.9, and –0.88 were obtained between CBR and sand, resistivity, and the amounts of fines, while a fairly strong correlation of R = –0.67 was obtained for the plasticity index. The high level of correlation implies that CBR can be predicted from geophysical data and other geotechnical parameters. The study has revealed that the advanced weathering of the underlying Mica-Schist to clayey/silty subgrades with unsuitable geophysical and geotechnical properties is a major contributor to the instability of the highway