ROAD INSTABILITY IN ONDO SOUTHERN SENATORIAL DISTRICT: A GEOLOGICAL AND CIVIL ENGINEERING APPROACH

Ondo Southern senatorial district covers areas underlain by loosely consolidated sand. This has led to development of gullies that starts as mere cracks which are mainly initiated by heavy rainfalls. Slope /mass movement occurs in areas of relatively steep topographic slopes underlain by unstable materials. Slides are known to result from high concentrations of soil moisture that lubricate the surface materials leading to increase in pore pressure and reduction in shear strength.

The soil mineralogical contents also determine the level of stability of the road.

The soils in these areas contain significant amount of smectite, kaolinite, illite and chlorite with the amount of smectite and Kaolinite increasing with depth while illite decrease with depth.

Swelling or expansive clays are a major geological and civil hazard causing extensive damage to infrastructures worldwide every year. The smectite family of clay minerals which includes the notorious montmorillonite, has the most swelling potential and accounts for most of the damage resulting from expansive clays. Smectite have an expanding structure which increases colloidal activity due to a significant increment of specific surface. All of the structure’s surfaces including inter-layers are available to hydration and cationic exchange. These characteristics give montmorillonite the capacity to swell between layers due to hydration.

The Atterberg limit tests were carried out on some samples from some locations. Further particle size and compaction test were made to access the geo-pedologic and hydrologic causes of road instability in the area. The plasticity Index revealed that the soils are non-cohesive and non –plastic because it ranged from 1.2% to 5.2%. Sieve analysis indicates that the soils from these sites are within the medium to coarse grain range with low percentages of silt/clay, therefore the soil is non-plastic. The compaction test also shows that the optimum moisture content ranges from 13.5% to 15.2%. While the maximum dry density ranges from 1.74mg/m³ to 1. 98mg/m’. The maximum dry density values are generally low which indicate that the soil is not compact but loose.