Abstract:
An integrated geophysical investigation was carried out at some portions of Ogbomoso-Ilorin dual
carriage way that is located within the Basement Complex area of Southwestern Nigeria, with a
view of delineating the cause(s) of road pavement failure observed along the road. Three failed
portions and one control stable segment of the road pavement were studied. Very-low-frequency
(VLF) electromagnetic (EM) and ground magnetic measurements were obtained at a regular
station interval of 5 m along traverses established parallel to the road pavement. Vertical electrical
sounding (VES) using Schlumberger array and 2-D resistivity imaging using dipole-dipole
electrode array were employed for the electrical resistivity method.
The results of the VLF-EM and magnetic methods identified presence of near surface linear
features which are typical of fractures and fault zones within the unstable segments. The
geoelectric sections generally delineated four geoelectric layers. These include the topsoil with
resistivity values ranging from 73Ωm to 903Ωm and thickness values of 0.5m to 2.5m, weathered
layer with resistivity values that vary from 10Ωm to 340Ωm and thickness values ranging from
1.9m to 16.6m, partly weathered/fractured basement with resistivity values that range from 78Ωm
to 447Ωm and thickness values ranging from 2m to 6m and the fresh basement which characterized
by resistivity values that vary from 228Ωm to 3083Ωm and infinite thickness.
The near surface subsoil on which the road pavement is founded within the failed segments is
predominantly characterized by low resistive materials (generally <150Ωm) typical of
incompetent clay formation, series of resistivity highs and lows, undulating topography, a poor
drainage system and shallowness of the saturated zone to the subsoil on which the road pavement
is founded. Also, major geologic features which are typical of fractures, faults, joints and
cavities/voids were identified from the 2D Dipole-dipole sections beneath road pavement within
the unstable segments and classified stable segments. The control stable segment is however
generally characterized by near subsoil with relatively high resistivity values (>150Ωm) and a
locality that is characterized by relatively flat topography and well drainage system. Therefore, the
geological factors responsible for the susceptibility of the pavement to failure are clayey subgrade
soil beneath the road pavement, lateral inhomogeneity, near surface geological structures and
changes in elevations due to fluctuation in the saturated zones.
