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Integrated geophysical study involving 2D resistivity survey was combined with geotechnical tests in order to gain an insight into the influence of the underlying subgrade soils on the failed Ajaokuta-Anyigba Highway. Resistivity data from twelve (12) 2D traverses using Dipole-Dipole Array were complemented with geotechnical tests on twenty-one (21) subgrade soil samples from the unstable and stable sections of the highway. The geophysical results revealed that the unstable and stable sections are underlain by a continuous stretch of subgrade soils with low (< 100 Ohms.m) and high resistivities (100 – 1000 Ohms.m) interpreted as clayey and silty-sand respectively. Results of Geotechnical tests showed that the subgrade soils of the unstable segment have higher fines (52 – 64%), higher liquid limit (21.3 – 46. 0), higher plasticity index (12.7 – 38.8), lower amount of sand (36 – 48 %) than subgrade soils of the stable section; fines (20 – 29 %), liquid limit (23.2 – 25.5), plasticity index (0.4 – 8.1%) and sand (71 – 80%). The subgrades of the unstable section classify as A-6 and A-7-6 clayey soils with poor rating while subgrades of stable section are A-2-4 silty-sands of excellent to good rating. Compaction test revealed that subgrade soils of the unstable sections have poorer compaction characteristics: Optimum Moisture Content, OMC (9.62 – 18.7%) and Maximum Dry Density, MDD (1.7 – 2.03 g/cm3) than the subgrades of the stable section; OMC (10.4 – 13.2%) and MDD (1.89 – 2.69 g/cm3). Unsoaked CBR (4 – 9%) and soaked CBR (2 – 4%) of the unstable section fell below the Federal Ministry of Works and Housing Standard while the stable section has higher strength with 15 – 17 % and 7 – 10 % at unsoaked and soaked states respectively. The study showed that the low resistivity and poor geotechnical properties of the subgrades are major geological contributors to the instability of the pavement.
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