Application of 2D Electrical Resistivity Surveys and Geotechnical Studies for Pavement Failure Evaluation along Ajaokuta-Anyigba Highway, North-Central, Nigeria

Abstract

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.