Comparative Studies of Subsurface Layers’ Competence Evaluation using TOPSIS and AHP Models at Ilaramokin, Southwestern Nigeria

Abstract

The incessant collapse of buildings associated with geotechnical incompetence in different parts of Nigeria and the rapid growth of  Ilaramokin town near Akure Southwestern Nigeria motivated this work. Two multi-criteria decision analysis approaches were used in  integrating geoelectric parameters (topsoil resistivity, weathered layer resistivity and bedrock resistivity), static water level measurements  and geology in evaluating the subsurface geotechnical competence of Ilaramokin. Thirty (30) vertical electrical sounding,  eighty-six (86) static water level measurements and geological maps were used. The vertical electrical sounding (VES) results delineated  three to five geoelectric layers which correspond to four geologic layers namely; the topsoil, weathered layer, partially weathered/ fractured basement and presumed bedrock. The resistivity of the geologic layers varies respectively from 48 - 701, 31 - 1065, 14 - 139, 132  - 6582 Ωm, while their thickness varies from 0.4 - 4.1, 1.3 - 11.6 and 4.0 - 20.1 m in the three upper layers respectively. The VES results were  presented as topsoil, weathered layer and bedrock resistivity maps. The VES results, static water level measurement and geology were integrated using both the Technique for the Order of Prioritization by Similarity to Ideal Solution (TOPSIS) and Analytic Hierarchy  Process (AHP) models to produce geotechnical competence maps. Consistency test and grain size analysis were carried out on 10 soil  samples obtained across the area to validate the geotechnical competence model maps produced using both TOPSIS and AHP models.  The validation showed that the geotechnical model map produced from the TOPSIS model has a higher percentage (90%) of correlation  with consistency tests and grain size analysis compared to that of the AHP model (70%).