Application of TOPSIS and AHP models in subsurface layers’ competence evaluation - case study of Ilaramokin, Southwestern Nigeria

Abstract

Incessant collapse of buildings associated with geotechnical incompetence in different part of Nigeria and the rapid growth of Ilaramokin town near Akure Southwestern Nigeria provided the motivation for 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 measurement and geological map were used. The VES results were presented as topsoil, weathered layer and bedrock resistivity maps. The vertical electrical sounding (VES) results delineated three to five geoelectrical layers which are the A, H, K, KH, QH and HKH. The resistivity of the topsoil, weathered layer and bedrock varies from 48–701, 31–1065, 14–139, 132–6582 Ωm respectively while their thickness very from 0.4–4.1, 1.3–11.6 and 4.0–20.1 m in the three upper layers respectively. The VES results, static water level measurement and the geology were integrated using both TOPSIS and AHP models to produced geotechnical competence maps. Consistency test and grainsize 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 geotechnical model map produced from TOPSIS model have higher percentage (90%) of correlation with consistency tests and grain size analysis compared to that of AHP model (70%).

Relationship of Resistivity Value with the Ground Material Information Obtained from Borehole Data: Case study

“…Resistivity values alone may not fully explain or interpret data in a geotechnical perspective, especially in areas with weathered materials. This is because resistivity can still be high in immersed soil or rock, similar to unsaturated areas, and this can be attributed to various factors (Adedotun et al, 2022). It is important to note that resistivity and SPT-N values are different parameters that provide information about different aspects of the subsurface.…”

“…The resistivity of each layer, including topsoil, weathered rock, and bedrock, is relevant in assessing the geotechnical competence of the layers. In general, higher resistivity values indicate better geotechnical competence (Adedotun et al, 2022). It is also important to understand how resistivity and soil suction interact when subjected to seasonally varying water content.…”