Subsurface Soil Investigation for Urban Development: Implication on the Greater-Jos Master Plan, North-Central Nigeria

The stability of engineering structures fundamentally depends on understanding subsurface conditions, particularly in reclaimed mined-out areas where ground instability persists. This study investigates subsurface soil characteristics within the Greater-Jos Master Plan area to provide critical subsurface data for sustainable urban development. Integrated geological mapping, 75 vertical electrical soundings (VES), and geotechnical analysis of 38 soil samples were conducted. The area is underlain by Jos-Bukuru Complex rocks (predominantly biotite granites) with lateritised Older Basalts. Resistivity values ranged from 29–2802 Ωm for topsoil (0.5–8.8m thick) and 103.20–891.51 Ωm for weathered layers (1.90–46.60m thick), with overburden thickness varying from 1.2–87.9m. Geotechnical analysis revealed low to medium plasticity (plasticity index 7.4–21.1%), low to medium swelling potential, and variable shear strength parameters (friction angle 12–43°; cohesion 0–41 kN/m²). Based on integrated resistivity and geotechnical characterisation, the study area was delineated into low (2.6%), moderate (63.2%), and high (34.2%) competence zones. Consequently, the resistivity values are seen as strong determinants of the subsurface condition thereby characterising the study area into low, moderate and high competent zones. The geologic/lithologic classification of each layer based on this characterisation has shown that the clayey material encountered is incompetent for engineering purpose. The weathered layer and laterite are rated moderately competent while the fresh granite and compact laterite is rated highly competent. These findings provide essential baseline data for foundation design, land-use planning, and sustainable infrastructure development within the Greater Jos Master Plan area. It is recommended that Foundations should be designed to sit comfortably on competent bedrock or by employing suitable foundation methods such as piling to ensure stability of structures.