Geotechnical Subsurface Characterization and Foundation Design Recommendations for a Proposed Six-Story Structure in Port Harcourt, Nigeria

Abstract

The influence of groundwater on foundation performance, poor soil characteristics, and insufficient subsurface foundation depth, which can lead to structural settlement, are some of the factors responsible for foundation failure. This work presents a geotechnical investigation carried out in part of Port Harcourt, Nigeria, to assess subsurface conditions for the establishment of a multi-story structure. In situ field and laboratory investigations, including geotechnical and geochemical analyses, were carried out on soil and water samples collected from the proposed construction site. Geotechnical laboratory analysis included natural moisture content, Atterberg consistency limits, and particle size distribution to classify the soils. Seven boreholes were advanced to a depth of 30 m employing the rotary boring method, with groundwater conditions examined. During and after the drilling exercise. Standard Penetration Tests (SPTs) were executed at uniform spacing to measure soil resistance through N-values documented on borehole logs. Results showed grey sandy clay of intermediate plasticity overlying medium- to coarse-grained dense sand. The soil pH values ranged between 5.1 and 7, suggesting neutral to slightly acidic soil conditions that may influence long-term structural integrity. At a shallow subsurface depth of 1.5 m, estimated settlement of 36.4 mm indicates potential instability. Subject to heavy structural loads. Resultantly, a piled foundation extending to Dense sand layers below 30 m is advised, substantiated by pile load testing for design validation.