Geoelectrical Characterization and Volumetric Assessment of Rockfill in Swampy Terrains: A Case Study of the Moyamba Junction to Moyamba Town Highway, Sierra Leone
Abstract
Abdul A. Koroma, Phodie M. Kamara and Michael Kingsley Afful
This study presents a comprehensive geoelectrical investigation of rockfill quality and volume along the newly constructed highway from Moyamba Junction to Moyamba Town, Sierra Leone. The primary objective was to verify the structural integrity and compute the precise volume of rockfill deposited at 14 critical locations, including swamp crossings, box culvert beddings, and weak soil segments. Utilizing the ASTM G57-06 Wenner Four-Electrode method, Vertical Electrical Soundings (VES) were conducted to delineate the subsurface resistivity profiles. Field resistance data were processed using EarthImager 1D inversion software, yielding layered models with high reliability (predominant RMS errors < 5%). The results reveal a consistent high- resistivity signature for the rockfill, ranging between 1000 Ωm and 2000 Ωm, which is diagnostic of high-strength, unweathered crystalline rock. A sharp geoelectrical contrast was observed at the interface with the underlying native subgrade, which exhibited significantly lower resistivity (<200 Ωm) typical of saturated swampy clays. By integrating interpreted depths with surface dimensions, the total volume of engineered rockfill was determined to be 45,404.50 m3 . These findings confirm that the rockfill effectively stabilizes the highway’s vulnerable segments and provides the Sierra Leone Roads Authority (SLRA) with a verifiable, data-driven metric for project auditing and long-term structural monitoring.