Geophysical Assessment of Two Collapsed and Concealed Septic Tanks at a Residential Site in Zaria Area, North-Western Nigeria

Gn Egwuonwu,So Ibe,Ib Osazuwa

doi:10.4314/fje.v6i1.68324

Abstract

A geophysical study was carried out in the vicinity of two collapsed and concealed septic tanks to delineate the structures responsible for their failure. Two-dimensional resistivity and seismic refraction tomography data were collected along a profile laid across the septic tanks. The data were collected using ABEM Terrameter SAS 4000, aided with LUND imaging equipment and seismograph, Terralloc MK6 respectively. The data were processed using REFLEX-W and RES2DINV software respectively. Pseudosections obtained show that the vicinity of the septic tanks is characterized by resistivity of 10–5200 Ωm and p-wave velocity of 320–2400 ms-1. Results show that these ranges encompass those of clay and sandy clay at shallow depths, and granite and gneiss in a shallow and undulating basement. The zones where the tanks were apparently sited show very low p-wave velocity of < 600 ms-1 and on the contrary, very high electrical resistivity of >5000 Ωm which suggest the presence of trapped air column. Based on the interpreted results, the tanks are estimated to be with lateral extent of about 6.5 m and depth of about 2.9 m. The delineated clayey soils at shallow depths suggest that there could be seasonal soil swelling and shrinkages due to seasonal variation in moisture content of the clay. These most likely led to annual ground movements, cumulative soil creep and the subsequent collapse of the septic tanks.

Highlights

Empty, filled or buried sinkholes and tunnels, whether natural or artificial pose a geohazard to buildings, highway, bridges and dam construction as they may be subject to subsidence, soil piping and erosion
The results suggest that the tanks most likely have average lateral extent of about 6.5 m and average depth of about 2.9 m
Integrated geophysical techniques involving two dimensional electrical resistivity and seismic tomography were used for the study

Summary

Introduction

Empty, filled or buried sinkholes and tunnels, whether natural or artificial pose a geohazard to buildings, highway, bridges and dam construction as they may be subject to subsidence, soil piping and erosion. These features are often filled or obscured at the surface, requiring the use of geophysical techniques or remote sensing for their assessment. Buildings, landfills, unregulated dump sites, waste lagoons and septic systems have often been sited within or above unstable soils and rocks In some places such as karstic environments, they may be ignorantly sited where cavities or sinkholes containing soil pipes exist. Some important structures in most cities of the world are currently at high risk of collapse due to the presence of both natural and artificially buried structures at shallow depths in the subsurface (Tejero et al, 2002)

Results

Discussion

Conclusion