Canal structure subsidence investigation using ground penetrating radar and geotechnical techniques

Abstract

Channel structure settlement along a canal in Lagos, southwestern Nigeria, was investigated using the ground penetrating radar (GPR) and geotechnical techniques with a view to establishing the cause(s) of the structure failure. GPR probing was carried out along eight parallel traverses in the E–W direction using the GSSI SIR-3000 200 MHz monostatic antenna. Geotechnical investigations involving sieve analysis, Atterberg limits, moisture content, organic content and consolidation test were carried out for soil classification and competence test. X-ray diffraction (XRD) studies were conducted to determine clay mineralogy of the peaty soil. The GPR data were processed using Radpro software. Four lithological layers consisting the topsoil (sand), peat, clay and sandy clay were delineated from the processed GPR data. Peat occurred at shallow depth to depths deeper than the foundation depth of the engineering structure investigated. The geotechnical results revealed that the peat has 125–167 % moisture content, high liquid limit and plastic index values of 109–174 % and 60–100 % respectively and organic content of 5.12 %. The sieve analysis showed that the peaty soil is fine grained having up to 98 % passing at 0.004 mm sieve opening exhibiting high compressibility and low consolidation coefficients. This peat was classified as organic clay with peat using the unified soil classification system. The XRD analyses showed the dominance of kaolinite, illite and quartz minerals in the peaty soil. This study revealed that the peaty soil underlying the investigated area has geologic and engineering properties not suitable to sustain the canal construction.