GEOTECHNICAL INVESTIGATION OF BLACK COTTON SOIL AROUND DEBA AREA, NORTH-EASTERN NIGERIA

Abstract

This geological and geotechnical study was carried out to investigate the engineering properties
of black cotton soil around Gombe – Deba area and environs. Geological field survey was
carried out around the study area and geotechnical laboratory analysis comprising of Atterbergs
limits, linear shrinkage, natural moisture content, CBR, UCS, and compaction test was
conducted on four different soil samples collected from the field. The study area is underlain by
the Pindiga and Gombe formations and the main lithological units are limestone, shale,
sandstone with large superficial deposits of Black cotton soil covering the study area. The
results of the geotechnical analysis revealed low unconfined compressive strength value of
116.6KN/m2 as the maximum of the four samples analyzed, CBR values for the soaked test are
3%, 4.6%, 4.5% and 4.3% and the unsoaked values of 9.1%, 8.5%, 8.8% and 7.5% for pits no.
1,2, 3, and 4 respectively. The OMC values of 24.03% and 19% with corresponding MDD
of1.33mg/m3 and 1.53mg/m3 were obtained as the maximum and minimum values respectively.
The soil of the study area show a high liquid limit with an average value of 62% and high
plasticity index having average of 35%. These results indicates that the black cotton soil of the
area fall within the CH group and A-7-6 group and are considered as weak materials in
engineering construction. In addition these soils are highly expansive; hence need to be suitably
stabilized to upgrade the engineering performance of the soils.