Building collapses have become a recurring incidence in many parts of Nigeria.In view of contributing to the forestalling of future occurrences, the Nigerian Geological Survey Agency (NGSA) as part of its statutory role had embarked on a systematic Geotechnical mapping of the major cities in Nigeria. This is aimed at providing relevant geotechnical information that would aid in mitigating these problems.The project area, Benin City and its environs, is the capital of Edo State located in the south-southern part of Nigeria. The project area is defined by longitudes 005º 30′ and 005º 45′E and latitudes 6º 15′ and 6º 30′N with a total area of approximately 625km2. The area lies within 1:50,000 Sheet 298Benin City N.W. The area cuts across four Local Government Areas namely Oredo, Egor, Ikpoba Oka and part of Uhunmwonde. The main town in the area is Benin City. Other settlements include IkhueNiro, Ute, Obanyator, Etete, Utoka,Ahor, etc.
The study area is underlain by the coastal plains sands of the Niger Delta region of the country.The project primarily involved soil sampling at various depths within a total area coverage of 625km2. Soil samples were collected from pits dug to 1m and from boreholes drilled to 6m.
Samples collected were analysed in the labaoratry for soil acidity, particle size distribution, Atterberg limits, Triaxial Compression. In-situ Standard Penetration testing was carried out in boreholes drilled at sixteen locations. Samples were collected from the boreholes at 1.5m intervals for Consolidation test.
Percentage fines ranged from 12.10% to 57.50%. Soils were classified as clayey, silty sand. Liquid limit ranged from 21.50% to 47.50%, Plasticity Index ranged from 3.40 to 24.20 while Shrinkage Limit ranged from 7.70 to 15.30. The soils showed no swelling potential and are highly erodible. The soil pH ranged from 4.36 to 5.42, Specific gravity ranged from 2.61 to 2.76 and dry unit weight ranged between 417.40g to 473.50g. Permeability ranged from 6.82 x 10-7to 4.80 x 10-4 which is very low.Maximum Dry Density ranged from 1756kg/m3 to 2078kg/m3. Shear strength ranged from 91.3 kPa to 170 kPa. Areas suitable for location of waste disposal sites covers about 38% and about 25% is suitable for development of housing estates.