The collapse of buildings is a re-occurring disaster in several Nigerian cities for many years, resulting in the loss of lives and properties. This explains the rationale for this project undertaken by the Nigerian Geological Survey Agency with the following major objectives and deliverables:
Produce engineering geological maps which can be used to select areas suitable for:
(i) Housing and industrial estate development etc.
(ii) Waste disposal, cemetery, pit latrines etc.
The methods of study involved field geological mapping, shallow borehole drilling/pitting and laboratory analysis. The field and laboratory geotechnical tests carried include SPT and particle size distribution, hydrometer analysis, Atterberg’s limits, compressibility, permeability, shearbox, PH and bulk density respectively.
The study area is within the latitudes 80 25’ – 80 39’E and longitudes 70 36’ – 70 50’N with a total area of about 625km2 and consists of Makurdi town at the center and parts of four (4Nos) Local Government Areas in Benue State (Makurdi South, Makurdi North, Gwer, and Gwer west Local Government Areas). The drainage system comprises of River Benue and many smaller rivers, and streams. The topography is slightly undulating and the elevation ranges from 70 to 163m above mean sea level. The climate is transitional savannah characterized by a long rainy season (April-October) and a short dry season (Nov. March). And rainfall ranges from 1120-1800mm
The entire project area is mainly underlain by the cretaceous Makurdi and Eze-Aku sand stone Formations.
The soils derived from these rocks were classified in to six (6) soil types according to the British Soil Classification System: sandy clay (CLS), lateritic gravelly clay (CIG), silty clay (CLM), sandy silt (MS) silty sand (SM) and Clay (CI).
The summary of the geotechnical properties is presented below in accordance to the various soil types:
Dark grey to grey, stiff to hard residual sandy clay (CLS).
Medium bearing capacity (150300 kN/m2) at shallow depth, very low compressibility (<0.05 m2/mN), low swelling potential (SP <0.5), intermediate plasticity and low permeability (10-5–10-7m/sec). Very good foundation material at shallow depth.
Dark grey to grey, soft to firm alluvial silty clay (CLM)
Low bearing capacity (qa<150kN/m2) at shallow depth, very low compressibility (<0.05 m2/mN), low swelling potential (SP<0.5), low plasticity and low permeability (10-5–10-7m/sec). Poor foundation material at shallow depth
Dark grey to grey, medium dense to dense residual sandy silt (MS) and silty sand (SM).
Medium bearing capacity (150<qa<300kN/m2) at shallow depth, non plastic and medium permeability (10-3–10-5m/sec). Good foundation material at shallow depth.
Dark grey to grey, stiff to hard residual clay (CI)
Medium bearing capacity (150<qa<300kNm2) at shallow depth, very low compressibility (<0.05 m2/mN), intermediate plasticity, low swelling potential (SP <0.5), and low permeability (10-5 – 10-7m/sec). Good foundation material at shallow depth. All the soils are slightly acidic (PH = 5 -6) and therefore do not pose any serious engineering, environmental or agricultural problems.
The maximum depth of investigation was 5.1m, bearing capacity was found to increase with depth from the depth of 2.5m in all the 16 drill holes, groundwater table was encountered in all the 80 pits and 16 drill holes and all the side walls of the pits and drill holes were not very stable.
The soil geotechnical properties (bearing capacities, permeability and soil types) together with the location co-ordinates were linked to Arc-view Geographic information system (GIS) soft wire to produce the following land-use maps:
i. Foundation soil map
ii. Soil bearing capacity map
iii. Soil permeability map The foundation soil map shows that the map is divided into six (6) portions according to the six (6) soil types: MS, SM, CI, CLS, CIG and CLM.
The bearing capacity map shows that the map is divided into 3 portions; low bearing capacity (qa>150kN/m2), medium bearing capacity (150<qa<300kN/m2) and high bearing capacity (qa>300kN/m2) portions.
The permeability map shows that the map is divided in to two portions: low permeability (10-5–10-7 m/sec) and medium permeability (10-3 – 10-5 m/sec) portions.
The soil foundation, bearing capacity and permeability maps can be used at the preliminary stage to locate suitable sites for housing/industrial estate developments, other lightly loaded civil engineering structures, waste disposal sites, cemetery, pit latrines etc.