Geotechnical Mapping of Maiduguri and its Environs

The project area is bounded by latitudes 110 41’ 30” to 120 00’ 00”N and longitudes 130 00’ 00” to 130 44”E and covers an area of 1,118 Km2. The area covers three local governments; Maiduguri, Jere and Konduga local government areas. Maiduguri town, the capital of Borno State is at the centre. The topography of the area is virtually flatlying, with visibility possible to several kilometers. It has an average elevation of 315m above sea level.

The area is within the semi-arid climate region and temperature varies from 25 to 360c seasonally. The area is characterized by a long dry season (October-May) and short rainy season (November-April) and the rainfall varies from 330 to 1020mm. The area is Generally drained by River Ngadda which has its sources from southern Borno.

The project area falls within the south-western part of the Chad Basin and is wholly underlain by Chad formation, a sequence of lacustrine and fluviatile clays and sands of pleistocene age. The top soils covering the study area within the depth range of 0-1m consist of very loose to loose slightly clayey silts covering 49% (548 Km2) of the area, very loose to loose slightly sandy silts covering 31% (346 Km2 ) of the area, soft silty clays covering 12% (134 Km2) of the area and firm to stiff slightly silty clays covering 8% (89 Km2) of the area.

Four different soil types between the depths of 1 and 2 m covering Maiduguri and its environs, which were derived from the Chad formation, have been identified and mapped because of their importance as foundation soils for light-loaded structures and as a link between the top soils and the soils at depth. The four soils mapped consist of medium dense clayey silts covering about 76% (850 Km2) of the area, firm silty clays covering about 12% (134 Km2) of the area, stiff slightly silty clays covering about 8% (89 Km2) of the area and medium dense silty sand covering 4% (45 Km2) of the area.

Five geotechnical factors were evaluated for the project area; soil bearing capacity, soil expansivity, soil compressibility, soil permeability, and soil acidity.

The clayey silts, the silty sands and the silty clays are of low bearing capacities at shallow depth (<1m) and high bearing capacities at depth (>1m); while the slightly silty clays have high bearing capacities even at shallow depth. This makes the silty sands, clayey silts, and silty clays bad founding materials at shallow depth (<1m) but good founding materials at depth (>1m). The slightly silty clays are good founding materials even at shallow depth because of their high bearing capacities, low compressibility and low swelling potential.

The silty and slightly silty clays of this area are of low swelling potential and inactive. The silty clays are of high compressiblities at shallow depth (<1m) and low to very low compressibilities at depth (>1m); while the slightly silty clays have low to very low compressiblity even at shallow depth.

The high compressibility of the silty clays at shallow depth is responsible for the structural and architectural distress of many residential buildings at Bulunkutu Abuja area of Maiduguri town. These occurred due to excessive differential settlements of the buidings in the silty clays.

The low to very low permeability of the clayey silts, silty clays and slightly silty clays make the soils very much suitable for sitting waste disposal, cemetery and pit latrine sites, while the medium permeability of the silty sands makes the soil not suitable.

The soils in the study area are slightly acidic with a pH range of 5-7. The acidity is not strong enough to cause any environmental, agricultural and engineering problems. Therefore, ordinary Portland cement could be used for foundations in the soils.

The percentage area of coverage, which is 8×10-6 %, is too small to give realistic values of the engineering properties of the soils in the area considering the heterogeneous nature of soils. However, the properties being presented can be used as first approximation of the likely values of the engineering properties of the soils tested. This will give a general over-view of the engineering properties of the soils in the area investigated. For specific sites, comprehensive site investigations must be carried out.