The geotechnical/ engineering geological mapping of Sokoto and Its environs is a positive step of the Nigerian Geological Survey Agency to attempt to address the present problems of foundation and other structural failures rampant in the burgeoning Nigerian cities. It will also generate data bank of the geotechnical properties of soils in all the big towns in Nigeria for future land use planning and expansion. The project area is enclosed by Latitudes 12̊ 57′ and 13º 11’N and Longitudes 5º 08′ and 5º 22’E. It measures approximately about 25km by25km i.e. about 625km2 in area.
Geologically, Sokoto and its environs fall within the sedimentary formations of the South Eastern extension of the Illumeden Basin which has its centre in the Niger Republic.The project area is underlain by the Eocene Gwandu Formatiion, the Palaeocene Kalambaina and Dange Formations, the Maastichtian Wurno, and Maastrchtian/Lower Cretaceous Wurno/Taloka Formations.
The geotechnical mapping focused primarily on the geotechnical properties of the soils derived from these geological formations down to the depth of 10m. Out of the 100 cells to which the area was grided, 82 test pits were dug each to a depth of 1.5m and the soil profiles studied and documented. Sixteen (16) Bore Holes were drilled and Standard Penetration Tests recorded at predetermined depths. Dutch Cone Penetrometer Tests were also carried out close to each of the 16 bore holes.
Two Engineering Services Companies – Kabia Laboratories Ltd based in Abuja and ADIBEKS Engineering Services based in Kaduna drilled the bore holes and also analysed the soil samples from the project area.
The results of the field soil profile study and the laboratory tests have indicated that:
(I) The excavability of soils in some parts of the project area can be quite difficult because of the lateritic boulders and cobbles within the soil.
(II) Lateritic overburden can be as thick as 3 to 5m in some localities in the project area.
(III) Stiff platy grey shale or marly and crystalline limestone may underlie the lateritic overburden in most places.
(IV) From the ground level (0m) to 0.5m, the soil can be moist, grayish brown with rootlets and worm burrows. Down to 1.5m, the soil becomes greyish, compact, with firm plastic clay and a lot of phosphatic nodules (characteristic of soils derived from the Kalambaina and Dange Formations).
(V) For soils derived from the Wurno/Taloka Formation, the soil varies from yellowish brown at the top to moist, reddish brown, intact,cohesive homogenous sandy clay down to 1.5m in the trial pits.
Two types of soil samples were collected for laboratory analyses:
Bulk Test pit samples (collected at 1.5m) and standard penetration test samples (SPT) (collected at specified depths) down the bore hole.
The acidic/alkaline tests performed on the 48 test pit samples in the field, indicate that near surface soils are slightly acidic but more alkaline down into the ground.
The results of Grain size distribution and Atterberg Limits tests used in soil classification showed that under the American Association of State Highway and Transportation Officials (AASHTO) the soils in the area classify as A-2-4, A-4, A-7-5, and A-7-6.
On the Unified Soil Classification System (USCS) all the soil samples classified as CL.
On the Modified Plasticity Chart for use with USCS,all the soil samples plotted above the ‘A’-Line and left of the 50% Liquid Limit line.
The bulk density of the sub-soil ranges from 1.30g/cm3 (BH 26) to 2.23g/cm3 (BHs 95,97) whereas their natural moisture content varies from 2.0% (BHs 22,95,97) to 23% (BH 26 @ 1.5m).
The maximum dry density of the compacted soil varies from 1.41g/cm3 to 2.0g/cm3 while the optimum moisture content(omc) varies from 11.0% to 28.0%
The consolidation test results showed a material of LOW compressibility. From the tests carried out, 50% consolidation occurred within 4minutes indicating immediate settlement upon application of load which is due to the fine nature of the dominantly silty sandy material.
Linear shrinkage test results showed a zero shrinkage for the dominantly silty sand and a shrinkage of 1.35% to 12.6% for the silty clay materials.
Bearing capacity values were computed based on the Standard Penetration Test (SPT), Cone Penetration Test (CPT) and laboratory Triaxial Compression test.
Results from SPT tests at Bore Hole locations IFS 9, 22, 50, 63 and 97 gave N-values of between 1-5, 3-48, 8-78, and 9-100 with corresponding calculated corrected N-values (N1) of between 1-15, 3-32, 8-47 and 16-58 for 0.5m, 1.5m, 3.0m and 4.5m depths respectively. All Boreholes mentioned above terminated at 4.5m depth, except Boreholes IFS 26 which terminated at 6.0m and IFS 97 that ended at 7.5m depths. Bearing capacity values for all the Boreholes ranged between 56KN/m2 and above 500KN/m2 at between 0.5m – 7.5m depth.
Bearing capacity computation based on the laboratory. Triaxial Compression Test gave values of between 180KN/m2 to 705KN/m2 at 1.5m depths and between 356KN/m2 to 1114 KN/m2 at 3.0m depths.
In the Cone Penentrometer Test, the allowable Bearing Capacity ranged from 29KN/m2 (BH 2) to 400KN/m2 with average value of 83.9KN/m2
The results of the California Bearing Ratio indicate that soils classified as A-2-4 gave CBR of as much as 24%, but A-7-5 and A-7-6 soils have CBR values of between 3 and 5%.
In designing for a building, the bearing capacity of 29KN/m2 the CPT will be employed for shallow foundations. i.e Strip Footing.
Both the field in-situ and laboratory tests have shown that the soils of the project area conform to standards for good sub-base and sub-grade materials for highway construction.
For the construction of buildings, the foundation depths (Df) should not be less than 1.5m below finished ground level. But for high rise or multistory building requiring pressures in excess of those recommended for strip/square footing, raft footing may be employed. To check the possible adverse effect of swelling and shrinkage by the soil / underlying shale, iron wire guaze should be laid in the foundation before blinding with a rich dense mix of Portland cement concrete.