Geotechnical Mapping of Sokoto and environs

The project area is bounded by Latitudes 12 57' and 13º 11'N and Longitudes 5º 08' and 5º 22'E. It measures approximately 625km2 in area.\r\n\r\nGeologically, Sokoto and its environs fall within the sedimentary formations of the southeastern extension of the Illummeden Basin which has its center 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.\r\n\r\nThe 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 gridded, 82 test pits were dug each to a depth of 1.5m and the soil profiles studied and documented. Sixteen (16) boreholes were drilled and Standard Penetration Tests recorded at predetermined depths. Dutch Cone Penetrometer Tests were also carried out close to each of the 16 boreholes.\r\n\r\nThe 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.\r\n(ii) Lateritic overburden can be as thick as 3 to 5m in some localities in the project area.\r\n(iii) Stiff platy grey shale or marly and crystalline limestone may underlie the lateritic overburden in most places.\r\n(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).\r\n(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. \r\n\r\nThe 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.\r\n\r\nThe 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.\r\nOn the Unified Soil Classification System (USCS) all the soil samples classified as CL.\r\nOn 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.\r\nThe 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).\r\nThe 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%. \r\nThe consolidation test results showed a material of low compressibility. From the tests carried out, 50% consolidation occurred within 4 minutes 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.\r\nBearing capacity values were computed based on the Standard Penetration Test (SPT), Cone Penetration Test (CPT) and laboratory Triaxial Compression test. .\r\nResults 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 that 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.\r\nBearing 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.dpths.\r\nIn 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%. \r\nBoth 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.\r\nTo check the possible adverse effect of swelling and shrinkage by the soil, Iron wire gauze should be laid in the foundation before blinding with a rich dense mix of Portland cement concrete.\r\n\r\n

Date (Publication): 2007

Presentation form: Digital document

Status: Completed

Point of contact

Organisation name	Individual name	Role
		Author
A&EG, NGSA		Publisher
A&EG, NGSA		Custodian
A&EG, NGSA		Point of contact
	Director, A&EG	Owner

Keywords

Soil Strength, Soil Permeability, Soil Type, Slope Stability, Landform, Building Collapse

Other constraints: Internal only

Denominator: 50000

Language: English

Topic category

Description: Nigeria, Sokoto State, Sokoto

Reference system identifier: 4326

Distribution format

Name	Version
.doc

OnLine resource

Protocol	Linkage	Name
		Geotechnical Mapping of Sokoto and environs

Statement: Inserted in the reports are the following maps (1:50,000): Location map, Geological map, Housing Suitability map, and Dumpsite Suitability map.