Geotechnical Mapping of Owerri and environs

Reported cases of collapsed buildings have become a common feature of our daily news. The failure of these infrastructures has resulted to huge losses in revenue and human lives. \r\n\r\nThis project which covers an area of 625km2 encompasses Owerri and environs. \r\n\r\nThe objective of this project was to investigate the subsoil in order to determine its engineering properties, its suitability as foundation for industrial and residential houses. Additionally, it aimed at sourcing suitable building materials for road embankments, earth dams and leeves, and to select and design suitable shallow foundations of structures. The causes of development of cracks and collapse of buildings will also be determined and possible preventive measures proferred.\r\n\r\nSeventy-one (71) test-pit bulk samples were taken for the determination of California Bearing Ratio (CBR), Atterberg Limits, and grain size distribution. Sixteen (16) boreholes were drilled for Standard Penetration test (SPT) and Ten (10) Cone Penetrometer Tests (CPT) were carried out. \r\n\r\nGeophysical method was also employed to assess the presence of any incompetent layer like clay, which could expand or shrink due to application or removal of moisture. Unacceptable degree of expansion or shrinkage affects the foundation, thereby causing the development of cracks. \r\n\r\nResults of laboratory analyses show that the soils can be classified as clayey gravel (GC), clayey sand (SC) and silty sand(SM). They are found to be highly permeable.\r\nTriaxial test results show that the soils have cohesion values ranging from 20KN/m2 in borehole 6, to 75 KN/m2 in borehole 9.\r\nCompaction test recorded maximum dry density (MDD) values ranging from 1.64 Mg/m3 to 1.75 Mg/m3 in boreholes 2 and (1,3,7,8) respectively.\r\nSoil strengths of 301KN/m2 to 621KN/m2 were recorded in boreholes 6 and 12 respectively. The presence of gravels imparted high strengths, low compressibility, poor consolidation, though the dominance of clayey sands and fines lowers the bearing capacity. The predominance of the fines is also responsible for the degree of instability witnessed. Their California Bearing Ratio (CBR) values are low, from 3 to 6, requiring that soil-improvement measures would be necessary for stable structures.\r\n\r\nThe soils are erodible, excavatable, very loose to loose, dispersive, and moderately plastic, thereby leading to significant deformation under load. Inundation caused critical changes in soil strength, consolidation characteristics, and bearing capacity. The influence of these changes manifested in somewhat excessive settlement and foundation failure. No serious slope instability problem existed.\r\nDynamic rollers used by road construction companies generated vibrations through the foundation, thus increasing the stresses in the ground. As the stresses exceed the soil strength, failure in form of cracks occurs.\r\n\r\nStable foundations could be achieved through compaction of the material prior to construction, and by increasing foundation width and depth. Reinforcing steel can also be added to foundation, while soil improvement measures are adopted.\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, Imo State, Owerri

Reference system identifier: 4326

Distribution format

Name	Version
.doc

OnLine resource

Protocol	Linkage	Name
		Geotechnical Mapping of Owerri and environs

Statement: Inserted in the reports are the following maps (1:50,000): Geological map, Structural map, Landform map, Potential Flood Prone Zones map, and Housing Suitability map.