The geotechnical and engineering geological investigation of Owerri and its environs had been studied. The project covered an area of 625km2 encompasses Owerri and environs. The focus of the research was to investigate the subsoil in order to determine its engineering properties, its suitability as foundation for industrial and residential houses. It was also aims at sourcing suitable building materials for road embankments, earth dams and leeves and to select and design suitable shallow foundations of structures.
The study involved collection of seventy (70) bulk soil samples from test pit soil at a depth of 1.5m for determination of California Bearing Ratio (CBR), Atterberg Limits and grain size distribution test. Standard Penetration test (SPT) test was done on sixteen (16) boreholes to a depth of 10m and undisturbed samples collected at depths of 1.5m, 4.5m and 7.5m in each hole for Standard Penetration test (SPT) for the determination of the bearing capacity of the soil from triaxial test.
The result of the California Bearing Ratio (C.B.R) for the soaked samples showed that the values varies from 3% to 6%. These values fall below the Nigerian General Specification for Roads and Bridges (1997) that states that soil for sub-grade, sub-base and base material should not be less than 15%, 30% and 80% respectively. Therefore, the soils in the study area are not suitable for sub-grade, sub-base and base materials, unless the soil is improved through soil improvement measures.
The result of the Atterberg Limits and grain size distribution of the soil samples showed that ninety-three (93%) of the samples belong to the clayey sand of low plasticity (SC) class, while three (3%) and three (3%) respectively belongs to the sandy clay of low plasticity (CLS) and silty sand of low plasticity (SM) class based on Unified Soil classification scheme. The result also showed that the samples contain relatively high amount of fines, which can contribute to some level of instability of the soil. In terms of strength, these soils have medium strength that can support housing estate development. Improved stable foundations on these soils can 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 can be adopted.
The result of the permeability test from sieve analysis showed that the coefficient of permeability of the soils varies from 8.1 x 10-5 to 1 x 10-4 cm/s. These values indicate that all the soil in the study area falls within the medium coefficient of permeability class of Terzaghi and Peck, 1967, Kulhawy and Mayne, 1990 and Ele Engineering Geology classification. These values indicate that the soil have good drainage characteristics. These soils are not be suitable for construction of sanitary landfill, core of earth dam, recharge pit, location of underground mines or good site for burial ground, because of their permeability characteristics. However, they are generally suitable for the shell of an earth dam.
The bearing capacity values of the soils vary from 300.75 KN/m2 to 664.89 KN/m2. Ninety-one (91%) and ten (10%) of the soil samples respectively fall within the high and medium bearing capacity of the Nigerian General Specification for Roads and Bridges (1997). Therefore, majority of the soil in study area are suitable for housing estate development in terms of their bearing capacity.
However, majority of the soil are erodible, excavatable, very loose to loose, friable, dispersive and poorly consolidated. These characteristics made them to be susceptible to flooding and erosion.