A Proposed Dam Site Investigation Using Integrated Geophysical And Geotechnical Methods; A Case Study Of Loko Area, Northeastern Nigeria

Abstract

The need for dam construction in the Loko area of northeastern Nigeria becomes necessary due to consistent flooding in the area and the demand for water for agricultural and domestic purposes. It is therefore very important to carryout adequate pre-construction investigations aimed at investigating the suitability of the area for dam siting. Geophysical and geotechnical investigation was carried out along River Loko. Detailed geologic mapping carried out showed that the area is 75% covered with alluvium while basalts and fine – medium grained granites underly the smaller portion of the area. A total of 10 Vertical Electrical Soundings were carried out using Schlumberger array. Disturbed soil samples were collected at a depth interval of 0.3m, from the surface to 1.2m, from three trial pits. Results of the geophysical survey show a 3 – 4-layer model consisting of H, HA, AH-type curves, interpreted with Interpex IX1D software. Results of the particle size distribution show that most of the samples are very uniform having a coefficient of uniformity of less than 5 according to (AASHTO) classification all the soil samples collected at the three trial pits are specifically A-3 soil group, which indicates that the material are fine sands. The Atterberg limit results indicate that most of the soil samples fall within the medium plasticity index. Soils with high plasticity (22.35) and medium plasticity (9) are usually semi-impervious to impervious, hence, suitable for dam foundation. The result of the unsoaked CBR compared with the standard, according to AASHTO, indicates that all samples fall under 7-20 which shows a general rating as ‘fairly soil’ and can be used as ‘sub-base’. Compaction results show that for Pit 1, the sample at 0.3m, with MDD 1700kg/m³ at OMC 20.0%, is the best sample. For Pit 2, the sample at 0.9m, with MDD 2400kg/m³ at OMC 10.0%, is the best, and for Pit 3, the sample at 0.3m, with MDD 1800kg/m³ at OMC 11.0%, is the best. The interpretations of the undrained shear strength result indicate that all the samples have ‘soft consistency’ since their values range from 23.0 to 25.7 KN/m². Based on the availability of construction material, perennial discharge from the river in addition to suitable geology and engineering properties, the area is feasible for an earth dam construction. Deep foundation is recommended for a massive rock filled dam, and some remediation is required in the design process to reduce the threat posed by thin fractures which may possibly cause subsurface seepage.