f Integrated Geophysical Tomography Around Buildings Having Foundation Defects At Some Residential Sites In Zaria Area, Northwestern Nigeria

Integrated geophysical imaging technique has been applied in the detailed investigation of foundation of buildings in order to identify the geologic formations and mechanisms of rocks and soils causative to the identified defects. The study was also aimed at the estimation of the dependent elastic constants of the subsurface formations which is useful to engineers. The study area is part of the Zaria Batholith which comprises series of granites that intruded the country rock gneisses. It falls within the Nigeria Basement Complex which is underlain by Precambrian rocks at the elevation of about 670 m above the mean sea level. It has a Tropical-continental climate with distinct wet and dry seasons. Structurally, the study area is characterized by deformations which are mainly due to NW-SE and NE-SW fracture patterns in the Nigerian basement complex caused by a trans-current movement. The study area precisely is located within 70 35’ 17’’E and 70 41’ 17’’E longitude, and 110 7’ 50’’N and 110 11’ 22’’N latitude on the Nigeria National grid. Three buildings affected by such defects were selected for investigation. Two of them were located at the Staff quarters of Ahmadu Bello University, Zaria, Nigeria and the other, at the students’ hostel of the same University. While the former were solely characterized by severe foundation-based cracks with unknown cause at sight of superstructure, the latter is a case of severe crack due to leaning having a relative rotational angle of about 5.50. Modern geophysical field equipment, namely; 24-Chanel Seismograph (Terraloc Model MK6) and Lund Imaging system (Terrameter Model SAS 4000/1000) with its electrode Selector (Model ES464) having 42 take-outs, were used for the data collection of the seismic refraction tomography and the electrical resistivity imaging respectively. Eight profile lines were laid parallel and close to each of the buildings during the investigation. While the geophone spacing of 5.0 m was used in the seismic refraction method, the take-out spacing for the electrodes ranged between 1.5 m and 5.0 m in the electrical resistivity method. The travel-time data for the p-wave velocity and Electrical Apparent Resistivity data were collected and processed with computer software namely; REFLEX-W version 3.0 and RES2DIV version 3.4 respectively. Both software were based on a high resolution inversion modelling techniques for data processing. Spearman’s formula for rank correlation analysis was applied on the pair of interpreted tomograms. The rank correlation results obtained from the two methods showed that there is positive correlation in the range of (0.3170 – 0.9883 0.0001) between the structural displays of their lithological stratification. The interpreted results from the seismic refraction tomography technique show that the compressibility of the soils within the foundation depths (0-5 0.1 m) ranges from 2.21797 x 10-09 to 1.45565 x 10-10 ( 0.08 x 10-09) Pa-1), while the estimated allowable total foundation bearing pressure ranges from about <4.5 x 108 Pa to < 1.31 x 1011 Pa ( 0.08 x 10-09) Pa). The depth range to the bedrock is 7 – 18 ( 0.1) m, while the overburden and the weathered basement thicknesses have been estimated to be in the range 3 – 26 0.1 m and 7 – 30 0.1 m respectively. The aquiferous zones based on the interpreted results, mostly occur both at the lower depths of the overburden and within the weathered basement. The integrated interpretations show that expansive soils such as clays, predominantly characterize the near-surface of the three sites. The clays usually undergo seasonal swells and shrinkages which have posed danger to the structures. Also based on the results, fractures, synclinal structures of shallow bedrocks, weakness of intact rocks, structural arrangement of the subsurface lithology, soil creep/crawl and to some extent, the growth of tree roots, have partly contributed to ground movements and differential settlements of most of the buildings in the area. These have resulted in the structural defects on the buildings. Conclusively it was observed that while two of the sites (sites 1 and 2) show low tendency of progressive structural failure, the other (site 3), poses a high tendency of a progressive structural failure.