Oghenero Enivwenaye AVWENAGHA

Warri and its environs have been recently challenged with incidences/imminence of building collapse with frequency of four incidences in the past four (4) years. Hence, geophysical, geotechnical and mineralogical investigations were conducted in an area of failed building and other areas in Warri with a view to comparatively analyse the geotechnical and mineralogical properties of subsurface soils for future building and road developments in Warri. This involved the drilling of ten (10) boreholes in the failed area and fifteen (15) holes in other areas which were all complimented with twenty five (25) Cone Penetration Tests. Samples from boreholes were subjected to geotechnical index/foundation analyses and X-ray Diffraction analyses (XRD). Geophysical and geotechnical investigations showed that the failed area had a laterally heterogeneous two-three layer soil profile which from top to bottom consisted of sandy silt/silty sand (3m thick), clayey silt (15-17m thick) and fine-medium grained sands (2m thick). This was at variance with the other areas which had a relatively homogeneous three layer soil profile which from top to bottom consisted of loose sandy humus top soil (0.25m thick), reddish brown silty sand and fine to medium grained sands (3-8m thick). Classification characteristics using the American Association of State and Highway Transport Officials (AASHTO) showed that the superficial soils in the failed area were mainly of A-7, A-6 and A-4 characteristics with California Bearing Ratio (CBR of 3-20.5%, average: 6.25%) indicating they are competent subgrade materials but incompetent sub-base and base course materials for road construction. Similar characterization in other areas reflected soils of A-2, A-6 and A-3 characteristics with CBR (10-17.9%, average: 15.02%) indicating similar competence and deficiency to that of the failed area. Sand stabilization increased the subgrade quality of the superficial soils in both areas. Cement and composite stabilization improved all soils to sub-base and base quality materials respectively. Foundation studies showed that superficial soils in the failed area are of lower foundation quality (bearing capacity of 184-229kpa, compression indices; 0.12-0.62) than the other areas (bearing capacity of 185-575kpa, compressibility indices; 0.31-0.34). Superficial soil mineralogy showed that the failed area consisted of quartz (75.33-94.20%), kaolinite (5.79- 11.99%), smectite, muscovite and microcline which is consistent with the other areas except for the absence smectite. This showed that structural failure in the challenged area was due to soil lateral variation and poor foundation quality.