Geotechnical Investigation Of A Collapsed Building Site For Foundation Design And Construction In Port Harcourt, Rivers State

Abstract

Building failures and collapses have been on the increase in recent times in Rivers State of Nigeria. Recently, a four-storey building belonging to a school at Rumubiakani, in Port Harcourt, collapsed with casualties. Investigation was conducted to determine the causes of the collapse and make recommendations to forestall future occurrences. To this end, this study was carried out to investigate the geotechnical properties of the soil. In doing this, 3 boreholes; BH1, BH2 and BH3 were drilled using cable percussion equipment to the depth of 20m and soil samples were collected for laboratory tests (Atterberge limit, particle size analysis, consolidation etc) according to BS 1377 (1990). The soil stratigraphy revealed near surface dark brownish silty clay of high compressibility from 0-2m, grey light silty clay of medium plasticity to 6m depth for BH1, BH2 and BH3. Below the 6m is medium-grained loose sand to 12m depth which increases in density to a coarse-dense sand beneath. Result showed a L.L and P.L of 42% (<50%) and 22% respectively while over 46.83% of the soil containssilty clay. The compaction test gave a Bulk unit weight γ, MDD and OMC of 19KN/m3, 1.6g/cm3 and 12.80% respectively. The soil produced a coefficient of compressibility mv of 0.625m2/MN and Cc of 0.24, while the shear strength parameters of the soils were; c = 40KN/m3 and Ø = 40 which produced a bearing capacity (BC) of 108.9KN/m2and settlement of 32mm for shallow foundation at the depth of 3.0m under a water level of 2.5m. Finally, it was concluded that the soil is weak silty clay and unsuitable for strip foundation, and that a raft foundation at 2m was adopted which produced excess settlement exceeding the tolerant limit of the building and consequently resulting in the collapse. A raft and beam/slab-raft were recommended for bungalows and two-three storey buildings respectively otherwise pile foundation terminating at 25m stratum should be adopted.