GEOPHYSICAL AND GEOTECHNICAL INVESTIGATION OF SUBSOIL CONDITIONS FOR FOUNDATION STUDY

Oyedele K. F., Adegbola R. B., Adebisi N. O., Lawal A. F.

Abstract


In the past couple of years Lagos State, Nigeria has been experiencing collapse of buildings. This has necessitated the means for a proactive geophysical and geotechnical assessment of areas where buildings will be sited soon or sometime in the future. This led to the investigation of an open field at C.M.S Grammar school, Lagos to identify competent subsurface materials which can host stable, safe and durable structures. The geophysical and geotechnical investigation were carried out by acquiring eight (8) Vertical Electrical Soundings (VES), two (2) Wenner traverses and two (2) Cone Penetration Test points respectively. The electrical resistivity data were processed and interpreted using Winresist and DiproWin software. The results revealed three layers: clayey topsoil, dry/partially saturated sand and clay/sandy clay. The topsoil has resistivity values that vary from 8 to 67 ohm-metre, with thickness value that ranges between 0.5 and 1.4 m. The second layer shows resistivity values that vary between 158 and 797 ohm-metre which possibly suggests a saturated and or dry sandy lithology and has potentially the best mechanical property and competency. This sandy layer occurs at depth  whose value ranges from 0.5 m to 1.4 m and thickness value that ranges from 14.2 m to 56 m. The layer indicated that shallow foundation such as footings (spread, combined or strap) is possible within this area. Although the CPT carried out did not penetrate deep enough to properly sample this sandy lithology due to hardness of the ground, it was able to show that the very near surface with depth to about 1.5 m was of incompetent nature based on the cone resistance values and the calculated allowable bearing capacity. Hence, this region should be avoided.

Keynote: Assessment, competency, lithology, mechanical property, footings

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References


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