Soil Categorization Using Vs30 Mapping, to Evaluate Probable Structural Effects of an Earthquake on a Proposed Construction Site

This research work was carried out with the aim of using average shear wave velocity of the top 30 m (Vs30), technique to categorize the soil, and ascertain the probable resistance or damage that the structure placed on it could suffer in the event of damaging earthquake. In a bid to achieve this, downhole seismic survey was carried out in eight boreholes within the same site, all drilled to 30 m depth. The field procedure used involves lowering of hydrophone into the borehole at an interval of 1 m, and striking a sledge hammer on base plate at a fixed distance of 3 m to generate seismic signals. The data were processed and the results revealed that, the distribution of average shear wave velocity of the top 30 m (Vs30) is not always even within a given site; instead, you have regions with low Vs30, and regions with high Vs30. The results indicated the presence of overburden layer (lose sand layer) within the site under investigation, and also revealed that in most instances, regions with low Vs30 match up with regions with thick overburden, and regions with high Vs30 correspond with regions with thin overburden cover. The research has shown that, in the event of damaging earthquake the regions with low Vs30 and relatively thick overburden cover will experience more of the amplification of the low frequency seismic waves, and consequently more damage to structures erected within that vicinity. Hence, regions with low Vs30 and thick overburden cover will require engineering remediation to improve its resistance to shaking and load bearing capacity, and to reduce the amplification of low frequency seismic waves in the event of ground shaking induced by an earthquake.