Geophysical and Geotechnical Investigations for Proposed Dominica Airport

The results of geophysical and geotechnical investigations carried out at the site of the proposed International Airport at Melville Hall, Commonwealth of Dominica, West Indies, are presented. The geotechnical investigation confirms the findings of the previous geophysical investigation, which concludes that bedrock is not likely to be encountered within the proposed depths of excavation. The stratigraphic models of both geophysical and geotechnical investigations are compatible and suggest that the soil profile is one of deeply weathered pyroclastic tuff and ash deposits transitioning to a boulder conglomerate ash horizon. The main geophysical method used was seismic refraction, additionally ground penetrating radar, resistivity sounding and resistivity tomographic imaging were also performed at some of the sites. Analysis of the seismic data shows a gradual increase in velocity with depth for which a model has been determined. Ancillary models or predictions of porosity, density, and (natural) compaction with depth are given, based on the basic seismic model. The main geotechnical investigative methods comprised of boreholes to 30 m depth with Standard Penetrating Testing (SPT) and undisturbed Shelby tube, and disturbed Split Spoon soil sampling. Water content, plasiticity, and grainsize distribution characteristics are obtained from laboratory testing leading to a classification of elastic silts and elastic silts with sand using the Unified Classification System. Geophysical and geotechnical data correlations are presented. Seismic velocity and SPT-N blow counts appear to be well correlated by a linear model. A model relationship between SPT and seismic dynamic elastic modulus is developed derived from seismic velocity. SPT-N is better correlated with the dynamic elastic modulus than with seismic velocity. The results show that seismically derived dynamic elastic modulus can accurately predict soil strength as measured by SPT blow counts.