Project Description

It was proposed to realign the existing runway at the northern end of Mustique Island to minimize the effects of elevated terrain features near the flight path. The excavation of soil and rock from high ground in the central quarry area and engineered fill placement at the low lying east and west ends of the runway are required to provide an appropriate runway gradient. The existing topography along the centreline of the proposed runway changes from approximate Elev. 1m ASL adjacent to the sanctuary to Elev. 24m ASL in the quarry area. The topography descends to Elev. 2m ASL adjacent to the Rutland Bay seasonal pond in the northeast.

The geology consists of both soil and volcanic rock formations. The surficial deposits are weak clays and clayey silts near the western sanctuary. In the quarry area the overburden soils comprise red brown sandy silt with a trace to some clay. Soft clayey silts underlain by red brown sandy silt are present near the eastern seasonal pond. The bedrock consists of Oligcenic volcanic rock including coarse reworked and primary hylaclastic tuffs intruded by dikes. The volcanic tuff is considered to be weak and heavily fractured. It is underlain by less fractured, relatively sound, dark grey andesite.

Nine boreholes were drilled along the proposed runway and three boreholes were drilled along the existing runway and apron areas. The drill equipment comprised a CME45 with casing and washboring through the soil and soft rock, and Nx double tube diamond core barrel equipment in the bedrock. Test pits were completed in the west sanctuary and wetlands. Laboratory testing included soil classification, moisture content, unit weight, liquid and plastic limit, grain size analysis, Standard Proctor density, and CBR and consolidation characteristics.

The geotechnical reporting includes the subsurface investigation findings and recommendations for (a) subexcavation and/or soil improvement of the weak swampy materials comprising wet, soft clayey silt and organics in the west; (b) excavation of overburden soils and soft rock (volcanic tuff) and blasting and excavation of hard rock (grey andesite) in the middle high ground and quarry; and (c) additional subgrade preparation and engineered fill placement in the east section near the seasonal pond at Rutland Bay. Based on the aircraft loading conditions a design for the new asphalt pavement thicknesses, asphalt overlays for the existing apron and runway, and subsurface drainage requirements were provided.