Engineering geology of the proposed Darwin East  urban development area, Northern territory

Field mapping, augering, and rotary core drilling have defined the main geological constraints to urban planning in the proposed Darwin East urban development area. The area is underlain by a sequence of subhorizontal Mesozoic rocks unconformably overlying a gently folded sequence of Lower Proterozoic rocks, which include cavernous dolomite. The overlying rocks have in places subsided into the cavernous dolomite to form dolines. A highly plastic clay lining the floor of these dolines has transformed some of them into perennial and ephemeral lagoons. Geomorphological processes have developed four major land surfaces which contain the present-day physiographic features and determine the distribution of the soil types. Of the twelve soil types mapped, planosol and yellow lateritic podzolic soils are expected to be troublesome: they have poor drainage characteristics. The establishment of a piezometer network, and subsequent water-level measurements, indicate that the groundwater catchments for the springs on the southern escarpment are likely to be more extensive than previously thought because the groundwater divide does not correspond to the topographic highs. Of the proposed alternative drainage schemes, a conventional scheme which minimises infiltration is favoured. Some supplementary subsurface drains might have to be installed to effectively drain the ephemeral lagoons and broad flat drainage depressions. Maintenance of the water-levels in the perennial lagoons is preferred to draining them. A plot of earthquake data indicates that nearly all seismic activity originates in the Banda Sea area, north of Australia. A comparison of data recorded from earthquakes in this area since early this century with a more recent Banda Sea earthquake of known felt intensity suggests that an earthquake with a felt intensity of MM V in Darwin can be expected at least once every 50 years. Coarse aggregate is readily available in the area, and resources of fine aggregate, fine sand, gravel, and topsoil may prove to be substantial. Extraction could be integrated with a sanitary method of garbage disposal. Some foundation problems can be expected for large structures sited above subcropping cavernous dolomite or completely weathered mudstone and shale. Planosols in the area have low bearing strengths when they are saturated, but these should not be a problem once urban drains are installed. Excavation for services will be difficult and expensive where quartzite crops out or lies just below the surface. Some sites are worthy of preservation as geological monuments. Rock bars, suitable for the foundation of relatively inexpensive embankments, occur adjacent to East Arm and Elizabeth River. A possible alternative site for an airport, northeast of Knuckeys Lagoon, lies partly within and adjacent to the Darwin East urban development area. Scope exists for the planning of a marina in Hudsons Creek, and for the use of sewerage effluent to irrigate part of the Mitchells Creek catchment.

Tags: AU-NT, Bulletin, Earth Sciences, GA Publication, Published_External, geology
Organization: Geoscience Australia Data