Engineering geological mapping for urban development in low relief semi-arid regions (Outapi northern Namibia)

Abstract

Considering the current lack of engineering geological mapping in urban developments in Namibia, it was necessary to pioneer the input of engineering geology in the planning of infrastructure. The benefits of engineering geological mapping in sustainable development were highlighted and present methodologies were explored, looking at their applicability to Namibia. The early activities of engineering geological mapping include terrain classification, which groups areas of terrain that have similar conditions. It applies the principle of homogeneity such that areas falling under one terrain class are treated the same during planning, as they are deemed to have the same impact on engineering structures. The traditional way of doing terrain classification places much reliance on topography features, such as valleys, streams, etc. However, this becomes fairly difficult to apply in flat lying areas where topographical features may not be readily visible from aerial photographs. Therefore, soil variability mapping using colour and texture, vegetation mapping, and damage distribution mapping or structural damage techniques were recommended to aid terrain classification in flat lying semi-arid environments. Employing these techniques, Outapi Town in northern Namibia was mapped, identifying four terrain classes. With subsequent testing methods, these were upgraded to just three engineering geological zones. The main geotechnical constraints identified include the possibility of collapse settlement given the aeolian nature of the Outapi soils, susceptibility to flooding attributed to the flat nature of the area with no properly defined pathways, potential for dispersion as the soils are rather fine and saline, and, to a lesser extent, the presence of potentially expansive soils.