Characterised as expansive clays, black cotton soils have a high potential for shrinkage or swelling, subject to moisture variables. On this site, these soils reach a depth of up to 15 m deep with a silty-sand layer below.
Road transport would have been an alternative option, but the general state of infrastructure is poor to the point where the route becomes non-negotiable in wet periods, hence the reliance on aviation. So when planes couldn’t land, this meant that inbound logistics for the aid mission had to be supplied by military-scale helicopters. However, given the much higher fuel burn rate of rotary aircraft, this added a greater payload cost.
Need for larger planes
The client brief to WSP was to come up with a design solution that would restore and then extend the runway to accommodate larger transport planes, like Lockheed Martin’s C-130 Hercules and its civilian variant, the L-100-30 Hercules – all year round. These aircraft are equipped to carrying a cargo of approximately 19 tonnes. Prior to the upgrade, the Antonov An-26, with its approximately 5.5 tonne payload, was the largest plane that the airstrip could handle.
“During construction, a key requirement was for the airstrip to remain open to traffic,” explains Hercu du Preez, principle associate: Pavements – Transport and Infrastructure, WSP Group Africa, who led the project.
“For this reason, the works were split into two phases. The first phase entailed the rebuilding of the existing 800 m airstrip, followed in phase two by extension works to establish the final 2.2 km runway,” he continues.