The technically-complex KwaNobuhle Area 11–Link Sewer project at Uitenhage in the Eastern Cape still stands out as a sound example of the extent of JG Afrika’s skills and capabilities in wastewater engineering.
We were appointed by the Nelson Mandela Bay Municipality (NMBM) to provide civil engineering consulting services, including design, contract administration and site supervision, for the project. The new infrastructure supplements the capacity of an existing sewer pipeline that had reached its full capacity and could no longer cater to the increase in sewerage from rapidly expanding housing developments in-and-around KwaNobuhle.
Velisani Nyoka, a JG Afrika technologist, says that the sewer pipeline discharges to the KwaNobuhle Wastewater Treatment Works (KWWTW), located north-east of this large township on the outskirts of Uitenhage, Eastern Cape.
“The pipeline is approximately seven kilometres in length and comprises mainly unplasticised polyvinyl chloride heavy-duty pipe sections. High-density polyethylene plastic pipe was installed at the road crossings using pipe-jacking technology to minimise the impact of construction activities in built-up areas. Ductile pipes were used to cross a valley, with these 50 m-long sections installed on top of four 600 mm x 400 mm concrete columns,” Nyoka says.
One of the challenges in constructing the pipeline was the varying gradients of the terrain through which the pipeline traverses. They ranged from 0,25% to two percent in the flat areas and between two percent and 12% in the steep regions. It was, therefore, impossible to use deep excavations in the very steep sections to prevent high velocities.
An eight-metre-high and 250 m-wide gabion/ Terramesh retaining wall was built along the severely eroded and unstable embankment between KwaNobhule South and North.
This structure protects both the new and old pipelines that run along the river during heavy rains, while also successfully addressing previous environmental and public safety concerns.
Unsuitable material was cut back at an average length of eight metres from the face of the existing embankment. Using an excavator, about 7 300 m3 of material was removed during this stage of the construction of the wall.
The 3m x 3m x 1m Terramesh baskets were then installed on an in-situ concrete foundation that was built along the embankment to create a robust wall. It features a step of about two metres in height and 0,5 m sideways along the entire length of the face to the final level.
Nyoka says that the Terramesh baskets were placed on geogrids that were spread throughout the area that received G7 material. The geogrids were positioned at two metre heights of the fill material, with the geotextile membrane placed underneath and at the back of the boxes for filtration purposes and to prevent the loss of fines.
Macdrains were then installed along the full length of the embankment for drainage purposes.
Spread across the 70 degree-angled slope of the embankment, they separate the fill and in-situ material to enable seepage water to drain down to the 110 mm-diameter perforated pipes.
The water is then drained out of the wall via transverse pipes that were installed at 50 m intervals along the length of the face.
Security fence and guard rails were also installed in front of the wall as further protection measures.
This labour-intensive construction method provided many employment opportunities for members of surrounding communities.
A total of 18 small, medium and micro enterprises from various wards of the NMBM were also sub-contracted by the principal contractor to build the retaining wall.
The project, it was delivered according to standard, drawing and specification – a feat he attributes to outstanding participation of all members of the professional team right from the outset.