The completion of an R80-million slope stabilisation engineering project in Nelson Mandela Bay metropolitan region, in the Eastern Cape, has prevented the risk of damage to power cables and related blackouts in Uitenhage.

The South African National Roads Agency Limited (Sanral) said on Tuesday that the instability of the R75 on-ramp into Uitenhage had been undermining the structural integrity of pylons carrying overhead electrical cables supplying large areas of Uitenhage with power.

“Sloughing or erosion problems [of the slope] began in the mid-1980s, owing to the lack of maintenance. This, in turn, allowed moisture to seep into the slope. Previous methods of stabilisation were not effective as they only dealt with near-surface water or moisture contained within the slope,” said Sanral Port Elizabeth communications manager Michelle Ah Shene.

The R75 was declared a national route in 2011 and was being investigated by Sanral when the failure occurred. “Every time instability occurred, the failure zone encroached on these pylons, undermining the stability of the foundations,” she noted.

Extensive slope landmass movements were further triggered by heavy rains in the Nelson Mandela Bay metro during October 2012. The rains created a 50-m-wide and 300-m-long slip along the R75 on-ramp, resulting in the movement of soil to the lower parts of the slope along the failure zone, further exacerbating the risk of damage.

The project entailed the excavation of 112 000 m³ of soil, the construction of 5 700 m³ of gabions in three continuous walls along the R75 on-ramp inclusive of grouted soil nails, construction drains at the top of the walls, and the installation of drains conveying stormwater runoff down the slope to the existing drainage system.

The project also involved the construction of an earth catch water bank at the summit of the slope to prevent overland flow from the summit flowing down the cut face.

Sanral said the project was not without challenges, noting that the excavated face beneath the southern pylon – if completely excavated to its full height – would have been 10 m high.

“When it had been partly excavated some 6 m to 7 m, it was prudent to install a system of temporary grouted soil nails plus geotextile and steel mesh to prevent the slope from collapsing.

“Had the temporary measures not been installed, there would have been a danger of the collapse endangering the lives of any person working below and could have, quite possibly, brought about the collapse of the electricity pylon standing at the top of the face,” said Ah Shene.

Secondly, during construction, where the excavations opened the face of the cut slope, a geological fault in the material had been exposed, with considerable amounts of groundwater seepage from the fault.

“The fault could have led to further sloughing of the existing cut face onto the R75 on-ramp. Additional stabilisation works in the form of gabions, soil nails, slope rehabilitation measures (top soiling and grassing) and trapezoidal drains were authorised.”

The project created 216 employment opportunities for 13 small and medium-sized enterprises over a 15-month contract period. Training in tendering and pricing, safety, health and environmental aspects, concrete skills and first aid was also provided.