THE completion of an R80 million slope stabilisation engineering project in Nelson Mandela Bay has saved Africa’s automotive manufacturing capital from power black-outs.
The South African National Roads Agency Limited (SANRAL) announced this on January 17.
The instability of the R75 on-ramp into Uitenhage was undermining the structural integrity of pylons carrying overhead electrical cables supplying large areas of Uitenhage with power. It has been a source of concern for authorities and industry since 2012 when a part of the slope was eroded by heavy rains.
“Sloughing or erosion problems [of the slope] began in the mid-1980s mainly due 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 close to surface water or moisture contained within the slope,” said Michelle Ah Shene, Admin Manager/Communications at SANRAL.
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,” Ah Shene said.
Recently, extensive slope landmass movements were triggered by heavy rains during October 2012 in the Nelson Mandela Bay metro.
The rain created a 50m wide and 300m 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 exasperating the risk of damage to the cables and a potential power black-out of Uitenhage.
SANRAL said it concluded a slope stabilisation programme of the R75 on-ramp, and that stability of pylon foundations will no longer be at risk.
The programme entailed the excavation of 112 000m³ of soil, the construction of 5 700m³ 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 drains conveying stormwater run-off down the slope to the existing drainage system.
The project also included 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 met and overcame two engineering challenges on the project.
The first challenge encountered was the excavated face beneath the southern pylon; if completely excavated to its full height, it would have been approximately 10m high.
“When it had been partly excavated some 6m to 7m 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 up 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 lead 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,” she continued.
“Geotechnical engineering is one of the fastest growing fields of engineering, albeit a relatively young one. Many pioneers within the field have carved a path for those that follow to help fill in the gaps in our knowledge of the fascinating world that lies beneath the earth’s surface.”
Ah Shene explained that most engineering projects deal with near surface challenges and solutions are effectively engineered, whereas slope stabilisation projects deal with more unknown challenges.
The project created 216 employment opportunities for 13 SMMEs over a 15-month contract period. SMME training in tendering and pricing; safety, health and environment (SHE) aspects; concrete skills, and first aid were also provided.