R80m ramp project success
Stability of vital Uitenhage power pylons assured by huge Sanral engineering feat
SOUTH African National Roads Agency Ltd (Sanral) has completed an R80-million engineering project in Nelson Mandela Bay which has saved Uitenhage’s industrial and automotive manufacturing hub from potential power blackouts.
It involved stabilising the slope off the R75 on-ramp into Uitenhage, which has for more than four years undermined the structural integrity of pylons carrying overhead electrical cables that supply large parts of the area.
It has been a source of concern for Bay authorities and industry since 2012, when a part of the slope was eroded by heavy rains.
“Sloughing or erosion problems on the slope began in the mid-1980s, mainly due to a lack of maintenance,” Sanral southern region marketing manager Michelle Ah Shene said.
“This, in turn, allowed moisture to seep into the slope. Previous methods of stabilisation were not effective as they only dealt with water close to the surface and moisture contained within the slope.”
Ah Shene said the R75 was declared a national route in 2011 and that the issues around the slope were being investigated by Sanral when the failure occurred in 2012.
The October, 2012 rain created a 50m-wide, 300m-long slip along the R75 on-ramp, resulting in the movement of soil to the lower parts of the slope.
This further exacerbated the risk of damage to the pylons and a potential power blackout of Uitenhage.
After completion of the work, the stability of the pylon foundations were no longer at risk, Ah Shene said.
The project involved the excavation of 112 000m³ of soil, the construction of 5 700m³ of gabions in three continuous walls along the R75 on-ramp, and addressing water drainage.
Sanral overcame two significant engineering challenges to complete the project.
The first involved the excavated face beneath the southern pylon, which according to Ah Shene, if completely excavated to its full height, would have been about 10m high.
“When it had been partly excavated to about 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 anyone working below,” Ah Shene said.
“That situation could quite possibly have brought about the collapse of the electricity pylon standing at the top of the face.”
Ah Shene said where the excavations had opened up the face of the cut slope during construction, a geological fault had been exposed and considerable amounts of groundwater seepage from the fault had been experienced.
To remedy this second problem, which may have led to further erosion, additional stabilisation works were carried out.
The project created 216 employment opportunities for 13 SMMEs over its 15month contract period.