A major project to effect road repairs and slope stabilisation at the Franschhoek Pass in the Western Cape has resulted in an innovative solution being developed by a leading South African engineering consultancy.

The Western Cape Department of Transportation and Public Works originally called upon the expertise of SMEC to assist with its emergency planning in the event of flood or fire damage to any of the province’s road network.

“Our initial scope of work was quite general in terms of the specific project terms and conditions,” explains Charles Warren-Codrington geotechnical engineer.

However, major rockfalls and slip failures at the Franschhoek Pass from June to November 2013 resulted in the Department tasking SMEC South Africa to devise emergency remedial measures for the vital link between the towns of Franschhoek and Villiersdorp on the R45.

Penny Farthing was appointed as the main civil engineering contractor. It commenced work on SMEC’s preliminary solution in February 2014.

The plan was to drain the slope by inserting 20-m-long drains, in conjunction with grouted steel nails, finished with a shotcreted or sprayed concrete top layer to create some adhesion with the soil at the surface.

A considerable area had to be shotcreted, measuring 80m by 20m. About 10% of this part of the project was completed initially, with SMEC even including a pigment in the concrete mix to mimic the sandstone colour of the area.

However, this is a highly visible section of Franschhoek Pass, and SMEC felt that shotcreting would detract from its natural heritage.

An amended approach

“We entered into discussions with the department and contractor, and proposed an amended solution based on the aesthetics of the area,” Warren-Codrington says.

This meant replacing the shotcreting with a high tensile strength steel mesh with a diamond-shaped aperture of about 80 mm. The mesh was positioned between the soil nails, with both effectively tensioned at the same time.

“The mesh served the same purpose as the shotcreting in terms of local soil support,” Warren-Codrington adds.

Extensive environmental considerations

The main challenge posed by the project was the environmental considerations. Bush had to be cleared from the construction area, whereafter the vegetation had to be reinstated.

A biodegradable geo-membrane was placed between the subsoil and steel mesh to counter any soil erosion.

This layer also assisted with the revegetation process by facilitating seed germination and the development of root systems.

Six months upon its completion, the project has been declared a success. “The drains are working and there is an indication that water is coming through the slope, which is a good sign.

“In addition, we have grass growing on the slope. There have been no excessive soil movements or problems encountered thus far.”