Construction World April 2017

GEOTECHNICAL ENGINEERING

ROAD REHABILITATION: AKKEDISBERGPAS Akkedisbergpas is one of the most southerly mountain passes in South Africa. It is situated on the R326 regional road between Riviersonderend and Stanford in the Western Cape. Whether national or regional roads, the roads need to be maintained to keep motorists safe. All mitigation processes should be scientifically and geotechnically approached keeping in mind costs at all times.

Usually the mechanisms that attribute to slope failure and the integrity of the fill embankment is a combination of things such as loss of vegetation resulting in erosion of exposed grounds as well as changes in the consolidation characteristics of the fill material over time. Natural disasters like fires or heavy rains also contribute to the degradation of the area. Consulting firm AECOM was approached by the Western Cape government in 2015 to do a slope

stability analysis in the Overberg area of Akkedisbergpas and propose remedial actions to be taken to get the road back to a desirable state. Fires in the area destroyed much of the fynbos vegetation. Rains shortly after the fires caused more topsoil to be washed away, leaving the ground vulnerable to slope failures. Four different sites along the R326 road were identified where rehabilitation work was to be done. In the second half of 2016, the tender for the construction work was awarded to Penny Farthing

Engineering with the undertaking to complete the project by the end of March 2017 before the rainy season in the Western Cape starts. A combination of gabion walls, backfill, steel mesh and soil nails were proposed for rehabilitation along the R326 on Akkedisbergpas. Prior to commencing work, a geotechnical engineer inspected the area to ensure that the wall footing and mesh positions were correct. Geobrugg’s Greenax mesh was used as a permanent erosion control measure. Greenax is a three dimensional poly-propylene erosion control mat that is incorporated into high tensile steel wire mesh, providing an integrated, reinforced composite that is suitable for temporary and permanent erosion control. Greenax can be used on embankments and cuttings where vegetation is to be encouraged. The erosion control mat provides restraint to the surface, restricting water and wind erosion from occurring, whilst still providing an ideal environment for the vegetation to grow. The geotechnical design allowed for the Greenax mesh to be supported by summitry cable fixed to the crest anchorage. Mesh were securely anchored from the crest downwards along the slope with Geobrugg’s spikeplates and grouted soil nails. Adjacent mesh panels were clipped together with T3 connection clips leaving no overlaps of the mesh and reducing wastage of material. Gabion walls ranged from 1 m to 3 m height and had to be installed in maximum 5 m sections to minimise disturbance to the existing slope. Backfill material was specified to be course granular material selected from excavated material where possible. The backfill was to be sloped at 1V:2H (26 degrees) with erosion protection woven mat placed on the backfill material and positioned with wooden pegs. The rehabilitation along the road on Akkedisbergpas is a good example of integrating various methods for slope stability. 

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CONSTRUCTION WORLD APRIL 2017