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CONSTRUCTION WORLD
APRIL
2017
40
GEOTECHNICAL ENGINEERING
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.
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.