Reconstruction of a failed retaining structure at Whalerock Beachend, Plettenberg Bay

During the month of February 2013 we started on the reconstruction of a failed retaining wall. As for most of the failed retaining walls in this area this wall was also initially constructed by a local company with little or no sound knowledge in the field of civil construction.

The wall was split into a lower and upper tier beyond a certain point, widening away from each other. In order to limit the surcharge impact on the lower tier and makes the structure more stable, the distance which separates the upper tier from the lower tier should be approximately equal to the height of the upper tier. This is the case at its widest point where the bottom tier needs to follow the curve of the top tier in order to support the top tier fully, due to the outside curve and the weakening effect it creates on the structure of the curved section.

The entire length of the wall has been reduced by three rows of blocks in height, thereby lowering the toe pressures on the foundations to about 70kn/m squared which is well below the allowable bearing pressures.

All necessary and required DCP test was carried out by Outeniqua Geotechnical Services. An additional 3% cement mixture was placed into the back fill.

The founding depth as shown on the plan was 1200mm below natural ground level. The foundations have been constructed at least 1500mm below natural ground level from approximately the center of the wall to the southern boundary.

The deeper founded lower tier wall was further strengthened by the introduction of additional RockPc 50/50 geofabric. Tie backs were placed on top of every second row of blocks above natural ground level.