The new alignment of the National Route N1 between Florence and Worcester, South Africa has resulted in the excavation of four major cuts in Nardouw Formation quartzitic sandstones of the Table Mountain Group. The geology and topography of the slopes indicated inherent instability which was confirmed when minor rock slides occurred in two of the cuts. The geotechnical investigation indicated that simple planar failure and block toppling were the mechanisms of slope instability. Rock bolts were used to stop block toppling and rock anchors were installed to stabilise the exposed cut faces to the undisturbed rock below. Test anchors verified the anchor design but indicated, however, problems with anchor pad design due to the weathered state of the surface rock. It is concluded that the anchors themselves were suitably designed for the ground conditions in this area and that special attention should be given to the design of the anchor pads.
The new alignment of the National Route N1 between Florence and Worcester, South Africa (Figure 1) cuts through the base of a portion of the Slanghoekberge for a distance of approximately three kilometres. Excavation into the hillslope has resulted in the formation of four major cuts in the realigned section. The geology and topography of the slopes indicated inherent instability in the area. This instability Of the slopes was confirmed when minor rock slides occurred in two of the cuts. This Paper describes the geotechnical investigation and stabilisation of these four cuts.
The position of the four cuts are shown in Figure 2. Cut 1 is 580m long and varies in height from 4m to 10m, Cut 2 is 140m long and approximately 6m high, Cut 3 is 100m long reaching a height of 7m and Cut 4 is 80m long with a maximum height of 8m. The first three cuts have been presplit at an angle of 1:0,25 (76°) and Cut 4 to 1:0, 6 (60°).
All the cuts consist of Nardouw Formation quartzitic sandstones of the Table Mountain Group. The rocks are generally Yellowish-orange to white and vary from being relatively unweathered to highly Weathered and in places very friable. In many places initially moderately weathered medium hard rock broke down to form highly weathered soft rock after a short period of exposure to the elements. At the western end of Cut 1 highly weathered soft interlayered sandstones and shales outcrop.
(figure in full paper)
Dip measurements on the sandstone bedding planes indicate an average dip of 35° to 40° into the cut face.
The overburden at all four cuts consists of fine silty sand with numerous roots and sandstone boulders of varying sizes and degrees of weathering. When wet this material is highly unstable.
(figure in full paper)
The field investigation involved determining the physical properties of the exposed rock and undertaking a rock joint survey. Gouge on bedding planes was sampled and full indicator and shear strength tests were performed on this material.
