Chapman’s Peak Drive #7: Extraordinary Engineering pt1
This is the 7th in a series of 8 caches (incorporating 2 existing and 6 new ones) along this 9 km long, world famous, spectacular and award-winning cliff-edge mountain pass.
The short multi can be done in around 10 minutes collecting the required info from near or around this parking and picnic layby on the east side of the road. See GC9KPCD CPD #3 for toll tips.
Step 1: at the given coordinates, you will be standing by a roadside rock with several holes drilled into it – presumably during the road construction process. Most of these are complete circular holes but looking at the top edge of the stone you will see A that are not.
Step 2: move south to S 34 4.017 E 18 22.218 a short distance to the signpost in the back of which you will see a couple of long numbers. If you subtract the fax number from the phone number, the answer is 33B
Step 3: carefully cross the road to S 34 4.014 E 18 22.228 where you will see a number plate attached to the edge of the road – make a note of the last two numbers of this and make this = Y
Step 4: take a few steps over to S 34 4.016 E 18 22.231 where you will see a post with a sign prohibiting a popular activity the first letter of which has an alphanumeric (A=1 B=2 C=3 etc) value of E
Update 25/1/24: the sign seems to have gone - it depicted a braai!
Step 5: now cross over the layby road to S 34 4.025 E 18 22.226 where on the nearby wall you will see another number plate. Again, note the last two numbers and call this X
Then X – Y = CD which is in fact the distance between the two number plates in metres.
Step 6: finally head down the layby a short distance to S 34 4.022 E 18 22.249 where another post has a sign prohibiting something. The alphanumeric value of the 4th letter of this word, which is also the number of letters in the word = F
The cache is hidden in a nearby shady spot at:
S 34 4.(C-E)(B-4)(F-5) E 18 22.E(A+B)D
The Project Engineering Solutions
Rock Fall Protection Measures: these were selected based on what is considered to be current best international practice and consists of a half tunnel, catch fences, slope stabilisation and canopy structures. In 2004, CPD received an excellence award for the work done.
The scale of the design and construction of the works was formidable, with many of the activities required being new in South Africa. The design of rockfall protection measures involved sophisticated 3D computer modelling of the topography assessment of mountain and location of boulders on the slopes and cliffs above the road to predict the associated rockfall patterns.
This provided statistical predictions of the location, trajectory, frequency, bounce height and energy of falling rocks along route. These predictions formed the basis for determining the optimum size and shape positions of the rockfall protection structures and for their design.
Working conditions were challenging and up to 11 different mobile cranes were deployed, as the road was too narrow to constantly move the cranes up and down. Helicopters were also used to lift drilling and grouting equipment into position at heights above 65m, while abseilers completed the work in those inaccessible locations.
Terrain Modelling: the engineers, with Swiss specialists utilised a digital terrain model to generate 3D simulations of rockfalls. This was used with interpretation of high-resolution aerial photographs to determine the distribution of relative frequencies, bounce heights and energies of simulated rocks of various sizes.
Half Tunnel: This is the first time such tunnelling has been used in South Africa and entails cutting into the mountain at road level, then moving the road in under the protection of the resulting overhang. The 155m half tunnel was built at the base of Chapman’s Peak and the overhanging rock-mass is supported by 95t rock anchors. The soffit and side walls are supported by 150mm thick steel reinforced shotcrete lining.
Rock Shelters: In other places the solution to rock fall protection measures was to build a concrete canopy, similar to those at Lake Garda, Italy. These measures ensure that any rocks that do fall off the mountain will, at worst, land close to the old rock-faced guard wall, while traffic will travel safely under the canopy.
At two locations where the sandstone cliffs extend to more than 400m above the road, concrete impact protection canopies were constructed. These structures arch over the road and protect it by intercepting rock fall and debris or deflecting the material over the road and into the sea below.
A 40m long curved cantilever canopy, arching over both lanes was built on a tight bend at the confluence of three gullies. It is tied back into the cliff face at each end by 100t rock anchors and supported in the middle by 11 large pre-stressed counterfort ribs, as the fault resulted in stability concerns due to hard granite on the northside and soft sandstone on the southside.
A portal canopy coincides with the highest predicted rockfall energies and the structure is free of the cliff face. It is supported from behind by tapered columns with a row of circular sloping columns to support the front edge.
