When I arrived at the Stompdrift Dam site, construction of the camp had just started, which allowed me to design my own pre-fab house. The camp consisted of some 40 comfortable houses, a clinic and a recreation hall with tennis court. Electric power was supplied by the Electricity Supply Commission (ESCOM), as it was called then. In contrast to Ebenezer, we had power day and night. Water came from boreholes drilled in the Table Mountain sandstone near the site. The works buildings consisted of an office building on top of a koppie overlooking the site, a store, a mechanical workshop, a carpenter workshop, a laboratory, cement shed and a batching and mixing plant for concrete, with bins for the various aggregates. A compound was built for some 300 labourers on the right bank of the future lake.
The dam consists of three concrete arches or domes. The main arch (Nr 1) is supported by the left flank, which consisted of TMS and by the central buttress, which also supports the secondary arch (Nr 2). The central buttress contains the outlet works as well as the silt outlet. Between the secondary arch and the tertiary arch (Nr 3), there is the secondary solid buttress. Dome 3’s left flank is supported by an enlarged part of the right bank concrete gravity flank. The original design showed an earth flank. The site staff made representations to head office to change this flank to a concrete gravity section, arguing that everything on the site was geared for concrete placing, while an earth embankment would require completely different plant and staff. Analysis showed that the proposal was more economic, and was accepted by Head Office.
The design was carried out under the direction of RIDM Myburgh, then in charge of the Design Office, using his pioneering work which he carried out in the Pretoria West laboratory of Water Affairs. He used a steel wire grid weighted by weights representing the loads hanging on the structure. The grid then took up a shape to resist the forces by tension in the wires. That shape was approximated by mathematical formulas. Because the wires could only resist tensile forces, the same loads applied in the opposite direction would produce only compression in a dam structure with the same shape. Most of the detail design was carried out by Vis Fourie, later of Hydroconsults. More about the shape later.
The Resident Engineer was TR (Theo) Hooper. I was the Assistant RE. The general foreman was Nick Harding, the mechanical foreman At Smal and the carpenter foreman Kosie van Staden (The one from Mapochsgronden). Because there was no technician to run the concrete lab, I trained a retired carpenter by the name of Jan Nel, to do the necessary tests. Nor was there a surveyor. All the setting out was initially done by me, including the calculations, until after two years or so, John Matchett arrived to assist me.
When foundation excavation was started, the railway line from Oudtshoorn to Klipplaat was running slap through the middle of the site, and a gap had to be left until the line had been diverted by the SAR. This diversion took place simultaneously, under the direction of a senior SAR engineer Plantema. An upstream arched coffer dam and a gravity downstream coffer dam were built, with a diversion channel on the left bank, which allowed only for the occasional freshets coming down the river.
Setting out the most complicated shape of the domes and their intersections with the buttresses was a major problem because of the severe overhang of the arches. Points could thus not be marked on a previously cast lift. The shape, as determined by the model tests, and expressed mathematically was defined as a series of ellipses with continuously changing major and minor axes. The joints are vertical parabolas, while the transition from ellipse to the straight lines of the buttresses was circular. It had been suggested by Head Office to mark the joints on concrete strips at the bottom of the dam, and use plumb bobs to transfer the markings to the shuttering boxes. I considered this proposal impractical and devised a completely different one. I accurately measured a base line by tape, and established a series of setting-out points by triangulation. With the aid of an electric Olivetti Divisuma calculator, I calculated the co-ordinates of the upstream and downstream intersections of the joints with the intrados and extrados of the arches at the various pouring levels, as well as the exact centre of the extrados. This was before the advent of computers, and was a very laborious calculating task, involving the solutions of thousands of quadratic equations. In addition I calculated the co-ordinates of the intersection of two lines: the line from the left hand extrados intersection to the right hand intrados intersection and the line from the right hand extrados intersection to the left hand intrados intersection. To establish this point of intersection on the concrete of a previously cast box, angles were calculated from three of the setting-out beacons. The surveyor’s assistant – Elias, whom I brought along from Ebenezer, had a tripod fitted with an optical plumb bob. He was directed by me from the one beacon to mark two points along the calculated line, and mark that line by pencil on the concrete. I then set up on the second beacon, to enable the assistant to mark the intersection. I then went on the box with my tachy, orientated towards the beacons I had used, and checked whether I was on the correct position by pointing to a third beacon, thus not only checking the setting out, but also the triangulation calculations. After that, I had the assistant marking the direction of the lines towards the four corners of the box and to the centre of the extrados, which also had been previously calculated. The corner points themselves could not be marked directly, because three of the four corners were then in mid-air, because of the overhang of the arch, as well as the joints. The carpenters were supplied with a drawing showing the calculated distances from the setting-out point to the corners and the centre of the extrados, as well as the calculated offset at the centre of the extrados and other calculated dimensions, such as the thickness. These enabled the carpenters to check their work, as well as my own work, because any error would be shown, whatever its source. As can be imagined, all these calculations were very laborious indeed, and took up many man-months of high-level staff. Later, when I was transferred to head-office, I attended a computer programming course from IBM. As a practical problem, I calculated all these data for the Kat River Dam, then under construction, which greatly facilitated things there. Attached are the instructions I drew up for that purpose.
Shortly after the excavations under the protection of the coffer dams were virtually complete and the rock cleaned, a major downpour occurred in the Karoo, which flooded the works and left a liquid soup of silt, which had to be laboriously removed. The presence of an operating railway line through the middle of the works proved to be a bit of a hindrance, especially after concrete placing had started. A gap had to be left in one of the arches.
Aggregate was obtained from deposits of river boulders and sand. Here was one area where Minister PK le Roux profited, because he charged a few cents per cubic metre extracted from the river bed on his property. I am not sure whether he was entitled to this compensation, because, without the dam, these deposits had no value. The boulders were graded and crushed as necessary, while only some of the sand could be used because of the presence of shale particles. Sand was obtained from deposits near the road from George through the Langkloof. Very low slump, high-strength concrete was placed, using heavy vibrators. Liebherr cranes were used to place the concrete. Over part of the river channel, a bridge was constructed to carry one of these cranes.
The main buttress consisted of two parts, joined on the upstream side and at the bottom. In addition heavy slabs joined them at intervals. I designed frames using rails from the stretch of railway that was left after the diversion. The shuttering for these slabs were suspended from these frames, which also served as reinforcement. Similar frames were used to make a bridge carrying the tower crane on the steep left bank. The hollow portion between the parts were used for the silt outlet, operated by a radial gate and protected by an emergency gate upstream, manipulated by a gantry on top of the buttress. The outlet pipes were also housed in the hollow buttress, ending in sleeve valves. During construction the silt outlet had to be used to evacuate occasional small floods. At one stage there was a major flood overflowing the half completed dam without major damage.
At the upstream side of the two buttresses, holes were drilled through the buttresses into the foundation, to house pre-stressing cables. These are intended to increase the stability of the buttresses. In those days, only white operators were allowed to operate the percussion drills. At one occasion, all of them were sick or absent and their brown assistants continued drilling, showing that they were at least as capable. I considered this a great injustice at the time. They were not even allowed to drive trucks!
Grout holes were drilled in the foundation plinth upstream of the dam. A series of relatively shallow holes were grouted with the aid of a site-manufactured grouting plant. This was followed by a deep grout curtain. I trained a man who was working as a lorry driver to do the grouting. He was a recuperating drunk, who used to be an accountant, and thus good at keeping records. Unfortunately he relapsed about the time the job was finished.
Target tiles were installed in the arches that could be monitored for deformations and movement of the structures from accurately placed survey beacons. These could be serviced from a walkway all along the downstream face of the arches.
As soon as sufficient storage capacity was available to store occasional floods, the stilling basin downstream of the main arch was constructed. The secondary arch is fitted with an emergency spillway at a higher elevation.
Theo Hooper left me very much alone to solve the technical problems and restricted his work to management and finance. At one stage, he was on leave, which gave me the opportunity to start the stilling basin below the main arch, which I considered overdue. When he came back, he remarked that he should go on leave more often seeing that the work advanced so well! We got on very well with the family, which lived next-door to us, consisting of Theo, his wife Sybil and children Lindsay and Andy. At one stage, before the railway had been diverted and sections of the concrete work had been started all over the place, the two families took a walk together and were standing at the foot of the cliff on the left bank, which gives a good overview of the site, when Sybil remarked: “It will never fit!” This remark gave me nightmares afterwards! During our time there, my third daughter, Lize, was born. We were also friends of John Matchett and his wife June, whom he married not long after he arrived on site. Their son Marc was born at Stompdrift. I believe he is now a senior engineer on the Lesotho Highlands Water Project’s second phase.We made friends with a number of the local farmers, all well-educated. There were Gerrit (Swepie) le Roux, the son of PK and his wife Ann. I already knew Gerrit from university days. The two Schoeman friends were both Jurie and to tell them apart, were called Jurie Voëlsang (wife Bernie) and Jurie Vlakplaas, (wife Betsie) after the name of their farms. I played tennis either on site or at the De Rust tennis club.
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