The City of Cape Town has embarked on a project to bolster the City’s water supply by accessing groundwater from the Table Mountain Group (TMG) Aquifer in the area surrounding the existing Steenbras Dam. Situated, in the Steenbras Nature Reserve in the Hottentots Hollands Mountain range, the dam is one of several essential water reservoirs supplying the city.

The Steenbras wellfield is one of the City’s new water programme projects, involving the drilling of 12 production boreholes into a section of the TMG Aquifer on sloping land above the dam’s northern bank. The scheme will ultimately contribute a volume of up to 30 megalitres a day to the City’s water supply system.

Each borehole is being equipped with state-of-the-art electric pumps and control equipment , supplied by main contractor, Hidro-Tech Systems.  This equipment is securely contained in pumphouse structures constructed with reinforced precast concrete panels supplied by CMA member, Cape Concrete.

The pumphouse structures were originally designed as in-situ concrete structures, but this specification was altered by consulting engineers, Zutari (formerly Aurecon), in preference for precast panels. The reason for the design change was the potential damage which spilled concrete and contaminated water could inflict on the environmentally sensitive eco system of the Steenbras Nature Reserve.

Each pumphouse structure measures 34m² and consists of four separate chambers, constructed on a concrete base. These were cast in-situ in a very controlled manner to minimise damage to the surrounding vegetation.

Kiewiet Viljoen of Hidro-Tech said that besides being potentially harmful to the environment, activities such as batching on site and in-situ wall construction would have taken much longer.

“Substantial time savings were gained by opting for precast concrete. Once the flooring was cast it only took a day to a day-and-a-half to erect each of the pumphouse structures. What’s more, precast concrete panels offer exceptional strength – a very necessary feature in providing for vandal and theft-proof barriers.”

Cast in Cape Concrete’s yard in 13 sizes, the panels came with external steel brackets to facilitate a Meccano-style assembly of the pumphouse structures. The joints between panels were sealed with Jointex, a self-adhesive sealer which was used to create a tight waterproof seal. The roofs were also constructed with precast panels which were tapered to facilitate water run-off. The roof panels were sealed with a polyurethane sealant and a flexible membrane supplied by Sika.

Viljoen explained it was necessary for the pumphouse structures to be waterproofed because they hold medium-voltage transformers, LV motor control centres and industrial PLCs. They also include vent ducting to avoid internal condensation.

In keeping with the nature-based environment, the precast panels were clad with natural stone for aesthetic appeal.  Loose cladding was not used on the roofs as it would have most probably been damaged by baboons and would have the potential to harm other forms of wildlife and personnel working at the site, so instead they were painted green.

Precast panels were also used to construct 7m² underground flow meter and valve chambers adjacent to the pumphouse structures. The chambers house electronic flow metering equipment and the pipework, which connects the boreholes to the main feeder pipeline.

Members of the project’s professional team include: Zutari (formerly Aurecon engineering consultants); Hidro-Tech Systems, main contractor-mechanical and electrical; Baseline, civil and construction work; Cape Concrete, precast concrete supplier and pumphouse construction.