SA’s largest prestressed hollow-core concrete panel retaining wall project

The construction of The Houghton, a development comprising two luxury residential apartment blocks (Houghton 1 and Houghton 2) and a 180 room boutique hotel development (Houghton 3), has entailed South Africa’s largest deployment of prestressed hollow-core concrete panels as retaining walls to date.

Spanning approximately 2 000m at an average height between 3.2m – 5.0m, the panels are fulfilling a dual role of retaining embankment soil and providing walling for parking basements and below-ground level service areas.

Supplied by Concrete Manufacturers Association (CMA) member, Echo Prestress, the prestressed panels were manufactured in standard 1.2m widths varying in length from 3.2m to 5.0m. Panel thickness was a standard 150mm in most instances except where two slabs, one above of the other, were required to accommodate embankment depths of up to 10m, and in these instances panel thickness was increased to 250mm.

The precast slabs used on Houghton 2 were between 3.2m to 4.1m, whereas in the cases of Houghton 1 (Residential) and Houghton 3 (The Hotel), where the contours are more severe, there was a much greater variance in slab length.

Rogan Duffy of Pure Consulting, the consulting engineers responsible for the project’s structural engineering, says that where possible the objective was to achieve as much standardization as possible in panel sizing to render their manufacture and on-site deployment as simple as possible.

Foundation support for the Echo panels is provided by 250mm deep footing channels. Additional lateral support is achieved through thickening the edge of the surface bed on the support side of the footing channel to 300mm, thereby lifting the height on that side of the channel to 550mm. Moreover, a fully cantilevered application was avoided by bolting small right-angled steel sections to the first floor slabs. These provide the head of each panel with additional support.

“In some instances we have done away with the steel angles and have allowed the Echo panels to rest against the first-floor slabs. These receive no support from the panels and instead rest on concrete columns situated inside the basements,” says Duffy.

“We are used footing channels for support because they were very cost-effective. They were very similar to the footings used for the project’s brick walls which, because they were curved, were not suitable for Echo panels. Furthermore, by integrating the surface bed into the support channel, on-site productivity was significantly improved.

“The channel or recess which provides foundation support for the panels was constructed in the same manner as any standard footing. Before the concrete is poured, a steel reinforced cage and a metal wedge is inserted into the trench. Once the concrete sets the metal wedge is removed yielding a support channel with very smooth chamfered edges.

“As with most building projects a key objective was construction speed and in this instance the use of precast panels meant that the retaining walls were generally erected three times faster than in situ construction would have taken. We wanted to have the retaining walls erected before the first floor slabs were cast. This improved access for the main contractor, Murray and Roberts, and enabled it to work off the critical path.

“As a result, Murray and Roberts was able lay about eight linear metres of walling a day in either brick or reinforced concrete, and the panel contractor, Echo Prestress, has achieved in excess of 30m to 35m of prestressed paneling erected each day without any significant involvement from Murray and Roberts,” observes Duffy.

Approximately 98% of the retaining wall requirement on the Houghton project comprised Echo precast panels.

“In some instances we were unable to use the panels owing to on-site geometry. For example, on walls with tight radii, in situ concrete or brickwork was chosen. As it turned out brickwork was used in most of these instances.

“Another advantage of Echo’s panels is that they are very easy to cut, for example in creating space for the installation of air-conditioning units. The friction between the concrete and steel reinforcing in a prestressed panel ensures that the tension and structural integrity of the panel is retained after cutting.

“We have used Echo panels on one other project and were very happy with their performance. This was why we chose the technology for this project,” says Duffy.

The Houghton is being developed by ASVID Holdings, a company headed by Irishman David Nagle, an international property developer with other property interests in South Africa. Besides the 180 room five star hotel, the development will see a total of 320 high-end apartments being brought onto the market.

Architects, Boogertman & Partners have designed Houghton 1 and Houghton 2 with considerable internal design flexibility. Apartments, which began at 190m² and rise to 350m², can be combined to form single units where required. For example, units can be combined horizontally or vertically and each has a balcony with a spectacular view over the Houghton Golf Course.

The development includes a luxury spa and gym and the golf course is being upgraded by world-renowned golf champion, Jack Nicklaus. A new club house is being built in the middle of the course.

Construction work on The Houghton began in January. The hotel and the bulk of the apartments are due for completion at the beginning of 2010. However, some apartments will come on stream and be ready for occupation as early as April 2009.

The Houghton construction site, where two Echo prestressed hollow-core panels have been inserted into a foundation recess prior to the casting of a 300mm deep by 300mm wide surface bed edge on the right side of the channel.

Prestressed hollow-core panels measuring 3.2m x 1.2m x 150mm shortly after offloading at The Houghton construction site.

A parking basement at Houghton 2 clearly showing how the precast panels provide internal walling as well as retaining support.

A completed section of precast panel walling at Houghton 1 in the process of being waterproofed.

Precast panels at Houghton 1 are supported by steel poles while the casting of the first floor slab is taking place.