South Africa’s Echo group has had considerable success in finding and developing new markets for hollow-core, especially in security and retaining wall applications. As South Africa has one of the world’s highest crime levels, good security walling has become an unfortunate necessity. The first leg of this article highlights a recent security walling contract which used Echo hollow-core for its construction. The second outlines an innovative form of retaining wall which was jointly developed by an Echo engineer in collaboration with an external engineering consultancy.
Waterfall Estate Security Walling
When completed in 2020, Waterfall Estate will include the largest shopping mall in southern Africa, the Mall of Africa, as well as residential estates, retirement villages, business parks, a hospital, five star hotels a private school, and an inter-city train station.
It is situated in Midrand midway between Johannesburg and Pretoria and is being developed by Century Property Developments.
The security wall is situated on the eastern border of the estate and is just under one kilometre long and 4,2m at its highest point. It was constructed with prestressed hollow-core panels manufactured by Echo and forms a secure boundary between what will become a wooded parkland and a new public road.
The wall is an example of Echo’s full-service security-wall solution which involves column and foundation designs, civil and construction work and wall panel installation. It was a joint venture between Echo Prestress, Encon and V-Con Civils and was built in two sections, one at 540m and 3,6m high, the other at 400m and 4,2m high. This height variance is not readily apparent to the naked eye, mainly because there is no stepping and the wall follows the natural slope of the land. To facilitate this, ground-level panels were cut trapezoidally (at the same angle as the gradient).
Six metres long and 120mm wide, the prestressed panels were secured between H-section galvanised-steel columns which were bolted onto precast foundation slabs measuring 1 200mm wide x 2 300mm long and 250mm deep. No other foundation material was required to support the wall and this realised considerable savings in time, materials and money.
Echo Group marketing director, Melinda Esterhuizen, says there were several advantages to this type of walling, speed of construction and cost being major considerations.
“Eight to 10 bays or 48 to 60 linear metres were completed daily (eight hours). A conventional masonry wall would have taken two to three times as long with no advantage gained in strength or durability. In fact, because our panels have a compressive strength of 50MPa, they are virtually indestructible.
“The cost of building a security wall using prestressed hollow-core slabs is considerably more economical than an in-situ wall offering the same properties. Moreover, precast walling requires no shuttering or propping, onsite curing, formwork or grouting,” says Esterhuizen.
Commenting further Esterhuizen said that the Waterfall Estate wall had added architectural features such as gabion cladding on the inner-facing steel columns, textured painting and the attachment of electrical wiring on top of the wall.
“We are currently researching additional architectural finishes for future wall projects,” she said.
Esterhuizen concluded by saying that the concrete slabs and steel support columns are both 100% recyclable and re-usable.
“The wall itself has a very long life span, is maintenance free, and other than occasional cleaning, no other maintenance is required. It can also be dismantled and re-used.”
Green Cross Medical Centre
Using precast concrete hollow-core slabs in combination with geotextile membrane, this project represents a new concept in retaining wall systems, Gravity Retaining Walling. The wall was constructed at Green Cross Medical Centre which is situated in Lords View, a new industrial estate in Chloorkop, Midrand.
According to Echo’s technical director Daniel Petrov, the original intention had been to construct an in-situ retaining wall. However, Echo’s engineers believed that a precast concrete wall would offer a more cost-effective option and submitted an alternative design proposal based on the use of prestressed hollow-core slabs in combination with geotextile membrane.
The design offered substantial savings in reinforced concrete and formwork, not to mention considerably faster construction times. Moreover, because the walls were vertical there was a space saving over traditional gravity walls which are constructed at an angle.
When the decision to use prestressed slabs was made, the in-situ wall foundation together with steel starter bars were already in place. To minimise costs these elements were included in the hollow-core retaining wall solution.
The wall is 135m long and varies between 2,5 and 5,7m high. It was anchored via the starter bars and layers of geotextile material. The wall slabs were cast with hollow cores to accommodate the starter bars such that when the slabs were lowered onto the bars, the latter were encased inside the slabs. The bars were then grouted into position through access slots and this created a moment connection.
The geofabric was bolted to the wall using steel bracing and several layers extended Xm into the soil behind the wall. The use of geofabric reduced the size requirement of starter bar foundation material and it stabilised the entire structure by providing an additional anchoring.
The top of the wall was finished with precast concrete coping to add further aesthetic appeal.