SOUTH AFRICA’S LARGEST PERMEABLE PAVING PROJECT TO DATE:

In this instance, PICP is being used to pave a 13 000m² car park on the West Campus of the University of the Witwatersrand. Abutting one of Johannesburg’s major thoroughfares, Empire Road, the parking ground is split in two, a lower section comprising approximately 6 000m² and an upper portion covering some 7 000m².

Designed by consulting engineers, Arup, the car park is being paved with PICP to provide on site storm water infiltration and attenuation to avoid contributing additional run-off to the Empire road storm water system.

Emannuel Prinsloo, director, property and infrastructure management division, of Wits says the university opted for this technology in view of its environmental and storm water management attributes.

Concrete Manufacturers Association (CMA) director, John Cairns, says besides the prevention of flooding, PICP offers environmental benefits such as filtering out pollutants and the storage of water for re-use and notes that all these conditions apply in the case of the Wits project.

“The threat of flooding on Empire Road will be curtailed and the water that infiltrates beneath the paved surface will replenish the underground water table,” he says.

The PICP project at Wits comprised six material strata. Starting with the bottom layer and rising sequentially to the top they consisted of: 150mm of re-compacted in situ soil; Typar geotextile membrane manufactured by DuPont and supplied locally by DV Building; 250mm of 37mm quarried stone; 100mm of 24mm quarried stone; Inbitex membrane supplied by Concor Technicrete; 50mm of 6.7mm stone; and finally 60mm rectangular paving blocks rated at 30 MPa which were supplied by Concor Tecnicrete.

The Typar membrane prevents soil mixing with the first layer of stone, and likewise, the Inbitex membrane performs the same function. Approximately 90% of pollutants will be trapped in the upper 50mm stone layer, some of which will water trees planted in tree rings in the car park.

There are essentially three types of PICP, full infiltration where everything goes into the soil,  partial infiltration where some of the water is stored for re-use, and zero infiltration where all the water is stored for re-use. Apart from the pipes which feed the tree rings, the first option applies in the case of the Wits project.

The paving blocks were laid in a herring bone pattern, which according to Concor Technicrete’s product manager, Taco Voogt, is the simplest and most effective method for the heavy traffic conditions of a car park. Based on a European design, the pavers include vertical slots (3mm deep and 40mm wide) at either end through which the water infiltrates into the sub-base.

Prior to the commencement of the project, six ‘soakaway’ tests were conducted to ascertain the capacity of the soil to accept rain water. The tests established that the soil had a more than sufficient absorption capacity with the lowest result being 79mm/h. When newly laid, the absorption rate of the pavement will be approximately 4 500 ltr per square metre per hour.

Voogt says this rate will stabilise at 20% of the original rate after 15 years or roughly 900 ltr per square metre per hour. In other words 0.9 cubic metre of water will still be absorbed into the pavement in an hour, i.e. 900mm/h.

Weko Civils was the main contractor and as such was responsible for the earthworks and laying of the sub-base layers. The paving contractor was Mondo Meccano, a CMA contractor member.

A new car park situated on the West Campus of the University of the Witwatersrand, which is being paved with a permeable paving system. The pictures above show the various layers of stone, membrane and paving stone of which the system is comprised.

Close up views of the laid permeable paving stones at a new car park under construction at Wits University.

One of the tree rings which will be fed with infiltrated water at a car park surfaced with permeable paving at Wits University.