PICP - A CASE FOR REGULAR PERFORMANCE TESTING AND MAINTENANCE

Permeable interlocking concrete paving systems (PICP) established a tentative toehold on the South African landscape 13 years ago. The first known system was installed in 2007 at The Reeds, a townhouse complex in Fairlands, Johannesburg.

As with all PICP systems worldwide, the prime objective – then as now – was the prevention of flooding through the controlled discharge of stormwater into municipal drainage systems. This was to be achieved by creating a surface which facilitated stormwater infiltration, rather than run-off, as found in traditional impermeable paved surfaces. With the introduction of Cape Town’s Management of Urban Stormwater Impacts Policy in 2009, PICP was quickly adopted by local stormwater professionals to reduce the negative impact of impermeable sur faces on equatic ecosystems.

Gaining traction Notwithstanding dozens of installations countrywide, PICP still has some way to go before it can claim fail-safe status. It is hard to find a single installation without some design flaw. These include the use of dirty aggregates, inappropriate use of geotextiles, pavers with poor interlocking properties and the use of sand as the filler between pavers.

Then there is maintenance. Many property-owners appear to be unaware that PICP systems require maintenance, but regular removal of sand and other debris from between the pavers to prevent blocking is essential to their proper functioning.

Cape Town f aces the additional problem of windblown sand, which ac- celerates the blocking process. As yet, it would appear that there is no local installation which has been properly maintained, although the PICP system at Cape Town’s Blue Route Mall comes close. The results were inevitable; apart from new installations, no South African PICP installation is known to be operating optimally and some – such as the parking lot at Cape Town’s Grand Parade – are totally blocked. Now 10 years old, the parking lot was constructed shortly before the 2010 Fifa World Cup.

Working group Fortunately, this situation has been identified as a major problem by the University of Cape Town (UCT ), which is tackling it head-on. Over the past year, Neil Armitage, deputy director: “Future Water” and a professor in UCT’s Department of Civil Engineering, has been conducting research which aims to improve the sustained performance of all South African permeable paving installations. He is being assisted by a working group of professionals and civil engineering students at UCT and some of their findings have been discussed in previous issues of Precast. In this issue we take a brief look at some research on PICP performance testing and maintenance aspects undertaken by one of Armitage’s students, Cole Barnard, for a thesis which forms part of his BSc in civil engineering.

B ar nar d asser ts that b e c ause PICP technology is still relatively new, maintenance requirements have yet to be categorically established across the full range of installation types and conditions.

(Above): Measuring a SWIFT test wetted area.

Sustainable solutions “This lack of information as to how regular maintenance affects the longevity of PICP systems has been identified as a limiting factor in improving the wider use and acceptance of PCIP. My research is intended to contribute to the limited source material on infiltration capability and testing methods of permeable pavement systems in the South African context and thereby to promote sustainable counter-solutions to conventional pavement designs which, in turn, will play a role in solving SA’s current water management and shortage problems,” writes Barnard.

He tested infiltration rates on seven sites in greater Cape Town. Some of them had not been previously tested, while others had test results data dating back to 2017. Only two sites were tested in Johannesburg, as most systems were located in gated communities with restricted access.

Barnard’s research involved testing the infiltration rates using the ASTM Single Ring Infiltrometer – the American standard – and the Stormwater Infiltration Field Test (SWIFT ) method modified from its original Australian use. Although the Single Ring Infiltrometer test, which measures actual infiltration

“SA needs an overarching governing design code to ensure permeable paving systems are designed and installed correctly.” rates, is the “gold-plate” standard, it is impractical, as it is time-intensive. Therefore, part of his research entailed establishing a correlation between the ASTM and a modified SWIFT test, which – as its name implies – yields much quicker results than the ASTM Infiltrometer test. The rationale behind this was to give PICP owners and researchers a reliable method to establish infiltration rates with equipment that is easy and inexpensive to use. Testing required The original SWIFT test is based on the number of fully wetted bricks resulting from discharging six litres of water from an elevated bucket onto the PICP pavers. Barnard found counting the number of wetted bricks to be excessively timeintensive. Instead, he measured the length and breadth of the wetted surface and a p p r o x im a t e d i t s area by using the formula for an ellipse. The best-fit curve between the approximated wetted area and the infiltration rate was found to be: I = 1958 – 1 056In(A) Where: I = infiltration rate (mm/hr) A = π (a x b) / 4 = wetted area approximated by an ellipse (m²) a = longest wetted length (m) b = wetted length perpendicular to a (m) Infiltration data was collected from each site and evaluated against the catchment area which the PICP installation was serving. In order to determine the required infiltration rate needed to handle five-minute design storms with 50-, 100- and 200-year return periods, the effective rainfall was calculated by multiplying the design rainfall by the ratio of the total catchment area to the PICP area. Full study Barnard investigated how age and environmental factors affect PICP systems and determined that without maintenance, PICP typically loses about 25% of its infiltration capacity per year. This may be exacerbated by local conditions. The ultimate objective of Barnard’s research was to assist in the creation of industry guidelines for the construction and maintenance of PICP systems, thereby encouraging more use of PICP systems, their better care and improved lifespans.

His research led to several conclusions. He established that, provided they are installed correctly using the specified codes, PICP systems are efficient stormwater management systems. They are able to perform at higher-than-average drainage capability when properly maintained and can also perform at acceptable drainage capability for a while, even when they are not properly maintained. However, unless deterioration is monitored regularly, full blockage and hence unanticipated flooding can occur on site. Limiting debris Barnard also found a reasonably accurate relationship between wetted areas and infiltration rates. This facilitates the rapid estimation of the latter, which makes it easier to determine how a PICP system is performing and thereby the required maintenance frequency.

Barnard believes that PICP siteowners should investigate means of limiting debris – for instance, by preferring evergreen to deciduous trees and the use of retaining walls for flower beds to prevent the deposition of soil on the pavers. Regular site clean-ups such as litter drives will assist in maintaining site functionality. He notes that the most efficient means of maintenance is restorative vacuum-cleaning with the aid of specially designed equipment.

Barnard concludes by saying that SA needs an overarching governing design code to ensure permeable paving systems are designed and installed correctly, and that contractors use specified materials. Moreover, he cautions that PICP should not be installed at all if property-owners are not prepared to perform regular maintenance, as sites can become hazardous when fully blocked.