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Quick and easy PICP infiltration test

“Armitage believes PICP has an important role to play in stormwater control.”

(Above): The wetted patch of the Modified Swift test.

(Right): Six litres of water are poured into the Modified Swift test bucket. (Below): Twenty litres of water are poured during the ASTM Standard test.

One of the requirements for maintaining a permeable interlocking concrete paver (PICP) system is a reliable, user-friendly method of testing water infiltration rates. This was one of the reasons UCT Permeable Paving Workshop leader Prof Neil Armitage and civil engineer, Peter Wium, as well as other members of the PICP workshop, spent a morning in February testing infiltration measurement devices at Cape Town’s Blue Route Mall shopping centre. Videos of each test were also made for the benefit of Quick and easy workshop members unable to attend. Three devices were tested: the Modified Stormwater Infiltration PICP INFILTRATION TEST Field test (Modified SWIFT), the Modified Simple Infiltration test (SIT) and the American Standard test (ASTM Standard). They all gave At approximately 35 000m², the Blue by the water, the lower the infiltration rate similar values. Route Mall’s parking area is the country’s and vice versa. largest PICP site. Installed in 2013, it has also The wetted area is estimated by been the best maintained and, as such, was multiplying the length of the major axis of the perfect setting for the infiltration tests. the damp area with its right-angled minor The Modified SWIFT test is based axis (ie, the area of the smallest rectangle on Australia’s SWIFT test for permeable enclosing the wetted area) and then reading paving. It involves filling a bucket mounted off the associated infiltration from a plot on three 60mm legs with six litres of water linking this to infiltration. and placing the bucket over the area to be In the original SWIFT test, which tested. A plug at the bottom of the bucket is was developed by Prof Terry Lucke of pulled, allowing the water to drain onto the the University of the Sunshine Coast, PICP surface. The greater the area covered Queensland, Australia, the infiltration was

determined by counting only fully wetted pavers – the smaller the number, the higher the infiltration rate. However, as Armitage points out, this method is problematic, as counting pavers is hard to do in practice, leaving out partially wetted pavers seems somewhat arbitrary and – most significantly – pavers come in different sizes.

The Modified SIT test, developed at North Carolina State University, USA, uses a rubber-lined 1m² steel frame. It involves pouring nine litres of water into the ring and measuring the time it takes for the water to infiltrate into the paved surface. One of the problems with this test is that leakage occurs. Also, if there is any slope, the water ponds on the downstream side, distorting the flow patterns. However, as a relative test, it performs as well as the others.

The “gold standard” for PICP testing is the ASTM Single-Ring Infiltrometer Standard test, but this takes far longer to execute and uses much more water (20 litres). It involves pouring the water into a metal cylinder which is first sealed onto the paved surface with plumber’s putty. Water is poured at a rate which maintains a constant 50mm level at the bottom of the cylinder. The rate at which the water is poured equates to the rate at which the water enters the paved surface.

Using the least amount of water by far and much the quickest, the Modified SWIFT test is the most appropriate method for multiple on-site testing.

Commenting on the results, Armitage says this is good news for property-owners with PICP installations because in the Modified SWIFT test, they have a reliable,

(Above): Pouring nine litres of water during the Steel Ring test. easy-to-use testing mechanism.

“What these tests prove is that the Modified SWIFT test is a more than adequate measurement technique. It might not be quite as accurate as the ASTM Standard test, but that level of accuracy isn’t required for establishing whether maintenance is required – and, if it is, the level and intensity of the maintenance to be performed.

“The knowledge that there’s a simple and inexpensive method of testing PICP installations should encourage propertyowners to begin maintaining their sites.

“Currently, most South African PICP systems aren’t maintained. Among the few that are, only one or two receive the regular upkeep needed for sustained infiltration and stormwater control performance. Unless PICP systems are cleaned regularly by removing dust, sand and detritus which accumulates in the aggregate between the paving blocks, they clog up and are eventually no better than conventional paved surfaces,” says Armitage.

Peter Wium, who designed the Blue Route PICP installation, says the site is maintained regularly.

“Every year we clean about one-quarter of the site on an ongoing, rotating basis. The Blue Route site is unusual in that virtually all the roads and parking areas are covered with permeable paving at a run-on ratio close to 0 – ie, there’s negligible flow from surrounding areas. It provides very effective stormwater control. Most PICP sites have substantial run-on ratios, although I’d discourage anything greater than 1 (permeable paving area) to 3 (nonpermeable paving area).”

Armitage believes PICP has an important role to play in stormwater control.

“It not only manages stormwater flows, but – if it’s installed correctly – assists greatly with the removal of pollutants from stormwater run-off. And unlike asphalt roads, which require major repair or even complete rebuilds after 15-20 years, PICP systems can be rejuvenated at minimal cost,” he says.

For further details about the Blue Route tests, contact Professor Armitage at: neil.armitage@uct.ac.za.

(Left): Measuring the axes during the Modified Swift test.

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