| TECHNICAL |
UCT RESEARCH SHEDS LIGHT ON PICP
STORMWATER PURIFICATION
Permeable interlocking concrete paving (PICP) provides two functions: controlling the rate, as well as the quality of stormwater drainage from roads and parking areas in urban environments. The flow rate is managed by temporary storage within the PICP structure. The quality is improved through a variety of different mechanisms, depending on the pollutants, which include heavy metals, hydrocarbons, pathogens and nutrients. Sedimentation plays an important role in the removal of some of these pollutants, through deposition in the openings between the pavers. However, nutrients are generally soluble and thus tend to pass through the openings into the underlying aggregates from where they are released back into the environment. In Sout h A fr ica, PICP ’s nutr ient removal (ie the removal of orthophosphates and nitrates) has generally been treated as secondary to its drainage capacity: more as a nice-to-have than an essential function. However, nutrient removal plays a vital role in preserving urban stream- and river-based ecosystems and the more it is researched, studied and understood, the more effectively it can be achieved. Pollution control Engineers who design for pollution as well as drainage control must contend with factors such as the quality of the local stormwater, the type of pavers, the possible use and location of different geotextiles, the difference in performance between washed and unwashed aggregates, the impact of piped outlets and the timing of rainfall events. Studies on pollutant removal from PICP stormwater have been conducted worldwide, but no common agreement has yet been reached on the most effective designs. For example, there is is a lack of consensus on the inclusion of a geotextile to enhance water purification because its presence has been observed to accelerate the clogging of PICP systems. Furthermore,
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PRECAST | ISSUE ONE | 2021
conditions vary from location to location, A f inal layer of “pea-sized” gravel let alone country to country, so without (2-4mm quartzite/gritstone) was placed local knowledge on the type and nature between the pavers to provide a f low of pollutants and how best to deal with path for the surface water. Some cells them, design engineers are left shooting in included a layer of geotextile between the the dark; a situation which base and bedding layers, has prevailed in SA since while others did not. In the inception of permeable certain cells the aggregate paving in 2007 – until now. was carefully brushed clean, Kimberley Liu BSc (Eng), while in others, the stone a civil engineering Master’s was left in conditions more student at the University typical of installations in of Cape Town (UCT) and the field. In one case, there her supervisor, Prof Neil was a raised outlet to create Armitage, published a paper a permanent wet sump at in 2020 which sheds light the bottom of the cell. Three on local pollutant removal. types of pavers were used in Entitled The Link Between (Above): Kimberley Liu. the research: Aquaf low® Permeable Interlocking Concrete and Permealock® ( both Paving and Nutrient Removal, it covers research specifically designed for PICP designs) into the nutrient removal performance of 10 and a standard exposed aggregate paver PICP test cells constructed at UCT’s civil laid with gaps for water infiltration. Three engineering laboratory. types of geotextile were trialled: Fibertex® (non-woven), Kaytech® bidim (non-woven) Test cells and Inbitex (non-woven). Each test cell comprised a HDPE container In addition, field testing was carried out ensuring 1 200mm x 1 100mm x 400mm on the new engineering building parking lot (depth). They were fitted with perforated located on UCT’s upper campus in a bid drains with a valve-controlled outlet. Each to verify the laboratory investigations in a cell was filled with a 250mm sub-base layer more realistic setting. Monitoring chambers of 50-63mm aggregate, covered with a were installed at the underdrain outlets to 100mm base layer of 19-25mm aggregate allow for water quality testing. and topped with a 50mm layer of 2-6mm Infiltration tests were carried out to bedding gravel. determine the inf iltration rate of each
