Improving the design, construction, and maintenance of Permeable Interlocking Concrete Pavement (PICP) in South Africa

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Imperviousness caused by urbanisation leads to increased runoff volumes, flow rates, and contaminant loads. These may be mitigated by measures such as Sustainable Drainage Systems (SuDS) that seek to mimic the natural hydrology of a site. Permeable Pavement Systems (PPS) are a popular SuDS source control measure, with Permeable Interlocking Concrete Pavements (PICP) being the most common. PICP comprise specially designed concrete pavers, inter alia, with grooved and dentated sides forming ‘joints' to allow runoff to percolate into the underlying single-sized basecourse layers and potentially the subgrade. A geotextile may be fitted between the bedding and base layers and/or base layer and subgrade. However, many PICP installations in South Africa have prematurely failed through clogging. While some failures can be attributed to poor design, construction and site management, there is ongoing debate regarding the principal failure mechanisms, particularly in connection with the inclusion of a geotextile under the bedding layer. Field investigations were thus conducted at eleven sites in Cape Town and Gauteng to provide evidence-based data related to clogging. Additionally, accelerated laboratory tests that focused on the role that different pavers and upper geotextiles played in the clogging process were conducted at the University of Cape Town (UCT). The investigations identified four types of clogging mechanisms: i) Type I is when sediment fills up the top 30 mm of the joints, ii) Type II is when the sediment deposits on the bedding immediately below the joints, iii) Type III is when the sediment mixes with the bedding and clogs the geotextile, iv) Type IV is clogging of the underlying layers. The field observations indicated that rapid failure of PICP correlates with: high sediment loading from adjacent areas for example as a consequence of being linked to excessively large impervious surfaces, leaf and pollen drop from overhanging vegetation, abrupt braking at highly trafficked areas – in particular at intersections often widening joints, and lack of maintenance. The non-woven heat-bonded upper geotextiles that have been specially selected in the past owing to their ability to promote nutrient removal in PICP were found to be susceptible to puncturing in areas with considerable turning traffic. Maintenance trials demonstrated that air blowers could remove Type I clogging, but probably at the cost of accelerating Type II clogging. The presence of a geotextile was seldom a cause of clogging except in limited, specific circumstances. Accelerated laboratory tests showed that the smaller the paver joint opening ratio, the faster the joint was clogged by non-cohesive soils.