Maintenance and monitoring of permeable pavements

The research project consisted of two phases. The first phase of research aimed to identify if cleaning restored infiltration capacity. The second phase of the research aimed to test different cleaning methods and then to monitor these to understand the change in infiltration rates over time and assess what frequency cleaning is likely to be needed. The findings from the testing would lead to the development of a cleaning program that council maintenance staff can undertake, as well as what the costs of the on-going maintenance would be.

Locations assessed

Permeable pavement assets at the following locations were assessed for this project:

• Permeable asphalt – Eades Place, West Melbourne
• Permeable asphalt – Harris St, North Melbourne

Cleaning methods

One full day was used to conduct the cleaning and infiltration tests of the selected permeable pavement installations. In this project three cleaning methods were conducted by a private contractor (Tox Free) to understand their relative ability to restore asset functionality. The cleaning methods used were:

• Pressure washing
• Dry vacuuming
• Combination of dry vacuuming and pressure washing

These three cleaning methods were trialled at Eades Place to understand which method was the most effective in restoring asset functionality. Subsequently, only the superior cleaning method (Pressure washing) was used at the Harris St installation.

Infiltration testing methods

A methodology for infiltration testing was developed and confirmed with the City of Melbourne, using the Simple Infiltration Test (SIT) proposed by Winston et. al. (2016) and the standard test method issued by the American Society for Testing and Materials (ASTM) as a basis (Designation: C 1701/C 1701M – 09).

To assess the progressive change in infiltration capacity of permeable pavements following an initial clean over time with testing at intervals such as immediate, four, eight and 12 months.

For phase one of the research City of Melbourne tested the effectiveness of permeable pavements and how maintenance influences this. The results for infiltration testing prior to cleaning showed all test sites were compromised by surface clogging. Test areas were cleaned (Figure 1) and results for post-cleaning infiltration testing indicated much higher infiltration rates could be achieved. This confirmed that certain maintenance activities are effective in restoring functional performance (Table 1).

Figure 1. The image on the left shows the pavements before pressure washing. The image on the right shows pavements after pressure washing.

Table 1. Infiltration rates pre and post cleaning at Eades Place and Harris Street.

The results demonstrated that pressure washing (Figure 2) is the most effective cleaning method in improving performance. This method was able to restore infiltration rates beyond 12 months, with over 55% of the testing sites at Harris St providing greater than 300mm/hr.

Figure 2. Application of pressure washing to clean permeable pavement.

Testing also identified infiltration rates decline at highly variable rates spatially across a permeable pavement. Areas with higher loadings of sediment, granitic sand from garden beds and organic matter from trees decline more rapidly. Areas in the ‘middle’ of car parks tended to have higher infiltration rates. This indicates a need for testing of a range of sites to properly characterise condition of an asset.

Even impaired assets usually had sufficient infiltration capacity to infiltrate most of the mean annual flow volume and events in the order of 1 Exceedance per Year (EY) through to 5 or even 10 EY although highly clogged assets with almost no infiltration capacity were also observed (notably Harris St before cleaning).

The research provides evidence to support the installation of more permeable pavements and outlines the cleaning regime to ensure optimal asset performance. The research will not only assist in the ongoing maintenance of installed assets but also will inform future design and construction practise.

Key findings from this work

• Cleaning restores functionality of permeable pavements, with pressure washing the most effective method
• Cleaning should occur once a year post autumn leaf drop.