Eades Place permeable asphalt

Eades Place, like most standards streets, is designed with a slight elevation in the centre of the road so that when it rains the water flows to the edge of the street. Before the street was redesigned, this water ran in to the gutters and flowed straight into the stormwater drains.

We replaced regular asphalt along the side of the street, under the parking bays, with permeable asphalt so that the water is intercepted before it runs into the gutter. The water soaks through the permeable asphalt and into the new structural soil below, irrigating the trees and cleaning the water. A drainage pipe at the bottom returns any excess water to the stormwater drainage network.

Original budget:                      $190,000

Estimated cost:                       $360,000

Final cost:                                $720,000

The final cost of the project was significantly higher than original expectations. This occurred as a result of unexpected electrical cabling being found during excavation of the site. Although it was redundant cabling, it still caused delays and took resources to resolve.

Construction:  December 2011 to March 2012

The City of Melbourne designed and delivered this project.

The following tasks were completed during the research and planning phase of the project.

MUSIC modelling

We used MUSIC modelling to inform the design development and achieve the highest nutrient and flow reduction possible in the Eades Place context.

Concept designs

We created two different concept designs: one with a vegetated centre median and new trees along the western side of the street, the other without the centre median. These concepts were taken to community consultation.

Community engagement

Local residents were involved from an early stage in this project, helping to shape the final design outcome for the street.

Detailed designs

Based on feedback from local residents, the final design was drawn up without the centre median.

Eades Place is a wide residential street, measuring about 30 metres across. This expanse of asphalt absorbs radiation from the sun, increasing the local temperature. Thermal imagery shows that Eades Place is one of the hottest streets in the area.

We wanted to add as many trees to the street as possible to provide shade and atmospheric cooling.

This thermal image was taken in the middle of the night after several hot days in the summer. Red indicates hot areas, showing that Eades Place is hotter than surrounding areas.

Under the Melbourne Planning Scheme, we are responsible for public open space and streetscapes. As the works cost less than $1 million, we did not need to apply for a planning permit.

We engaged extensively with the community engagement on this project. Our original concept design included a central median, but this was altered following feedback from the community highlighting concerns about visibility and safety. The final design was a compromise between achieving the most canopy cover and cooling possible and the priorities of local residents.

Permeable asphalt

Dimensions: 700 mm deep, varied widths depending on bay sizes
Location: western length of Eades Place, underneath the car parking spaces
Structure: three layers of permeable material to replace asphalt

The asphalt is made up of the following layers that allow rainwater to pass through:

Structural soil

The bottom layer of the system is 600 mm thick and made up of structural soil. The soil is compacted on top of a geotextile lining. It provides a stable yet permeable base. The structural soil is made up of:

• 80% aggregate – 40 mm crushed basalt, washed and free from clay
• 20% filler soil – clay loam and 5% composted green waste, screened to less than 12 mm
• A number of additives, including iron, potassium, magnesium and gypsum.

Aggregate

A 75 mm thick layer of aggregate sits on top of the structural soil. The aggregate is 40 mm crushed basalt that has been washed to remove any clay or sediment.

Porous asphalt

The top layer is an 80 mm thick layer of open graded modified asphalt.

Tree islands

Purpose: increase canopy cover, providing shade and cooling the street
Species: lemon-scented gums
Location: 10 new trees planted in islands along the length of the porous paving
Dimensions: each tree island is about 700 mm deep, 4.5 m x 3 m

Permeable paving provides an excellent growing environment for street trees. When it rains, water that would usually flow into drains is absorbed through the paving into the soil.

Rainwater is able to soak deep into the soil, encouraging deep root growth and improving the trees’ stability and resilience.

Permeable paving is designed to be very low maintenance, but does eventually need to be replaced. The pores in the paving capture sediment, dust and grit. Over time, this will clog the surface. This will occur first on the road edge of the porous asphalt, gradually migrate towards the gutter and make the paving surface impermeable.

The design life should be 10 years or greater.

We are measuring the success of this project by the health of the trees. Since work completed, the trees have grown very well. This shows that the passive irrigation system is working well.