Urban Water » Projects » Permeability and infiltration » Eades Place permeable asphalt

Eades Place permeable asphalt

Permeable asphalt and new trees will help to cool this West Melbourne hot spot in coming years.

Eades Place is a residential street in West Melbourne. It is 250 metres long and has car parking spaces along both sides of the street. Thermal imaging shows it is one of the hottest streets in the city.

After many years of drought, several of the elm trees in Eades Place were in poor health and needed to be replaced. We took the opportunity to redevelop the street so that rainwater could be used to keep the new trees healthy.

We removed the dying trees and also the asphalt underneath the car parks on the western side of the street. This was replaced with permeable asphalt that lets water through.

We planted 10 new trees in beds within the paving. When it rains, water soaks through the permeable asphalt and into the root systems of the trees. This process passively irrigates the trees and also cleans pollutants from the water.

 

What benefits has the Eades Place permeable paving project provided?

  • Provided an ideal growing environment for 10 new trees, creating shade and helping to cool the street.
  • Increased permeability (unsealed soil) in the street by 482m2
  • Reduced stormwater pollution by 90 per cent.
  • Reduced peak stormwater flows by 90 per cent.
  • Captured up to 790,000 litres of water each year.
Permeability
& infiltration

Project location

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Project details

How the system works

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.

12981-CoM-Raingarden-infographic-Porous-Pavement-1090px

Cost

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.

Project timeline

Construction:  December 2011 to March 2012

Project team

The City of Melbourne designed and delivered this project.

PLANNING AND DESIGN PHASE

Research and Planning

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.

Design challenges

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.

eades place hotspot

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.

 

Planning approval process

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.

Community engagement

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.

CONSTRUCTION PHASE

System components

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.

 

ONGOING MANAGEMENT

Maintenance and monitoring

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.

To monitor the system, soil moisture or water level monitoring points need to be included during construction. Such probes cannot be retrofitted into structural soils.

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