SESL Australia recently completed work on the Melbourne Metro Rail Tunnel project as part of a consortium under the umbrella of Loci Environment & Place. Loci is a registered charity dedicated to disseminating information to the public via its Urban Well.
SESL Australia’s involvement was led by Declan McDonald, SESL’s Senior Soil Scientist. Declan developed a range of soil specifications that were included in the contract documentation. This ensured that tenderer proposals incorporated optimised soils for various installations. The specifications included provisions for the development of tree pits to maximise the performance of new tree plantings in congested urban soils. The use of tree pits reduces the overall footprint of tree planting. However, through shared root zones and access to native soils where possible, a substantially higher number of trees can be accommodated in a given area.
Another significant innovation in the project was the inclusion of passive irrigation for street trees. This required the precise specification of soils capable of receiving large quantities of stormwater during rainfall events, while also draining rapidly. These soils filter pollutants from the stormwater and retain sufficient water for optimal tree growth.
The islands in the project range from 1.4 to 4.2 hectares (3.5 to 10.4 acres) in area, with distances between islands averaging 100 metres. The project was unveiled in May 2003, and dredging of the marine sands began roughly four months later.
This major construction utilised 321 million cubic metres of sand and 386 million tonnes of rock dredged from the coastal regions of Dubai, which were displaced into the shorefront to form the sand banks. The limestone rocks used to support the structures were sourced from multiple quarries around Dubai.
SESL conducted a scientific review of the natural soil resources available for the project and provided recommendations for the soil and landscape work concepts. We also developed strategies for soil reconstruction.
The review detailed several recommendations to maximise the project's success. The location of this man-made landscape had high erosion potential due to its constant exposure to coastal currents.
To prevent the erosion of the islands, a water-permeable geotextile was added to the lowest layer of the breakwaters. One tonne of limestone sits on top of the sand, and another two layers of larger boulders cap and separate each structure. Additionally, the breakwater was designed to include two 100-metre openings on either side to eliminate stagnation in the islands' deep and narrow channels. These openings allow for complete water circulation every 13 days.