Updated: Oct 25, 2022
Tunnel spoil is the waste by-product generated during tunnel excavation operations such as the construction of major road tunnels, i.e., Cross City Tunnel, WestConnex and NorthConnex in Sydney, Australia. Depending on its geological origin, spoil will consist of shale, sandstone or sandy matrix and can include topsoil, subsoil, subgrade and bedrock. The NSW EPA landfill levee for disposal currently costs $147.10/tonne (2021-22).
There are many benefits to the use of spoil in landscape projects. Being excavated from depth, tunnel spoils are usually completely free of weed seeds. Additionally, most contamination occurs in shallower depths and most often the deeper layers are not affected. While contamination of in-situ geology at depth is rare, other issues such as acid sulfate soil hazards can also occur. Tunnel spoil should always be checked for environmental hazards.
There is an alternative to disposal, which is not only an environmentally sustainable solution but financially beneficial. Convert SPOIL into a SOIL using sound science!
Spoil is a mineral material void of life. Soil is mineral and organic material that is teeming with life and viable from a horticultural perspective. To convert one into the other, you first need to understand the characteristics of spoil by assessing its geology. chemistry and physical properties.
Figure 1. Tunnel boring operation.
Sydney can be divided into 4 main geologies: Hawkesbury Sandstone, Botany Sandsheet, Bringelly Shale and the occasional diatreme.
Sandstone is derived from freshwater origin. It typically has low fertility with loose sandy topsoils and gradual increases in clay content with depth. Sandstone has the potential for many soil landscaping applications from ornamental or vegetable gardens to passive amenity turf areas.
Sandsheet has a low effective cation exchange capacity (eCEC), low nutrients, low fertility & low water holding capacity. Sands are perfect for low fertility native species such as Eastern Suburbs Banksia Scrub, but can be ameliorated for higher fertility landscapes and sports fields.
Shales are derived from saltwater origin. They have a higher clay content and therefore a higher cation exchange, water & nutrient holding capacities. Sodicity increases with the depth of the soil profile, as well as increasing further inland, as there is less leaching in the drier climate. Shales are hostile to plants so a strict amelioration strategy is required to neutralise the spoil for horticultural use. Due to the high clay content, shale is recommended for ornamental landscaping applications, where high nutrient and moisture is often required to support plants.
Diatremes contain volcanic breccia. Although it is less readily available, breccia has high fertility and cation exchange properties making it an excellent source material for soil.
Chemical and Physical Properties
Depending on the origin of the spoil, the chemical and physical properties will differ. Chemically there can be variations in the pH, salinity, sodicity, magnesicity and nutritional content. Physically, the soil will differ in the level and grading of sand and the amount of silt and clay. All these properties determine how the soil should be amended and the landscape and planting typology.
How can I convert spoil into soil on my landscape project?
No matter the size of your project (swimming pool or road construction), if you are excavating into subsoil and are unsure what to do with the overburden, it may be worth considering its reuse potential to save on disposal costs..
A soil scientist can characterise the extracted spoil and provide an informed decision on its management reuse opportunities. If the spoil contains a suitable proportion of sand, then the reuse potential is vast. Note that the extracted product may not look like sand.
In Figure 1 (a) below, the spoil at 10 m depth was a liquid and initially gave the appearance of an unusable material. Once drained and dried, the spoil proved to be a viable base material for a range of landscape soils including ornamental gardens, passive turf, wetlands and even sports fields. The ratio of imported compost and sand differed with each landscape type. In Figure 1 (b) the spoil was blended with 20 % v/v site-won crushed sandstone bedrock and 20 % v/v imported green waste compost to develop an AS4419 (2018)*1 compliant sandy loam garden blend.
Figure 1. (left) Drilling for tunnel spoil samples. (middle) Saturated and sloppy spoil from 10m depth. (right) Spoil from sloppy spoil, dried and blended with 20% v/v crushed sandstone bedrock and 20% v/v compost.
We recommend characterising the spoil early in the project as this will inform your landscape process and planting typology.
By way of demonstrating the cost-benefits of soil re-use, we have provided an example of a 1-hectare site which is being excavated to a depth of 10 m. In Option 1, the soil will be disposed of and soil to a depth of 1 m will be imported. In Option 2, the site soil will be characterised and reused. Note that this pricing is approximate and does not consider labour costs.
Table 1. Cost comparison between spoil disposal and spoil reuse.
The conversion of tunnel spoil to soil can also apply to non-tunnelling projects. Barangaroo Headland was constructed using excavated sandstone from neighbouring sites and by applying innovative soil science strategies, converted it into the iconic landscape it is today. The aim is to better understand your site material to see how it can be best reused. The environment will thank you as well as your pocket.
If you are interested in determining the savings of your next development project, then speak to one of our professional and experienced environmental/soil scientists at SESL.
1* The Australian Standard Soils for Landscaping and Garden Use 2018 (AS4419_2019)