Ravenswood Soil Re-use

The Brief

We were engaged by Ravenswood Developments Limited to support a multistage subdivision development, providing geotechnical and environmental expertise through earthworks and construction. During earthworks for a later stage of the development, peat was encountered in the underlying soil profile, which required excavation for geotechnical reasons. 

Challenges Identified

Peat bogs are often associated with the presence of arsenic, as it can bind strongly to organic matter and become relatively immobile. An initial assessment of the peat indicated that arsenic was present at concentrations above established background levels, meaning the peat was unsuitable for disposal to off-site cleanfill sites. Reuse on-site was not possible, and alternative reuse options were limited due to the presence of large organic pieces. 

What We Did

We worked with Ravenswood Developments Ltd to assess the amount and quality of peat that needed to be excavated, analyse its properties and level of arsenic contamination, and explore options for its responsible disposal or reuse.

Approximately 10,000 m³ of peat needed to be excavated for geotechnical reasons. Instead of sending this valuable soil to costly and unsustainable landfills, we looked for better ways to reuse it. We worked with a local farm that needed soil to improve drainage and address flooding problems, and our planning and civil team navigated the necessary permissions to move the soil and repurpose it there. 

To ensure the reused soil met safety standards, we developed a method that involved screening the large pieces of peat, then mixed them with other unsuitable soil and surplus topsoil from the development. The goal was to reduce arsenic levels to match or be lower than natural levels. A pilot study was conducted to determine the right mixing ratios, and we performed continuous sampling and lab analysis during the four-month screening and mixing process. 

The project faced challenges involving the removal of the arsenic-containing peat from the development area. It was crucial to identify the source of the arsenic to demonstrate to the local council that the contaminant was naturally occurring and not from anthropogenic sources. Additionally, reusing the soil on nearby land would slightly change the ground level, affecting drainage patterns. We carefully assessed potential flooding risks to neighbouring sites and secured the necessary consents from the local council. Navigating these types of challenges is managed more seamlessly due to our thorough understanding of the applicable laws and working closely with regulatory bodies. 

Key Deliverables

Adhering to local regulations and obtaining consents can be complex and time-consuming, but we overcame these obstacles by being well-versed in the relevant laws and maintaining close collaboration with regulatory bodies. Soil reuse requires extensive stakeholder engagement in terms of consultation and approvals. 

The soil was repurposed effectively on a nearby farm, avoiding the need for costly off-site disposal that earlier consultants had deemed necessary. This shift in approach brought about cost savings in both disposal and transportation. Through careful screening and mixing, arsenic levels in the soil were successfully lowered to match or fall below natural levels. This approach had several positive outcomes, including the environmentally friendly reuse of soil within the same area and minimised soil disposal costs, as well as a reduction in flooding on the reuse farm, which in turn enhanced its productivity.