Innovative Solutions for the Redevelopment of Brownfields
Innovative Solutions for the Redevelopment of Brownfield Sites
Those who have been involved with brownfield redevelopment projects know there can be significant risks in removing impacted soils and groundwater from a site. These risks can include:
- risk to human health and the environment
- risk of ballooning costs where impacts are found in previously unknown areas
- risk on project schedules
Not even the most thorough subsurface investigation, can account for every localized area of impact that will be discovered when completing a full-scale remedial excavation. Largely based on mitigating the risk of these unknown conditions, managing impacted soils and groundwater in place has become a preferred approach at many sites.
It should be noted that the level of risk is site- and project-specific. Sites that have low concentrations of contaminants may be assessed based on generic provincial standards and usually only pose a risk related to the generation and disposal of excess excavated soils. Sites with higher levels of impact may require the completion of a site-specific risk assessment and implementation of risk management measures (RMMs) to protect human health and the natural environment.
It’s important to confirm that the impacts are not posing an adverse impact on human health or the environment, that there is no regulatory impetus for impact removal, and that the site development plans do not result in the client sustaining unplanned long-term liabilities. Given these constraints, managing the impacts in place can prevent the generation of cost and risk factors such as:
- excavation, hauling, and of disposal impacted soils
- environmental sampling and analyses
- over-excavation due to unknown impacts
- direct contact of impacts with construction staff and public (direct contact and airborne)
- no receiver site available or receiver not accepting clean excess soils due to stigma
- trucking soils great distances to find a suitable receiver site
- delays from waiting for analytical results
- soil disposal variances and variables
- regulatory hurdles for management of excess soils
- accidental transport of impacted soils to an inappropriate receiver (e.g., farmland)
Leaving impacted materials in place can present a redevelopment option for properties where a large-scale traditional dig and haul remediation would make development economically unfeasible. It is also a more sustainable approach, which is an important consideration to reduce unnecessary pressure on landfill capacity (for waste disposal) and aggregate for backfill.
Where Brownfield redevelopment involves the construction of settlement-sensitive structures or infrastructure on compressible ground, leaving impacted soils or groundwater in place requires the use of construction solutions that allow little or no impacted material to be moved. For these sites with both environmentally and structurally impacted soils, standard foundation systems in these situations may not work. The following are some of the methods that can be considered to deal with these sites:
- piling systems
- driven Ductile Iron Piles
- helical screw piles
- driven steel H-piles or pipe piles
- ground improvement by Rammed Aggregate Piers (RAP)
- replacement RAPs (requires some soil removal)
- displacement RAPs (full displacement)
- combination systems (requires some soil removal)
- rigid inclusions
- grouted displacement RAPs (full displacement)
- GeoConcrete Columns (full displacement)
- soil mixing (generally full displacement)
- ground improvements by in-place densification
- Rapid Impact Compaction (RIC)
- dynamic compaction
The solution that is chosen will largely be driven by ground conditions, structural loads, and associated costs. Many of the technologies outlined above are becoming common in the construction industry, but their applicability to Brownfield sites may not be considered by design engineers.
Most of the above-noted ground improvement and foundation methods result in no removal of soils. Piling systems, displacement RAPs, and GeoConcrete Columns are generally displacement methods that do not require excess soils to be handled or removed. Likewise, the in-place densification methods do not generate any excess soils. Those methods that do generate some excess soils would result in only a fraction of spoils compared to full excavation and replacement.
The ground improvement and displacement foundation construction methods can be used in conjunction with other risk management measures. However, it is important to consider that the installation of foundation elements can result in creation of preferential migration pathways for contaminants. The key in identifying the risk of preferential pathways is the depth to which the element needs to penetrate. While piling systems require end-bearing soils or rock which could be deep, RAP systems work in friction and therefore typically do not penetrate to as great a depth and may not need to cross natural underground barriers such as aquitards. Where penetrating through an aquitard (e.g., a water-barrier soil such as clay) to an aquifer (water-bearing soil) the use of grouted RAPs or GeoConcrete Columns in these situations may mitigate the potential for migration, and could be considered.
In summary, the following presents some scenarios where ground improvement or foundation methods in redevelopment of Brownfield sites may not be helpful and where you may need to consider other RMMs or impact removal:
- presence of non-aqueous phase liquids (NAPLs)
- potential to cause migration of contaminants into previously clean areas (i.e. cross contamination)
- presence of hazardous waste, or soil impacts that are acting as continuing sources of groundwater contamination
- potential to create a longer-term liability
The following presents some scenarios where ground improvement or foundation methods in redevelopment of Brownfield sites should be suitable:
- soil and groundwater do not pose an adverse impact on human health or the environment
- leaving impacted soils below a building or other structures does not result in an unacceptable future liability
- soil removal results in increased soil management and disposal costs
- foundation methods can be used along with RMMs to mitigate adverse impacts on human health or the environment
- soil removal requires dewatering, resulting in other potential environmental impacts such as forced migration from dewatering
- soil removal increases project’s environmental footprint (e.g., more trucking of soils, filling up landfill space)
In conclusion, navigating environmental issues on an impacted development site can be complicated and potentially costly. Using best practices promoting the management of soils and groundwater in place can provide the best outcome for all stakeholders including the general public. When faced with both environmentally and structurally problematic soils on a site, it’s important to consider dealing with these soils in a way that minimizes cost, schedule, and very importantly, any adverse human health and environmental effects. The use of ground improvement and piling systems that do not generate excess soils can be a winning approach for the “double whammy” that can hit you on Brownfield sites—structurally and environmentally poor soils at the same time.
Renato Pasqualoni, P.Eng. is a Vice President of GHD Limited, and the GHD North America Geotechnical Service Line Leader, with more than 30 years of experience in geotechnical and geoenvironmental consulting. Renato’s experience includes consulting on numerous Brownfield sites in Canada and the United States where the use of innovative solutions was key in making these projects successful. Renato can be reached at Renato.Pasqualoni@GHD.com