Soil Vaults Return Huge ROI in Quantifiable Results
Overview
Located in the inner eastern suburbs of Perth, Foster Park plays hosts to a community centre and provides space for recreational activities to the City of Belmont’s residents. With the design of the new centre’s facilities, a new car park was also devised with parking bays and trees planted in traditional tree wells to provide shade coverage.
Encountering the same microclimate challenges facing many global cities today, The City has an ongoing commitment to their Urban Forest Strategy realising the benefits canopy coverage has on their City’s liveability and sustainable population growth. In addition to the public health interests, The City have also recognized that the urban forest canopy is a tangible asset with a high propensity to deliver valuable ROI on their balance sheet if adequately managed.
Stratacell trees circled in red. Note shade coverage.
Photo taken: February 2017.
Photo taken: February 2013.
With this vision in mind, following a review of the initial car park plans and the performance of trees planted conventionally in neighbouring sites, concerns were raised that within the standard tree pits (tree wells) sufficient soil volumes were not being provided. With both insufficient soil volume and compacted root space due to planting into carpark subgrade, trees were not growing to their full potential as intended within the landscape design. The traditional planting method was compromising tree health and therefore yielding a significant reduction in final shading area.
To address this, it was decided that the car park would be redesigned to include vaulted soil cells to increase available rooting volumes. After an initial trial installation and the direct comparison of results with nearby conventionally planted sites, the soil vaults have been adopted across three car parks and achieved not only momentous but quantifiable success.
Challenges and Solutions
Challenges
- This green space was to be comprised of trees to provide shade coverage; a key design element significantly adding to the experiences of guests to the park and prolonging the life of the pavement.
- The original designs accommodated 88 bays and seven London Plane trees in traditional 1.5m2/16.15ft2 tree pits, however, having seen poor growth and shade output results from nearby conventionally planted sites, the Parks and Environment team agitated for vaulted soil tree pits.
- The cells would provide adequate, uncompacted soil volume and generate healthy trees with significant tree canopy, with minimum sacrificing of car parking space despite their increased footprint.
- Tree health being directly proportional to canopy size, the City’s arborist calculated that for every 2m2/21.53ft2 of shade desired, 1m3/35.31ft3 of loam soil was to be specified in the tree pit designs.
- Cost and perceived complexity of their installation meant the inclusion of cells were
Solutions
- The car park was reinterpreted to cater for additional soil vaults, minimising the loss of car parking to only two bays despite the inclusion of five trees and associated garden beds.
- Trees were planted in two garden beds, 2m/6.56ft wide, extending the length of the car park bays with a combined soil volume of 45m3/1,589.16ft3.
- The linear trenches were chosen for reason of installation efficiency and shared root zone. A locally available, tested soil mix was also utilised.
- The cells would accommodate a larger volume of uncompacted soil and provide aeration for healthy root growth ensuring increased tree life spans with capacity for maximum canopy coverage.
- The high strength engineering of the tree pits would allow the functional use of the surface above them for weight bearing car parking whilst also mitigating pavement uplift and root intrusion on surrounding infrastructure.
- The increased health and life spans of the trees would provide a sound investment for the City in line with their vision for urban forestry.
- An initial trial was agreed upon and following it’s success, a further two installations would occur.
Material Specifications
- Stratacell Module 30: Extending 1.5m2/16.15ft2 either side of planter beds and two layers deep
- Available soil within tree island = 45m3/1,589.16ft3 (9m3/317.83ft3 per tree), Stratacells added: 80m3/2,825.17ft3 of volume (16m3/565.04ft3 per tree), combined total = 125m3/4,414.33ft3 (25m3/882.87ft3 per tree)
- Soil: Amazon Stand Landscape Mix – Complies AS4419
- Trees: London Plane x 5
Outcomes
Healthier trees growing at unprecedented rates were recorded deeming the root cells a resounding success and thus undertaken across all three car park sites.
Comparatively, a tree planted in a traditional tree well in a nearby site, now 15 years old, displayed ongoing stagnated growth and signs of health decline.
To further illustrate the differing growth rates between methods, five soil vault trees were assessed and charted over a four year period against eight trees planted conventionally on the same site: The tree inspections were carried out on five separate occasions and measured for canopy cover and trunk diameter.
Results recorded here also indicated that shade from the cell root systems far outpaced that of the open planting methods.
Investment vs ROI
- To determine a quantifiable value for each tree set, the Burnley Method, a recognised tree appraisal approach developed by Dr Greg Moore at the Victorian College of Agriculture, was employed at both sites. The method measures for health, stature and shade output.
- The value of the soil vault trees grew exponentially over just a four year period delivering a huge return on investment at $17,500 per tree and an average canopy coverage of 50m2/538.20ft2.
- Trees planted conventionally (without cells) in a nearby car park site drew significantly lower results, with much older trees effectively worth replacement value only, at just $510 with 7.5m2/80.73ft2 canopy coverage after 15 years.
- The tree volume (or cone) data further revealed the drastic differences between each approach.
- The combined value of the soil vault trees was $87,500, essentially resulting in trees worth 34 times as much – in one quarter of the time.
“Trees were worth 34 times as much – in one quarter of the time.”
Considerations
- To ensure equal evaluation, each comparison uses an identical species of trees with equivalent climate and maintenance conditions.
- All sites were operated as car parks and relied on natural rainfall without the use of artificial irrigation.
- The conventional method used for planting involved a square cut in the pavement, curbing around the edges and soil loaded into the dug out cavity.
“In a short time, combined value of trees far exceeded installation costs.”
Summary
Whilst the initial outlay was more expensive, the Stratacells have enabled the trees to thrive at unparalleled rates resulting in a huge return on investment for the City both financially and environmentally.
In only a short period of time, the combined value of these trees has far exceeded the installation costs of the cells and will continue to substantially increase in years to come. Conversely, the value of the trees in the traditional tree pits, when studied both four and 15 years on, cannot improve and will largely remain irrelevant.
It is apparent that the additional rooting volume and uncompacted soil in the soil vaults has provided the space, air and water required for the trees to continue maturing and reach their full growth potential. With savings on tree maintenance, replacement costs and the environmental and sociological gains, the cells have ultimately benefitted both the City’s asset register and its residents.
Due the veritable success of the project, the City of Belmont plan to extend the Stratacell program and adopt the soil vaults as a pillar of their urban forestry strategy ongoing.
View the full report here
The Project Team
Owner: City of Belmont
Parks & Environment Manager: Warren Stephens, City of Belmont
Arborist: Vic Bijl, City of Belmont
Soil Vault System: Ben Gooden, Citygreen
Landscape Architect: City of Belmont
Engineer: City of Belmont
Contractors: City of Belmont