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New research – Trees, stormwater, soil and civil infrastructure

February 1, 2018

The debate between local government asset managers and their WSUD practitioner colleagues about the benefits and risks associated with permeable paving and infiltration systems, in close proximity to road pavements and footpaths, may be one step closer to reaching consensus. Historically, the use of permeable pavers has been constrained by the perception that infiltration associated with these systems, would exacerbate problematic ground movement in Adelaide’s reactive clay soils

Dr Tim Johnson has dedicated 20 years of his career planning and implementing projects that ensure trees in our streetscapes and public open spaces thrive under Adelaide’s harsh growing conditions. For the past eight years Tim has undertaken extensive monitoring and reporting of the performance of footpath assets, for both permeable and non-permeable pavements, and interactions with street tree roots, as part of his PhD thesis, Trees, stormwater, soil and civil infrastructure: Synergies towards sustainable urban design.

Dr Johnson’s research indicates that soil moisture and ground movement did vary beneath impermeable and permeable pavements on silty clay soils, and that the differences related to the interaction of several factors including pavement design, and the size and proximity of nearby vegetation, shading and season, and that under some circumstances ground movement was reduced by permeable pavement.

Serving as a working demonstration in an established suburban street situation, the permeable pavements used in Dr Johnson’s study have revealed acceptable footpath pavement performance over the five-year duration of the experiment. Several of the results of the experiments detailed in his thesis have significant implications for local municipal works, sustainable development and risk management:

  • Permeable paving with a gravel base layer has been shown to reduce the potential for tree root-related footpath damage.
  • Problematic ground movement did not occur due to infiltration of incident rainfall through permeable pavement into moderately reactive soil.
  • Although the compacted surface of the pre-existing footpath’s subgrade had been removed and the new subgrade surface had not been compacted, the permeable footpath pavement did not fail; the strength of the uncompacted silty clay subgrade was adequate to support the footpath pavement.
  • Greater annual soil moisture extraction and recharge associated with greater street tree cover nearby indicates that trees, urban soils and pavements can work in combination and contribute to in-situ stormwater management.
  • Abundant fine root development, rapid turnover and resulting creation of biopores in the subgrade beneath permeable paving suggests that tree growth may increase infiltration rates in soils of low permeability, thereby increasing their capacity for in-situ management of stormwater.

Dr Johnson will be presenting at the 13 March Water Sensitive SA seminar, Permeable paving and infiltration systems.

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