Stormwater Soakage Wells Do Not Affect Vertical Ground Movement in a Roadside Tree Site with Reactive Clay Soil

Abstract Background Sustainable cities will require water sensitive urban design (WSUD) that integrates street trees, urban soils, and engineered infrastructure. However, there can be a perception that these systems contribute to ground/soil movement that can be problematic for hard infrastructure. There is concern that diverting water into kerbside harvesting systems for street trees may exacerbate ground movement. Method Twenty-eight small kerbside stormwater harvesting systems that diverted storm runoff into roadside soakage wells were dispersed along a residential street in an inner suburb of the City of Adelaide, South Australia. The wells can be utilized with or without the presence of street trees, but greater utility and environmental benefits could be achieved with vegetation. Regular measurements of ground level were undertaken to determine ground movement over a period of 2 years, comparing sites with and without soakage wells. Results Results showed that stormwater harvesting and infiltration into the site’s reactive clay did not increase ground movement at the kerb or at the road surface during periods of above and below average rainfall. Kerb levels generally remained within +10 mm (heave) and –15 mm (settlement) of the reference datum, and movement at the top of the asphalt road seal was no greater than +7 mm or –6 mm. The maximum extent of ground movement (sum of the maximum heave and maximum settlement) at any point was 26 mm, but typical movement at 9 points, 6 of which were near inlets, was 4 mm. Ground movement was of similar amplitude near and further from infiltration points and was unaffected by stormwater harvesting. Conclusion The use of trees within WSUD, particularly in fine clay soils in which root growth increases water harvesting and storage, can substantially contribute to urban hydrology through canopy rainfall interception, hydraulic redistribution, enhanced soil conductance through biopore creation, and preferential flow along root channels. This research shows that the presence of soakage wells does not lead to increased ground movement in reactive clay soils.