Field monitoring and assessment of the impact of a large eucalypt on soil desiccation

Expansive soils, or reactive soils, experience moisture changes under the prevailing climate conditions, leading to shrink during dry periods and swell in wet months. Houses founded on such problematic soils are subjected to considerable ground surface movements, which can greatly affect the function and performance of buildings. Such movements will increase with the presence of a tree or tree groups nearby since tree roots can extract considerable amounts of water from soils, resulting in localized shrinkage settlement. In this study, a field site was selected with a large native spotted gum tree in a well-established eastern suburb of Melbourne, Victoria. The site and tree are fully instrumented. Surface and sub-surface movement pads, in situ soil psychrometers and neutron moisture meter access holes were installed to evaluate soil moisture/suction changes and subsequent ground movements. A sap flow meter that incorporated the heat ratio method was installed on the north side of the main trunk of the tree at breast height to enable sap velocity and sap flow volume to be closely monitored. Soil types, degree of reactivity and capability of soil water retention have been evaluated through a number of laboratory soil tests including shrink-swells, Atterberg limits, and soil water characteristic curve determination. This paper presents the preliminary monitoring results for a period of 6 months. Valuable field data such as this can help geotechnical engineers and practitioners to get a better understanding of the physical processes that drive tree root-expansive soil interaction and in time improve the current footing design guidelines.