Sustainable planting of trees in suburban environments on shrinkable clays
Creators: name: Sun, Xi
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400502
subject: Civil geotechnical engineering

description: Trees are provided by local government authorities to improve the landscape, enhance the environment, and increase land values. Trees, however, can also cause problems if they become too large for the streetscape. Trees can cause damage to lightly loaded structures such as pavement and residential buildings. Trees close to a residential building are responsible for imposing relatively severe suctions on the foundation soils beneath lightly loaded structures, leading to localised settlement. If the shrinkage settlements are significant, the buildings may deflect and result in structural damage. 

This study involves field monitoring of 8 well-established sites with 12 urban trees from three species (spotted gum, prickly paperbark and Brisbane box) in the City of Knox located in the eastern suburbs of Melbourne for a period of 3.5 years. High-quality data such as potential evapotranspiration, xylem pressure, tree transpiration rates and soil moisture state were obtained using the state-of-the-art instruments, including PSY1 Stem Psychrometer, SFM1 Sap Flow Meter, WP4 Dewpoint Potentiometer, CI-110 Plant Canopy Imager, and Neutron Moisture Meter. The geological investigation was carried out at selected tree sites. Soil samples extracted from a number of boreholes drilled in the vicinity and sufficient away from trees were used to conduct a series of laboratory tests, including Atterberg limits tests, shrink-swell tests and suction-controlled oedometer tests (Soil water characteristic curve, SWCC). Soil water budget curve consisting of precipitation and potential evapotranspiration was developed to show overall soil water status at the sites.

Parametric studies were conducted to assess the impact of various parameters on residential footing design and construction cost. The size of a stiffened raft footing was determined using parameters values introduced by AS2870 for different tree scenarios. The results showed that the presence of trees could lead to a deeper footing beam and additional rebar, and building a footing close to a tree would incur an additional construction cost when compared to the construction cost for a no tree scenario. The analysis of the results revealed that the calculated ys using field data is 58% higher than the value estimated using parameters outlined in the standard for the no tree case, while yt estimated using field data is 7% lower for the single tree scenario.

A case study was carried out to evaluate the effect of tree roots on subsidence and surface cracking of asphalt pavement at the southeast corner of a netball court in the City of Knox located 28 km east of Melbourne CBD. A total of four BHs were drilled near and away from a group of spotted gum trees to conduct the geotechnical investigation. The soil suction and water content test results for all BHs showed that the severe desiccation has occurred at 1.5 m below ground surface, where soils in BH 3 are the driest up to 1 m depth. Such severe aridity is most likely contributed by the impermeable asphalt pavements preventing ingress of rainwater. The desiccation would exacerbate with the presence of group spotted gums at the southeast corner of the netball court since a large quantity of water would be extracted by the tree roots, and consequently result in a surface crack and subsidence due to soil shrinkage settlement. A contour map was delineated around the southeast corner of a netball court using level survey data, revealing that the court has experienced significant settlement in close proximity to the group trees, which most likely contribute to the surface crack.

The analysis of the sap flow in twelve trees from three species revealed that the annual water use for single Brisbane box trees varied from 9.58 kL to 21.65 kL, which is the lowest when compared to the water use ranging between 18.28 kL and 52.78 kL, and between 18.28 kL and 52.78 kL for single spotted gum and prickly paperbark trees, respectively. Therefore, Brisbane box tree be preferred for street planting due to smaller discrepancies in monthly water use and its lowest annual water use. Monthly sap flow volume on the north side of the selected four trees from three species was compared with that at the south side for a period ranging from 7 to 12 months, and the results showed that all trees could transpire 8% - 44% more water on the north side than on the south side.

This study provides the building industry and civil engineering profession with guidelines to reduce the risk of accommodating trees in an urban environment, leading to an increased acceptance of trees within communities living on problematic clay sites. It is believed the findings in this study can promote a higher level of understanding of long-term water use of individual trees, which could be used by local councils to assess tree water use and develop a cost and time-effective urban tree planting management plan. As well, researchers could utilise the high-quality field sap flow data sets to formulate and calibrate root water uptake models.