Effect of Tree in Residential Construction
Trees in the property and surrounding areas play a major role in the determination of soil stability. Trees absorb moisture from the soil and change the moisture conditions of the soil. These changes in moisture conditions gradually cause abnormal moisture conditions in the soil and lead to soil subsidence over time and eventually differential settlement in soil and footings.
Removal/ chopping down of growing tree or fully grown tree abruptly increases the moisture conditions in the soil as water is no longer absorbed by the tree. So, after cutting down the trees, the roots and stumps left behind can add moisture as they release the stored/ remained water in them. Eventually, the released moisture dries and clay shrinks consequently. The leftover roots eventually biodegrade leaving voids. This causes soil subsidence and eventually differential settlement in soil and footings.
Hence the effect of trees is to be considered in the design of footings as per Australian Standards AS 2870 – 2011.
This blog summarises the influential radius of a mature tree as per AS 2870 – 2011.
The influential radius of a mature tree can extend from the trunk to up to 1-2 times the height of a mature tree or a group of trees. This is defined in Clause H2.6 from Appendix H and Figure CH2 from Appendix CH in the Australian Standards AS 2870-2011 (Image 1 and Image 2).
HTg is the design height of a group of trees in which each tree has a different height. In this case, HTg is taken as 0.9 times the design height of the tallest tree in the group.
Table CH 5.1 of AS 2870 – 2011 (Image 3) gives a better understanding of severity and distance up to which soil suction changes can be experienced due to the presence of trees.
The tree effect score helps to determine the distance of soil suction changes from the drip line of the tree (Refer to Image 4). Soil suction changes lead to abnormal moisture conditions and instability in the soil.
The tree effect score (Image 3) determines the influence distances (TD1 and TD2) from the tree based on climatic zones and tree effect score (Image 3). Refer to Image 5 for TD1 and TD2 values for various climatic zones. However, the table in Image 5 is applicable for predominantly clayey sites.
Image 6 represents the classification of climate zones
in and around Melbourne. These climate zones can be used for determining TD1 and TD2 distances (Image 5).
It is to be noted that the removal of the existing
tree can cause similar effects as those during the growth of a tree.
Let us consider an example to understand the soil suction changes due to a tree (indicated in red colour in Image 7) located in a property in Greensborough, Melbourne. Image 8 shows the size and extent of the tree observed during the site inspection.
The score for adverse conditions has been given ‘0’ based on the excerpt from clause CH5 of AS 2870 – 2011 (Image 10).
As none of the factors mentioned in the adverse conditions matches the condition of the property, the score for the adverse condition is taken as ‘0’.
Based on the tree effect score (Image 9) and location: Greensborough (Climate Zone 3: Refer to Image 11), TD1 and TD2 are taken 7 m (Refer to Image 12)
Hence, the influence area of the tree which is being discussed for soil suction changes is shown in Image 13.
It can be concluded from Image 13 that the soil suction changes due to the tree in the discussion can occur in the property and surrounding properties as well. Hence, this factor should be considered if any renovation or reconstruction work has to take place in these three properties.