Effect of Tree in Residential Construction

Tree

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).

Image 1: Clause H 2.6 from Appendix H of AS 2870 – 2011
Image 1: Clause H 2.6 from Appendix H of AS 2870 – 2011

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.

Image 2: Figure CH2 of AS 2870-2011
Image 2: Figure CH2 of AS 2870-2011

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.

Image 3: Tree effect score as per AS 2870 – 2011
Image 3: Tree effect score as per AS 2870 – 2011

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.

Image 4: Figure CH3 of AS 2870 – 2011 for determining the soil suction changes
Image 4: Figure CH3 of AS 2870 – 2011 for determining the soil suction changes

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.

 

Image 5: Table CH5.2 of AS 2870 – 2011
Image 5: Table CH5.2 of AS 2870 – 2011
Image 6: Climate zones in and around Melbourne (Image sourced from AS 2870 – 2011)
Image 6: Climate zones in and around Melbourne (Image sourced from AS 2870 – 2011)

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.

Image 7: A case study to understand the effect of a tree in a property in Greensborough, Melbourne (Image sourced from Google maps)
Image 7: A case study to understand the effect of a tree in a property in Greensborough, Melbourne (Image sourced from Google maps)
Image 8: Size and extent of the tree observed during the site inspection
Image 8: Size and extent of the tree observed during the site inspection
Image 9: Tree effect score of the tree
Image 9: Tree effect score of the tree

The score for adverse conditions has been given ‘0’ based on the excerpt from clause CH5 of AS 2870 – 2011 (Image 10). 

Image 10: Adverse condition definition as per Clause CH5 of AS 2870 - 2011

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)

Image 11: Climate zone of Greensborough (Represented by the red spot) – Image sourced from AS 2870 - 2011
Image 11: Climate zone of Greensborough (Represented by the red spot) – Image sourced from AS 2870 - 2011
Image 12: TD1 and TD2 determination based on the tree score and climate zone (table sourced from AS 2870 – 2011)
Image 12: TD1 and TD2 determination based on the tree score and climate zone (table sourced from AS 2870 – 2011)

Hence, the influence area of the tree which is being discussed for soil suction changes is shown in Image 13.

Image 13: Influence area of the tree in discussion for soil suction changes (Image sourced from Google maps) Conclusion:
Image 13: Influence area of the tree in discussion for soil suction changes (Image sourced from Google maps) Conclusion:

Conclusion

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.

 

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