"If the proposed encroachment is greater than 10% of the TPZ or inside the SRZ (see Clause 3.3.5), the project arborist must demonstrate that the tree(s) would remain viable. The area lost to this encroachment should be compensated for elsewhere and contiguous with the TPZ. This may require root investigation by non-destructive methods and consideration of relevant factors listed in Clause 3.3.4".
The team at E.A.R are currently working on a large scale tree protection plan and arboricultural impact assessment for a the site above, in which majority of the trees must be retained as they pertain high habitat amenity for the local Koala population. There are a number of homes in the plans which will encroach the TPZ by more then 10%, subsequently the E.A.R team spent Friday exploring root plates and carefully mapped the findings. These root map findings will help engineers to plan to avoid roots minimising the overall impact of the proposed development allowing the trees to continue to grow in a some-what altered environment which also complements optimum growing conditions.
If dealing with encroachments into the TPZ on any development site the following from AS 4970 is also extremely handy.....Remember once roots are severed the damage is irreversible and the affect can be life threatening to the subject tree. Initial tree management, sufficient tree protection measures with on going management by the project arborist can avoid unnecessary headaches and the trees amenity features will undoubtably complement the development.
3.3.4 TPZ encroachment considerations
When determining the potential impacts of encroachment into the TPZ, the project arborist should consider the following:
- Location and distribution of the roots to be determined through non-destructive investigation methods (pneumatic, hydraulic, hand digging or ground penetrating radar). Photographs should be taken and a root zone map prepared.
NOTE: Regardless of the method, roots must not be cut, bruised or frayed during the process. It is imperative that exposed roots are kept moist and the excavation back filled as soon as possible.
- The potential loss of root mass resulting from the encroachment: number and size of roots.
- Tree species and tolerance to root disturbance.
- Age, vigour and size of the tree.
- Lean and stability of the tree.
NOTE: Roots on the tension side are likely to be most important for supporting the tree and are likely to extend for a greater distance.
- Soil characteristics and volume, topography and drainage.
- The presence of existing or past structures or obstacles affecting root growth.
- Design factors.
Tree sensitive construction measures such as pier and beam, suspended slabs, cantilevered building sections, screw piles and contiguous piling can minimize the impact of encroachment.
When siting a structure near to a tree, the future growth of the tree, both above and below ground should be taken into account. Precautions should be taken at the planning and design stage to minimize potential conflict between trees and new structures.
When the root zone is reactive clay, techniques such as localized pier and beam (bridged), screwpile footings or root and soil moisture control barriers may be appropriate to minimize effects on structures.
NOTE: Collaboration may be required between the project arborist and the geotechnical or structural engineer. Further information is provided in the documents listed in Appendix E