Avoiding and Minimising Impacts

Hierarchy of controls

There are three key controls for avoiding and minimising the impacts of construction on trees. This hierarchy of controls provides an opportunity to retain trees currently proposed for removal or further reduce impacts to trees that will be retained. A summary of this hierarchy of controls is outlined below:

• Avoidance: The key factor for the successful retention of trees is providing adequate construction setbacks by maximising the distance between trees and infrastructure. This is achieved through design strategies such as the relocation or modification of structures, services, and hardscapes. Ensuring adequate distance between the tree and the construction footprint not only reduces the impact on trees but also reduces the likelihood of trees causing damage to infrastructure in the future.
• Minimisation: Minimising impact may be a viable option when there is little opportunity to relocate or re-align proposed works. When impacts on the trees or encroachment within the TPZ cannot be avoided, they may be minimised. Minimising impacts on trees can be achieved through tree-sensitive design techniques, where proposed works are designed in a way to reduce ground disturbance and impacts on tree roots. This tree-sensitive design strategy can include utilising alternative construction methods or alternative construction materials.
• Mitigation: Tree protection mitigations will be required to ensure that any proposed impacts on trees are strictly limited to those which are expected and have been assessed and approved. These tree protection measures form the Tree Protection Plan (TPP) and provide the safeguards to ensure that trees proposed to be retained are protected during construction and that any impacts on these trees are controlled and monitored.

Avoiding impact

Key strategies for avoiding impact are outlined below:

• Relocation of infrastructure to maximise the distance between trees and proposed works.
• Relocation of underground services to maximise the distance between trees and proposed works.

Minimising impact

Key strategies for minimising impact are outlined below:

• Design modification to maximise the distance between trees and proposed works.
• Bridging the root zone by utelising suspended slabs, elevated walkways, and boardwalks.
• Pier and beam footing design.
• Cantilevered design.

Mitigating the impact

Key strategies for mitigating the impact are outlined below:

• Implementation of a tree protection plan.
• Installation of tree protection fencing, trunk protection, and ground protection.
• Project arborist supervision and monitoring of trees during construction.

Methods to avoid and minimise impact

The table below provides a summary of major construction impacts and a range of methods for avoiding or minimising these impacts.

Construction impact	Methods to avoid or minimise impact
Cut and fill (grading)	Restrict stripping of topsoil within the TPZ. Utilise retaining walls as an alternative to grading or battering to maximise the distance between trees and excavations. Woody vegetation to be removed adjacent to retained trees should be cut at ground level and not pulled out by machinery or equipment.
Retaining walls	Design walls with pier foundations to bridge tree roots. Uteselise retaining walls with discontinuous footings to maintain natural grade as far as possible from trees. Relocate retaining walls to maximise the distance between trees and the construction footprint.
Roads	Use retaining walls at the edge of the roadway as opposed to grading or battering to maximise the distance between trees and excavations. Utilise suspended slabs or cantilevered sections. Limit excavation for sub-grades Construct at or above existing ground level where possible.
Pathways	Utilise suspended walkways such as boardwalks or pier and beam bridges. Utilise articulated pavement with a 100mm thick gravel subbase over tree roots. Split or narrow shared pathways to avoid trees. Use paving materials requiring a minimum amount of excavation such as reinforced concrete. Specify minimum subgrade compaction under pavement within the dripline. Utilise permeable paving materials such as asphalt or decomposed granite.
Underground services	Re-route underground services to avoid root zones. Consolidate utility trenches. Utilised manual excavation or non-destructive excavation. Thread conduits under, below, above, or around existing roots. Utilise directional drilling or boring to install services. Trench directly towards the centre of the tree to minimise segmented root loss.