Last edited 17 Sep 2019

Tree protection during construction


[edit] Introduction

Protecting existing trees is imperative in an age that recognises the beauty and contribution they make to the environment. As well as contributing to urban cooling through evapotranspiration, they also provide micro-climatic environments that can reduce energy demands in buildings. Trees also contribute to carbon capture and provide visual relief, softening the sometimes-hard effects of the urban environment.

However, trees can also be a constraint to development and some developers may be tempted to remove them to facilitate construction. This would be unlawful if trees enjoyed protection under a tree preservation order, or if they are within a conservation area. For more information see: Tree preservation order.

Tree preservation may even facilitate construction: protecting trees which might be threatened by construction processes can help in either gaining planning permission or discharging a planning condition.

Local planning authorities (LPAs) may request a tree survey or an arboricultural impact assessment (AIA) for trees on or adjacent to properties of land subject to planning applications. BS 5837:2012 provides recommendations relating to tree care, with a view to achieving a harmonious and sustainable relationship between new construction/existing structures and the surrounding trees.

Once it has been decided which trees to protect, there are a number of techniques that can be adopted.

[edit] General considerations

Before any materials or machinery are brought onto the site, trees that are required to be retained should be protected by barriers and/or ground protection as recommended in BS 5837:2012. Before the site is handed over to the contractor, it may be wise to seek advice from arboriculturist for vulnerable trees or those located close to the site storage areas, site huts, entrances/exits and so on which may have heavy traffic, or loadings / drainage that may affect the ground.

By using a variety of methods, trees can be encouraged to survive where construction activities have altered the adjacent ground level.

[edit] Lowered ground level

Where construction has resulted in a lowered ground level around an existing tree, the root system can be enveloped by a circular or square retaining wall made of brick, stone or rubble walling. Alternatively, an earth bank may be formed around the tree but this should extend gently, well beyond the root system and allow for future growth.

[edit] Raised ground level

Where the ground level is raised around the tree, a retaining wall of stone or brickwork built to enclose the tree will allow the existing ground level to remain within the container but a new higher ground level around it. Materials such as clinker placed around the back face of the container can assist with drainage, relieving hydrostatic build-up.

Alternatively, when raising the ground level, a hollow can be made around the tree. The ground should be contoured gently with a shallow slope extending well beyond the root system. As the hollow will tend to accumulate run-off, land drains should be installed around the tree with clean rubble as soakaways to dispose of surplus water. In addition, a 100mm-thick layer of pebbles around the base of the tree can facilitate drainage and prevent excessive drying out of the soil. All such constructions should be in accordance with BS5837:2012 and in keeping with good practice.

The maximum that ground level may be raised simply by depositing topsoil is 150mm. Soil or water should never be allowed to stack-up against the trunks. Furthermore, isolating a tree on an area of raised ground can make it susceptible to drought. In heavy soils, land drains should be considered.

[edit] Old trees

Particular care is needed when it is desired to retain old trees that will become enclosed by a development. Such trees may be less resilient than younger varieties and could die or become unsafe as a result of construction pressures. If that happens, their removal could be difficult and costly. Where the retention of large, mature or veteran tress is deemed desirable, they may be better conserved by incorporating them into open spaces or large gardens where they may have adequate space and a better chance for their long-term survival.

[edit] Tree damage

Trees can cause damage to buildings through:

Tree planting should be kept at an appropriate distance from the structure in accordance with the species of tree.

Throughout the design process, it is important to remember that new buildings should allow for the future growth of planted trees. Foundation design is therefore critical and should be in accordance with BS5837:2012 to allow for reasonable future vegetation. Furthermore, the BS also stipulates recommended distances between young trees or new planting to avoid direct damage to a structure from future tree growth.

More detailed recommendations on tree care during and after construction can be found in BS5837:2012.

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