Trees on development sites are more often seen as a hurdle to overcome than an opportunity to add value. Yet this needn’t be the case. There are various solutions of how to retain and integrate quality trees into close proximity of a project. Doing so can not only lead to a more straightforward passage through the planning process, but if undertaken early enough, can even increase the floor area of the project itself.
Why seek to retain trees? There is a long list of benefits that trees provide, and which helps explain why almost any species of trees can be protected: their shade reduces the amount of heat absorbed and stored by buildings. Tree canopies slow down winds thereby reducing the amount of heat lost from a home. They reduce stormwater runoff by canopies intercepting and holding rain, and through the uptake and evapotranspiration of water. Trees play a role in mitigating the effects of climate change by sequestering atmospheric carbon. They reduce noise and the visual impact of traffic. The list continues.
Trees improve air quality, biodiversity, human health and wellbeing. Quite simply, we enjoy being around trees and they are good for us. Most pertinently for our sector, trees also increase surrounding property value.
Local and central government have actively protected the tree stock for decades – and this is unlikely to change. For example, in London the Mayor’s RE:LEAF work is targeting that 25% of London’s land has tree cover by 2025. Too often, proposed developments show the removal of trees the local authority considers valuable. The effect of this approach manifests in a project stalling at planning application, and the design team being faced with making rushed changes to the plans, at the expense of floor area and overall building design.
The first time to consider trees is before acquiring the site. It is essential to always conduct a tree preservation order search. Developers should be mindful that it is an offence to cut down, wilfully damage or destroy a tree protected by that order without the authority’s permission. Equally of note, owners remain responsible for trees covered by tree preservation orders, their condition and any damage that may be caused. This is actively enforced, with local authorities empowered to serve fines of up to £20,000 for damages to a protected tree.
Once a site is acquired, seeking expert arboricultural advice enables the site’s trees to be evaluated before the outline design is undertaken, and for the local authority’s possible viewpoint to be sought and assessed. This runs counter to a traditional approach that assumes the removal of trees as necessary to allow for construction facilitation. This rationale spans from access and construction easement to installing utilities or car parks. Discussions with the Tree Officer at this point will provide an understanding of the parameters of what is acceptable, and time and cost can be saved by incorporating these conditions into the initial design from the outset.
A tree survey, conducted by an arboricultural consultant, must be submitted where there are trees within, or adjacent to, a proposed planning application site, which could be affected by the development. Eight Associates offers a free tree survey tool so that a member of the design team can carry out the survey independently, before then requiring the expert input of one of our arboricultural consultants to assess the health and quality of the trees on site. This information forms an important part of the impact assessment, and is subsequently used by the local authority to draw conclusions about planning permission.
It is damage to, as much as removal of, trees that the design team needs to be wary. Trees need light, water and aerated soil to remain healthy. The most common form of damage to trees is to the roots – and hence the term ‘Root Protection Area’ (RPA). Tree roots uptake water, minerals and absorb oxygen. They are an anchor to the tree and a store of energy through non-photosynthesising periods. Roots are generally found in the top 800mm of soil, and typically extend radially from the stem via the path of least resistance. The BS5837:2012 Trees in Relation to Design, Demolition and Construction – Recommendations, which all planning documents and Local Authorities adhere to, provides a calculation for establishing the area around a tree in which roots are likely to be found. The RPA can be calculated by multiplying by 12 the stem diameter, at 1500mm from ground level, to provide a radius from the stem.
Two regular causes of damage to trees include soil compaction and root severance. Both oxygen and water are held in the pores between soil particles, and there is a risk to the tree’s overall health where the pores are small, for example in heavy or clay soils which have been compacted. Compacted soils are caused by vehicular or pedestrian traffic through the RPA, squeezing the oxygen held in the pores of the soil. This restricts the growth of fibrous roots and prevents free draining of water causing waterlogged soil. Root severance occurs during excavation works within the RPA. The BS5837 will allow the severance of roots <25mm in diameter, if carried out with a sharp tool and overseen by an arboricultural specialist.
Early arboricultural advice can enhance the design by showing how light construction could take place safely within the Root Protection Area (RPA) of trees. There is a choice of a few technical solutions which may be applicable depending on the type of trees and the site constraints:
An increasingly popular solution is Cellular Confinement Systems (CCS). This is a ‘no dig’ solution that is both time and cost-effective, and enables car parks, driveways and pedestrian pathways to be constructed over the RPA – with the end result providing a pleasant environment in and around mature trees for residents and users. CCS involves a versatile hard plastic mesh being overlaid on soils, which has the effect of spreading the load laterally – and drastically reducing soil compaction created by pedestrian and vehicular traffic.
A key benefit of its ‘no dig’ nature means the existing top-soil undergoes minimal preparation before installation, beyond some preliminary site clearance of shrubs or bushes. Initially a non-woven geotextile, capable of trapping hydrocarbons and heavy metals, is laid over the subject area. The CCS mesh is laid over the existing soil and filled with a clean, granular sub-base (typically 25-50mm non-frost susceptible quarried rock) to allow for soil aeration. This is then levelled and compacted, ensuring the granular fill is proud of the CCS mesh. A further layer of geotextile is laid over this, followed by a sub-base, and then it is finished with either a permeable surface such as porous asphalt, porous block paving, grass, loose and bonded gravels.
With its successful track record of deploying CCS solutions to a range of settings, Infra Green’s Managing Director Phil Tomlinson recommends a good quality, three dimensional mesh as the key to successfully integrating and retaining trees through the use of CCS.
It is well understood how the traditional strip footings of piling rigs can result in substantial root loss. This damage can be minimised by using the smallest practical pile diameter when piling is to be installed near to trees. This reduces the chances of striking substantial tree roots, and causing considerable damage, and reduces the size of the rig required to sink the piles.
The insertion of piling rigs to create specially engineered structures within the RPAs may be justified if it enables tree retention. Firstly the viability of such an approach must be carried out through an exploratory investigation and a compressed air soil displacement study undertaken. This method involves carefully excavating a cylindrical hole within the RPA of a tree, avoiding substantial tree roots (>50mm diameter) and creating a number of concrete columns to support a cantilevered slab, followed by the careful excavation within the RPA to create a number of concrete columns to support a cantilevered slab. Consideration needs to be given to ensuring there is a ventilated air space between the underside of the slab and the existing soil to enable gaseous exchange through the soil. A specialist irrigation system should also be used (e.g. roof run-off redirected under the slab). Specialist engineer and arboricultural advice is needed to ensure the effect of the load-bearing properties will endure the redirected roof run-off. Where piling is to be installed near to trees, the smallest practical pile diameter is used, to reduce the possibility of striking substantial tree roots, and the size of the rig required to sink the piles. These and CCS solutions must adhere to the BS5837:2012, advice from a suitable engineer, specialist arboriculturalist and the local authorities’ requirements.
Before you dismiss this as too complicated, it’s worth considering ‘the size of the prize’. The ability to integrate trees into close proximity of the project has been shown to increase floor area by more than 10%. In the worked example, the drawings show a site as it is acquired, with boundary trees that have been identified as good quality and a prominent landscape feature.
Trees should be pragmatically considered as assets that have the potential to add value to a project on many levels. Rather than an issue to be overcome, the early valuation of trees on the development site can lead to a more attractive design, with a maximised footprint, being given planning consent. Working with an experienced arboricultural consultant at the outset of the project, can deliver a viable design in close proximity to trees. Keeping that relationship going through construction, could lead to the smoother delivery of a more desirable, and marketable, final development.
