Tree protection on construction sites

Introduction

Trees are deemed as being an asset by Oxfordshire County Council (OCC) and their benefits include those of economical, sociological and environmental nature. Development and its related works can cause serious, sometimes fatal, damage to trees located on or near the construction site. Such damage is easily and cheaply avoided and this advisory note sets out the recommended best practice for avoiding unnecessary damage to trees on OCC construction sites.
What Oxfordshire County Council expects from developers
OCC manages many properties and highways, and undertakes a considerable amount of construction and maintenance work. OCC considers it the developer’s responsibility to protect all existing trees throughout the whole development process. As trees are a material consideration in the UK planning system, it expects developers to take all necessary precautions to ensure all trees are retained long after the development has been completed.

Certain developments will require an arboricultural report. OCC expects developers to submit a full and detailed arboricultural report, in accordance with British Standard 5837:2012 Trees in relation to design, demolition and construction - Recommendations. This will will include all trees likely to be affected by the development, the arboricultural constraints and impacts of the site, an appropriate tree protection plan, and a detailed arboricultural method statement (if requested) for the works prior to any development commencing on site.

Detrimental impact to trees caused by development

Direct physical damage to the crowns and stems of trees is avoidable. Any physical damage associated with development activities will injure the tree and require additional resources being expended for the healing process to take place Furthermore, instances of injury will offer an entry point for tree related pathogens. This can lead to a reduced lifespan and/or ultimately compromise the integrity of parts, or the entire structure of the tree.

Tree roots are mostly below ground and all but invisible to designers of schemes or buildings. A tree relies on its roots for support and to provide it with water and nutrients from the soil. Trees are supported by the larger structural roots, near the main stem, but most of their water and nutrient needs are taken up by the fibrous roots which are located within the top 600mm of soil. Damage to any of these roots, or to the soil they are growing in, will injure/kill the tree or compromise its structural integrity.
British Standard 5837:2012 Trees in relation to design, demolition and construction – Recommendations
For these reasons above, all trees retained on or near any development or construction site must be protected in accordance with British Standard 5837:2012 Trees in relation to design, demolition and construction - Recommendations (BS5837). This document sets out the current best practice with regard to tree protection and development, which must be considered during the planning stage and in place for its duration; from commencement until completion. Appropriate protection must comprise of the establishment of a Construction Exclusion Zone (CEZ), derived from the calculated Root Protection Area (RPA).
BS5837 clearly states how the RPA is calculated. For single stem trees, the RPA should be calculated as an area equivalent to a circle with a radius 12 times the stem diameter, measured at 1.5m above ground level. For all other commonly encountered trees (twin-stem, multi-stem, irregular swellings, low branch unions, sloping ground) alternative formulae are advised. Please consult BS5837 or contact the County Arboriculturalist for further guidance.
In the case of hedges, avenues, or rows of trees, the dimensions of the largest trees must be used to calculate the required width of protection areas for the avenue.
For woodlands, top height of the tallest trees in the woodland should be used and an appropriate protection area established, all round each woodland, measuring from the woodland boundary.
General requirements for tree protection during development works for all OCC sites
A statement of best practice
1. An Arboriculturist must be involved from the early part of the design stage.
2. All development work on OCC sites where trees are present must be carried out in accordance with British Standard 5837:2012 Trees in relation to design, demolition and construction –       Recommendations.
3. No part of any development, including landscape works involving excavation or change of ground levels, must be within the tree Root Protection Area (RPA) of any retained tree(s).
4. Any access facilitation or general pruning works required for development must be agreed with the County Arboriculturalist prior to commencement and completed in accordance with British Standard 3998:2010 Tree work. Recommendations.
5. Site operations involving exceptionally wide or high sided vehicles which may inadvertently cause direct physical damage to trees must be supervised by a competent banksman.
6. RPAs must be identified at the design stage and clearly marked on all plans.
7. RPAs must be protected by establishing a Construction Exclusion Zone (CEZ) around all retained trees on the site before the contractor takes possession. This must remain in place throughout the contract and may only be removed on completion of the development.
8. The boundaries of the CEZ must be marked by good and substantial fencing consisting of a vertical and horizontal scaffold framework, well braced to resist impacts. The vertical tubes will be spaced at a maximum of 3m intervals and driven securely into the ground. Onto this framework, welded mesh panels will be securely fixed with wire ties These boundaries should be marked with an all-weather notice clearly stating ‘CONSTRUCTION EXCLUSION ZONE – NO ACCESS’
9. There must be no access to any part of the CEZ by a contractor's plant, personnel or vehicles. This prohibition includes access on foot and by private motor vehicles belonging to contractor's employees or to other persons visiting the site. Instances where access is required must be agreed with the County Arboriculturalist prior to entry.
10. There must be no storage of plant or materials within the CEZ.
11. Drains and other underground services must not pass through any tree RPAs unless with prior agreement with the County Arboriculturalist.
12. Above ground utilities and apparatus (CCTV, lighting, aerials) must be positioned so as to avoid any necessary pruning. Appropriate consideration should be given to the expected future growth of any trees and how this will impact on utilities or apparatus functionality.
13. The installation of scaffolding, walls, fences, hard surfaces and other structures must be avoided within the tree RPAs unless with prior agreement with the County Arboriculturalist.
14. Demolition of structures, including structures below ground, within tree RPAs must be carried out as specified by and under the direction and control of a qualified and experienced arboriculturalist and must conform to the recommendations of BS 5837.
15. Fires are prohibited within, or within 25 metres of the boundary of, all tree RPAs.
16. Application of herbicide is prohibited within, or within 2 metres of, any tree RPAs except unless agreed in principle with the County Arboriculturalist.
17. Some construction sites will require a full written arboricultural report which must include an arboricultural impact assessment, tree protection plan, and an arboricultural method statement, as per BS5837. For further information please contact the County Arboriculturalist.
18. The above recommendations should be included, as appropriate, as conditions in any contract for building construction, alteration or maintenance of County Council buildings where trees are present and in need of protection.
In cases where it is unavoidable to achieve desired development outcomes without having access to trees RPAs during construction, consultation with the County Arboriculturalist must be sought. Please contact the County Arboriculturalist (0845 310 1111), Highway.Enquiries@Oxfordshire.gov.uk, should you require advice on any such works not outlined above and / or should you require further advice concerning any of these recommendations.