This blog describes the techniques for the construction of access drives, which may avoid or lessen the damage caused to trees. Compaction of the soil, pollution, changes in soil levels in the root zone and cutting roots can all kill existing trees.
Most tree roots are contained within the top 900mm/3ft of soil. and many of the trees feed roots within the top 100mm/4”. It may not be wise to make even shallow excavations within the root zone of existing mature trees which are to be retained. Where Planning Permission is needed Local Authorities may make special construction techniques mandatory for any paving and drainage, etc. in what are known as Tree Protection Zones (TPZs).
The objective is to avoid damaging roots, preventing compaction of the soil and to allow water and oxygen to permeate through any construction. Details are shown in the current edition of BS 5837 or APN12 e.g. using a geogrid fabric, Type 1 sub-base and a porous block pavers or a no-fines gravel topping. The area affected should be restricted to >20% of the root zone.
APN12 is a very useful document for all landscape designers and architects and I would recommend all design practices own a copy of APN12 in the design library. APN12 embraces the principles first published as 'Driveways Close to Trees' (APN1) and reviews where the principles may be applied in practice.
