Care should be taken when backfilling excavations within a root protection area. Tree roots are particularly affected when soil is compacted and they can also be damaged by the mechanical action of tamping down. Backfill material around exposed roots should consist of a fine granular material which provides a high proportion of air space. Builders’ sand must not be used as it is toxic to tree roots. The material should be compacted using hand-tamping methods which maintain a degree of aeration and enable tree roots to survive. This method will allow the backfill material to be compacted to the required level for footway construction.
Compacting a road base close to tree roots is more complicated due to the load-bearing requirements of a road. If a road carriageway is to be constructed within a tree’s root protection area and tree roots more than 25mm are present, the forestry officer should be contacted to identify special measures to protect tree roots. Non-paved areas must be backfilled using the previously excavated soil, which should be only lightly firmed and left proud to allow for natural settlement.
To survive, tree roots need water and oxygen within the soil. Surface material should also be flexible to allow for soil movement and future root activity. Granular surfacing, such as chippings or other permeable materials, laid on to a geo-textile membrane would be appropriate in this case.