Arboraeration was asked to investigate the rooting conflicts between three trees – all protected by tree preservation orders – growing in the rear of a residential property in London. The patio area of the property had been damaged by the roots, causing the paving slabs to lift and pose a trip hazard. Initial investigations were undertaken using an airspade tool around the perimeter of the patio, directly in front of the trees. This was undertaken to evaluate the amount of rooting activity directly beneath the terrace and assess the feasibility for root pruning and root barrier installation. A large amount of rooting was discovered in the top 40cm of the soil meaning that root pruning and root barrier installation would not be possible. In light of the above, we were asked by the client to suggest two alternative solutions to reinstating the patio area without causing undue damage to the trees on site. While the below options are for a patio area they can also be successfully used for other buildings and construction within the RPA of trees.
Option 1: Cellweb TRP Geogrid System
Cellweb TRP is a 3D cellular confinement system produced by Geosynthetics Ltd (http://www.geosyn.co.uk/) that is specifically designed and approved for use within the root protection area of trees. The system is designed as a ‘no dig’ solution to be laid directly on top of the existing ground level thereby causing minimal disturbance to tree roots. Following the installation of the Cellweb, the existing paving would then be re-laid on top of the system. This would cause a slight raise in the overall height of the terrace, by up to 75mm.
Installation Method
Using hand excavation methods, the existing terrace and subbase/hardcore should be removed down to ground level. Any roots that are growing within this layer of subbase will need to be root pruned by a suitably experienced arborist. Cellweb can then be installed as per manufacturer’s guidelines. The paving slabs can then be laid on top of the Cellweb system.
Conclusion
Utilising Cellweb in this capacity (as a foundation) will cause minimal damage to the trees’ rooting structure.
Of the two options, the Cellweb will be the most economical and can be installed by most competent landscape/groundworks contractors. Only the root pruning will need to be undertaken by specialists.
Option 2: Screw Piles and Suspended Slab
Screw piles are a type of foundation that is pre-engineered and screwed into the soil. There is no concrete or excavation required. The suspended slab is either a precast, or cast in situ, slab of concrete that is suspended above the ground so to leave a small void. Utilisation of mini piles and a suspended slab in this context would involve more investigations prior to their installation, to determine the location of the pilings without causing compaction or physical damage to the tree roots.
Installation Method
The rough locations of the pilings, as based on the structural engineer’s drawings, would be initially excavated using an airspade to a depth of 60cm. The potential locations of the pilings can then be moved where required until suitable locations, without any tree roots, are found. This process could be completed prior to lifting the whole terrace, with the preferred piling locations being marked and the paving slabs re-laid after. This would allow for minimal disruption to the use of the terrace whilst awaiting any other project requirements to be met. Once approved, the paving slabs would be removed, pilings inserted in the pre-marked locations, and the suspended slab would then be installed on top of the pilings prior to the paving slabs re-laid.
Conclusion
The screw piling method will cause minimal damage to the existing tree root system, especially as there will be no root pruning required. Indeed, it may improve rooting conditions beneath the void as the suspended slab will allow for some root gas exchange, as well as some surface water absorption, neither of which is currently possible.
The costs of this option will be far greater than that of using the Cellweb product, however, this is a permanent engineered solution that will not suffer from any movement, or damage, from the tree roots moving forward.