Terram - a Berry Plastics Company

Redrow homes farnborough


Courtesy of Terram - a Berry Plastics Company

Tree root protection is a task faced by many developers as they seek to provide cost-effective housing whilst preserving the surrounding environment in the face of large scale developments.

A perfect example is Redrow Home’s exciting new housing project, Farnborough Central located within 2.5km of the Thames Basin Heaths Special Protection Area (SPA), a site requiring adequate measures for an area of such significant historical interest.

Recommendations in a detailed arboricultural study of the site meant Redrow had to source an alternative, non-invasive solution to the conventional methods used in the construction of many of the site’s roads, drives, and pathways - with particular focus on the root protection of the area’s existing trees, placed under a Tree Protection Order (TPO) as part of the planning approval by Rushmore Borough Council.

Standard practice for constructing traffic-bearing surfaces involves excavating soil, allowing the installation of a sub-base that will adequately support traffic loads. Unfortunately this method of construction can badly damage trees since a byproduct of the excavation is root severance.

Trees are extremely sensitive to disturbances in the soil around them reason being that, contrary to popular belief, trees do not have massive roots that go down deep into the soil but rather have lots of relatively small roots which spread out from the tree very close to the soil surface for quite large distances. About 80-90% of all tree’s roots are in the upper metre of soil and if even relatively small roots are severed, for example by digging a trench, the tree can begin to suffer symptoms of drought stress as it is no longer able to obtain all its water needs. In addition the tree may become unstable as cutting the roots is a bit like cutting the guy ropes on a tent.

It is not only root severance that may harm trees but also compaction of the soil. If the root zone of a tree is not protected during development then the soil may become compacted by vehicles repeatedly moving over the ground. The effect of com paction is to close up pores in the soil which contain air and water. The tree's roots then begin to suffer from both a lack of oxygen and a lack of moisture, and, as the soil becomes denser, roots find it hard to penetrate the soil. All this can lead to a dieback of the root system and frequently dieback of the tree. The raising of soil levels has a similar damaging effect as it deprives roots of oxygen and creates a build up of harmful carbon dioxide around the roots.

Redrow’s answer to root protection for their Farnborough scheme came in the form of an above ground, no dig method of construction using Erocell, from Terram’s cellular system product portfolio.

Conforming to recommendations within the British Standards Institution BS5837 ‘Trees in Relation to Construction’ and the Arboricultural Advisory and Information Service’s practice note APN12 in relation to constructing driveways close to trees, the use of Terram Erocell enabled Redrow to carry out construction work within the specified root protection areas of the site - development which would not have been permitted using traditional construction techniques due to the unacceptable levels of soil disturbance and root damage caused by such methods.

Installation of the solution proved simple and highly effective. Each of the root protection areas were cleared of grass and other vegetation and then the upper layer of soil was removed by hand to limit any possible soil compaction of the remaining surface. A layer of Terram 1000, a permeable geotextile, was laid over the area to prevent any surface fill materials from penetrating the soil. The Terram Erocell was then pegged out on top of the geotextile, and the cells filled with a permeable aggregate. A permeable wearing course was than placed on top of the Erocell, separated by a second layer of Terram 1000 in areas where block paving on a bed of sharp sand was used to prevent the sand from mixing with the aggregate below.

Terram Erocell was the optimal solution for this project due to the nature of it’s strong and flexible cell structure, designed to provide even load distribution and a stable traffic base for the site with loads of up to 300kN/m2 revealing only minimal deflection (<5mm) of the surface of the filled cells. Terram Erocell is constructed from a geotextile which is permeable, allowing lateral movement of air and water.

In total Redrow Homes deployed 1200 m2 of Terram Erocell and 2500 m2 of Terram 1000 in the designated root protection areas across the 5.16 Hectare Farnborough site.

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