Root protection: Preserve and protect

Cellular confinement systems (CCS) are three-dimensional geogrids designed to reinforce aggregate within the cells in order to disperse loadings laterally through the system as opposed to vertically to the existing ground below. The use of CCS also reduces the amount of subbase required when compared with typical construction build-ups.

In accordance with the Arboricultural Practice Note 12 and BS 5837 2005 and 2012, three-dimensional CCS have been commonly used as a ‘no dig’ solution in the construction of new hard surfaces within root protection areas (RPA).

Most people within the arboricultural sector have become familiar with cellular confinement and they have confidence that the use of an open structured granular infill material offers vertical and lateral water permeation and enables continued gaseous diffusion between the rooting environment and the atmosphere.

More often than not, they are happy to use the system on relatively level sites, however there is a perception that the system is unsuitable for anything other than level terrain.

In this case study we will look at how Cellweb TRP has been used to overcome significant changes in levels within the RPA, while maintaining a healthy environment for tree roots.

Our technical team was approached by JPP Consulting Civil and Structural Engineers regarding a new residential development on land off Keats Way, Rushden, Northamptonshire. The area of land to be developed could only be accessed from Keats Way and a new access road would need to be constructed. The only feasible route for the new road would be to pass through the RPA of a large beech tree. The beech was considered to be of high amenity value and was to be retained within the new development, which meant that a ‘no dig’ tree root protection system would need to be used for the construction of the access road.

Figure 1 shows the proposed route of the new access road passing through the RPA of the beech, as outlined by the red circle. The blue hatched panels across the width of the access road denote panels of Cellweb TRP.

The site posed a second challenge in the form of its terrain.

Over the length of the access road there was a fall in levels of 2.4 m from the southern end of the road at Keats Way to the northern end of the road in the new development.

The cross section in figure 2 shows a longitudinal section showing how the existing ground level changes along the route of the proposed new access road. The hatched area represents the area where the ground level needed to be built up inside the RPA.

Any increase in levels within the RPA should be carried out and constructed to ensure continued water permeation and gaseous exchange to and from the rooting environment and the atmosphere. If this is not maintained and anaerobic conditions are created, it can result in root death and ultimately bring about the demise of the tree.

To overcome this change in levels it required a build-up of 2.4 m above the existing ground level at the most southern end of the RPA, reducing to 1.2 m above the existing ground at the most northern point.

When providing technical recommendations, the engineering team considers the existing soil, looking at both the CBR (California bearing ratio) value and whether the soil is granular, cohesive or peat. It also needs to know the heaviest loadings anticipated over the area as well as the number of axles and the load per axle as this will affect the angle of load distribution. Lastly, it looks at the final surface over the Cellweb and the depth of this surface as this could potentially be used to offset any sub-base requirements.

The engineering department at Geosynthetics calculated a site-specific technical recommendation and proposed the solution that can be seen in figure 3.

The proposed solution shows how layers of 200 mm depth Cellweb TRP can be stacked on top of each other to increase levels. The system can then be terraced to create a gradual reduction in levels as the route progresses north towards the new development. This solution was adopted and a full design was carried out by JPP Consulting Civil and Structural Engineers.

Photo 1 shows the first layer of Cellweb TRP being installed, with the retained beech tree on the left. Minimal excavation was carried out on the far southern extremity of the RPA, at the entrance from Keats Way. This was carried out to create a flat surface to which 2.4 m of Cellweb TRP would need to abut. The Cellweb TRP system is pegged out over the top of one layer of Treetex Geotextile. This acts as a separation layer, preventing the infill aggregate from migrating into the subgrade below. This also acts as a pollution control measure in accordance with BS 5837 2012.

Photo 2 shows the progression of the installation and the layering of the Cellweb TRP. To the right in the photo and next to the tree you can see the beginnings of the formation of a Cellweb TRP embankment. This batter is designed to have a gradient of one in three and is infilled with the same 4–20 mm clean angular stone used to infill the rest of the system. The end cells of each layer of Cellweb TRP are left empty and are later filled with topsoil and planted to create a vegetated embankment.

Photos 3 and 4 show the completed access road and development eighteen months later. In photo 3 it can be seen that the Cellweb TRP section of the new access road is surfaced using permeable blocks, allowing continued water permeation and gaseous exchange. The remaining road outside of the RPA is constructed using a conventional sub-base with an asphalt surface. In photo 4 we can see the completed batter which passes around the main stem of the retained beech, allowing enough space for the future incremental growth of the buttress roots. The end cells of each layer of Cellweb TRP have been infilled with topsoil and are now planted to create an attractive vegetated bank.

This case study shows that with thoughtful design, Cellweb TRP can be utilised on far more challenging sites than previously thought. This is particularly poignant at a time where we see increasing demand for development land, which places ever increasing pressure on our national tree stock. The benefits of trees are becoming increasingly recognised and it is important that we utilise engineered solutions to enable us to retain healthy trees as part of future developments.

We have been monitoring the health of this tree since 2012. We have made regular visits to site and tree vitality assessments were carried out by Geosynthetics’ in-house LANTRA-qualified professional tree inspectors. When assessing tree vitality, Visual Tree Assessment (VTA) techniques were used to assess the tree’s condition. Observations were recorded and photos were taken. The key indicators of tree condition we look out for are changes in leaf/needle/bud coverage, leaf/needle size, shape and colour, the presence of dead wood and the evidence of bacterial or fungal colonisation; when these key factors change this can indicate that a tree is under stress conditions and is experiencing physiological changes. As of our latest inspection it can be seen that this tree continues to exhibit no signs of reduced vitality or ill health caused by this installation.

With careful planning, new tree-lined routes can be constructed utilising established trees without compromising their health or longevity, even in areas where difficult levels need to be achieved. The utilisation of established trees provides an immediate beauty, difficult to replicate with newly planted trees.