Tree Root Protection Systems – Are we on the road to success?

With increasing pressure to construct internal road systems and driveways on new development sites amongst retained trees, Alister Rankine investigates how the Geosynthetics Limited ‘CellWeb’ root protection system is instrument in putting the no-dig theory into practice.

As arboriculturalists we are all familiar with that bible of no-dig construction APN1 ‘Driveways Close to Trees’. As a consultant I have included this worthy publication many times as an appendix to a tree report or arboricultural method statement. I have to confess, however, that I have never had the opportunity to be involved in the actual ‘hands on’ construction of a new road or driveway using the method outlined in APN1.

I thought therefore, that it would be helpful to make some enquiries and to see if I could discover a case study that shows theory translates readily into practice. My investigations led me fairly swiftly to the products of a company called Geosynthetics Limited based at Hinkley in Leicestershire. A very helpful chap there, Roy Partington provided me with some very interesting facts and figures, and was only too willing to send me details of a project to construct access to a new car park at Ambleside in Cumbria.

I would like to share this information, which is not intended to be a definitive specification, in the hope that it will help to illustrate the various stages and procedures involved in the construction of a no-dig roadway.

The project was for the Lake District National Park Authority. ‘Our CellWeb Tree Root Protection System was specified by the architect.’ says Roy. ‘The product specified was our 200mm deep system due to the amount of traffic and the fact that the client needed to run construction traffic over the area.

Figure 1

The line of the new road was along an existing track through a wooded area, with a number of mature trees lining the route (see Figure 1.)

Preliminaries
Before going into the details, it would be useful to remind ourselves what the benefits are to the trees of employing this system.

Traditional road construction entails excavation and inevitable changes in levels which can lead to physical severance of roots or raising soils causing compaction. There are other risk factors to consider. These include:

• Creating an impermeable surface, thus depriving the roots of essential moisture
• Construction causing a rise in water table thus creating anaerobic conditions, starving roots of oxygen
• Contamination of subsoils from run-off and materials used in construction process
  

Soils vary in their type and therefore their capacity to bear loads. The potential for compaction to occur has to be assessed for each site where a road or driveway is to be constructed close to trees. The following factors and information should be considered to enable a load-bearing structure capable of supporting traffic to be proposed:

• The California Bearing Ratio (CBR) – this is a standard test method for measuring the soil strength
• Soil type
• Location of the water table
• Maximum permissible load with respect to vehicles likely to use the road

There are two broad approaches to new road construction close to trees. The first option is o consider stripping the ground surface in order to allow the proposed geogrid system to be placed without raising the finished surface above the existing ground level. This method will invariably damage some root structure prior to construction, and in my opinion rather defeats the no-dig concept.

The second option, and one that I believe as arboriculturalists we should be advocating every time, is to have a literal no-dig, which elevates the road structure thus requiring edge protection.

Step 1

• Remove the existing surface vegetation. If this is predominantly herbaceous in nature a proprietary herbicide can be used providing it is not harmful to tree roots.
• Any existing woody vegetation can be removed using mechanical mowers or flails, providing there is no soil surface disturbance or physical damage to any surface roots.

Figure 2

Step 2

• Place the geotextile separation filtration layer over the prepared ground surface. Use a Fibretex F4M non woven geotextile, with dry joints overlapping by 300mm.

Step 3

• Place the Cellular Confinement System (CellWeb) over the geotextile and anchor open during the infill operation. The three-dimensional cell structure of the CellWeb is formed by ultrasonically welding perforated polythene strips / panels together to create a three dimensional network of interconnecting cells. A high degree of frictional interaction is developed between the aggregate infill and the cell wall, thus increasing the rigidity of the system. (see Figure 2 above)

Step 4

• Place edge constraints along both sides of the new road. A treated timber edging is usually acceptable here.

Step 5

• Place backfill material in the open cells of the CellWeb system. This is usually a no fines granular fill, typically 20-40mm (see Figure 2 above).

Step 6

• Lay chosen surface over the backfilled cells. There are a number of surfacing options.

Block Paving

• Lay a second layer of Fibretex F4M Geotextile separation fabric over the backfilled CellWeb sections
• Lay sharp sand bedding layer compacted with vibro compaction plate to recommended depth
• Place block paviors as per manufacturer’s instructions

Tarmac

• Place 25mm surcharge of the granular material above the CellWeb system and lay bitumen base and wearing courses (see Figure 3 above).

Loose Gravel

• Place second layer of Fibretex F4M Geotextile separation fabric over the backfilled CellWeb sections
• Construct a treated timber edge to restrict gravel movement
• Place decorative aggregate to required depth

Grass Blocks

• Place second layer of Fibretex F4M Geotextile separation fabric over the backfilled CellWeb sections
• Place 50/50 rootzone bedding layer to the required depth over the fibre to the required depth
• Lay recycled ‘Duo Block’ 500 Grass Protection System infilled with 50/50 rootzone mix
• Seed as per architect’s instructions. (Alternatively, the grass blocks can be filled with gravel. (see Figure 3 above)

Roy Partington says, ‘The use of cellular confinement reduces the bearing pressure on subsoils by stabilising the aggregate surfaces against rutting under wheel loads. Comparisons between cellular confinement and traditional aggregate and grid-reinforced structures demonstrate a 50% reduction in construction thickness.’

The Finished Product

The access road to the new car park at Ambleside was 150m in length and 3.5m wide. It took approximately 5 days to complete. ‘although I do not have a cost for the whole project, the final cost is estimated to have been 30-40% less than conventional construction.
The final product was a road able to carry substantial construction traffic without causing damage to the existing trees (see Figure 4 above)
It is worthwhile remembering that a conventional road in this location would not have been given planning consent by the local authority!

Summary

I found this case study a great help in illustrating how the familiar concept of APN1 can be achieved on the ground.

I hope that my article has answered some questions, and not generated too many new ones. If, however, you have any queries may I suggest that you get the information direct from the experts
Roy Partington at Geosynthetics can be contacted on 01455 617139 or roy@geosyn.co.uk.

Alister Rankine is a fully trained forester and arboriculturalist with more than thirty years’ experience and runs his tree and woodland management consultancy, ‘Hillside Trees’ at Chilcompton in Somerset. alistair@euroarb.eu.com