Fig 1: before flexi-pave
The RBG Kew is home to some 14,000 trees, a world-renowned collected. Over the past year or so, I have shared about some of the notable specimens. This month, I want to do something a little different. I am going to explore one of the management challenges we face as we look after the trees. This article is about some solutions we are evaluating with managing the issue of root heave in footways around the garden.
Fig2- After flexi-pave
This is an issue not only in public gardens, but also for local authorities. We only need to look at Sheffield and the council’s decision to remove trees which are causing an issue with footways.
Here at Kew we currently have some major infrastructure projects underway to maintain our estate. Some of this work is to replace footpaths, installation of a new family landscape, and the replacement of restaurants. Kew is in a conservation area; therefore, we are bound to plan with specific sensitivity to any redevelopment works.
We are currently working on design solutions for our footpath, not only when replacing footways, but also installing new ones.
One of the solutions we have been using over the last couple of years is flexi-pave. This material is made from ground-down used car tyres into rubber granules, about 3-6mm, which forms the basis of the material. This is mixed with multi-faceted stone granules of 3-6mm; this is bound to together with a polyurethane bonding agent. This bonding is what gives this material its flexibility which helps to prevent cracking and allows for movement with root growth. We have found this material hardwearing to foot traffic and vehicle traffic and is highly porous which is also good for drainage and allows water to penetrate to the tree roots. This material can be laid over existing ground without any sub-bases which is a great advantage when dealing with tree roots. The downside to this product is that it is very time consuming to install, and dry weather is also needed for the installation.
Fig 3: Cell web prior to use
We found the best solution to use this material is to use it in sections throughout the footways of Kew. We intend to use this product when root heave of the footways is a major problem. If removal of the existing surface is to take place, care must be taken, and Arboricultural supervision is needed to ensure the works is carried out in line with BS: 5837:2012. Implementing this has been the most difficult part of the project; ensuring contractors understand the need to carry out the work by hand. I have specified the use of an Air spade to remove the debris and other material from within the Root Protection Areas to limit the damage to the roots. One thing I have noted throughout the replacing of damage footways, is the roots are hydrotropic with a build-up of material under the existing surface on the path edge.
Fig4: Installation of Cell Web Cell Webb
The other material we have been using for many years is a compacted sandstone gravel. We use this form of footway material in our more natural areas. For any of you who have been regular visitors to Kew, this was also used for the footway for the Gingko tree.
The downside to this form of path is that it is not very hard wearing, and the dreaded pot holes appear. Also weed growth can also be an issue. The material is if a loose surface therefore picked up and carried away on soles of shoes and on vehicle tyres. There is also a need for a subbase, therefore there is an elevated risk of root damage if any excavation is needing to install the subbase.
Fig 5: Air-spade used to excavate
We are currently in the initial stages of the construction of the new children’s garden. This is an interesting project with the construction being within the Root Protection Areas of no fewer than 60 trees. This represents an interesting challenge to me as the Arboricultural supervisor for the project and working to very strict planning restrictions. Due to the complexity of the site, the footways must be installed first to allow for access across the site. This is all ‘no dig’ installation due to the restrictions presented by the Root Protection Areas of the existing trees. Therefore, the sward is to be sprayed off with an herbicide to prevent any digging.
Fig7: Cell Webb installed for root protection
To achieve this, a cellular confinement system is to be used with an increase in levels needed to accommodate this solution. The levels are to increase by 215mm, with top soil being used to grade the path edges down to the existing ground level.
The cellular system above is now being standard approach for ground protection for all access passing across Root Protection Areas. In the past, track boards were laid over woodchip to a depth of 150mm with geotextile as a barrier between the woodchip and soil. As recommended in BS5837:2012 (6.2.2.2 note B) this form of ground protection needs to be maintained and regular inspections are needed for this to provide adequate protection.
Fig 8: A lesson learned: wood chip ground protection didn’t work here!
With Kew having many upgrades to its estate over the next five years, my role as Manager of Arboriculture has needed to adapt to this. A large part of my time is now spent monitoring and supervising construction works on Kew projects. I know the frustration that Arboricultural consultants have had dealing with aspects the construction industry. Kew is not exempt from this, I have had some interesting discussions with friends in construction and trying to understand why they cannot carry out work in Root Protection Areas without supervision from an arboriculturist.
Fig10: Site monitoring
To try to overcome some of the frustration on both sides, I have created tool box talks on the issues with trees and constructions site. All staff, before working on Kew construction site, must attend a talk box talk on trees. I have found this does help to cover some middle ground between the two parties trying to reach the end goal.
Accompanying this is a Kew new policy for dealing with construction site in relation to trees (Schedule of Construction Site Monitoring for the Protection of Trees at the Royal Botanic Gardens, Kew). This helps formalise the arboricultural supervision and records all the comments and recommendations with time scales. I found this has helped all those involved to take BS5837:2010 seriously and not just regarding it as a box ticking exercise. For example, when the project to replace footpath was in its infancy, I requested a method statement for the removal of the existing hard surface. The first method statement I received was far from acceptable! After some time, the contractor submitted a method statement which covered my concerns. However, when they arrived on site it was clear that they were using this as a box ticking exercise. On my first site visit they decided the RPA of an Oak on site made a lovely storage area- this was soon repositioned, and the hand digging was with the use of a 3T digger to remove the hard surface within the RPA.
Fig9: site monitoring of Root Protection Areas in line with the Tree Protection Plan
This is all too common after arboricultural consultants leave site and the consent for planning has been received. We all know the importance of good report writing, and fancy CAD plans to complement our AIA’s TPP and so on. However, if the site is not monitored throughout the duration of the project and recommendations are not followed, then all our arduous work at report writing is a waste of time, effort, and money. Sadly, some people just treat the information as a box ticking exercise needed to get planning. The real work starts when the site is set up and the works starts on the site along with the monitoring of construction site and enforcement of the Tree Protection Plan and Arboricultural Method Statements. The arboricultural industry needs more powers from the local planning authorities and the construction industry itself, with the urban environment receiving more pressures with climate change. Green infrastructure is going to become invaluable in providing shade and cooling the environment which we all inhabit.
Article by Kevin Martin, Kew Gardens