New guidance on hard surface installation next to trees has generated a wave of controversy. This at a time when real progress is being made in deve-loping better solutions and identifying priorities for research. In practice, damage to trees can result from either use of an incorrect system or insensitive installation. A new approach is now beginning to emerge that addresses issues of both design and implementation. This is good news for the trees and everyone involved.
Arboricultural Practice Note (APN) 12 was unveiled in September at the National Arboriculture Conference in Warwick.
Produced by the Tree Advice Trust and designed to replace earlier guidance in the form of APN1, it attracted criticism from some in the profession. Jeremy Barrell of Barrell Tree Consultancy for one is disappointed with the new document.
The guidance just hasn’t embraced current thinking and ideas, he argues. Frankly, I think it should be withdrawn until a proper process of consultation can inform a revised document.
Publication of APN1 in 1996 constituted a significant breakthrough and established important principles. The critical factor — as with current systems — was the no-dig approach, with levels being built up and no excavation taking place. Type 1 road stone was laid on a pre-stretched two-dimensional geo-grid and capped with a final surface. Compaction of the underlying soil was less than with a conventional construction method and there was greater permeability to water and oxygen.
APN12 further develops the ideas presented in APN1 and now refers to three-dimensional cellular confinement systems (CCS). Although these are generally considered preferable, co-author of APN12 Ben Holding explains why continued reference is made to the earlier geo-grid approach. There is still a place for two-dimensional systems and they can be particularly useful where the ground is uneven, he says. We have not seen any clear evidence that CCS systems are more effective and we at the Tree Advice Trust are happy that APN12 accurately reflects the current situation.
Barrell, however, feels that inclusion of two-dimensional geo-grid systems is insupportable. He says: Three-dimensional systems have been successfully specified as the method of choice for some years now and there has been an almost universal move away from ineffective earlier methods.
This is reflected in the 2005 update of BS5837 relating to trees and construction, which clearly states that two-dimensional systems are not recommended. Roy Partington of Geosynthetics — manufacturer of CCS products — agrees with Barrell and is perplexed by the continued reference to two-dimensional systems in the new APN. It has been demonstrated that the CCS approach is more effective in limiting compaction and preventing rutting of the sub-soil, he says. APN12 is sending out a mixed message which is likely to cause confusion.
So is the current CCS approach a panacea for all potential conflicts between hard surfaces and trees? Unfortunately, probably not. Although such systems reduce impact, they do not eliminate it altogether. Research conducted by the Environmental Protection Group (EPG) on behalf of Geosynthetics indicated that in one of the three trials involving a 100mm CCS over sandy soil, the critical soil bulk density at which tree root damage occurs was exceeded.
Surrey-based consultant David Challice says: In practice, there will be situations where a conservative stance is warranted and even a CCS cannot be recommended. A mature hornbeam on a clay soil with unfavourable level changes, for example.
Barrell agrees: Trees will generally tolerate some change in rooting environment, but in cases where zero impact is required, alternative approaches such as bridging may be favoured.
Partington feels that a CCS approach can be designed to cater for most eventualities. With a depth of aggregate and CCS carefully tailored to cater for the anticipated vehicle loadings and sensitive installation, we can provide solutions that will allow for tree survival and growth. One of the problems here is that the required depth of the system may be greater than can be accommodated within the development proposal.
So do we have any other tools in our armoury and are we sunk if a CCS is unsuitable? Yes and not necessarily, according to Richard Nicholson of Borough of Poole and Arbsolutions. He feels we need to adopt a more open-minded approach from the outset: What we don’t really want to see is consultants taking it upon themselves to specify CCS solutions without appropriate engineering input and design advice.
Nicholson is a firm advocate of the emerging approach, where the arborist provides a performance specification comprising a list of arboricultural requirements the new surface must meet. In a nutshell, this entails preserving the soil characteristics that trees need to survive. An engineer then assesses the particular site characteristics and uses the performance specification to devise an appropriate design.
This approach has several inherent advantages. From the arborist’s perspective, it takes out the guesswork and provides a clear framework to operate within. It allows an initial assessment by the engineer of which system is most likely to meet the requirements of the surface. This may or may not be a CCS.
Nicholson has witnessed the installation of several reinforced concrete surfaces, supported on mini piles. This can be an excellent design solution in cases where use of a CCS is not favoured, he says. Necessary soil attributes are preserved and features can be incorporated that allow sufficient passage of water and oxygen. I think we may well see an increasing trend towards designs of this nature. Furthermore, the approach avoids the pitfalls of the arborist straying beyond his or her area of expertise, which includes liability, should the specified surface not be up to the job.
Arboricultural method statements encompassing installation of hard surfaces are typically produced by arboricultural consultants to submit to the Local Planning Authority (LPA) in connection with planning applications for development.
These are increasingly being required at the application stage and one of their objectives is to demonstrate to the LPA that retained trees will be afforded an adequate degree of protection. The performance specification approach is more likely to satisfy the LPA as it is clear that a logical thought process has been followed and appropriate professionals have been engaged in arriving at a final design.
In terms of presentation, the specification determined by the engineer may be incorporated within the arboricultural me-thod statement or presented separately in a construction method statement. In the latter case, the details need to be approved by the arboricultural consultant and LPA prior to works commencing.
Mindful of the trend towards involvement of an engineer to advise on site-specific solutions, Geosynthetics — in collaboration with EPG — has developed an enhanced design service that was explained by Partington at the recent arboricultural conference.
Although there is a cost implication for us, our service is free, he says. We recommend clients use it as a matter of course to achieve a correct specification. It involves providing some site-specific data, including soil type, current level of compaction and anticipated axle loads of vehicles using the new surface. An appropriate depth of CCS is then recommended and advice given on suitable infill materials and final surfaces to provide the requisite degree of permeability.
Partington adds: On most soils, with anticipated axle loads ranging from light domestic traffic to fire engines and skip lorries, we find a 150mm CCS will fit the bill.
The design service offered by Geosynthetics is, in effect, a variation of the performance specification approach and adheres to the same principles.
One problem is that current methods are relatively recent and have simply not been around long enough for us to fully assess their worth. To address the current paucity of research, Barrell — in collaboration with Geosynthetics — is carrying out studies on the subsoil beneath a CCS installed two years ago. An opportunity presented itself to carry out some basic research, he explains. We are looking at compaction and evidence of root activity beneath a surface which has been extensively used by construction traffic.
Barrell believes the CCS approach will be favoured in most cases and is keen to contribute in any way he can to further know-ledge in this area. The results of our studies so far are encouraging and show that the soil beneath the CCS is not excessively compacted and is supporting live roots of several tree species.
Partington would like to build upon the work already undertaken with EPG and Barrell: We are currently seeking opportunities to engage with those in the profession to assess the practical uses and limitations of our cellular confinement systems.
Holding agrees more research is necessary and would like to see comparisons of all the different approaches. APN12 sets out the main requirements and parameters and is a useful starting point, he says. Research will help clarify which methods actually are most effective and APN12 will evolve over time as new information comes to light.
The Tree Advice Trust is hosting a seminar entitled Through the Trees to Development at Wyboston Lakes Conference Centre, Bedfordshire on 6 December. Arboriculturists, planners and engineers will be on hand and delegates will have the opportunity to explore the complex issues relating to hard surface installation and trees. We invite those who have been critical of APN12 to attend our seminar and engage with us to move things forward, says Holding.
Whatever system is used, the way in which it is installed on site is critical. Challice has been refining his method statements to ensure that retained trees are safeguarded at all stages in the process. If we are serious about protecting the trees, he says, we need to think about every detail. Sequencing, effective briefing of contractors, fencing, ground protection and arboricultural super-vision are all key.
LPAs have an important role here, first in approving appropriate designs and method statements and secondly in being serious about enforcement. Challice adds: Effective implementation relies on open communication between the LPA, the consultant and the developer. If LPAs are on the ball and have a consistent, fair and robust approach towards developers, then successful tree protection can be achieved. Challice embraces the performance-specification approach championed by Nicholson as this ensures that the design is fit for purpose and provides a starting point for effective tree protection during implementation.
The current British Standard recommends surfacing be permeable if it is wider than 3m within the root-protection area (RPA) of a tree or covers more than 20 per cent of the RPA area. Despite advances in permeable paving and an increase in the range of available surfacing options, debate continues on what constitutes a permeable surface. This applies in particular to permeable Tarmac and asphalt, which, research suggests, is permeable when first constructed but can lose its permeability within a short period of time, owing to internal changes and silting.
Having reviewed some current literature, Nicholson is not convinced we can be confident in specifying permeable surfaces of this nature: The evidence is inconclusive and at the moment we probably need to be looking at alternative surfaces such as dry-jointed brick paviors or permeable paving blocks.
Permeability of the aggregate layer is also important and Type 1 road stone is no longer recommended as it has a high fines content. Partington says that 20-40mm or 4-20mm clean angular road stone is the norm. Gravel can be used as long as it is over 4mm in diameter and angular enough to create a positive interlock with the CCS. We are in the process of producing guidelines on infill materials to clarify this, he adds.
Whatever the shortcomings of the new APN, it may just have done the industry a favour in stimulating important debate. Arboriculture has come a long way in the past 10 years since APN1 appeared and the future under the surface looks promising. Not that we should overlook the obvious.
As Challice observes: In some cases, removal and replacement of the trees is the most cost effective and environmentally sustainable option. In others, the best solution is simply to move the hard surfacing outside root protection areas. L
For details of the Through the Trees to Development seminar and a booking form, call the Tree Advice Trust on 01420 22022 or email firstname.lastname@example.org.