Christopher Wright
Managing existing trees in the context of development is so context-specific that it is difficult to know how to approach the topic with any generic guidance.
Indeed, it seems that this topic has become a bit of a blind spot for the arboricultural industry – specifically, with regard to how individuals may confidently approach the subject. It is, in my opinion, unteachable in any meaningful way at an academic level.
Thus, the industry finds itself in a bit of a bind. Everyone is busy and there isn’t necessarily the time for those who are less experienced to be taught on the job (which to my mind seems to be the only viable way in which the topic of managing trees in the context of development can be learned).
Therefore, as a bridge that tentatively links the abyss between theory and practice together, above the murky waters of applied pressure, I am sharing some of my observations of how trees affect design decisions in the context of new developments.
Levels (formation and finished)
Everything has a finished level and everything has a formation level – as above, so below. In almost all instances, finished levels need to meet an existing access point’s finished level (e.g., an existing road). Every finished level has a formation level, which will depend on an array of factors that are context specific.
When we see no-dig approaches specified (e.g., a cellular or mini-piled system), it must be established that this actually means no digging will occur. However, before this can be confirmed, one must review the requirements of the formation of the structure. For instance, if the existing spot level of the ground is 21.00 and the finished spot level needs to be 21.15, the structure needs to be, from formation to finish, no thicker than 150mm. If it is any thicker than this, unless 21.15 can be pushed upward, one must dig downward – and thus we enter the realm of the tree roots. At this point it is best to advise on investigations that may be required to confirm whether excavation is viable – and certainly before the 21.15 finished spot level is confirmed.
To understand levels, one needs a topographical survey. Generally, these are invaluable; they certainly make consultants’ lives easier and therein help to de-risk a project by reducing the number of unknowns (there will be more on that later). It is rarely wise to assume.
In this case, it will be the architect and landscape architect that are probably the best persons of contact.
Attenuation tanks/SuDS
Nobody likes flooding – unless you are cultivating cranberries! Cranberry diversion aside, storm drains can only cope with X amount of flow per second, which means that water may need to be temporarily attenuated elsewhere in order to reduce the peak load on them. Usually, the water is attenuated within the site being developed – and pretty much always underground.
One perfect place for attenuating water is under car parks and other hard surfaces – or just beneath turfed areas if you’re fortunate enough these days to have a garden. Regardless of where the tanks go, it is essential to ensure that they don’t undermine the entire purpose of ‘no digging in location X’. Tanks come in various shapes and sizes, and some come as blankets that can make the arboriculturist quite uncomfortable. Larger tanks may require significant excavations to fit them in – which brings us back to the formation level issue discussed above.
In this case, the drainage engineer is probably the best person of contact.
Any formation level in the area around this plane will need to consider the presence of the surface root.
Delivering a scheme with an attenuation tank (or similar) within this narrow area presents arboricultural challenges.
Access to the elevations of this building will likely require some lateral pruning of the adjacent trees.
Uncovering roots within an urban park, to inform designs for a new footpath access.
Access and logistics
However much I’d love to see a prefabricated development get dropped in from above by a helicopter, I think I’m more likely to see a musk ox in the Sahara. Contractors need space to do their thing and build the things they have tendered to build. Designs also tend to maximise the built space, often going as close to trees as is possible before the arboriculturist starts to lose sleep and eat a few too many biscuits. Plus, everything comes at a cost, and some means of access are riskier than others.
One has to be mindful of the space needed for (in no particular order): scaffold, piling rigs, deliveries, site cabins, contractor parking, excavators, pedestrian walkways, tower cranes, and so much more – perhaps, also helicopters. There is little use in designing things that consider trees and then undermining it all with a blind spot about construction logistics.
In this case, a construction consultant or contractor will probably be the best person of contact, though it really depends on the context of the project and a lot of this is left until the packages have been tendered.
Known unknowns, unknown unknowns, and knowns
There is a lot that we don’t know at the moment, and there will be a lot that will forever remain unknown. There is also a lot that we do know. For example, the rooting system of a tree is an enigma (at least, to me it is), though we know every tree has roots. Where we have unknowns, we have risk. We are able to place a measure against some of them so they are no longer unknowns. Where we cannot place a measure, we know they are unknowns.
Using tree roots as an example, in my experience, nothing quite beats opening up a trench (carefully) and looking to see what is there. Granted, this isn’t always possible (e.g., if there’s a footpath in the way), but where it is, this can be very informative. For instance, in a recent project a trench was dug along the edge of a proposed building to search for roots, which were subsequently plotted onto the topographical survey by the topographical surveyor (including their crown and invert levels, amongst other things). This information was then incorporated into plans, including for the specification of foundation designs and pile locations. We put a measure to an unknown and from that we de-risked this particular item down to a more manageable level.
In this case, wherever the unknown resides it will be the judgement of the arboriculturist and other consultants to identify how to proceed – if it is deemed efficient, effective, or practicable to proceed at all. Sometimes, one carries the known unknown risk forward, though it must be understood that this is therefore a known blind spot. Sometimes, there are monsters in the shadows.
Concluding remarks
I must clarify that, at all times, one ought to remember to value logic and perspective. X always means something in relation to Y; and nothing exists in a vacuum. Decisions have effects, and those effects have effects that affect decisions, and the effects of your decisions will influence your future decisions in turn – it is our experience that improves our decision-making, assuming we stay alert.
For the sanity of everyone, including oneself, it is preferable to keep things simple. This is certainly easier said than done, though there is little utility in reinventing the concept of the wheel, as is there little utility is chasing archetypes (and pots of gold at the ends of rainbows).
All that being said, keep the wheels of the mind turning and keep the blade sharp on both sides – though do remember to hold the blade at the hilt.
Christopher Wright is an arboricultural consultant who works for Tim Moya Associates (now referred to as TMA) and focuses on the management of trees in the context of development (often, in the London area).
This article was taken from Issue 196 Spring 2022 of the ARB Magazine, which is available to view free to members by simply logging in to the website and viewing your profile area.