Hannah Cook at GreenBlue Urban emphasises the importance of green infrastructure within the road network as towns and cities across the UK expand.
With increasing urbanisation of our towns and cities, the pressure to plant trees is huge.
Priorities are under challenge, our road systems, with increasing traffic, need to provide safe pedestrian access suitable for all, and the plethora of below ground constraints create almost impossible hurdles to overcome.
Only as we give our green infrastructure some sort of parity with grey infrastructure in our urban conurbations, will we truly build resilience and create sustainable cities for future generations.
GreenBlue Urban encourage design with canopy cover in mind, as mature trees have multiple benefits; promoting a healthy, biodiverse, and the sustainable city which includes sequestering carbon, mitigating the effects of climate change, and improving our sense of biophilia minimising stress.
In 2015, a collaboration between Highways England, Keir, Treeconomics, Evans Associates, Davey and Forest research published an i-tree eco survey stating that trees planted in South West Region Highways network remove 29 tonnes of pollutants every year.
GreenBlue Urban worked closely with Glasgow City Highways and Civic Engineers on the scheme: Sauchiehall Street in Glasgow. This project was implemented due to the considerable decline in the road’s retailers, so renovation to consolidate the main shopping centre to heart of the city, encouraging customers to visit the shops in person was put into action.
The ‘Green Avenues Plan’ was the driving force to increase canopy cover, with Sauchiehall Street being the perfect pilot to highlight how green infrastructure can be used to change human behaviour, challenging the dominance of vehicles in the public realm.
Twenty-eight specimen trees have been planted in full GreenBlue Urban ArborSystem tree pits provide a strong visual segregation between vehicles and pedestrians/cyclists. Designed as “linked” they provide maximum rooting space in uncompacted soil, giving the trees the best opportunity of attaining species potential.
Beneath the streets lie a labyrinth of interconnected utilities, foundations, and other construction constraints. Many of these constraints are not anticipated, as much of the infrastructure below ground is not well documented.
Guidelines given in the DMRB (Design Manual for Roads and Bridges) HD24 HD26: Pavement Design and Construction, state that the requirement of the road design and surface layer works are to be based principally on a traffic assessment figures. These are expressed in terms of million standards (80kN) axle loads (msa) carried during the design life of the construction. This traffic loading together with the quality of the subgrade dictates the selection of the surfacing and depth of a road pavement required.
The clue to successful tree planting in and around utilities is collaboration and understanding the risks to the utility provider. Trees are not inherently damaging, but they are exploitive of rooting volume, and tend to find the easiest route to nutrient, water, and air. When the initial planting zone has become insufficient for the tree health, the tree roots will explore further areas in search of these items, and in doing so can find small fissures or holes in service ducts and drains.
The complexity of utility runs beneath the paving means that the GreenBlue soil cell system; Rootspace can intertwine around these features, directing the roots into the uncompacted soil chambers. Joining the tree pits beneath ground allows the soil volumes to be reduced, as the trees share the available nutrients and become more resilient than standalone trees would.
The Challenges:
- We often receive the question: Can you access the tree pits if required after installation? If utilities need to be accessed in the case of an emergency, the contractor simply must excavate to the top of the cell system, remove the RootSpace Lids and hand dig or vacuum excavate through the rooting zone to access the service.
- Footway constrictions can also be an ongoing issue. GreenBlue advise to use ReRoot Barrier away from these potential issues and into the Cells below the road, allowing root penetration into optimum soil conditions.
- Adoption of tree planting is a complicated issue, and many local authorities find it hard to maintain the balance between efficiently managing their scant resources and greening up urban areas. Commuted sums vary so it is advisable to be clear from the outset.
Well designed highways networks can deliver economic, social and wellbeing benefits, tree pits for the hard landscape are very much embedded within the narrative of green, blue, biodiverse thoroughfares through the effective use of green infrastructure.
New tree planting is required if we are to ensure a continuum of canopy cover for future generations. Urban tree planting needs emphasis on quality rather than quantity.