Whilst many city centres have avenue trees, most of these are of a formal uniform, single species, single variety type. Our client, Glasgow City Council, was seeking to deliver a flourishing Avenue that would emulate a naturalised environment, with an all-season aesthetic appeal, as well as taking account of the city’s flood protection requirements. Sauchiehall Street Tree Avenue is the pilot scheme for the “Avenues” projects across the city centre, within the overall £1billion Glasgow Region City Deal for infrastructure development. The project is working in advance of the other 16 “Avenues” within this Glasgow place-making scheme.

As the first City Deal Region in Scotland, Glasgow is seeking to demonstrate the importance and harmony of good landscape design, civil engineering innovation, and structural design in achieving a sustainable urban greening and flood protection vision. The method of construction offered by idverde and adopted by the client had never been done before, but it is one that the Client intends to replicate across the other Avenues. Similarly, the naturalised aesthetics, from the choice of trees with their year-round appeal, balanced against the climatic challenges, to the sustainability of the trees with the consideration of effective SUDs, is a first.

idverde was awarded the contract as Principal Contractor, with design responsibility of 12 years and build phase responsibility of five years.

The scope of works consisted of

  • Installation of geotextile membrane to prevent contamination with existing soil
  • Construction of root cell structure to incorporate 28 trees from Rose St. to Elmbank St.
  • Installation of new irrigation system with 100mm perforated carrier drain
  • Importation of sub-soil to provide the optimum environment for the trees to thrive
  • Tarmacadam and Type 1 sub-vase layer to support the new Caithness paving
  • Species assessment, landscape design and installation of 28 trees along the avenue
  • Installation of bespoke tree grills
  • Laying of Caithness paving slabs along the length of the Avenue.

The Council initially proposed a list of approximately 30 street trees of different species, varieties, and growth habits. From this list we prepared a Tree Risk Register, categorising each tree as red, amber or, green, and shortlisting 8-10 species and varieties of trees. The final selection basis included colour and interest, suitability for the latitude, climate and wind parameters, resistance to Dutch Elm Disease, avoidance of lateral spread at road intersections, and being less attractive to aphids to avoid residue dropping. We have trialled the Japanese species, Ginkgo Biloba, positioning them in the most suitable location for survival. These will be closely monitored, having never been tried in Scotland.

The final five species selected promote colour variation throughout the seasons and differences in form, maintaining public interest throughout the year:

  • 6 x Acer platanoides ‘Deborah’
  • 5 x Acer campestre ‘William Caldwell’
  • 6 x Ulmus ‘Columella’
  • 5 x Carpinus betula ‘Fastigiata’
  • 5 x Ginkgo biloba

Working with Greenblue Urban, we arrived at a bespoke solution for the design and installation of load-bearing soil cells (crates). This creates structurally adequate soil systems that are conducive to root growth, provide adequate root spread and nutrients and support the longterm health of street trees. The Council re-routed utilities services to allow each tree to be planted in a self-contained 2.5m deep pit of double-layered crates, with 8-10m between each pit.

Sauchiehall Street comprises multiple road intersections. The Council’s road department required assurance that the loading from road traffic would have no sideward displacement effect on the rooting zone. This could result in a void being filled by road material causing the road surface to rut. We worked with Plean Precast to design a concrete retaining wall structure to absorb the forces from the road and verify the stability of the tree zones. However, once excavations were exposed at the road intersections we found unexpected communications utilities encased in concrete. These would have interfered with the design for the retaining structures, meaning our proposed solution could not be incorporated.

Additionally, the Health & Safety Executive (HSE) was concerned about the depth of excavation and its effect on the stability of adjacent footpaths and buildings. After various engineering testing we moved from a discrete single pitch design to a single layer of crate that spread continuously across 3 or 4 trees without spacing, collectively achieving a volume of 22m3 per tree. With this reduced depth of excavation, the concrete retaining structure was no longer required as the concrete-cased communication ducts were enough to satisfy the retention of the single create layer of the tree zones at carriageway intersections. The excavated area was also reduced to satisfy the HSE.

As part of the new SUDs system, there is a requirement for water to run off the paving, over the tree pit, and find its way below. Our design required sufficient attenuation capacity of our rooting space and the design of outfalls to remove over-capacity at a controlled rate into the sewers. We also needed to ensure the protection of services drains and basements from root invasion, the safeguarding of our growing medium from contamination from adjacent soils, and the irrigation of our trees, while preventing waterlogging. To ensure adequate surface run-off from adjacent public-realm we used a porous jointing product between paving units. To support the strength of these joints, we used a proprietary porous concrete as a base material to the paving surface, ensuring the paving construction was robust. We carried out infiltration tests to demonstrate water moved away quickly enough.

We trialled our first engineered designs off-site to test the design principles and programme predictability. Trees were selected 18 months in advance and air-potted to generate healthy fibrous roots. We manufactured a root zone, matching closely with the trees’ indigenous soil, and used manufactured, sterilised soil to give the best possible health for the trees to grow.

idverde was delighted to receive a British Association of Landscape Industries (BALI) National Award for this project in 2019.