Chelsea Garden Project
Chelsea, London Hospital of St John & St Elizabeth Landscape Planning Limited C M Maintenance
ProtectaWeb™ TRP 200mm,
Tree Planting Boxes,
Root-tex™ 10 & 30 geotextile,
GT500 geomembrane and tapes.
The Hospital of St John and St Elizabeth in London is one of the UK’s largest independent charitable hospitals.
This project involved a need to solve the design constraints for the relocation of a Chelsea Flower Show garden to a hospice, part of the hospital site.
There were three main constraints to overcome in implementing the work. The first involved a large Lucombe Oak adjacent to the hospice and covered by a Tree Preservation Order (TPO), with over half its defined Root Protection Area (RPA) encroaching across the required site for the Chelsea garden.
Secondly, the garden had to be level with the hospice glass door to allow wheelchair access.
Finally, it was important not to cause undue damage to the TPO tree roots from compaction, raising of the ground level, contamination, or restriction of available precipitation and to allow gaseous exchange with tree roots.
A successful conclusion was reached using Wrekin’s geosynthetic products and expertise and thanks to a close working relationship between arboricultural consultants Landscape Planning Ltd., engineers at ESL and contractor C M Maintenance Ltd.
Wrekin provides geosynthetics products, the expertise of a specialist team led by Roy Partington and on-site product installation supervision to devise innovative solutions.
Protectaweb is a proven no-dig method of reducing the imposed loads on the soil and tree roots beneath. It is a matrix that contains and confines granular infill. Its perforated cell walls permit lateral drainage, thus minimising hydrostatic build-up and ensuring the maximum supply of water to the tree roots.
The Arboraft tree planting system combines a high-strength pre-formed void creator with a specially designed soil. It prevents compaction of the sub soil ensuring the roots are not inhibited and growth is unrestricted. Tree anchors and irrigation can simply be incorporated into the system to provide a total tree pit solution.
An aerated and lightweight solution was provided to raise the garden level over the RPA of the constraining oak tree using Arboraft crates and 3D cellular confinement ProtectaWeb in layers.
Gaseous exchange to underlying tree roots has been facilitated using air funnels and pipes, located every 2m around the edge of the 11m x 11m garden. The garden edging was supported by high quality green oak sleepers bolted into the Arboraft crates and supported by hand-dug and lined concreted oak sleeper posts.
The crated garden base was lined with specially treated permeable geotextile with ‘wicking’ properties that enable it to ‘hold’ water. This will prevent the 300mm garden topsoil from drying out and allow water to percolate to lower levels.
Below the wicking geotextile, a thick dpc plastic sheet ‘tray’ is located to within a crate’s width of the garden edge to capture and hold water. This prevents drying out but also allows excess water to drain away around the edges to the tree roots below.
Finally, 11 specially designed Wrekin ‘Tree Boxes’ were used to form part of the edging of the garden to allow the planting of 11 topiary yew and four mop headed hawthorn. A base has been created which has allowed the stunning Flower Show garden to be relocated and installed for the patients and their families to enjoy at St John’s.
The construction was consultant-led and supervised by arboricultural consultants from Landscape Planning Ltd. with the help of Roy Partington, using specially selected contractor C M Maintenance Ltd.