Development collaboration in Central Oxford
About This Project
Client:
Site:
Central Oxford
Introduction:
Tree Frontiers were asked to carry out a survey of trees at the Oxford High Preparatory School site in advance of planning applications for major works at the school. During the survey we identified one tree which would have its root protection area (RPA) impacted due to the proposed works. We proposed that below ground investigations be carried out to understand the extent of roots within the proposed development area. During these works a previously undocumented foundation was discovered and we concluded that this significantly affected the spread of roots. The design for this area of the site could be adapted to fit along the existing foundation, therefore limiting the potential for harm to the tree through encroachment of the RPA.
The Client:
The Girls’ Day School Trust oversees a group of independent girls’ schools, including Oxford High Preparatory School in central Oxford. The prep school site is located in north Oxford, and sits within the North Oxford Victorian Suburb conservation area. We were initially approached by Potter Raper, the construction consultancy who would be project managing the proposed development of the site. Having successfully assisted them in the initial planning and design during the first phase of the project, we were instructed again to further assist for a second phase.
Background:
One of the challenges to be faced was the balance between the need for new construction against the preservation of the existing tree stock. The proposed development included the demolition of an existing unused building and the construction of a new assembly hall with an annex and lobby, as well as alterations to the boundary wall, gates, and access points. Ensuring that these changes did not adversely impact the trees on site was a critical concern.
The Opportunity:
This phase of the development consisted of both demolition and construction. It was therefore vital that the constraints posed by the trees on site be accounted for within the site design in order to limit the potential for damage to or encroachment of their rooting environment. Following a detailed survey of the site following the guidance provided in BS5837 ‘Trees in relation to design, demolition and construction’, we were able to identify ten trees which presented a constraint to the scheme and which would need to be considered during the design phase. One tree had the potential to have its RPA significantly impacted by the proposed plans.
In collaborative discussions with Potter Raper and Webb Yates (the project engineers) we proposed that further investigations be carried out along the line of the proposed new foundations in order to better understand the extent of roots present in this area, and therefore the potential impact on the tree.
The existing asphalt surface was carefully removed under supervision, using a mini excavator to break the surface and pull sections away, revealing the sub-base.
Clearing the asphalt
Geo-Tree was engaged to undertake an air spade investigation. This approach is deployed using high pressure air to dislodge the underlying material, which consisted of sandy gravel with a small amount of organic matter, minimizing damage to the root system by avoiding any severance of roots. During the investigation, a significant below-ground structure was uncovered almost along the axis of the proposed assembly hall.
Footing revealed after air-spade works
It was found that there were substantial roots, up to 80mm in diameter, most likely from a horse chestnut that was growing in close proximity to the site. However, to the side of the uncovered foundation away from the tree, only a small quantity of roots was found, having passed below the existing foundation at a depth of about 500mm.
Roots to east of footing
The Results:
As a result of the investigation, the design of the new assembly hall was adjusted to follow the route of the old foundation, ensuring no works would occur within the RPA of the retained tree. The main access route to the new assembly hall would pass over ground above the RPA and therefore special measures were required to ensure that the potential for any harm to the tree was minimized. Initial design concepts were to install a three-dimensional cellular confinement system but this was found to be impractical due to the required build-up of a geo-grid system and wearing course, which would result in an increase in levels that would not tie into the new and existing buildings. An alternative solution was required that would have a shallower build up, but not require any excavation. A no-dig resin bound permeable surfacing solution called Terrabase Rustic, manufactured by Addagrip, was chosen. This solution involved laying a 2.4mm geofabric onto the ground, overlaid by a 7.1mm Terrabase Addagrip, and a 30mm wearing course, creating an overall depth of 35mm, matching the existing surface level. This approach ensured that the rooting area below the surface remained viable and had no impact on the health of the horse chestnut.
Measuring the sub-base
The collaboration between the members of the project team resulted in a successful planning application that met both the development needs and tree protection requirements. The proposed scheme did not require the removal of any trees, and special measures were implemented to protect the rooting environment of a significant horse chestnut.
The project demonstrated the capabilities of Tree Frontiers to work in a collaborative way and to develop innovative solutions to a complex problem. The outcome demonstrates compliance with both national and local planning policies, ensuring the sustainable integration of trees into the urban environment. If you encounter similar challenges and aim to achieve comparable results in your development projects, please contact Tree Frontiers Ltd.