Establishing the extent of peat for the Royal Horticultural Society

In 2015, the Royal Horticultural Society (RHS) announced its intention to develop a 63-ha garden in the heart of the Manchester conurbation at the site of the former Worsley New Hall. The project, the first RHS garden in North-West England, spans a mixture of green and brownfield development sites, includes plans for the construction of an impressive visitor centre with associated learning space, parking spaces for up to 1,000 visitors’ cars and associated access roads and service yard infrastructure.

In 2016, RSK company RoC was appointed to assist with the technical design of the project and quickly identified that a significant portion of the site, including most of the 1,000-space car park, access road and visitor centre, was located across an area of peat associated with the nearby Chat Moss.

Although the rough extent of the moss was indicated on records from the British Geological Survey, further work was needed to establish its true boundaries. To do this, we designed and conducted a programme of ground investigation work to define the peat boundary and establish its depth across the site. During the design stage, we recognised that construction across the peat was still possible utilising a range of techniques ranging from wholesale bulk excavation and disposal to in-situ treatment and stabilisation. We concluded that to simply excavate and replace the soils was neither sustainable nor economically viable. In-situ treatment of the peat was determined to be the most cost-effective solution to road and car park construction, particularly given the size of the potentially affected area. Therefore, before starting the on-site work, we began discussions with specialist contractors to assess the viability of the peat treatment and establish the initial costs. This enabled us to tailor ground investigation works to both meet the requirement for the delineation of the peat extent and enable specialists to collect bulk samples for mix trials, thereby allowing a more accurate estimation of stabilisation costs.

The results from the initial ground investigation work identified an affected area of approximately 6 ha. RoC’s strategic land services team’s ability to recognise the potential abnormal costs that the peat presented at the pre-engagement stage, as well as to suggest potential treatment solutions, was one of the driving factors behind our subsequent appointment to the project.