Extensive cut & fill earthworks carried out across several phases at Story Homes development at Edgehill Park, Whitehaven had generated several stockpiles comprising 18,000m3 of natural topsoil and subsoil and included a historical stockpile comprising 20,000m3 of predominantly anhydrite gravel and cobbles. Whilst several phases of the development had been carried out in "greenfield" zones and consequently site won natural soils could be re-used without requirement for an Environmental Permit, the final phases of development included the anhydrite stockpile and zones of "brownfield" including a former depot and infilled former reservoir where made ground was known to exist. In addition, an adjacent development phase required import of suitable fill materials to achieve agreed planning levels.
Re-use of made ground or import of natural subsoil requires an Environmental Permit and it was concluded that a Materials Management Plan (MMP) prepared in accordance with the CL:AIRE Definition of Waste Code of Practice (DoWCoP) was the most appropriate regulatory option, despite material being already excavated. In order to demonstrate DoWCoP was appropriate, it was necessary to first present a detailed chronology of the site's planning history & corresponding earthworks activity to confirm the site won natural origin of the topsoil and subsoil stockpiles and that the stockpiles were not waste. In addition a rigorous chemical and geotechnical testing programme was undertaken to eliminate any environmental and geotechnical risks associated with re-use of both natural and made ground soil and to inform an earthworks specification that enabled the natural subsoil to be re-used as structural fill.
A broad range of chemical and geotechnical testing was undertaken to establish the soil properties: These established that the subsoil stockpiles comprised predominantly Class 2C stoney-clay (Figure 1), confirming the stockpiles to be relatively homogenous and of Glacial and weathered Coal Measures origin.
The properties of the anhydrite stockpile however, proved much harder to define. It had been anticipated at the time of deposition, that following exposure to the elements (principally moisture) that the anhydrite would convert to gypsum, undergo volume change and achieve a relatively "stable" state. However detailed chemical and mineralogical testing (XRF & XRD) to establish phases of gypsification demonstrated that despite over ten years exposure, a minimal volume of the anhydrite gravel and cobbles had begun conversion. Due to its inherent geotechnical instability and high sulphate content the anhydrite could not be re-used in proximity to development, however, could be re-used to form landscape-screening and POS where any long-term physical changes would not impact upon surface stability or concrete structures.
Following detailed discussions with the Environment Agency, presentation of the Site Earthworks and Planning Chronology and approval of a detailed Remediation Strategy, it was confirmed that DoWCoP was applicable and furthermore that as the adjacent development requiring import formed part of the same development (one "site of origin") one MMP encompassing the entire development was also applicable. A MMP Declaration was made in October 2023.
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