Case Study 2 - Hunt’s Green Farm, Buckinghamshire
Site summary
The site at Hunt’s Green Farm contains evidence of an Iron Age boundary ditch called Grim’s Ditch, part of a larger monument that continues outside the site area.
The ditch to the south-west (see Figure 1a and 1b) of the reburial area is scheduled. Other scheduled and undesignated sections are located throughout Buckinghamshire. The area being preserved is not designated, because it was not known about before the HS2 excavations.
The site, shown as area C21052 on Figure 2 is part of the HS2 project and is located on land required for the construction of landscape mitigation earthworks. Archaeological information below summarised from Hughes and Stafford (2022), engineering information and the reburial design from Hallett and Williams (2024).
Reburial objectives
The reburial objectives for this site are permanent reburial beneath an up to 4 metres-high earthwork (embankment). The ditch (truncated by agricultural activity) was located during site evaluation.
A decision was taken to preserve the ditch in this area beneath the embankment rather than excavate it, in line with HS2’s Heritage Memorandum, which states:
The nominated undertaker, so far as is reasonably practicable, will seek to reduce harm to the historic environment (HS2 2017).
Prior to the construction of the earthwork, the topsoil was removed from most of the footprint of the embankment in area C21052. This enabled its course to be plotted and for two sections to be excavated – to better understand the ditch and its contents at this location in advance of reburial.
Significance
The Iron Age boundary ditch is scheduled in several locations, including one section of visible bank and ditch adjacent to the reburial site. The site is, therefore, of national importance and high significance.
Also present on site are a small number of Iron Age and Roman cut features, which have not been excavated.
To the north of the reburial site is an area of Romano-British settlement, with industrial features including furnaces and corn-drying ovens. Part of this area has been excavated, while another area (shown on Figure 2 as C21054) will also be preserved under the embankment; this area was evaluated and will be subject to the same strategy as described below, although in this instance the topsoil has not previously been removed.
Condition assessment
Across the Hunt’s Green Farm site, the ditch is present only as a buried, in-filled ditch, the bank having been removed by previous ploughing. The ditch is approximately 5 metres wide and 1.3 metres deep.
In the scheduled area to the south-west of the bank, it is visible as a bank and ditch earthwork feature. The local geology at the surface is clay-with-flints, into which the ditch is cut and with which it is filled.
To better understand the area being reburied, the topsoil was removed and the visible features were mapped and recorded. A small number of features, including two sections of the ditch, were excavated. Finds were limited to small fragments of robust pottery, largely post-dating the assumed Iron Age construction date of the ditch.
Overall, other than the historic plough truncation, the ditch feature and finds were assessed to be in good condition. The main vulnerabilities that needed to be considered for the reburial design were those associated with loading (the weight of the embankment) and the methodology for the embankment construction.
Risk assessment
The main risks to the archaeological deposits were considered to be damage from construction plant and / or construction methodology, and excessive settlement (both total and differential) due to the load imposed by the placement of fill material for the proposed earthworks.
HS2’s specialist supply chain undertook an assessment of the settlement. This demonstrated that the ground containing the archaeological deposits is anticipated to experience a small amount of compression, limited to 10mm. The overall settlement caused by the earthworks is predicted to be <35mm.
A protective starter layer and construction control measures have been designed to ensure the movement occurs evenly across the area, and to prevent damage from construction plant.
Reburial design
Environmental criteria
As the features being preserved were quite simple, well-understood and robust, with few finds recovered, there were not many environmental criteria that needed to be considered for this site.
The embankment would cause a decrease in surface water infiltration to the area underneath it, but this was not identified as a particular issue because no archaeological remains were preserved as a result of waterlogging and any reduction of water in the deposits under the embankment were unlikely to impact any of the remains.
The embankment, when constructed will be placed directly on top of the replaced topsoil, which came from the site so is chemically similar. The embankment will be constructed from a range of materials including soil excavated from across the HS2 scheme. As the archaeological finds are robust, the impacts of using potentially chemically different construction materials, in this instance, were not viewed as a high risk.
In relation to the other environmental criteria outlined in the guidance, having a large embankment on top of the remains will provide thermal protection and will restrict any animal burrowing activity, plant growth, or erosion of the archaeological features in the future.
Functional criteria
The main functional criterion is to accommodate the construction of landscape mitigation earthworks above the reburial. A 4 metre high engineered embankment will be constructed 7 metres away from the top of a cutting (in which the railway will be constructed). The embankment, which will face the railway (termed the HS2 shoulder), has been designed to be stable for 120 years.
Landscape fill (consisting of materials excavated on site) is placed behind the embankment to create a gentle mound which retained open views from the East, see Figure 3. Some fencing will be required around the site, but this will be located away from the main areas of significance.
In terms of other functional criteria outlined in this guidance, having a large embankment on top of the site will reduce the need for maintenance and reduce the risk of unexpected damage. Prior to the construction of the embankment the preservation area is fenced off with signage indicating the presence of archaeological remains and access tightly controlled.
Regular inspection visits will ensure the temporary protection is being maintained. The embankment construction will be archaeologically monitored.
Programmatic criteria
The actual reburial will be long-term/permanent, but because there was a number of years between the completion of the archaeological works and the start of construction in this area, the topsoil needed to be replaced, following investigation and recording. This soils replacement could be viewed as a period of temporary reburial, prior to the construction of the embankment.
Summary
Prior to work, the area will be strimmed to remove any vegetation that has grown.
A protective starter layer and set of construction control measures have been designed to reduce differential settlement, which might occur in archaeological features if their fill is not as compact as the surrounding soil. This is a particular risk in those areas where the ditch, and other features were excavated and backfilled.
To reduce this risk the protective starter layer will be constructed immediately beneath the landscape mitigation earthworks. This comprises a layer of geogrid (Tensar TriAx TX150), laid directly on top of the topsoil, followed by a 300mm minimum depth layer of Class 6C fill (natural gravel / crushed clean stone aggregate, up to 125mm in size). This protective starter layer will distribute the load of the embankment and landscape fill evenly across the whole area (see Figure 4).
The geogrid acts as reinforcement to the fill to distribute the loading by the earthworks fill, reducing the overall settlement. A layer of non-woven geotextile (Terram T1000) will be placed above the aggregate, to prevent finer particles from the landscape mitigation fill migrating into the Class 6C starter layer (as this could potentially impede the layer from draining freely).
The first 600mm of embankment and landscape fill will be placed in layers using low ground pressure (LGP) plant, and compacted with non-vibratory plant.
Stakeholder consultation
There has been strong stakeholder engagement with Historic England and the local authority planning archaeologist throughout the development of this reburial scheme.
Maintenance and monitoring
Work to place the embankment materials during construction will be monitored to avoid disturbance and damage from heavy machinery. Once construction is complete, no monitoring or maintenance will be required for the reburied site because it will be below a large embankment.
Maintenance of the embankment and any vegetation cover will fall to the railway operator, and further operations will be unlikely to impact the below-ground remains.
Documentation
Once the reburial is complete, details will be placed on the Archaeology Data Service as part of the HS2 Historic Environment Archive Strategy and recorded on the county Historic Environment Records.
References
HS2 2017 High Speed Rail (London–West Midlands) Environmental Minimum Requirements Annex 3: Heritage Memorandum. [Accessed 4/6/2024]
Hallett, M and Williams, D 2024 Design Technical Note for Hunts Green Farm Archaeological Sites Requiring Preservation In Situ. Unpublished HS2 document: 1MC05-ALJ-GT-NOT-CS03_CL05-710056
Hughes, V and Stafford, L 2022 Post-Excavation Assessment Report for Archaeological Recording at Hunts Green Farm. High Speed Two Ltd.
