Case Study 4 - Harvil Road, Buckinghamshire
Site summary
A 17th–18th-century brick and tile kiln was identified and partly excavated during evaluation work (Horsley 2020), see Figure 1. It was located on the alignment of a new road being constructed as part of the HS2 project (a high speed rail link between London and Birmingham). The road was to be built on a large embankment (required for engineering rather than archaeological reasons).
Following discussion and assessment of the feature, it was decided to preserve the kiln beneath the embankment.
Reburial objectives
The objective was to protect the buried remains of a post-medieval kiln found during evaluation excavation. As the site was going to be reburied under a large road embankment, it was a long-term reburial. In this instance, reburial was chosen over excavation 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).
Significance
The remains comprise the buried lower parts of a post-medieval brick and tile kiln, including the oven chamber and lower walls. The clay floor of the chamber was reddened from repeated firings.
The kiln had been set into the hillside at the time of construction to make use of the slope and presumably to help retain the heat. It is one of a number of kilns in the surrounding area, likely constructed near sources of raw materials (clay and fuel) and close to buildings requiring bricks and the road for access to other markets. Trenches were excavated over the kiln to better define the significance, character and survival of the feature. These were subsequently backfilled.
Condition assessment
The kiln had been demolished after going out of use. The walls and roof (where it existed) were collapsed in onto the kiln and the area backfilled and compacted.
The kiln was constructed from robust materials, and most of the loose material in the trenches came from the demolition of the structure or wasters from firing, all of which were robust. The structure was not stabilised prior to the trenches being backfilled with the excavated material.
Risk assessment
The main risks were considered to be damage from the embankment construction and the potential for excessive loading. These risks were mitigated by assessing the loading impacts and carefully controlling the construction work. For example, no excavation was allowed in the area of the kiln and the construction methods for the embankment were designed to be monitored to ensure compliance.
Reburial design
Environmental criteria
Material excavated during the evaluation was backfilled into the trench, ensuring that the fill was chemically and physically compatible with the buried feature. Most of the other environmental criteria discussed in Section 2 were less relevant at this site or were mitigated by having a large embankment on top of the kiln.
Functional criteria
The key functional criterion was to accommodate activity above the buried feature, in this case a large road embankment.
In relation to the other functional criteria discussed in Section 2 of the guidance, this reburial design required minimal maintenance of the kiln and the feature was not at risk of theft or vandalism because it was buried under an embankment.
Accidental damage prior to or during the construction was a greater potential risk. This was managed by appropriate control measures, such as safety fencing and method statements.
Programmatic criteria
Given the scale of the embankment (more than 10 metres high), the intention was that this would be a long-term permanent reburial. However, the retention of information about the kiln in the Historic Environment Records and in the HS2 Historic Environment Archive would allow the kiln to be located again in the future in the unlikely event that the road and embankment were removed.
Summary
Overall, this was a pretty simple reburial project, with very robust remains. These were subject to fairly limited levels of settlement, which were unlikely to harm the significance of the site.
The following design information is drawn from the method statement (Cathro and Tetlow 2021) produced by HS2’s specialist supply chain. It was developed in consultation with the relevant stakeholders.
To ensure a stable base on which to construct the embankment, topsoil was removed from the area below the embankment site to reveal the natural substrate. In the area of the kiln, topsoil was removed to the depth reached in the evaluation excavation. A geotextile separator membrane was put down, and Type 1 stone (crushed stone or recycled material, 0–40mm size) was spread to a depth of 150mm. This was rolled with a small ride-on roller, and further stone was added in 150mm layers up to 600mm.
A second geotextile separator membrane was installed, and Type 2 backfill (slightly finer aggregate materials than Type 1, 10–20mm size) was placed above and compacted with larger earthworks plant. The areas adjacent to the kiln (not covered by the stone layer) were built up with Type 2 backfill until they reached the top of the stone layer. The embankment construction then proceeded as normal across the whole area.
The use of two geotextile separator membranes was entirely for engineering reasons, to stop the mixing of fills. In particular, the upper membrane was installed to stop fine particles from the Type 2 fill filtering down into the stone layer and potentially destabilising the embankment.
Settlement calculations were made. A maximum settlement of 60mm at the centre of the embankment and 10mm at the edge was felt to be acceptable, given the robust nature of the remains. As the stone layer covers the whole kiln and spreads the load across the area, this ensures that any settlement is evenly distributed and does not cause any localised deformation.
Stakeholder consultation
Stakeholder consultation on the proposed method statement for reburial was undertaken by HS2’s specialist supply chain, with Historic England and the local authority planning archaeologist.
Maintenance and monitoring
The initial backfilling of the evaluation trenches took place at the end of the evaluation excavation. Compliance with the reburial method statement was provided by HS2’s specialist supply chain, who monitored the works for construction, quality and technical assurance purposes.
There will be no ongoing maintenance or monitoring required because the site is deeply buried beneath the road embankment.
Documentation
Details of the reburial methodology will form part of the HS2 Historic Environment Archive Strategy, and the location of the kiln will be recorded on the Greater London Historic Environment Records.
There are no particular responsibilities for the upkeep and long-term management of the reburied feature due to the depth of burial and inaccessibility.
References
Horsley, K 2020 Area South Assessment Report on Archaeological Trial Trench Evaluation at Copthall North Field A and Field G. High Speed Two Ltd. Document no: 1EW02-CSJ-EV-REP-S002-000117
Cathro, J and Tetlow, E 2021 Measures for Preservation in situ of a 17th–18th-Century Tile Kiln, Harvil Road, Hillingdon. Contract Lot S1 Method Statement. High Speed Two Ltd. Document no: 1MC04-SCJ-EV-MST-SS05_SL08-000001
