Case Study 1 - Curzon Street Round House, Birmingham

Published 29 November 2024

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Site summary
Reburial objectives
Significance
Condition assessment
Risk assessment
Reburial design
Stakeholder consultation
Maintenance and monitoring
Documentation
References

Site summary

This site contains the remains of the world’s earliest round house engine shed. It was designed by Robert Stephenson, built in 1837 and demolished around 1852. The location was known from maps, but the level of survival below ground when the site was excavated was unexpected. The Round House is located within the site of HS2’s new Birmingham terminus, part of the high speed rail link between London and Birmingham.

Photograph of an archaeological excavation showing the foundations of a 19th-century circular train shed. A mechanical excavator sits on a mound of soil. A modern train is visible in the background.

Figure 1: Curzon Street Roundhouse during excavation. The concrete turntable is visible in the centre of the image. Image courtesy of HS2 Limited

Reburial objectives

The main objective, following the excavation, was the permanent reburial to protect archaeological remains during the construction and operation of the HS2 Birmingham terminus.

Significance

The site is internationally important and of high significance due to the surviving layout of the engine shed and the early use of concrete for the turntable base.

Condition assessment

This assessment was carried out by HS2’s specialist supply chain. It identified areas of damage to brick culverts and that the turntable base was constructed of weakly bonded early concrete. Some of the culverts needed protection during backfilling to reduce their potential for collapse, but no consolidation was undertaken to any of the remains prior to reburial.

Risk assessment

There were three main threats and risks to the structures of the Round House:

A small area at the north end of the site would be impacted by foundations for a viaduct carrying the HS2 line into Birmingham. These areas of direct impact were excavated in advance of construction.
There was potential for damage to the more fragile elements of the Round House during reburial.
The area above the reburial site was required as a lay-down/storage area for construction materials. There was a risk, therefore, that construction activities taking place above the site after it had been reburied would cause damage.

Plan of the Roundhouse and the new railway line with piles and piling caps shown. A hashed area shows the area of the Roundhouse that were reburied and preserved. The remaining parts to the northwest were excavated and recorded.

Figure 2: Plan showing the area of the Roundhouse that was preserved under the reburial area (hashed area). The northwestern part of the structure, outside the hashed area needed to be excavated for the new railway viaduct pile caps. Image courtesy of HS2 Limited

Reburial design

Environmental criteria

Given the relative robustness of the construction materials (brick and concrete) and limited number of other finds on site, there were few environmental criteria to consider in the reburial design. The fill was thick enough to provide thermal protection; this was probably the greatest environmental risk, albeit a low one.

Functional criteria

The reburial design needed to protect the remains during reburial and facilitate the use of the area above for storing construction materials.

Fill materials were used to cover the remains and to create a raised platform that could be used for lightweight construction activities (such as material storage). The purpose of the additional fill was to spread the load of any activity above the remains.

Fragile areas (the turntable and brick vaults) were protected prior to reburial. Concrete blocks and slabs were installed to create boxes around the structures. The fill comprised a layer of sand, with recycled construction materials above (pulverised/graded material from the demolition of the previous building/hardstanding from the site).

Signs were placed around the reburial site to indicate the presence of archaeological remains. Permitted activities in and around the reburial area were carefully controlled by barriers, signage and regular site briefings.

Programmatic criteria

Final landscape design plans for the area following the completion of the construction of the HS2 viaduct were not available at the time of reburial and at the time of writing this case study, the Round House is reburied within an active construction site. However, it was recommended that the reburial scheme (at least the lower fill and concrete blocks) was designed as a permanent long-term reburial.

It will be possible to remove some of the upper layers of engineering fill (which were installed to protect the site from activity above) to reduce the height of the area so it can be integrated into any subsequent landscape design.

Summary

After the condition assessment, it was agreed that fragile areas – a series of brick-built vaulted culverts and the turntable base – would need to be protected. Concrete blocks were placed either side of the fragile features, with another reinforced slab placed on top. These spread the load of the fill away from the fragile areas. The areas within the blocks were filled with uncompacted sand. Sand was also placed over the whole reburial site to a thickness of 150mm above the top of the upstanding Round House features.

Technical drawing (not to scale) of a brick arch, surrounded on the top and sided by sand and concrete blocks.

Figure 3: Idealised section drawing showing how fragile culverts would be protected using concrete blocks, slabs and sand. Image courtesy of HS2 Limited

Sand was available from the site, as part of the local geology. The designers wanted to use this sand for reburial as it would reduce the financial and environmental cost of importing sand to site. The sand did not meet the criteria set out in Section 4.3 Sand (and Annex 1: Finding a suitable sand for reburial). However, the chemistry of the sand reflected that in which the site was constructed, and although the sand contained more staining oxides than would usually be acceptable, it was decided that these would be unlikely to impact the bricks or concrete being buried.

The particle size distribution of the sand was also not quite in line with the recommendations. The sand contained a number of slightly larger pebbles/cobbles, so the reburial design stipulated that any inclusions within the site-won sand should be no larger than 30mm when used within 150mm of the brick features, and no larger than 10mm for sand placed near the concrete. This required the sand to be screened to remove large clasts. The reason for this was to eliminate any pebbles/cobbles that might cause point-loading damage to the buried features (and also so that the fill materials would be in intimate contact with the features).

These stipulations meant that those constructing the reburial had a choice. They could either bring in a small amount of appropriate sand for the areas near the archaeological features, or they could screen site-won sand for the vulnerable areas, and then use unscreened site-won sand for the rest of the reburial.

A layer of geotextile was placed above the sand as a separator membrane, to stop the mixing of fill materials. The specification for the geotextile was set out in the backfill specification document (Smith 2021), which included the following requirements:

Minimum tensile strength shall be 8.0kN/m in all planar directions.
Tensile elongation at maximum load shall be 60 per cent (+/-20 per cent).
Minimum water permeability shall be 90l/m2^s (-27).

Above the geotextile, a variable thickness of Class 6F2/6F5 fill (crushed recycled aggregate produced from on-site demolition work) was placed to level up the site to a set height AOD, with a minimum thickness of 300mm above the top of the sand. A geogrid was placed 150mm below the finished level to improve the performance of the capping layer. The Class 6F2/6F5 fill was placed in 400mm layers and dead rolled to attain a minimum 5 per cent California Bearing Ratio (CBR) surface stiffness at the final surface (Smith 2021).

Discussions were also held about using another geotextile as a separator membrane between the sand and the concrete turntable (to facilitate potential re-excavation at a later date). Following the principles set out in this document, it was not included in the reburial design.

Stakeholder consultation

Frequent stakeholder consultation took place throughout the design process. The HS2 team and their supply chain, including design, construction and archaeological specialists, Historic England and the local authority planning archaeologist were involved in regular design discussions and document review/sign-off.

Photograph showing a mound of soil with safety barriers. A trainline is behind.

A site visit by staff from HS2 and Historic England once the reburial scheme had been in place for some time showed how the protection measures were working. Some tensions relating to the interface between the reburial site and adjacent construction areas were flagged up, caused by the close proximity of a raised protective cover over the remains. This specific design aspect was not part of the reburial design discussions, which highlights the importance of having as much information as possible about subsequent use of the area when reburial plans are developed.

Maintenance and monitoring

Maintenance requirements for this site are low. The areas of greatest sensitivity, where structural elements are closest to the surface or in areas with vertical built shafts, are fenced off to stop accidental access. Across the area signs indicate the presence of archaeological remains and the area is currently used for light material storage only.

Photograph of a construction site with metal bars in the foreground, a tall fence in the middle distance and a scaffolding and crane in the distance.

Figure 5: Photograph taken standing on area of raised ground protecting the roundhouse building, the most sensitive parts of which are fenced off completely. In the foreground, materials are being temporarily stored. The scaffolding for the construction of the viaduct piers is visible behind. © Historic England

The reburial site is noted in the risk assessment method statement and it is mentioned in site inductions/briefings, so that everyone working on site is aware of its presence and the need to avoid excess loading above it. The site team carries out regular monitoring to ensure that only permissible activities take place in the reburial area.

Documentation

The reburial design documents are stored on the HS2 document management system and will be archived on the Archaeology Data Service as part of the HS2 Historic Environment Archive Strategy.

References

Smith, R 2021 Curzon St N4 Station – Roundhouse Backfill Specification – SCEW Series Appendices 6/1 to 6/15 – Enabling Works North. High Speed Two Ltd. Document no: 1EW04-LMJ_DJV-EV-SPE-NS08-016001

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