Digging for victory

Caroline Raynor outlines how sustainability was placed front and centre during the archaeological excavation of St James's Gardens in Euston

Archaeological mitigation can all too often be regarded as a preamble to construction activities, rather than a mainstream element. This has been further exacerbated by the fact that archaeological works no longer fall under Construction Design Management Regulations 2015 in the UK. At the HS2 Enabling Works in Area South (Euston Station to Colne Valley), the Costain-Skanska Joint Venture Construction team is seeking to unify construction and archaeological processes in order to maximise learning, innovation and environmental impact management.

A huge programme of archaeological work is under way at St James's Gardens, Euston. This former cemetery site was in use between 1789-1853 and closed as a result of the Metropolitan Burials Act of 1852. Although only in use as a burial ground for 64 years, the site is believed to have received around 61,000 burials. An extensive programme of archaeological investigation will generate a large number of artefacts and human skeletons, which will be assessed and studied by a team of osteologists.

The St James's Gardens Engineering and Construction team has identified a number of ways to ensure that the site is safer and more sustainable while assisting the archaeological works, supporting the archaeological subcontractor and helping to change perceptions of how archaeological work can be carried out. The key mandate is to ensure the works are carried out with care, dignity and respect for those interred.

All works are being carried out beneath a bespoke 11,000m2 encapsulation structure that fulfils a number of functions, the foremost of which is legal compliance with requirements set out in the High Speed Rail Act (London-West Midlands) 2017 and the undertakings and assurances linked to the act.

Sustainable structures

A collaborative approach when optioneering the design allowed the team to identify a number of sustainability benefits. The erection of the structure using Systems Scaffold, rather than a modular building made of bespoke components, allows for flexibility in design. The structure can be broken down into its component parts and reused, allowing the materials to be returned to market quickly and simply.

A birdcage scaffold was constructed on site and the roof structure was erected using two working platforms and a winch. This cut the number of person-hours spent working at height and mitigated the need to bring a crane to site, reducing plant and pedestrian interface, emissions and the number of large pieces of construction plant being brought into a busy area. It also provided reassurance to Network Rail, with whom there is a shared site boundary.

Ballast used to support the construction of the 15 tent towers will be removed during the excavation process and recycled for use on haul roads and within a gabion wall being constructed along the site boundary.

The structure encloses the site to ensure that the works can be carried out with dignity, in line with the legal requirements and expectations of the Church of England and Historic England. The 110m x 90m canopy protects the site from adverse weather and reduces flood risk by channelling rainwater into 11 bespoke 25,000-litre attenuation tanks, where water is harvested for use elsewhere on site or allowed to flow at a controlled rate into the nominated discharge point via three 100ml connections, which control the flow and mitigate the need for a flow metre.

The structure is designed with a number of key features that support the archaeological excavation, including lifting equipment (reducing the number of pieces of plant required) and LED lighting (to permit working during winter months and provide the correct lumen levels for shadowless archaeological photography). The exterior of the tent is made of Monoflex and Kingspan, which act as insulation and provide an acoustic baffle.

Within the structure, archaeological works are taking place in compact London Clay. A huge volume of material (around 29,000m3) will be excavated as part of the process. Around 10% of the site was contaminated with Japanese knotweed; careful mitigation measures, on-site training and a watching brief by an ecologist had a positive impact, proactively managing this invasive species and reducing the amount of contaminated material being removed from site. All materials generated by the archaeological excavation are screened to ensure that only London Clay leaves the site, and this material is put through extensive sampling to ensure it can be repurposed for a number of functions within the works.

Innovation and integration

In support of the archaeological works, it has been key to provide user-friendly and environmentally sound plant and equipment to manage spoil removal. The team has deployed six mini electric tracked dumpers and six electric wheelbarrows with ruggedised wheels to improve stability. These have saved time and money, as well as reducing the impacts of manual handling as they replace traditional small hand barrows. There is also a two-tonne electric dumper, a five-tonne electric telehandler and a 20-tonne hybrid excavator. The latter operates on battery power while slewing to load muck-shift wagons, reducing emissions and fuel consumption by 40% in comparison to a standard 20-tonne 360°-tracked diesel excavator. The environmental focus of the encapsulation structure at the design stage allowed for charging points at key locations across the site to allow plant to be charged on a nightly basis without needing to be moved off site.

The bespoke archaeological research facility and lab that supports the off-site works was designed by the Costain-Skanska team to exacting EPC specifications (EPC B-rated) with insulation, PIR lighting and specialist flooring, implementing lean and ergonomic design. All vehicles involved in the delivery and construction of the facilities were Euro 6-compliant. The team has even gone to great lengths to integrate new members of a more sustainable supply chain for contracts such as sanitary waste disposal, by choosing a new self-managed sustainable waste system. This has reduced the number of vehicle movements by 104 per year to St James's Gardens alone.

An integrated approach and a passion for innovation is driving this unique project. The benefits are clear and measurable: improved working conditions for the team and greater control over the way archaeological mitigation is carried out ensures that the impacts on the environment are greatly reduced.

Caroline Raynor is principal archaeologist for Costain-Skanska Joint Venture and work package manager on the archaeological programme at Euston for HS2 Enabling Works