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Case study

Safe by design

Costain-Skanska JV’s work on the High Speed 2 rail link’s central London site has changed perceptions on sensitively delivering urban archaeology projects while pleasing two disparate professions.

05_archaeological excavation in foundation shafts
© John Zammit Photography

Archaeological excavation is taking place on a huge scale at Euston, London, as part of the enabling works for the new station on the High Speed 2 (HS2) rail project. The site, at St James’s Gardens, is a former post-medieval cemetery and is estimated to contain up to 61,000 burials. The Church of England interred bodies there between 1789 and 1852 and it served as an extra-parochial burial ground for the church of St James’s, Piccadilly. But the Metropolitan Burials Act 1852 put an end to this function after connections were drawn between the spread of communicable diseases such as typhus and cholera, contaminated water and the presence of over-full burial grounds in a fast-growing city.

This site is unique in both scope and scale and has presented the ideal opportunity for principal contractor Costain-Skanska JV to integrate the disciplines of archaeology and engineering to deliver a project of unparalleled size and academic significance. Applying engineered design, detailed planning and lean and ergonomic processes to the archaeological works has increased performance, while ensuring that all remains within the burial ground are treated with care, dignity and respect.

04_aerial views showing completed structure in relation to wider enviroIt has also significantly reduced risk, while permitting scope to introduce safety, environmental and technological innovations. This has benefited the workforce and stakeholders, as well as increasing safety, efficiency and productivity when compared with standard archaeological processes used around the UK.

Archaeological works, from evaluation to mitigation, are often regarded as an inconvenient preamble to many construction activities rather than a mainstream part. Archaeological subcontractors working in commercial archaeology are also regarded as “non-standard” construction functions.
This issue is compounded by the 12_22 tonne hybridfact that the Construction (Design and Management) Regulations 2015 do not cover archaeology or the onsite activities associated with it.

Clear advice for safe working processes in archaeological evaluation and mitigation is rarely tailored to archaeological works or those who lead or plan it (see box, below).

The magnitude and complexity of archaeological works being undertaken on the 230 km route as part of the enabling works requires a very different approach, particularly in central London.

Work involves co-operation with significant national infrastructure owners and stakeholders, including Transport for London, Network Rail and London Underground. There is a need to spearhead a drive in excellence and collaboration between archaeology as a discipline and the construction industry to complete works safely, on time and to budget.

At St James’s Gardens, the work centres on the burial ground and the surrounding residential and business areas in the London Borough of Camden.

At the site all possible avenues of innovation were explored in the planning to ensure the works were “safe by design” from the start. This involved identifying and designing out all major safety and occupational risks. The design of the encapsulation structure (a big tent) that now covers the burial ground is an important example of this process.

Encapsulation structure

Undertakings and assurances associated with the High Speed Rail (London – West Midlands) Act 2017 require that the 90 m x 110 m site was enclosed from public view before the excavation work started. The aim was to encapsulate it within one large structure to maximise the open area of the dig and limit the need for confined space working or the need to strike, move and reassemble smaller tents.

The scaffold design and construction removed the need to transport a large crane through an urban environment

This approach provided the most favourable environment to protect the privacy and dignity of the remains, while permitting the safest, most efficient method of working. Provision of the largest possible open working area allowed the archaeological works to be carried out with minimal confined space work (and therefore the least possible work at height and at depth) and afforded a safer environment to the workforce of about 200. By promoting a collaborative and integrated approach great benefit was gained: in terms of allowing the archaeological team to understand and experience the engineering processes as well as promoting shared learning on the value of archaeology and heritage with the construction and civils teams.

08_erection of steel tower on siteUnder the detailed sifting process five engineering designs for encapsulating the site were “optioneered”. Those involving sheet piling or creating large caissons (watertight retaining structures) or coffer dams were quickly discarded. The site lies directly above London Underground’s Northern line, which increases the possibility of prolonged working within a confined spaces environment.

The selected design involved the archaeological excavation of trenches for 15 steel and concrete foundations with associated steel towers, in a grid arrangement. These were subject to a robust engineering design (sheet piles and frames executed using hand dig and dig and push so there was no impact on the human remains) to ensure that maximum depth could be safely achieved. These support the encapsulation structure’s twin-apex roof and ancillary equipment. By designing for a minimal number of localised footings, situated below the level of the soils where human remains are present, there was a greatly reduced risk of undermining the structure during the main archaeological mitigation phase.

Confined spaces

A team of field archaeologists subcontracted from MOLA Headland Infrastructure excavated the foundation trenches and all human remains were recorded and retrieved by hand. These areas were classed as confined spaces so a full regime of confined space working, including an emergency rescue team, was in place throughout this phase.

Since the burial ground was in use at the peak of London’s industrial expansion, it is populous and some deep burials were identified requiring a proof dig of 8.2 m below ground level. The grid arrangement of the footings spans the full extent of the site on an east-west alignment. This proved highly advantageous to the archaeological team which was able to develop a detailed picture of population density, levels of preservation and class differences present across the “four grounds” (upper, middle and lower class and pauper’s ground). This in turn helped to inform the archaeological sampling and research strategy for the main mitigation phase. More than 1,100 burials were excavated from the 4 m x 4 m x 6 m footings over 16 weeks.

Before works commenced on site, scaffolding contractor Palmers Scaffolding UK erected a 1:1 mock-up section of the encapsulation structure to trial the main rollout mechanism. Built on site at Chester airfield, the trial allowed the scaffold and engineering teams to improve the original design, while identifying and eliminating risk before work started in London.

Safe working processes

Select advice on health, safety and environment issues is available from public forums including the British Archaeological Jobs Resource and the Chartered Institute for Archaeologists.

Information on the Health and Safety Executive website regarding management of excavations, confined spaces, plant and temporary works should be adhered to rigorously, although this is not aimed at an archaeological audience.

Reinforced concrete foundations were cast 6 m (17.3 m Above Ordnance Datum) below the ground with a thickness of 0.75 m. Additional kentledge was provided on the top of the foundation. The sheer area and height of the structure (11,000 m2 and more than 13.5 m tall) made it imperative to prevent uplift and overturning due to wind loading. The foundation design had to account for these forces on the structure, which will stand for up to two years. Steel work for the foundations was constructed on site at ground level by Costain-Skanska JV steel fixers. The bespoke design integrated a table-top arrangement with pre-placed shims. This was lowered directly into the foundation before the concrete pour, with minimal work at depth because the fixings for the tower attachments were already in place, checked and determined to be true. 

The steel towers were transported to site using Euro 6 compliant vehicles to minimise emissions and ensure environmental best practice. During initial optioneering, the design and materials were explored for construction of the 15 towers. Based on the span and scale of the structure, each tower would be required to bear a load of about 200 tonnes.

The initial design proposal allowed for the use of scaffold to form the towers. This would have required  64 scaffold standards and necessitated prolonged periods of working in confined spaces, lifting activities and manual handling. This prompted the design team to examine other options, including slim-shore and mega-shore systems. Slim-shore would not have tolerated the loads generated by the huge roof. The mega-shore option could take the load, but the components were heavy and would have presented manual handling challenges and risks for the scaffold team therefore it was important to remove these risks.

The team identified that a bespoke steel tower was the best solution to replace a traditional scaffold approach and provide appreciable safety benefits. By changing the design to steel towers, these key components could be prefabricated in the UK and transported to the site for erection in two stages. This activity required only two lifts for each tower and most of the work could be carried out safely at ground level. Only one person was then required to enter the trench to bolt the towers to the foundation, an activity that took less than one hour. Use of the bespoke tower legs provided a robust solution to eliminating multiple risks associated with manual handling, lifting and working within a confined space. By using this method, an estimated 5,000 hours of work at height and 5,000 hours’ manual handling were eliminated from the construction of the towers.

Environmental benefits

Rail ballast was used to mass-fill the 15 excavated shafts, permitting removal of the sheet piling and reducing the risk of falls from height into an open excavation. This method also reduced the temporary works inspection regime and removed the need for hot works within the structure (the standard method for removing sacrificial sheet piles). This material was chosen for its ready availability, self-compacting properties and the fact it looks visually different from the in situ cemetery soils which allowed better management of the archaeological works as the dig progressed. The ballast was recycled on site to reduce the project’s carbon footprint and reduce the number of vehicle movements needed to remove it in line with undertakings and assurances on carbon management and sustainability.

2_ Chest tomb in situ in St James's GardensOnce the towers were in place the process of erecting the roof structure began with the construction of a bird-cage scaffold at the eastern terminus. The design and construction of the scaffold eliminated the need for a crane to be used next to live Network Rail assets. It also removed the need to transport a large crane through an urban environment occupied by numerous sensitive stakeholders including schools, medical centres and homes. The scaffold was used to erect a large working platform for constructing and launching the three spine beams that support the roof trusses. The beams were constructed using system scaffold in a cassette formation and launched from east to west using a tirfor and guide tower mounted on rails. This helped to control the movement of each beam, giving a greater level of control during the launch. The apex roof structure was erected in sections on the working platform and winched into place using a hoist integrated into the scaffold. Once mounted on the spine beam, the roof sections were pushed out from the scaffold on nylon rollers. This greatly reduced manual handling time and eliminated around 9,000 person hours of work at height on the roof construction.

An extensive package of mechanical and electrical systems (M&E), including lighting, fire suppression, lifting equipment and high-level mobile access gantries for maintenance, was installed within the structure. These features were added to promote the safe and efficient maintenance of the build while maximising the safety and efficiency of the archaeological works. The structure was completed with an exterior cladding of kingspan insulation panels, erected from a cherry picker. These provided thermal and acoustic properties and limited the noise emitted during the works.

Electric plant

Working within an enclosed space led the team to seek innovations in new-to-market electric plant and equipment. With most site activities taking place over nine months inside the encapsulation structure, it was important to ensure that the site remained largely emission-free. Coupled with the undertakings and assurances specified for air quality monitoring in the London Borough of Camden, this was a key driver for the team. The St James’s Gardens engineering and construction team worked hard to ensure that implementation of electric and hybrid plant was mandated from the inception of the project design. The encapsulation structure M&E was designed to support multiple charging points housed in the tower legs.

Archaeological works are often regarded as an inconvenient preamble to many construction activities

This location ensured that all M&E was successfully excluded from the dig area and mitigated cable strike risks. The distribution of the towers on a grid system across the site also permitted ample locations for the charging of various electric plant. Key to delivery of the works were nine 1 tonne all-electric tracked dumpers and six small electric wheelbarrows. These improved productivity for the archaeological team and ensured fast, efficient and emission-free spoil management across the localised excavation area.

Use of traditional wheelbarrows was reduced significantly. This improved manual handling and minimised the risk of musculoskeletal issues among the archaeological team. The site also provided an excellent proving ground for first-to-market plant, including a 22 tonne hybrid 360 excavator and the first two all-electric non-umbilical 1.7 tonne 360 excavators. A two tonne all electric dumper and an all-electric front loading excavator were also trialled during the works. By working closely with the supply chain, the team identified opportunities and mitigated risk while providing valuable feedback on innovations. The archaeologists benefited from maximising the use of small electric plant which reduced physical excavation and spoil management. In total, 8,869 person days of hand digging were saved through the careful deployment of small electric plant. Small standard diesel excavators were also deployed but were fitted with scrubbers to reduce output of fumes.

Project credits

Costain-Skanska JV St James’s Gardens team credits: Paul Snelson, David Frodsham, Andrew Alexander, Mark Ballard, James Todd, Ben Lennon, Michael Eborn, Myles Johnstone, Shannon O’Keeffe, Arty Tailor, Richard Martin, Ben Rowan, Ette Roberts, Kearney Sullivan, Aaron Murphy, Alganit Tabir, Abid Al-Noori, Amy Kwok, Charles Nicholson, James Beard, Michael Ridley, John Greggains, Woan Ten Lee, Jason Bicknell, Andrew Foster, Robbie McCabe, Simon Mintoft, Isobel Simpson. Scaffold credits: Paul Duggan, Colin Duggan, Dave Siddell and the Palmers Scaffolding UK team. Civils and groundwork support credits: BCL. Archaeological fieldwork team credits: MOLA Headland Infrastructure.

Innovative design

Lean and ergonomics had never previously been applied to archaeological works and the effects and improvements (removal of eight wastes) were measured using a bespoke control board, which captured key outputs daily. Costain-Skanska JV identified an opportunity to implement a series of changes in the way the archaeological working area and laboratory spaces were designed and ultimately used. After visiting the archaeological contractor’s offices and consulting site staff, Costain-Skanska JV lean specialists identified a number of potential improvements. The first was to provide the archaeological team with a crew of support staff to assist in spoil management. This removed the more labour-intensive element of the works. The second was the implementation of the electric plant.

Within the laboratory space designed by Costain-Skanska JV, lean and ergonomic principles were applied to ensure the optimum space was provided to manage the flow of about 100,000 archaeological artefacts, which were retrieved from the site for analysis. This included bespoke storage and washing facilities with consideration given to the correct heights of shelving and washing stations. Ergonomic tool sets placed on shadow boards were sourced and provided to assist in manual handling and the cleaning of artefacts. Specialist anti-slip flooring was sourced to mitigate risk of slips, trips and falls and anti-fatigue matting was laid to prevent fatigue when working in a standing position in key areas of the lab.

As the archaeological phase of the project nears completion this month it is gratifying to look at the statistics and outputs and see how engineering and lean processes can be applied to archaeology to create a safer, more efficient working environment. The success of the project can be attributed directly to the effective blending of two previously disparate disciplines and approaches. By planning all aspects of the work the Costain-Skanska JV St James’s Gardens team has been able to change the perceptions of the way in which large urban archaeology projects can be delivered and open pathways of communication and routes for shared learning that benefit both groups.

 

CAROLINE CATRIONA RAYNOR
Caroline Catriona Raynor is project manager and principal archaeologist at Costain-Skanska JV

Comments

  • Very interesting piece a

    Permalink Submitted by Amanda Way on 5 August 2019 - 06:22 pm

    Very interesting piece and nice to see both professions coming together and working so well together. It true that archaeology can be seen as a preamble to a construction project but hopefully, as this piece has shown, it will now be seen as part of the overall project both in necessity and as part of the project itself.

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