Deep in the medieval backlands, Parliament House, Edinburgh
Urban archaeological requirements may lead to archaeologists working in very different conditions, even to other construction industry workers on the same site. For instance, while the Main Contractor of an urban development site may only need to operate machines at ground level when excavating deep foundation trenches or driving down piling, the archaeologists are more required to bodily reach the depths of any excavation. Obviously, the deeper an excavation trench the more unstable the sides become. The easiest way of overcoming this is to step the sides, thereby maintaining a stable angle to the trench sides. However, within a small urban location, this is not always possible.
One way such a problem can be overcome was demonstrated at AOC Archaeology Group’s evaluation of the former medieval graveyard of St Giles Cathedral, now located behind the Scottish Law Courts at Parliament House, Edinburgh. The evaluation was within a small courtyard surrounded on three sides by five-storey buildings, set on the steep south slope of Edinburgh’s Old Town. From the start, AOC Archaeology was faced with problems in assessing the spatial distribution and depth of archaeology here. As the depth of the 19th century foundations of surrounding buildings was unknown, it was imperative that the evaluation should not destabilise the surrounding walls. This was compounded by results from earlier boreholes, which indicated that potential archaeological deposits four metres deep were overlaid by an overburden of made ground that was also four metres deep.
The solution was to step the ends of the trench to maintain a stable 45º angle between the base of the evaluation trenches and the adjacent walls. The sides of the trenches were supported with man-hole boxes 3m x 3m. These were pushed down as the excavation continued, the archaeologists excavating no more than 30cm below the box plates to ensure that no archaeological remains were damaged as the boxes were pushed down.
Trenches 8m deep raise other problems of course. One is the removal of significant amounts of spoil. At this site, this was overcome using a tray lowered by the site machine excavator, onto which spoil could be loaded. However, because of the operation of a machine within such a limited space, access into the trench had to be carefully managed; access routes were demarcated and scaffolding erected to enable the archaeologists to enter and exit safely. Another problem of the sheer depth was that the base of the trenches effectively became confined spaces, with a significant potential hazard from the build up of gases. Gas monitors were constantly employed to ensure that if a critical level was about to be reached, the archaeologists would be alerted. The archaeologists were also required to wear harnesses so that they could be quickly and safely brought to the surface.
In situations like this a proper programme of staff training pays dividends. Because AOC Archaeology Group staff have undertaken confined space training, the company can not only react in a competent manner to situations that develop during a long term project, but also plan ahead. AOC Archaeology Group’s training programme enables its staff to deal with any problem that a site can throw at them and this enabled the team to record significant medieval remains within the heart of the Old Town Edinburgh World Heritage Site. The archaeological investigation revealed the 17th-19th century Meal Market of Edinburgh and part of the late medieval graveyard that underlay this, from which 96 skeletons were excavated. Preliminary interpretation indicates that these burials formed part of the southward expansion of St Giles’ Church graveyard from the later 15th century and is consistent with the cessation of burials in St Giles graveyard in 1566. The burial ground itself overlay a series of middens spread over this part of the medieval backlands of Edinburgh. The findings of the evaluation are currently undergoing post-excavation analysis and preparation of a publication.
AOC Archaeology Group Project Manager: Ronan Toolis
AOC Archaeology Group Excavation Director: Michael Roy
Main Contractor: Laing O’Rourke
Project Manager: AMEC
Client: Scottish Court Service
