Digging for victory

Work is underway across the Salisbury Plain Training Area to prepare for the Service personnel returning from Germany this year under the Army Basing Programme (ABP). This work has uncovered a unique network of First World War front line trenches and tunnels on MOD land in Larkhill, earmarked for over 400 new Army family homes. The trenches represented British and German front lines, built for realistic training prior to sending troops to fight on the Western Front.

Cundall’s involvement with this project began when a soil strip operation undertaken to allow the project archaeologists to investigate the former trench network led to an area of ground collapsing to reveal entrances to tunnels below the trenches. The presence of the previously unknown tunnels, potential archaeological significance and risk of Unexploded Ordnance (UXO) presented the project team with a set of seemingly insoluble problems and the potential for significant programme delays.

Initially appointed to provide advice regarding gaining safe access to the tunnels for archaeologists and UXO specialists, we instead suggested employing a set of complementary and integrated survey techniques. These techniques would ultimately succeed in mapping the orientation, inclination, extent and volume of the historic tunnels with pinpoint accuracy, while still allowing the most thorough archaeological records possible along with effective management of the UXO risk.

In this discussion, Jim Allen, Geotechnical Partner, discusses the innovative survey techniques used when working on the trenches.

Our Survey and Modelling Approach

One of the challenges for this army base was a sprinkling of unexploded ordnance (UXO) on the site. We wanted to minimise intrusive survey techniques to avoid risks as well as being able to map where ordnance might be located.

We designed a two phased approach, using integrated and complimentary tunnel survey techniques. This comprised a Geophysical survey, including Electromagnetic Conductivity (EM), Electrical Resistivity Imaging (ERI) surveys, laser scanning and topographic surveying. Effectively, the EM surveys give a plan view of the site and ERI gives detailed sections. The laser scanning enables a very detailed image of the discovered tunnels, techniques which complement each other very well.

See the images below for a typical resolution!

The combination of EM and ERI meant we could discover tunnel entrances, which could then be excavated, and laser scanned to make detailed records of the tunnels.

For the deeper tunnels, we used rotary probe drilling in conjunction with downhole CCTV and laser scanning to trace tunnel paths.

Outcomes

• Integrating engineering, archaeology and UXO surveys has enabled the most comprehensive record possible of a development site with significant and unique heritage.
• We have enabled effective management of ongoing (and previously unknown) UXO risk to ensure safety of construction workers, and future residents.
• Detailed 3D mapping/scans of sub-surface features to supplement the archaeological record and enable an effective engineering strategy to be developed.

By enabling the archaeologists to gain safe access to the shallow tunnel network, our methodology allowed them to record and extract substantial historical information. This included personal effects and numerous pieces of graffiti, one of which recorded the name of an Australian soldier who trained at the site and went on to be awarded a Victoria Cross.

The site presented a unique set of circumstances and problems, which played to Cundall’s great strength as a highly collaborative technical leader, and particularly our ability to integrate with diverse and already-established teams. We are proud to have developed a solution that resulted in the recording of so much detail about the tunnels, and the lives of the soldiers who constructed and trained in them before facing the horrors of the Western Front.