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Unexpected WW1 tunnel find digitally mapped

Digital mapping has located and recorded an array of World War 1 practice trenches and tunnels on a Ministry of Defence (MoD) construction site in the south of England.

The area in Larkhill on the MoD’s Salisbury Plain training area is being prepared for a housing development for returning service personnel. But the site was once used as an early 20th century practice ground to prepare troops for trench warfare.

Discovery of the tunnels prompted a full scale archaeological programme to map and record the underground locations of the features and recover artefacts.

Archaeologists had been able to survey and subsequently backfill the trench system at ground level. However,it was more difficult to map safely a network of tunnels underneath the trenches, so consultant Cundall was appointed to record the deeper tunnel network.

Cundall partner and geotechnical lead Jim Allen said only small depressions could initially be seen on the vast sloping landscape, giving indications that there were collapsed tunnels underneath. A layer of flint in the chalk ground, meant that ground penetrating radar and gradiometry could not be used. Instead, the team used resistivity surveying to locate the tunnels from ground level.

“The resistivity imaging involves putting out an electrical sensor along a linear array with electrodes at fixed intervals,” explains Allen. You then shoot a current into the ground and you measure the resistivity of the ground. Where you have a void or an air space, it’s much more resistant and less conductive than rock would be.”

To further map the tunnels Allen, suggested using digital technology.

“At first the MoD were asking us to provide a man riding cage, so they could excavate and then send someone in, but we advised not to go with that but that seemed overkill,” said Allen. “So we suggested if the tunnels were relatively shallow we should excavate at the point of collapse, up to 5m deep we make it safe, put a person in there and then use a laser scanner to map the tunnel’s inclination, orientation and extent.”

The ground was excavated around the collapse points and a laser scanner was fired through the tunnel. When a wall was hit, its position was marked on the ground surface and the process repeated to see if it was a turning point or the end of that particular system.

For tunnels deeper than 5m, a rotary drilling rig was used to create a bore hole and a down hole CCTV camera was used to explore the caverns underneath. A down hole laser scanner was used to map the deeper tunnel.

Although this method could have been used across the site, Allen said allowing archaeologists to gain access to the shallower tunnels meant artefacts could be retrieved and recorded.

“The archaeologists found there was a lot of graffiti on the walls,” said Allen. “On one tunnel a list of Australian soldiers was found, one of them they found, went on to get a Victoria Cross for the Battle of the Somme.

“The ammunition also provided a significant challenge.”

As the area was used as a practice ground, the team also came across live ammunition which had failed to detonate at the time.

“A UXO [unexploded ordnance] specialist called Dynasafe Bactec took control of excavations until they felt it was safe for our staff to enter,” said Allen.

“It was predominantly hand grenades and rifle grenades which were found on site, and the problem was that these were the ones which had been discharged and were fully live, but for some reason didn’t go off.

“One of the guys said to me, if you hit it with a shovel hard enough, you’ve probably got five seconds to run as far as you can.”

The ordnance were deemed too dangerous to move, so when some was found, the surrounding area was instantly isolated and at 4pm on most days the army bomb squad would arrive to detonate it. In one week around 45 live hand grenades were found.

In preparation for the housing development the shallow tunnels have now been backfilled and the deeper tunnels will be stabilised with drilling and grouting methods once the final layout of the development is known.

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