SMHS project: Excavation


11 Mar 2012

Excavation

Having cleared the vegetation from around the dragon's teeth, we wanted to conduct an excavation of one of them to determine construction detail.

An initial precaution was taken to rope the block to be excavated to its neighbour until we were sure it wouldn't move as we partly undermined it. The geology proved to be stable enough, however, and the block did not move at all.

We dug down around one corner of the block and exposed the foundation, which was found to be 85cm deep.

A metal detector sweep of the area was conducted and some minor excavations carried out. These revealed some corkscrew pickets that would have been used as part of the barbed wire entanglements of the Cripp's Corner nodal point perimeter.

This was useful evidence, as we hadn't expected the barbed wire perimeter to have been so close to the dragon's teeth.

It appeared that the pickets had simply been bent over in situ and buried after the war. We also found a number of them in nearby hedges.

Construction

A full survey of the seven blocks was taken, each one being accurately plotted on a scale plan. Even the marks left by the wooden shuttering (the mould into which the concrete was poured) was measured, allowing the construction technique to be identified.

Excavation

As the local geology was firm enough, the foundation pit did not need to be lined with wood. The wooden shuttering was placed over the hole and the concrete poured in stages.

Our survey data allowed us to construct the 3D model seen at right.

This shows how the excavated block looked during and after construction. Take a moment to consider the enormity of the construction of an estimated 800 of these blocks around Cripp's Corner; although shuttering could be reused, the construction of one set took time and materials.

Then the concrete needs to be mixed and poured. This was done in stages to avoid putting too much pressure on the shuttering. Even so, many of the blocks are not truly square and bulge outwards!

The concrete would take days or maybe weeks to cure enough for the shuttering to be removed. This entailed taking it apart and cleaning it off before it could be reused.

We estimate from our data that the dragon's teeth here in the vineyard (about 1.3m high) weigh about 5 tons each. Some of the blocks elsewhere at Cripp's Corner are taller and may weigh an extra ton or so.


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