From the Yateley Society Newsletter No.43 November 1990

YATELEY HISTORY PROJECT

Archaeological Investigatiobs at Chaddisbrooke

by Geoff Hoare

As you are all probably aware, an excavation is planned on the Ramons/Chaddisbrooke site prior to the building of the St Peter's Worship Centre. Obviously the first area to be investigated will be the garden which lies between the wooden outbuildings and Pipson Lane; but before the "dirt archaeology" can start however there is a large amount of preparatory work to carry out as follows:

1. The position of the site must be located in relation to the Ordnance Survey grid so that it can always be pinpointed in the future, and particularly by anyone to whom "behind Ramon's DIY" might mean very little: This is an easy procedure in an urban context and just needs a large scale map, We also have to be able to take levels at all stages of the work, and a site datum is set up and all levels related to it. A11 the levels have also to be related to sea level, and for this we have to use the nearest Bench Mark. The 0.S. surveyors marked a height datum at countless points across the country and it is placed on a building or structure considered to be permanent, for re-use later as required. Churthes are a favourite location and there is such a bench mark on the NE corner of the chancel of St. Peter's. These are marked BM on 1:2500 OS maps with their actual height above sea level alongside.

2. Apart from levelling to record where everything is located in the vertical plane, we have to be able to locate everything in the other two dimensions. A measured grid has therefore to be marked out on the ground itself and this transferred to the site plan. Usually on a small site such as ours this consists of measuring out a series of contiguous squares with 5 or 10 metre sides. to form the grid, which must be related to the reference points on the site. Thus the position of anything on the site can be identified by its coordinates and transferred to the plan. Care must be taken to ensure the accuracy of the measurements and cross-checking of diagonals carried out to check that all right angles are true. A grid inaccurately laid out cannot be corrected once work has started, so everything depends on its initial accuracy.

3. So far quite a lot of work has already been carried out, but we're still not ready to start digging., It's very useful to have some idea of what lies under the surface, and to this end we will be carrying out a Geophysical Survey. There are various ways of doing this with much of the equipment used being expensive, some prohibitively so, but we are fortunate in NEHAS with one of our members, Charles Alexander having fabricated his own equipment for carrying out a Resistivity Survey. As I mentioned above, there are other and also more expensive ways of achieving this, including Ground Pulse Radar, but at Sutton Hoo it has been reported that the best results were obtained by the type of survey we will be using. Briefly four metal probes are inserted in the ground in a line and a constant alternating current passed through the outer two probes, the voltage, measured across the two inner probes, with a simple calculation, giving the electrical resistance. The probes are advanced down one line of the grid and then down the next, and so on, until readings have been taken across the whole site. The values obtained are not absolute, but comparative, and generally speaking a higher resistance will indicate a buried wall or something similar, and a lower value a ditch or pit. This way a plan of some of the underlying features can be built up.

4. NEHAS have been carrying out a programme of Dowsing in parallel to a Resistivity Survey, and we will continue the programme in this instance, utilising the same grid of course.

5. A sweep with a metal detector will be carried out, but as part of the controlled investigation. The position of significant signals will be noted on the site plan for subsequent examination during the excavation.

Once the above is completed and analysed we will be ready to excavate!

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