Wednesday, April 28, 2010

How deep are the Boreholes?

27 April : I have been trying to find ways to demonstrate the proportions of the house and the boreholes beneath the house. This is for the poster for the Open Day on 16 May. The holes are 48m deep, but of course, what matters is their 'reach', i.e. how much of the soil around will be thermally responsive to our cooling and heating.
    Initially, one imagines deep Cylinders, but in all likelihood, the shape is more like a tapering Bottle shape.
    Near the ground surface, the ground temperature varies with the seasons and the days, so only the volume immediately round the pipes is relevant - so it looks like the neck of the bottle.
    As you go deeper, the radius of active soil around them increases, perhaps to 3.6m, so it looks like a Bordeaux bottle. As it deepens, it widens to something like 5 metres depending on conductivity, the delta-T that we are working with. The deeper parts are no longer affected by the seasonal changes in the upper layers. At the bottom of the borehole, the bottle-shape is likely to be domed at the bottom.
  This model is absolutely to scale (using ArchiCAD 3D), but it's a perspective projection, foreshortening the bottle-shapes.
  The model also assumes consistent conductivity. All we know is from what the drillers told us, who reported that for the entire depth, it was pretty consistent 'Marl', a mixture of dense clay and broken limestone - a glacial mixture. It sounds right, there was a huge amount of clay slurry coming up, and much of it is still splattered on the wall. If the density varies, the bottle would have thin and wider zones, indicating the range that is affected by our 'interfering' with the natural ground temperature.
  The GDL model tells me that the volume is 3600 cubic metres which amounts to about 8,000 tons of dense conductive marl.

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