The cavity between the two leaves of masonry will be filled with LECA (lightweight expanded clay aggregate) which is both an insulation and a free draining material. Any moisture that gets into the cavity will drain down through the cavity. The LECA does not hold moisture or wick moisture up. In the bottom of the cavity we therefore needed to make provision to get rid of any water that drained through the LECA.
Originally we had planned to insert short sections of pipe approx 150mm diameter into the ground at 1metre centres, the top of which would be flush with the top of the lime mortar. This would allow the lime mortar to be cast around the pipes. However, given the hardness of the chalk ground we decided that it was just not possible to get the pipes in the ground without compromising the chalk bearing around the pipes. The only way to break out the chalk is to pickaxe the ground and this is not very precise or accurate.
Instead, we used a piece of plastic pipe to cut holes out of the mortar once it was cast. The sections generally came out quite well and we then just went round with a gardening trowel and cleaned out the bottom of the holes.
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