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Nigel is absolutely right.. I would add that the shaft would need to be capped at rockhead level, or at a level acceptable to the Coal Authority, so any material above would need to be engineered back up again, suggesting either a raft or a piled solution for your properties. The general requirements for the shaft cap is twice the shaft diameter, designed to support the surcharge loads and the fill over. It is usual to include a shear key in the shaft to help resist the effects of suction. You would probably require a 450-600 deep cap dependent upon the diameter of the shaft (1-6m diameter), with a 500mm shear key. It is usual to place property foundations at a line of 45 degrees from the side of the cap to the edge of the foundations. H&S is a serious issue when investigating shafts, and special measures are required... Hope this helps. If you need any more information, please do not hesitate to contact me... We do lots of shaft caps over here in Yorkshire Kind Regards Andrew Summerton BSc (Hons) CivEng Senior Structural Engineer