Abstract
In one proposed geothermal energy scheme [1,2], a large vertical main crack is produced in a hot dry rock mass by hydraulic fracture. To be able to remove heat from rock mass which is remote from the crack face, it is necessary to induce by cooling a secondary crack system normal to the wall of the main crack. Significant heat removal is possible only if the opening of secondary cracks is sufficient to allow rapid water circulation in them. The crack opening is wider, the larger is the spacing of cracks. The rate ~ of beat removal from secondary cracks by non-turbulent water circulation is roughly proportional to wS/h, where w = width of cracks and h = their spacing; w is, in turn, proportional to h, and so ~ ,h ~. Likewise, crack spacing is of importance when dealing with shrinkage cracks in reinforced concrete, for the opening of such cracks has a decisive effect on the rate of corrosion of the embedded steel reinforcement and on the shear transfer capability of aggregate interlock on rough crack surfaces. Other problems in which crack spacing is of interest include the vertical cracking of lava beds extruded and solidified at ocean floor [3], as well as cracking of mud flats and permafrost soils caused by drying [4,3].
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