Most leaks in pools occur through cracks in floors or walls, but some develop through highly porous or honeycombed concrete. When pools are at least partly below the groundwater table it is possible for leakage to occur in either direction. In such circumstances water leaks out until the level in the pool is the same as that of the water table. Whenever the pool is drained and the drain valve closed again, outside water will seep in until the level is as high as the water table. Some causes of leaks are: errors in structural design- these result in the most serious kind of leak, and the most expensive to repair; settlement of the pool shell- settlement due to ground movement is likely to cause cracking in walls or floor or both and is usually expensive to repair; cracking from thermal or shrinkage stresses- cracks occur when stresses exceed the tensile strength; cold joints or weak construction joints- such joints in walls or floor will allow seepage unless special precautions have been taken to seal them; improperly sealed working joints- any joints that are not well sealed will permit leakage; weak, porous or honeycombed concrete- high slump concrete, low cement content, or poor workmanship and curing are likely to produce low quality concrete that allows water to pass directly through it. Specific repairs consist in: for structural design errors or settlement of the pool- repair of leakage from these causes requires consideration of means of strengthening the pool; if the pool is large, the services of an experienced engineer may be required. Some of the more common repairs are: bonding external piers or buttresses of reinforced concrete into the wall; providing new interior linings of reinforced shotcrete, cast-in-place reinforced walls and floor bonded to the shell, or cast-in-place reinforced walls and floor unbonded to the shell; bonding external piers or buttresses combined with a shotcrete lining; or installing an aluminum inner pool shell. For thermal or shrinkage cracks- this kind of crack can usually be repaired with epoxy resins. Construction joints that leak should be cut out for the length of the defective sections and repaired. All other joints that failed will have to be resealed.