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• maximum subsidence that will occur after pillar recovery. It can be reasonably assumed that the initial subsidence is negligible compared to the final, maximum subsidence. While subsidence of small magnitude will occur during mining of entries and rooms, the majority of the subsidence will occur during and after pillar recovery. The rate of subsidence at this time will be comparable to subsidence rates above a long wall panel advancing at the same rate as the pillar line retreats. Subsidence rates above longwall panels are described in detail by Kratzch (19E3). Subsidence is generally described as occurring in three phases. At the time the working face (retreat line) passes under a point (Phase I), usually only about 10~ of the total subsidence has occurred. As the face advances beyond this point, subsidence occurs rapidly, typically by as much as 20 mm per day shortly after passage of the face. By the time a length of panel equal to 1.9 times the overburden depth has been extracted, 90% to 95~ of the total subsidence will have occurred at a point above the center o£ the panel during this phase II. The time for this degree of subsidence to be achieved depends on the rate of mining and the depth of the panel. The main subsidence, which in total extraction includes • phase I, initial subsidence, and phase II, accelerated subsidence, is referred to as "active" subsidence. "Residual" or "delayed" subsidence accounts for the remaining 5~ to 10 ;. There are few guidelines as to how rapidly this occurs, but since the subsidence magnitudes would be small, their effects will be minimal. Gradual subsidence may occur over areas where pillars have been left as the pillars gradually yield. Total sub- sidence will be less than subsidence above total extraction zones, since the coal remaining within the pillars will eventually be spread throughout *_he abandoned workings, preventing complete closure of the roof and floor. The rate of surface subsidence will depend on the rate at which *_he pillars yield. Large barrier pillars may be too wide to ever yield entirely, with the result that there would be minimal subsidence of the overlying ground surface. Small pillars left for local roof support will fail soon after *_he main pillar line retreats beyond them; they will not significant- ly delay the rate at which surface subsidence occurs. For the case of partial extraction ~..~ith yielding permanent pil- lars, the rate of surface subsidence will correspond to the rate of yielding of the pillars. • - °- 9 -