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• Zone 4 -This zone is characterized by fissuring of the near-surface overburden strata in areas where surface soils are <br />thin. These fissures aze caused by extension deformation within the subsidence trough. These fissures normally <br />close-up or are sealed as time passes. Because overburden subsidence effects in Zone 4 are essentially transitory, <br />these effects will have no significant or lasting impacts. <br />Zone 3 -Zone 3 can only form where the overburden is thick enough to allow the complete development of the <br />underlying Zone 1 and Zone 2. Minimal subsidence effects characterize zone 3. As a result, overburden subsidence <br />effects in this zone will not have any significant or lasting impact. <br />Zone 2 -Zone 2 is characterized by possible separations along bedding planes. As the mining face advances, the bed <br />separations close-up behind the face at some point (Ropski and Lama, 1973). The continuity of the beds is <br />maintained to a large degree, although cracks can develop. Maximum separations varying from 8 to 15 millimeters <br />have been observed during active subsidence in other areas. After the overburden has stabilized, separations in these <br />areas were reduced to just a few millimeters. Because the overburden subsidence effects in Zone 2 are essentially <br />transitory, these effects will not have any significant or lasting impact. <br />Zone I - Zone l is characterized by disruption and heavy fracturing of the overburden as a result of caving of the <br />near-roof strata into the mined-out void. Zone 1 represents the most significant overburden subsidence effects. The <br />height and distribution of Zone ]caving and fracturing can most directly impact the water-bearing characteristics of <br />the overburden strata. <br />Notable work on the disruption and fracturing of the near-roof overburden as a result of caving has been done by <br />Ropski and Lama (1973), Kenny (1969), Wilson (1978), Wardel (1970), Dunrud (1976, 1980), Sovine (1974), <br />Harstnik (1971), Nakajima (1976), and Williamson (1978). Various researchers have attempted to define the height <br />• of Zone I caving and fracturing. Ropski and Lama (1973) indicate that Zone 1 caving and fracturing is from 3 to 3.5 <br />times the extracted seam height (where m =extracted seam height). Nakajima (1976) indicates 3m to 4m as the <br />height of Zone 1 caving and fracturing. Thomas (1978) indicates 3m to 8m. Abel (1977, Personal Communication) <br />indicated that at the Thompson Creek No. 1 Mine near Cazbondale, Colorado, mining experience with the "A" Coal <br />Seam and the "B" Coal Seam indicate that the Zone I caving height is less than Sm. Bovine (1974) correlated the <br />height of the Zone 1 cave zone with extracted seam height through the following equation: <br />Where: <br />h =height of caving <br />m =extracted seam height <br />g =swell factor <br />and <br />g = DUDC <br />Where: <br />DI = in situ density <br />DC = caved density <br />Bovine's equation indicates that the critical factor controlling the height of Zone 1 caving and fracturing is the swell <br />factor of the cave material. This agrees with observations by Mining and Subsidence Engineering Company (MSE) <br />on longwall panels in the western United States and Thomas (1978), Wilson (1978), Kenny (1979), Wardel (1970), <br />and Ropski and Lama (1973). This is also consistent with the results obtained from subsidence monitoring at the <br />CEC Mine complex. <br />• The value of the swell factor is a function of the lithology, structure, and strength of the near-seam roof strata in situ. <br />Values for swell factors for various rock types obtained from the literature are presented in Table 85, Typical <br />Overburden Swell Factor Values. Specific swell factor estimates from Northwest Colorado obtained from the <br />Permit Renewal No 3 2.05-61 7/15/98 <br />