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Page 14 In the Census, eight mines are classified as subsidence interactions where the longwa(1 ~ operation is mining above an underlying mine. These aze cases that could be considered similaz to the White River Nahcolite, Inc. case because we aze concerned about the effect of mining in the Boies Bed, a lower bed, on the minability of the overlying bed of the "Mahogany Zone." Four of the eight mines reported subsidence-related ground problems, and in all the cases, the in- terburden is less than 200 ft and the mining height in the lower seam is 5 ft or greater. Fractured strata leading to roof control problems in setup and recovery rooms and in gate entries appear to be a common problem. Subsidence generally produce two types of ground failure that might affect minability of overlying strata. In the first type, the strata bends in response to subsidence, leading to the for- mation of a trough. The second type, known as interseam shearing, occurs when subsidence pro- duces highly inclined shear or shear-tensile failures. The magnitude of roof problems in the subsidence trough is dependent upon the extent of the tension zone. This zone is associated with the formation of fractures and opening of joints which lead to poor roof conditions. The amount of damage subsidence can cause to an overlying ~ strata is largely dependent upon five factors: (1) the lower seam mining height, (2) the interbur- den thickness, (3) the angle of draw and the caving angle, (4) the time or age of the workings, and (5) the geologic chazacteristics of the interburden. According to the movement characteris- tics, the damaged overburden is divided into three zones: caved zone, fractured zone, and con- tinuous deformation zone (Figure 4-1). The caved zone is normally two to eight times the mining height depending on the properties of the immediate roof and the overburden (Peng 1992). As for the fractured zone, the height is approximately nine to eleven times the mining height for a soft and weak rock overburden, and may reach twenty to thirty times the mining height for the hazd/strong type overburden (Peng 1992). The fractured zone can be further di- vided into, from top to bottom, minor, regulaz, and severe fractured zones. In terms of height, the minor fracture zone occupies one third of the entire fracture zone, while the regular and se- vere zones occupy the other two thirds. Interseam shearing can be very damaging to mining in the upper seam because the strata tend to sheaz and displace as shown in Figure 4-2. Zhou and Haycocks (1986) reported that this 1 interaction is more likely to occur when the operations of both the upper and lower seams aze Agapito Associates, Inc.