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SUBSIDENCE RESULTS NORTHWEST SECTION The final subsidence profiles for the Northwest section arc shown on figure 9. The figures show a baseline (zero line) averaged from two surveys performed in 1981, a profile from a level survey performed in 1985, and a profile from a level survey performed in I991. The mW - mum subsidence measured in this section was approxi- mately 3.0 ft located at station C6. This maximum subsid- ence value was approximately 40 pct of the mining height of 7 ft. The subsidence profiles (fig. 9) show a lack of Os -0 s -1 0 -1 5 -2.0 -25 -3.0 KEY Q/s; 85 6/24/91 - A-1 ins -2 -3 c� Crl � ne 1 2 3 4 5 0 7 8 9 t0 11 12 13 14 1LMUTS, 100-11 spat i nq F19we 9,Subsidw4a pre#ts tw f4Mhw*a ilec*m. 11 subsidence above the chain pillars; it is apparent, there- fore, that the chain pillars between the panels had not crushed. The shallow overburden in this section (200 R) and the relatively flat topography had a minimum effect on the subsidence profiles. The angle of draw for this section was calculated from subsidence values taken from monu- ment A2 through A6, B2 through B13, and C4 through C13 (where the monitoring network lines passed from mined panels to barrier pillars or chain pillars). The average angle of draw for these sections was 11° to 141. The monument spacing of 100 ft adversely affected the cal- culation of the angle of draw because the distamce between the monuments made choosing a specific point where sub- sidence ended a partially subjective process. The actual rate of subsidence was not obtained for this section be- cause of a limited number of surveys. Figure 10 shows the locations of several tension cracks over the area that were mapped during the survey in 1985. Several cracks were apparent (fig. 11), with up to six parallel cracks appearing between monuments B5 and 136. These tension cracks ranged from approximately 1 to over 12 in wide in some locations, and varied in depth from 6 in to 1 ft. Observation of the affected area during the survey of June 1991 showed only one crack (fig. 12); this crack was approximately 1 to 2 ft wide and ranged in depth from I to 9 ft. The orientation of this crack was similar to the sets of cracks shown in figure 10, crossing the A-line be- tween AS and A6, and crossing the B-line between B5 and B6. The length of the crack was approximately 200 h, with the deepest section in the middle of the length. The crack occurred in the tensile zone of the subsidence profiles for both the A- and B -monitoring lines. The subsidence pro- files show no appreciable change from 1985 to 1991. The limited amount of vertical movement, with an increase in the width and depth of the crack, indicates that the crack may not have been affected by horizontal displacement. It is possible that erosion had acted as a catalyst to increase the depth and width over a period of 7 years. Erosion could have also'hcaled' the other smaller cracks that had appeared in 1985, but were not apparent in 1991. SOUTHWEST SECTION The subsidence profiles for the southwest section arc shown in figure 13. The till material that was encountered in this section during development limited miming, and no significant subsidence occurred from 1981 to 1985 over the southwest section. The survey that was performed in June of 1991 also showed no appreciable subsidence. The