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112 • Fourth International Symposium alt-Northern Ohio Geological Society <br />raeLE w <br />Prohlems analyzed-Case II <br /> <br />Prohlem Height of fladius of Slip <br />K y <br /> Cavity Ift.l Cavity (ft.l Layers o pcf <br />1 8 1080 None 1.0 136.2 <br />2 8 1080 7layers 1.0 136,2 <br />3 16 1080 7layers 1.0 136.2 <br />4 B 1080 111ayers 1.0 136.2 <br />The subsidence becomes smaller as the distance from the <br />center of the cavity increases. <br />Comparison ojcomputed and observed subsidence pro- <br />files. The calculated subsidence profiles for various cases <br />analyzed, as well as the observed, are presented in Figure <br />]0 and Table IV. For a cavity with a height of 8 feet, the <br />maximum surface subsidence occurring at the center of <br />the cavity was calculated to be 0.?9, ?.98, and 3.40 feet for <br />the cases with none, 7, and 11 assumed weak bedding <br />planes, respectively. For the case with the cavity IS feet <br />in height, [he subsidence is essentially the same as that of <br />the cavity 8 feet high for the same number of the weak <br />0 <br />2 <br />.. <br />W <br />U <br />z 3. <br />0 <br />ut <br />m <br />0 <br />u, <br />~ 4 <br />a <br />u <br />i <br />6 <br />bedding planes assumed. This is to be expected because <br />the cavity is situated at the great depth (1560 feet) below <br />the surface, relative to the height. When the case with <br />seven weak bedding planes was iterated once for no-ten- <br />sion analysis described previously, the subsidence within <br />the mined area increased ten percent. It is expected that <br />the subsidence will increase with increasing numbers of <br />the iteration for no-tension analysis. As shown in Figure <br />10 when the number of weak bedding planes were in- <br />creased from seven to eleven, the calculated subsidence <br />increased from 2.98 to 3.40 feet. •$ecause of the type of <br />rock (primarily shale) present at the site, larger numbers <br />of x~eak bedding planes may be present in [he strata. Thus, <br />the subsidence may be expected to be larger within the <br />mined area. <br />The observed subsidence was 5.5 feet at the time of <br />survey in November, 1969. The modulus values used in <br />the computation were based on 3-D velocity logs. Consid- <br />ering the overall rock mass, it is generally agreed that <br />these values are probably on the high side by a factor <br />Ivhich may be as high as two. With a modulus o(one-half <br />DISTANCE FROM CENTERLINE OF CAVITY (F7) <br />0 200 400 1100 800 1000 1200 1400 1600 ]800 2000 <br /> <br />- - <br />-- <br /> / - <br /> <br /> / <br /> / <br /> N07F5 <br /> . - ~ OBSERVED SUBSIDENCE PROFILE <br /> (BETMEEN BLOCKS J ANO L). <br /> CALCULATED SUBSIDENCE PROFl LES <br /> FOR IDEALIZED DISC SHAPED <br /> CAVITY OF RADIUS R = 1080 FT. <br /> / ~ _ _ THICKNESS OF CAVITY = 8 FT <br /> MI TMOUT SLIP LAYERS <br /> / O--[1-O THICKNESS OF CAVITY = 8 FT <br /> MI TH 7 SLIP LAYERS <br /> 2ND ITERATION OF "NO TENSION <br />" <br /> j ANALYSIS <br />MI TH 7 SLIP LAYERS <br /> / <br /> o-CF--O TMI CKNE55 OF CAVITY = IS FT <br />~ MI TM 7 SLIP LAYERS <br />~ ..v.. THICKNESS OF CAVITY = 8 F7 <br /> MI7H ll SLIP LAYERS <br /> <br />Figure 10. Suhsidence proliles, Case I1. <br />