1981-11-13_PERMIT FILE - C1981013 (40)

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!.1 DBSCRIPTIVB THEORIES 239 X POINT sy01lACE A DATUM 0 1s•s x' 30 .o n soy- to ass s0 90 INLI sl•s% Iw10 O•s 0•6 0•4 O•a O O•a 0•4 04 04 10 1•8 FACE ADVANCE IN TE11Ms Of tl Fit. 9.2.6 Typical subsidence development curve (12). Courtesy U.K. National Coat Board. function of the face location, which is advancing toward the right. Subsi- dence at P begins when the face is 0.7h ahead of P and increases linearly us the face udvunces. Subsidence uccelerutes when the face is about 0.3h ahead of point P, reaching 15.5% of maximum when the face is directly below P. It accelerates rapidly when the face passes beyond point P and reaches 97.5% when the face is 0.7h beyond P. At this point active subsidence is complete; the remaining 2 to 3% is due to gradual compac- tion of subsided ground and is known as residual subsidence or time- dependent subsidence.It should be noted that 0.7h is roughly one-half the critical width for the point P if the angle of draw is taken as 350. Residual subsidence also occurs simultaneously with the instantaneous subsidence. It can be measured by precise leveling or automatic photo- graphic survey for various points along the subsidence curve. Figure 9.2.7 shows the instantaneous surface subsidence (solid line) and subsidence profile (dashed line) 21 days after the face stopped. It is seen that maximum residual subsidence of 99/o occurs at half-maximum subsidence and that it decreases toward both ends, that is, the amount of residual subsidence is proportional to the rate of subsidence of the surface points. Residual subsidence is also associated with the physical properties of the overlying strata. Strong rocks exhibit more residual subsidence than weaker ones. Thus we see that, according to the British investigations (12) time- dependent or residual subsidence can amount to a maximum of 9%of the total subsidence if the face is stopped within the critical width of the