2015-02-26_REPORT - C1982056 (3)

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3 SUBSIDENCE PREDICTION APPROACH A computerized version of the graphical prediction method originally described in the Subsidence Engineers Handbook (NCB, 1976) was adopted for use at the Twentymile mine. The speed and capabilities of modern personal computers has facilitated the entry, storage and use of the large quantity of empirical data available from subsidence monitoring programs at the mine. To date, data have been collected for panel widths of 640 and 840 -feet with depths ranging from 700 to 1,200 -feet. Figure 2 shows the layout of the first three, 640 - feet wide panels and transverse and start monitoring lines. The mining height and panel lengths averaged S•5 -ft and 9,000 -ft respectively. Subsidence resulting from the extraction of the first of these panels has previously been reported by Stewart and Shoemaker (1992). Figure 2 Layout of the Case 1 Workings Overburden at the monitoring site ranges in thickness from 1,000 to 1,100 -ft and consists of shale (75 %) and sandstone (25 %); a large proportion Of the sandstone occurs in a single bed approximately 800 -ft above the mine floor. Bedding dips approximately 5 degrees to the northwest. The subsidence monitoring system consisted of casing/rebar, steel spikes, and wood stakes surveyed using a total station instrument. The original survey contract was set up with a required survey accuracy of 0.3 -ft, however, statistical analysis of repetitive survey results indicates that an accuracy of about 0.1 -ft was actually achieved. 3.1 Profile Data Figure 3 provides the subsidence values measured at each of the monitoring stations following extraction of each of the first three panels. The maximum subsidence after Panel I extraction of 44 -in subsequently increased to 54 -in after Panel 2 excavation and 57 -in after Panel 3 extraction. Comparison with predictions from the Subsidence Engineers Handbook (NCB, 1975) indicates that this method would have over- predicted maximum single panel subsidence by approximately 20 percent. However, the profile, adjusted for the difference in maximum subsidence and relative survey accuracy, closely resembles the profile predicted from the Handbook. Figure 4 illustrates a comparison of measured and calculated transverse strains after Panel 1 and 2 extraction. Strains were calculated from the second differential of subsidence using the numerical method contained in the Subsidence Engineers Handbook. Rey parametric values obtained from the collected data include: • Subsidence over the rigid -yield pillar pair increased from 30 -in after Panel 2 extraction to 36 -in after Panel 3 extraction. • Maximum subsidence for Panel 2 is the same as for Panel 1 after Panel 3 extraction. • Maximum transverse horizontal displacements of +5 and —10 -in after Panel 1 extraction; +7 and — 1 I -in after Panel 2; and +8 and —4 -in after Panel 3. • Maximum strains of +0.003 and - -0.006 after Panel l extraction. Maximum compression located over Panel 1 centerline. Maximum extension located over Panel I rib. • Maximum strains of +0.003 and —0.005 after Panel 2 and 3 extraction. Maximum compression located over Panel l and 2 centerlines. Maximum extension zones located over Panel I and 3 ribs. Start subsidence profile survey results are shown in Table 2. Again, the measured profile closely compares with that predicted using the methods described in the Subsidence Engineers Handbook. Similar correlations were obtained with the displacement and strain data, which are summarized below: • Maximum horizontal displacement of 6 -in. • Maximum strains of +0.003 and —0.002 • Maximum extension zone located over the start room.