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• The construction of the survey control monuments is illustrated on <br />figure 4,6.1.5.4-3. <br />Monument Spacing <br />The monument spacing selected for the subsidence/strain monuments is 80 <br />ft. The selection of this spacing was dictated by the overburden depths <br />along the monitoring lines and the results of a similar subsidence moni- <br />toring program in New Mexico (King, 1980). <br />The leveling points are installed between the subsidence/strain points. <br />They are placed on 20-ft. centers. <br />• Monitoring Schedule <br />The monitoring net was installed during the summer of 1978. It has been <br />monitored on an annual basis since that tune to provide a check on monu- <br />' ment stability. <br />The frequency of monitoring subsidence/strain points once active mining <br />commences will be a function of four major factors: <br />1. The distance between a line of subsidence monuments and <br />the mining face <br />Z. The level of mining activity <br />3. The rate of subsidence <br />4. Type of monument <br />• <br />30 <br />