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/C <br />. Main CressonSlopeEvaluation AdrianBrown <br />5. STRUCTURE <br />5.1 Overall Structural Conditions <br />The overall structure of the rock materials is indicated in Plate 3. This plate has been prepazed from data <br />measured in the mine walls in the ongoing structural mapping- conducted in the first 3 years of mining. <br />The data is continually updated by surveys conducted by the Cresson Mine Geological Division. The <br />data is input into the mine computer database, and is available for evaluation from that database. The <br />data has been divided into a number of regions, as discussed below. <br />The Main Cresson Mine structural data for the period up to May, 1997, were downloaded from the <br />database for use in the analyses performed for this stability evaluation. This data is presented in <br />Attachment 2 of this report. A total of 2,277 structural features had been measured up to May, 1997. The <br />data is presented by joint type: <br />F -Fault <br />J -Joint of less than 25 foot (one bench) persistence <br />J2 -Joint of more than 25 foot and less than 50 (two bench) persistence <br />J3 -Joint of more than 50 foot (two bench) and less than 75 foot (three bench) persistence <br />• J4 - Long j oints that span more than three benches, and often more than one face <br />JT -Joint sets <br />All the data are presented in stereographic form in Figure 3'. The frequency of occurrence of mine wall <br />structures with different lengths (persistence) as a function of dip angle has been computed from the <br />database, and is presented in Figure 4. The distribution of dip angle in the South Mine wall as a function <br />of bench elevation is presented in Figure 5. <br />Based on the above information, the structural system in the mine area as a whole is generally <br />characterized as follows: <br />1. The dominant strike orientation of joints is NNW/SSE (Figure 3). This is consistent with the <br />regional structural fabric in the vicinity of the Main Cresson Mine. Almost all of the faults in <br />the mine azea also strike in this direction. <br />2. The dip of most joints is near-vertical (Figure 3). This is indicative of the structural history of <br />the volcanic rocks. Columnar jointing is also evident, but is a minor component in the <br />vertical joint system. Almost all faults are also neaz-vertical. <br />3. There are very few intermediate-angle joints, that is which dip in the range from 30° to 60° <br />(Figure 5). This is significant from a stability point of view, as these fractures would most <br />4 A polar stereographic plot is a presentation of the data which is created by plotting the points where a line normal to each joint plane and <br />passing through the center of the hemisphere (the pole of the plane) penetrates the surface of the Lower hemisphere. Poles which plot <br />• near the center of the hemisphere represent low-angle joints, white poles which plot near the rim of the hemisphere represent high angle <br />joints. Poles which plot near the north (top) of the plot (for example) represent joints whose strike is approximately east-west, and <br />whose dip direction is approximately south. <br />1385D.980612 12 <br />