2008-04-15_REVISION - M1980244 (193)

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l~ • Main CressonSlopeEvaluation AdrianBrown Based on these results, the unconfined compressive strength (UCS) results for the principle rock types in the Main Cresson Mine are generally indistinguishable from each other'. Table 4 presents a summary of the statistics of the unconfined compressive strengths from all tests, subdivided by rock type. Table 4. Unconfined Compressive Strength Results Material Type Units Phonolite Porphyry Phonolite Dike Volcanic breccia All rock types Geological Symbol Tbx1L Tphd TphK No. Tests 12 3 12 27 Mean psi 18,246 17,098 16,583 17,379 Standard Deviation psi 5,402 10,689 5,677 5,951 SD of mean psi 1,629 7,559 1,712 1,167 l~phd -Phonolite Dike; Tbx1L -Phonolite porphyry; TphK - Breccia Based on these data, the following is concluded about the unconfined compressive strength of the principle mine materials (excluding latite dike material): 1. The average unconfined compressive strength of the mine materials is approximately 17,400 • psi for any intact material in the Main Cresson Mine area. 2. The standard deviation of the mean unconfined compressive strength of the mine wall materials2 is approximately 1,200 psi; the 95% confidence interval of the average unconfined compressive strength is from 15,000 psi to 20,000 psi. 3. The standard deviation of the population of the unconfined compressive strength of the mine wall rock samples is approximately 6,000 psi; the 95% confidence interval for the unconfined compressive strength of any single intact material sample taken from the mine is approximately 5,000 psi to 30,000 psi. These generalizations exclude latite dike material, which was not tested in this program, but is observed to be friable and have little shear strength in outcrop in the mine. The UCS can be used to classify the rock strength. Hoek and Bray (1990) present a rock strength classification based on the UCS, which is summarized in Table 5. Based on this classification, the unconfined compressive strength of the Main Cresson Mine wall rock samples are "Moderately Strong Rock (R3)" to "Strong Rock (R4)". I Note however, that there are no tests on the dike material, due to the difficulty of obtaining an intact core sample to test. Accordingly, the dike material is expected to have an unconfined compressive strength in the order of 1,000 psi or less. As the latite dikes which have been identified to date aze generally vertical or sub-vertical, the relative weakness of the material poses little threat to the overall stability of the mine walls. • 2 The actual value of the mean is uncertain, as it is estimated from a finite number of samples. The standard deviation of the mean is computed by dividing the standard deviation of the sample by the squaze root of the size of the sample minus one. It is an estimator of the standard deviation of the mean around the estimate of the mean derived from the sample. 138SD.980612 ~