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4.0 SUMMARY OF PREVIOUS STRENGTH TESTING <br />Shear strength of coal mine refuse at Bowie has been characterized by three consolidated <br />undrained (CU) triaxial shear tests with pore pressure measurement and by several <br />unconsolidated undrained (UU) triaxial tests. These samples were collected by BGI in 2005 and <br />are described in detail by BGI (2006). The results from the CU triaxial shear tests are <br />summarized in this section and the test results are presented again in Appendix C of this report. <br />The UU triaxial shear test results are not applicable to the updated stability evaluations <br />presented in this report, and are therefore not presented. <br />All samples of refuse were obtained from the CWDA No. 2, as no compacted refuse was <br />available from CWDA No. 4 at the time of the original stability evaluation. Since coal refuse is <br />generated from the same source and processes for the two disposal areas, properties of refuse <br />at CWDA No. 4 are considered similar to those at CWDA No. 2. Two of the CU triaxial shear <br />tests were conducted on thin - walled (Shelby tube) samples (SH -1 and SH -2) of previously <br />placed coal mine refuse at the adjacent CWDA No. 2, while one sample (BS -3) was conducted <br />on remolded samples of refuse from CWDA No. 2. Confining pressures for the CU triaxial tests <br />were selected to span the in -situ vertical effective stresses anticipated for the ultimate refuse <br />bank. Failure was selected at the maximum pore pressure for all samples. <br />• <br />Three points were tested for sample SH -1 at confining pressures of 20, 40 and 80 psi. After <br />consolidation, the samples were sheared at an approximate rate of 0.003 in /min. The effective <br />angle of internal friction, �', was determined to be approximately 34.3 with zero cohesion. <br />Only enough sample was recovered from sample SH -2 for two points. These two points were • <br />sheared at confining pressures of 40 and 80 psi. The samples were sheared at an approximate <br />strain rate of 0.003 in /min. The effective angle of internal friction, �', was determined to be <br />approximately 33.6 with zero cohesion. <br />Sample BS -3 consisted of three remolded points from a bulk sample of coal mine waste. The <br />three points were remolded to approximately 90% of the maximum dry density as determined <br />by the standard Proctor moisture contents of approximately 16.5 %. These remold parameters <br />were intended to represent the lowest density that meets compaction specifications at a <br />relatively high moisture content. Sample BS -3 was consolidated at confining pressures of 20, <br />40, and 80 psi, and subsequently sheared at an approximate rate of 0.003 in /min. The <br />effective angle of internal friction, �', was determined to be approximately 34.0 with zero <br />cohesion. <br />The results of the CU triaxial shear testing are presented in Table 2. The effective friction <br />angles for the three tests are consistent for tests conducted on both remolded and thin - walled <br />samples. The average friction angle for the three CU triaxial tests is 34.0 degrees. <br />n <br />U <br />Bowie No 2 Refuse Pile No 4 2009 Revised stability.doc <br />Project #06- 481 -003 <br />Page 6 of 12 <br />