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Page (2 <br />approximately 8% of lime (CaO; Section 4, Waste Characteristics), which acts as a pozzolan in <br />aiding the fly ash to gain strength in long-terin in the presence of moisture. Due to the lime <br />content, this fly ash is classified as F -fly ash. An evaluation of the shear strength characteristics <br />had indicated that the Craig Station fly ash had cohesion of 1,728 psf and friction angle of 33.8° <br />(Appendix F, Geotechnical Considerations), which indicate the fly ash is a stronger material <br />compared to the spoils used in AAI's analysis. The spoils' cohesion ranged from 475 to 700 psf <br />and friction angle ranged from 26° to 34°. The following are the direct reasons why AAI believes <br />simulating a fly ash matrix in the Ash -Pit will result in an even higher Safety Factor against <br />global slope failure relative to the spoil backfill models. <br />(1) Appreciably higher shear strength parameters are associated with the Trapper fly ash <br />relative to the spoils. Especially, the contrast is the highest with the lowermost spoils <br />layer assumed in the study, whose friction angle (26°) is nearly 25% lower than the <br />friction angle of the fly ash (33.80). The lowermost spoils layer is most consequential for <br />the global stability of the in -pit backfill, as it sits on the weak shale/clay layer along the <br />inclined pit floor, experiences maximum depositional stresses at depth and is the seat of <br />sliding movement, per AAI's modeling results. Unlike the spoils, the shear strength of the <br />fly ash is likely to be much more uniform throughout the fly ash backfill matrix given the <br />latter's uniform particle size distribution and absence of segregation during placement. <br />(2) The pozzolanic behavior of fly ash aids in strength gain over long periods of time in the <br />presence of moisture. Pozzolans such as F -fly ash, which include both lime and silicates, <br />form stronger Calcium-Sillcate-Hydrates in the presence of water, which is a slow but <br />definite strength -gaining process for the ash backfill. The Trapper fly ash typically <br />contains 10-15% moisture by mass. Physical property testing has indicated a moisture <br />content of 11 % in the ash (Appendix F, Geotechnical Considerations). Given the fly ash <br />deposits in the Ash -Pit have received moisture during and after placement (through <br />infiltration), the ash matrix has likely gained strength, which is supported by anecdotal <br />observations during digging of several test trenches in the pit. Therefore, the fly ash <br />backfill likely possesses greater shear strength characteristics compared to its laboratory <br />test parameters, which were obtained from testing as -received fly ash that hadn't been <br />allowed to benefit from long-term pozzolanic strength gain. <br />(3) The pozzolanic behavior of fly ash has likely improved the strength of the weak <br />clay/shale present along the pit floor of Ash -Pit, due to contact between the two <br />materials. During pozzolanic activity in presence of moisture, the calcium ions present in <br />the lime tend to replace Sodium and Potassium ions present in the weak clays, making the <br />latter stronger in the process. Given AAFs modeling results indicated that the weak <br />clay/shale floor layer along the pit floor provides the seat of maximum movement, their <br />strengthening is likely to have resulted in a stronger overall backfill configuration <br />compared to the models presented in AAI's study report. <br />(4) The Craig Station fly ash has an as -received density of approximately 77 pcf (Appendix <br />F, Geotechnical Considerations), which is significantly less than the spoils' density <br />(approximately 110 pcf), which means the backfill matrix will be acted upon by <br />significantly less gravitational loading down the pit gradients, due to lower overall self - <br />weight. Therefore, the fly ash backfill is likely to have a greater overall safety factor <br />compared to the spoils backfill. <br />