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