2020-06-30_REVISION - M1974086 (6)

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designs was to provide a compacted earth structure that was stronger than the pre-slide tailings fill. The tailings fill had been placed on the slope via dozer push instead of placing organized lifts. This led to it having high variability in conditions and strength in addition to variable water transmission. The approved TR 02 design eliminated both flaws of the tailings fill by building buttresses of consistent material and construction and by installing a drainage system below the buttresses. The data gathered on the pre-slide earth materials onsite is shown in Section 4. In summary, the tailings fill and colluvium have similar density but different friction angles and cohesion. Ground Engineering identified in their report in TR 02 and in Amendment 03 (AM 03)to the Division that the principal time of slope failure to be anticipated on this slope would rotational movements (i.e. circular failures)through both tailings and colluvium. The main control of these failures appears to be the presence of perched groundwater. Therefore,the subgrade drainage system was developed to ensure that the slope would not have perched groundwater presence. 2.3.1 Construction Challen-aes The construction of the lower portion of the drainage pipe(Z3B Low Pipe)that runs transverse across the slope (Figure 2)brought to light challenges that would continually confront the work in Zone 3B. First,the depth of tailings and colluvium in this area was significantly greater than preconstruction test holes had anticipated. Also, the material being excavated was not able to hold up very long and frequently construction crews worried about recently excavated near vertical highwalls sloughing off into the work areas. Slope laybacks took place as much as possible. Unloading the upland slope was attempted, but the nature of such unloading limited it: it was not practical to remove all material from Zone 3B down to the shale to comprehensively unload the slope.Nonetheless,APC and its earthwork contractor worked diligently and installed the Z313 Low Pipe to the design. APC adjusted the longitudinal collector pipe location slightly south to ensure that it was at the lowest grade point of all the installed pipes to ensure successful flow. Once the Z313 Low Pipe was installed,the first stage of the buttress could be built. APC and its contractor laid material in compacted lifts according to the TR 02 approved specifications on the Z313 Low Pipe. APC worked to install the upper portion of drainage pipe (Z3B High Pipe). However,the depth issue identified during construction of the Z313 Low Pipe proved to be an even greater issue in the construction of the Z313 High Pipe. Excavation failed to reach the shale layer at>40' of depth in the Z3B High Pipe area. At such an excavation depth,APC and its contractor had serious concerns regarding the safety and practicability of placing the sub-grade drainage pipe at the required depth. It was not safe to excavate much deeper at the toe of highwall that had demonstrated sloughing. Furthermore, similar to the installation of the Z313 Low Pipe, unloading the upland slope was not practical. The scale and scope of material above the Z3B High Pipe location was such that removing it would have involve disturbing a large area of materials that had not been disturbed or affected by any of the earthwork or previous slide. APC thus decided to install the Z313 High Pipe at the depth that had been reached; based on the reports of the technical support from Ground Engineering that was present, this is 5' deep into the colluvium and further to the west. Ralston TR 03 5 Gnu 1,"icki and Asw ialcs,PLIA