2014-12-24_REVISION - M2004051 (3)

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The slurry mixing pond was constructed immediately south of the previously mined area and immediately east of the QA/QC trailer. Ground water (which had infiltrated the previously mined area) and dry bentonite was pumped through a jet shear mixer to prepare the bentonite slurry. Once the bentonite slurry was prepared to project specifications, trash pumps were utilized to deliver slurry to the trench via HDPE pipe. The slurry wall trench was excavated with a Komatsu PC 800 LC trackhoe excavator, with extended boom and stick. The trench was excavated through the alluvium and keyed into the underlying sandstone or claystone bedrock. The bentonite slurry was maintained within a few feet of the surface in order to keep the trench open and minimize sloughing from the sides. Smaller trackhoes (Kobelco 350 and/or Doosan) provided initial soil - bentonite backfill mixing. The backfill mix consisted of previously stockpiled supplemental fines (finer grained soils), approximately 1 percent dry bentonite, slurry from the trench, and select cuttings (mostly overburden and softer claystone) excavated from the trench. A Cat D8F dozer performed final soil - bentonite backfill mixing. Once the backfill was prepared to project specifications, the bull dozer slowly tracked the backfill into the trench from the surface. The backfill slumped beneath the bentonite slurry. The as-built slurry wall alignment and stationing is shown on the as -built plans (included in Appendix A). Photos of the construction are provided in Appendix B. Aggregate Industries preferred that the slurry wall construction commence on the northeast part of the site in order that the wall be complete where the conveyor enters the pit and to allow dewatering in the previously mined area. This plan was generally followed, but was complicated by the need to excavate under the Public Service Company of Colorado (PSCO) power lines. License agreements to work in the PSCO right of way were obtained. PSCO was able to deenergize the lines allowing for safe construction. Envirocon had to utilize several lead in trenches and had to vary the direction of excavation because of scheduling issues in obtaining the license agreements and times that PSCO was able to deenergize the lines. Construction commenced in the northeast corner at approximate design station 48 +77 where the slurry wall meets at approximate right angles with 10 foot overlaps. The trench was excavated downstation (with the 10 foot overlap) to approximate Station 48 +60. The following day the excavation proceeded upstation on the other leg (with the 10 foot overlap) from approximate station 48 +77 to approximate station 49 +30. From approximate Station 49 +30 the excavation formed a lead in trench at a slope of approximately 0.5:1 (horizontal to vertical, H: V) that reached the surface at approximately station 49 +50. At this point the crew started placing backfill and trenching progressed downstation until approximate station 45 +90. The crew then started constructing upstation from station 49 +30 to 51 +80 after receiving guidance from PSCO as to how close the excavator could approach the power lines. The crew then reversed direction again progressing down station from approximate station 45 +90 to approximate station 41 +10 until PSCO was able to deenergize the lines. The power lines were deenergized on April 8 and April 11 to 13, 2013. In this period, the excavation proceeded to approximate Station 59 +00 and PSCO deemed that it was safe to - 2 - August 2013 P:\23514\1 33-23514-13003\Docs\Reports\W attenberg_Construction _Report_2013. doc