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6. RCC AS FOUNDATION REPLACEMENT The design process continued, following the initial construction bidding and award, as the overall completion moved forward. As the design progressed towards about 60% complete, a decision was made to modify the RCC portion of the chute to a foundation replacement only, which would provide a similar design approach, using reinforced-concrete for the chute slabs and cantilever retaining walls for the entire FCO Spillway chute. The construction contractor was confident that the RCC foundation replacement could be completed within the necessary timeframe, but indicated that the single-sided reinforced- concrete walls originally envisioned for the RCC portion could not be completed during the 2017 construction season. As a result, an intermediate design modification of the RCC chute replacement was developed that included RCC as foundation replacement only so that the rebuilt chute would have similar reinforced-concrete slabs and cantilever retaining walls throughout the spillway. This 60% design concept is shown in Fig. 5. Fig. 5 Intermediate RCC Design Concept (60%) The advantage of this concept was a simplified RCC construction process without the walls, resulting in a shorter RCC placement schedule. The challenge would be to get the RCC placed in time ahead of the reinforced-concrete work to allow that work to meet the critical November 1st completion date. There was some concern about the early strength of the RCC to allow proper anchoring of the chute slab. Early tests in the RCC Test Fill #1, however, showed that sufficient RCC strength would develop to allow proper anchorages. 7. INTERIM RCC FOUNDATION REPLACEMENT CONCEPT The design and construction work continued to move together at a fast pace between April 15, 2017, when the construction contract was awarded and the anticipated start of field work on June 1, 2017, following final gate closure of the FCO Structure. During this time, the contractor was evaluating the post-incident condition of the foundation with regard to the clean-up needed for the RCC and conventional concrete work. In this same period, project geologists determined that a considerable volume of rock foundation under the remaining non- eroded FCO Spillway chute was unstable and needed to be removed before foundation clean- up could be started in the deepest upper hole” 93