Seite - 153 - in Book of Full Papers - Symposium Hydro Engineering

other side. The dowels were shown just below the reinforcement mat near the top of the slab. Although all formed joints were intended to be keyed, formed joints without keys were observed at a few locations during the post-incident forensic investigations. Based on the design drawings and specifications, as well as an interview, the design intent was for the chute slab to be founded directly on moderately weathered rock or better, which could not be further removed by heavy-duty power excavating equipment. The excavated surface was to be pressure-washed to remove all mud, debris, and loose or unsound rock fragments. However, the foundation requirements were substantially relaxed during construction. In some places, the prepared foundation appeared to have high points of reasonably sound rock surrounded by areas of what appears to be rock weathered to soil-like material, as shown in Fig. 5. In other locations, the prepared foundation appears to have been almost entirely rock that had weathered to soil-like material, as shown in Fig. 6, which was taken in the area of the initial chute slab failure. Fig. 5 Example of the clean-up effort for the service spillway chute prior to concrete placement. The chute slab concrete was specified to have a 28-day compressive strength of 21,000 kPa (3,000 lb/in2), and all concrete cores tested in the post- incident field investigations had compressive strengths higher than the specified strength. Type II cement was used for the concrete, and the coarse aggregate was up to 15.2-cm (6-in) size. While large size coarse aggregate can be beneficial in thicker placements, the 15.2-cm (6-in) maximum size would seem to be too large 153