Page - 150 - in Book of Full Papers - Symposium Hydro Engineering

2.5. SERVICE SPILLWAY CHUTE DESIGN AND CONSTRUCTION Understanding the service spillway chute design and construction is critical to understanding the chute slab failure in February 2017. The nominal service spillway chute slab design thickness was a minimum of 38.1 cm (15 in), although the nominal chute slab thickness as-constructed was significantly greater in many locations. However, as discussed below, the chute slab thickness over the chute’s underdrains was much less than 38.1 cm (15 in). The design included an underdrain system consisting of herringbone drains beneath the chute slab, connected to collector pipes outside of and parallel to the service spillway chute walls. Each herringbone drain was oriented across the spillway with the pipes on either side of the spillway sloping downstream for drainage, from a point beneath the spillway centerline, as shown in Fig. 2. The herringbone drains were spaced at an interval of 7.6 m (25 ft) along the flatter upstream section of the chute and at 6.1 m (20 ft) along the steeper downstream section. When viewed in plan, the pattern of these drains looked like fish bones or herring bones, as shown in Fig. 2, hence the name. Sets of herringbone drains connected to separate collector drain pipes on either side of the chute. The collector drain pipes extended downstream to outfall locations near the tops of the chute walls. To accommodate all of the herringbone drains, there were 12 collector drain pipes on each side of the chute, for a total of 24 collector drain pipes. Fig. 2 Plan view of headworks (top) and upper service spillway chute showing herringbone drains and collector pipes In the original design/bid drawings, the herringbone drains were indicated as 10.2-cm (4-in) diameter perforated vitrified clay pipes (VCPs) surrounded by gravel, and the collector pipes were indicated as 20.3-cm (8-in) and 15.2-cm (6-in) 150