Page - 151 - in Book of Full Papers - Symposium Hydro Engineering

diameter VCPs. However, construction records indicate that the herringbone drain size was increased to 15.2-cm (6-in) diameter and the collector pipe size was increased to 30.5 cm (12-in) diameter, which is consistent with the as-built drawings and the pipe sizes observed during the post-incident forensic investigations. In both the design and as-built drawings, the herringbone drains are not located entirely beneath the chute slab, but rather they protrude up into the slab section, as shown in Fig. 3, taken from the as-built drawings. This location of the drain pipes substantially reduced the thickness of the slab immediately above the drains. The drawings noted the slab thickness above the drains as a minimum of 17.8 cm (7 in). From available information, it appears that, in most locations, even when the as-constructed slab thickness increased, the drain pipe locations were maintained near the top of the slab. Fig. 3 Herringbone drain VCP pipe protruding into slab section As designed and constructed, the chute slab included a single layer of reinforcing bars near the top of the slab, as shown in Fig. 4. The reinforcing bars were 15.9 mm (No. 5) bars, spaced at 30.5 cm (12 in) each way. The chute design also included 35.8 mm (No. 11) anchor bars, spaced at 3 m (10 ft) each way in plan view, to extend 1.5 m (5 ft) minimum into the foundation or 5 ft (1.52 m) minimum into sound rock and to be grouted into the foundation. The anchor bar detail is shown in Fig. 4. From construction records and photographs, it does not appear that anchor lengths were adjusted when “sound rock” was not located at foundation grade, nor when the concrete slab was thicker than 38.1 cm (15 in). Hence, the anchors likely extended a maximum of about 1.5 m (5 ft) into the foundation. 151