Page - 113 - in Book of Full Papers - Symposium Hydro Engineering

headworks structure would need to remain in place [3]. However, for the design team and dam safety regulators to be comfortable with this situation, many explorations were performed within the upper chute to better understand the actual thicknesses and quality of the concrete slabs, foundation rock quality, concrete- foundation interface conditions, and so on. To address design concerns, the chute slab concrete was cored at 68 exploration points within the upper half of the FCO Spillway chute, upslope from the initial scour hole. Of the 68 locations, 24 were additionally cored into the foundation rock to assess foundation rock conditions. Falling-head permeability tests were completed in 18 locations to test the bond between the slab concrete and the foundation rock. Ground-penetrating radar (GPR) was also used to image the concrete and the underlying foundation rock in the upper chute; the results from the GPR imaging were used to guide some of the physical exploration locations. Eight wireless vibrating-wire piezometers were installed in six locations to determine groundwater pore pressures beneath the chute slab during spilling and non-spilling events. In addition to explorations to address design concerns, DWR performed forensic studies that aimed to address the following potential physical factors raised by the Independent Forensic Team in their preliminary report [10]: 1) thickness of the chute slab above herringbone underdrains; 2) variations in slab thickness, 3) characteristics of weathered foundation rock and intensely-weathered rock; 4) foundation rock bonding with the concrete slab; 5) effects of extended drought impacts on foundation materials; and 6) groundwater pressures exerted on the chute slab. Four areas of the undamaged concrete-lined FCO Spillway chute were evaluated using saw-cut trenching to examine the foundation conditions and potential physical factors that might have contributed to the damage of the FCO Spillway chute. Each area of the saw-cut concrete chute was carefully lifted away from the surrounding slab concrete to minimize disturbance. Each location was then photographed in detail prior to performing any investigative work. Once the area was thoroughly documented, the gravel surrounding the perforated underdrain pipes (vitrified clay pipes placed during original construction) was hand- picked from around the pipes. Any non-rock materials mantling the foundation rock, such original excavation debris, were measured, sampled, described, and then cleared off for final geologic mapping of the foundation rock surface. The findings from the FCO Spillway chute explorations indicated that – where measured – the concrete slab was greater than 18.3 cm (7.2 in) thick above the underdrain pipes; the concrete slab thickness varied from about 38 cm (15 in) to over 2.1 m (7 ft); there were not large extents of intensely-weathered to decomposed rock; there was generally good bonding between the rock and the concrete slab; and groundwater piezometric pressures within the chute’s foundation rock were typically below the concrete-foundation interface. 113