Page - 109 - in Book of Full Papers - Symposium Hydro Engineering

it was rapidly determined that the foundations of the spillway structures needed to be further explored to better understand the site geologic conditions, provide parameters for the design team (e.g., range of rock shear strength, rock and dowel anchor designs, and filter compatibility), investigate the FCO Spillway chute that would likely remain in place, and to anticipate construction issues (e.g., rippability, areas prone to slope instability, and groundwater conditions interfering with excavations). 3.1. EXPLORATIONS TO UNDERSTAND GENERAL SITE GEOLOGIC CONDITIONS The Oroville Dam site was one of the most studied dam sites of its time. That said, many of the eroded and scoured areas around both the FCO Spillway chute and the Emergency Spillway exposed fresh geology that clearly indicated the need for additional explorations to better understand and define the geologic site conditions. The FCO Spillway chute failure initiated in a portion of the chute founded on intensely-weathered to decomposed amphibolite rock (Fig. 4). Within this initial scour hole, the depth of rock scoured out ranged up to 30 m (100 ft). Intermittent spillway discharges resulted in the creation of a large erosion channel leading away from this initial scour hole and running downslope to the left of the damaged FCO Spillway chute (Fig. 4). Within this left erosion channel, the near vertical eroded rock faces were as tall as 60 m (200 ft). The left side of the scour hole and erosion channel generally consisted of a thin veneer of soil or colluvium at the top, about 6.5 to 10 m (20 to 30 ft) of moderately-weathered amphibolite, with slightly- weathered amphibolite below (Fig. 4). There was no intensely weathered to decomposed rock observed in this left channel exposure. The zone of intensely 109