CS25B - Concurrent Session 25B: Assessing Uplift at Existing Spillway Chutes in a Post Oroville World

The failure of the principal spillway concrete chute at Oroville Dam due to uplift from various causes has triggered many dam owners to examine their spillway chutes for the potential of an uplift failure. While there is a wealth of information on design for a new spillway chute to address uplift, there is little guidance on assessing the uplift potential of an existing spillway concrete chute. What causes uplift underneath the spillway chute? How does one assess uplift if some of the recommended defensive design measures for uplift are not in place?

Current design of spillway chutes for uplift assumes a saturated subgrade with a conservative pressure distribution applied over the entire slab. Some load cases allow for a reduction in uplift for an assumed drain effectiveness. Issues of groundwater infiltration, soil or rock porosity, extent and sizing of the underdrain system, if present, and slab jointing and integrity are not part of the assessment of uplift on the slab. This paper will examine the various forms of uplift – localized, slab and systemic uplift. The cause for each of these various forms of uplift will also be discussed along with the impact of defensive measures. These causes for uplift include stagnation pressure, hydrodynamic pressure, inadequate underdrain spacing, capacity and porosity; and underdrain redundancies or lack thereof. Key takeaways from this paper are understanding of slab uplift processes, ability to assess potential failure modes for uplift at existing spillways, and to incorporate good design practices to mitigate uplift for new spillway concrete chutes.