CS9C - Concurrent Session 09C:A New Concrete-Lined Spillway Chute Concept, Part 2 – Addressing Uplift Through Seepage Measures

Abstract Description: Assessing uplift beneath concrete-lined chute spillways that is above tailwater levels and with minimal ground water seepage is not well defined. Past approaches have assumed the phreatic surface as equal to the flow depth, as a percentage of this phreatic surface if drains are present, using seepage analysis for the subgrade materials (such as spillway chutes constructed on embankment dams) or using seepage studies and examples derived from experience with concrete lined canals. Quantifying uplift beneath spillway slabs is further complicated by flow-induced hydraulic phenomena when the spillway us operating including induced flow through cracks, hydraulic jacking due to stagnation pressure at slab offsets into the flow, and cavitation damage due to surface roughness and irregularities.

In a previous paper (Part 1), the authors described a new spillway concept that would consist of a series of integral spillway segments beginning downstream of the control structure to upstream of the energy dissipator. Each integral segment would place concrete continuously across the spillway width between upstream and downstream transverse joints with longitudinal contraction joints already preformed. Each segment would be separated using a “lapstrake” transverse joint, which overlays the downstream slab creating a step. Consequently, there is no exposed transverse joint to generate stagnation pressures. Also, the step allows for air entrainment to mitigate potential cavitation damage downstream.

In addition, this step is tall enough to provide underdrain outlets that daylight on the exposed face and discharge to the downstream slab. Consequently, each segment can be constructed with its own drainage system. This has several advantages:

• Drains can be inspected directly from these openings in the step.
• No collector drains between segments are needed.
• The drainage system can be tailor for each segment for slope and subgrade conditions.
• Air can be introduced at the step to mitigate potential cavitation damage.
• Seepage can be readily observed from the segment drain outlets.

The paper will present drainage system concepts and their design for an individual segment for concrete lined spillway chutes. Analysis procedures to estimate the amount of seepage for each segment would be described. Hydraulic design, layout and selection of drains will be presented including drainage examples for hard rock subgrades, “soft” rock subgrades and soil foundations as well as steep and shallow slopes.