Effects of Joint and Crack Geometry on Hydraulic Jacking in Lined and Unlined Spillways

Abstract

Offsets into the flow at joints or cracks can cause hydraulic jacking of spillway chute linings or remove rock blocks from unlined chutes. Previous articles established experimentally validated relations for uplift pressure and flow into square-edged joints and cracks oriented perpendicularly to the flow and chute surface. This paper addresses chamfered and rounded edges, tilts into or away from the flow, skewed orientations, irregular midslab cracks, and bevels or relief slots used to remediate existing offsets. Chamfers and rounding reduce uplift by diminishing the effective offset height. Tilts and bevels away from the flow reduce uplift and joint flow in proportion to (cos ϕ)1/2 where ϕ = tilt angle, whereas tilts toward the flow cause marginal increases. Very flat beveling is needed for major uplift reduction; as a result, relief slots that shift the offset downstream from the joint may be more effective remediation alternatives. For skewed joints, uplift is reduced in proportion to (cos ψ)2.5 where ψ is the deviation from perpendicular orientation. Flow through various irregular joints was reduced compared with that through regular square-edged joints. Existing offsets into the flow at joints or cracks in spillway chutes create the potential for hydraulic jacking failures that can destroy a spillway chute and potentially lead to uncontrolled release of reservoir storage. This study used laboratory experiments to evaluate several treatments that can reduce the risk of hydraulic jacking failure, including chamfering, rounding, and beveling of offsets or milling slots to shift the offset further downstream from an open joint. If the goal is to reduce uplift to 30% or less of that caused by the untreated offset, very flat beveling is needed and relief slots are more effective than bevels that involve the same volume of material removal. The study also shows that skewed joints (i.e., twisted so that their axis is not perpendicular to the flow direction) can reduce uplift pressures by about 55%, although this may not be an economic or practical construction approach for most spillways. Data collected in the study also aid in the estimation of uplift pressures at rounded and chamfered joints and midslab cracks, and at natural joints in unlined rock chutes.