Internal Stability (Stresses)

Learn more about the need to consider uplift pressure when designing a gravity structure at DamFailures.org

"Water pressures caused by reservoir water and tailwater occur within the dam and foundation in pores, cracks, joints, and seams. The distribution of internal water pressures along a horizontal section through the dam or its foundation is assumed to vary linearly from full reservoir pressure at the upstream face to zero or tailwater pressure at the downstream face in the absence of drains or a more detailed analysis."^[1]

"The internal water pressure, also called uplift, acts to reduce the compressive stresses normal to a horizontal section through the dam. Including a line of vertical formed drains within the dam and parallel to the upstream face serves to reduce the uplift force. The uplift reduction is dependent on the size, location, and spacing of the drains.^[1]

"Forces from water pressures also occur within the foundation. Uplift forces in the foundation decrease the normal forces occurring on potential sliding planes. Water forces occurring on high angle joints increase driving forces on foundation blocks. Both of these reduce foundation sliding stability.^[1]

"The uplift forces within the foundation and along the foundation-dam contact can be reduced by a line of drain holes drilled into the foundation from the floor of the foundation gallery. The internal pressure distribution through the foundation depends on depth, location, and orientation of the drains, rock permeability characteristics, jointing, faulting, and any other geologic features that may modify the flow. The line of drains should be located a distance downstream from the upstream dam face that will ensure that direct connection from the reservoir will not occur."^[1]