Piping failures in small dams

Abstract Only - Storm water management pond dams have become an integral part of most commercial and infrastructure development projects. Although these dams are generally designed for shallow normal pool depths of 5 to 15 ft, sloping ground conditions often substantially increase the hydraulic head differential within the dam. Numerous failures continue to occur due to use of ineffective design approaches and in appropriate specifications.
Unfortunately, these small earth dams are often designed with internal seepage control consisting of anti-seep collars around a spillway conduit, with no cradle. Although dam safety engineers have known for years the inadequacy of anti­seepage collars and misfortunes related to improper soil compaction around conduits, this knowledge has not been broadly adopted by counties or state agencies who still show anti-seepage collar designs with no outlet pipe cradle in their erosion control manuals.
The passage of water through earthen dams is inevitable. Following filling of the dam, a wetted surface permeates the dam embankment materials as the hydraulic head forces the water through the soil voids. The surface of the wetted embankment is termed the phreatic surface, and is a measure of the hydraulic head within the dam. This surface decreases towards the downstream toe as hydraulic head is lost due to friction from flow through the soil mass.
The inclusion of rigid or semi-rigid pipes through a dam embankment is problematic in that, if not properly designed and constructed, may create a preferential flow path for water through a dam. This preferential flow path (typically void space along the annulus of the pipe) allows the passage of water with very little head loss.
A high water head along the downstream toe, if sufficient, will produce seepage forces and exit velocities high enough to cause erosion of the downstream embankment soil. This erosion continues back into the core of the dam, choosing the path of least resistance. As the erosion continues, the flow path length through the soil is reduced, increasing the hydraulic gradient (the water head, or pressure, divided by the length of the flow path). Because of this increased hydraulic gradient, the erosion accelerates until the dam breaches.
Early dam designs (up until the mid-70s) considered concrete collars, called anti­ seepage collars, to be the method of choice, to prevent piping along conduits through dams. It was originally thought that decreasing the hydraulic gradient by increasing the flow path along the conduit with seepage collars would be a sufficient means to prevent piping.
Following observation of many piping failures of dams with anti-seepage collars, the U.S. Army Corps of Engineers and Bureau of Reclamation both declared decades ago that anti-seepage collars were not adequate to prevent piping failures through dams. Inadequacy stems from void space created along the pipe due to inadequate compaction around and beneath the pipe as well as the collars, and the notion that voids may be created due to differential settlement of the embankment and foundation.