Seepage Analysis & Filter/Drain Design

A diagram illustrating the effects of seepage and erosion on a dam. (Pocket Safety Guide for Dams and Impoundments (FEMA P-911))

Image Source: USDA.gov

Nearly every dam experiences seepage to varying degrees. Depending on the type of dam and its unique site conditions, seepage may go through, underneath, or around the dam. When left unchecked, excessive seepage can lead to problems such as internal erosion and eventual failure of either the dam itself or other vital components of the dam. According to the Association of State Dam Safety Officials (ASDSO), the second most common dam failure incident driver between the years 2010 through 2019 was “internal erosion”.^[1] “Erosion and piping can occur when hydraulic gradients at the downstream end of a hydraulic structure are large enough to move soil particles. Analyses to compute hydraulic gradients and procedures to control piping are contained in EM 1110-2-1901”.^[2]

The effects of seepage should be evaluated for all dams. "The evaluation must address all potential embankment and foundation seepage related failure modes, including the potential for internal erosion, erosive flow along defects, internal instability, and uplift pressure to damage the embankment, its foundation, and appurtenant structures. The evaluation should be commensurate with the complexity, function, and hazard potential classification of the structure. Seepage control and management must be adequate to accomplish the intended reservoir function, provide a safety operating structure, and prevent damage to downstream property. Design and construct or rehabilitate existing embankment dams with sound defensive measures to reduce, filter, collect, and discharge seepage that are representative of current practice”.^[3]

Seepage Mitigation Measures

Cross-section of a seepage filter/drain system. (Montana DNRC)

Image Source: Montana DNRC

“Filters are placed in embankment zones, foundations, or other areas of hydraulic structures for two purposes:

"To intercept water flowing through cracks or openings in a base soil and block the movement of eroding soil particles into the filter. Soil particles are caught at the filter face, reducing the flow of water through the cracks or openings and preventing further erosion and enlargement of the cracks or openings.
"To intercept water flowing through the pores of the base soil, allowing passage of the water while preventing movement of base soil particles.^[4]

"Without filters, piping of susceptible base soils can occur when seepage gradients or pressures are high enough to produce erosive discharge velocities in the base soil. The filter zone is generally placed upstream of the discharge point where sufficient confinement prevents uplift or blowout of the filter. Drains consist of sand, gravel, or a sand and gravel mixture placed in embankments, foundations, and backfill of hydraulic structures, or in other locations to reduce seepage pressure. A drain’s most important design feature is its capacity to collect and carry water to a safe outlet at a low gradient or without pressure buildup. Drains are often used downstream of or in addition to a filter to provide outlet capacity. Combined filters and drains are commonly used. The filter is designed to function as a filter and as a drain”.^[4]

Special considerations are required when filter drains include a collector pipe to carry flow to the outlet. In a paper presented at the ASDSO 2004 Annual Conference titled "Seepage Collection and Control Systems: The Devil is in the Details", John France discusses drainpipes embedded in drains. It is recommended that drainpipes not be embedded in sand zones. There are cases where migration of the filter sand has resulted in clogging of the slots or perforations in the pipe. Use of a geotextile fabric wrapped around the pipe to prevent clogging by filter sand is not recommended, since clogging of the fabric may occur with the result that flow into the pipe will be reduced. If pipes are needed to carry the anticipated flow then they should be surrounded by gravel envelopes meeting recommended filter criteria. A suggested construction sequence is included in Figure 2 of the referenced paper.^[5]

Also, when pipes are embedded in filter drains, it is recommended that provisions be included for future inspection and cleaning. Collector pipes should not be smaller than 6-inches in diameter to allow access by cameras. Any change in direction of seepage collector pipes should be gradual, preferably not more than 22.5 degrees. Sufficient straight sections should be provided between multiple bends so that cameras can negotiate the resulting configuration. It is recommended that the collection pipes be inspected immediately after installation and several times during fill placement to address any construction damage in a timely manner. If damage to a drainpipe is found after dam construction is complete then repair or replacement may be difficult.^[5]

Installation of seepage barriers such as cut-off walls or injecting grout under pressure at key locations underneath and around the dam can help to reduce the amount of seepage at a dam. The use of these methods increases the seepage path lengths, increasing head loss, and reducing the amount of flow. However, installation of these seepage barriers can sometimes lead to increased hydraulic gradients in key locations underneath or around the dam and lead to the erosion of soil and piping. Careful analysis of the effects of installing a seepage barrier should precede its construction/implementation.

“Design seepage reduction measures to limit seepage and embankment saturation as necessary to address seepage failure modes, provide adequate static and dynamic stability and limit water loss to the extent required by project function”.^[3]