Underseepage

"Flow of water through soil can lead to movement of the soil grains. Continued movement is erosion. Many factors affect whether soil grains will move or not, including hydraulic gradient, soil plasticity, particle size, capillary tension, cementation, and others."^[1]

"A review of historical dam failures indicates that nearly half of all failures of embankment dams have been a result of seepage-induced internal erosion. Additional failures due to high pore pressures or saturated slopes, both attributable to seepage water, add to the list of seepage-related historical failures. Since all soils are erodible to some extent, embankment dams are potentially susceptible to failure due to seepage. In order to evaluate new or existing dams, with respect to safety against seepage, and design defensive measures to mitigate the effects of seepage, it is important to understand the various modes of failure that can occur due to reservoir seepage acting on an embankment or its foundation."^[1]

"Observations related to seepage through an earth embankment or its foundation may give an early indication of the nature, location, and severity of potential failure modes (PFMs) that may already have initiated or could initiate at higher reservoir levels. For example, vigorously flowing sand boils discharging sediment near the downstream toe of an embankment may indicate that backward erosion piping in the foundation has already initiated and that it has the potential for rapid progression. Likewise, concentrated, clear seepage at an abutment groin may be an indication of a preferential flow path through the abutment, which could provide an avenue for internal erosion at higher gradients."^[2]

"Seepage collection and monitoring systems should be designed with all failure modes in mind that may reasonably be anticipated at a particular structure site, given its unique geologic characteristics and design features. Ideally, all needed seepage measurement and monitoring systems should already be in place and available to collect seepage information from the first filling on, but of course new seepage areas can emerge at any time. Once water is impounded in the reservoir, all seepage-related phenomena should be evaluated with reference to PFMs of which they could be indicators. Early identification of PFMs that may be at work can facilitate the undertaking of emergency or remedial actions that need to be performed on short notice."^[2]

"Measuring seepage quantity and quality provides several important benefits:

Comparison with Design: Measurements of actual seepage can be compared with seepage rates assumed or calculated during design to evaluate whether the dam and its various seepage control systems are functioning as intended (e.g., a foundation cutoff system or an embankment or foundation drain).
Identifying Changes with Time: The appearance of unexpected seepage or changes in seepage quantity or quality over time can alert the dam owner to potential problems occurring within the embankment or foundation and allow for the implementation of corrective measures in a timely manner."^[2]