Seepage Control in Earthen Dams

Most dams in active use today exhibit seepage of one form or another. The location, rate of flow, and turbidity (clear or murky) are the critical factors when evaluating the seriousness of seepage from a dam. Seepage is the continuous movement of water from the upstream face of the dam toward its downstream face, and is a major minor problem when it comes to the life span of dams and embankments. It is a major minor problem because if controlled the affects are minor and not hazardous, but if not controlled it can become a major problem and possibly result in failures.

-Conditions of Seepage-

Most dams have some seepage through or around the embankment as a result of water moving through the soil structure. If the seepage forces are large enough, soil can be eroded from the embankment or foundation. Seepage can also develop behind or beneath concrete spillways or headwalls. The signs of this type of problem could be cracking or heaving. Freezing and thawing will amplify the affects of seepage on concrete structures. The rate at which water moves through the embankment depends on the type of soil in the embankment, how well it is compacted, and the number and size of cracks and voids within the embankment. Saturation of embankment soils, abutments, and foundations due to seepage generally result in reduced soil strengths leading to sloughing, sliding and instability. In the worst case, seepage can result in total embankment failure if situations are not monitored. Many seepage problems and failures of earth dams have occurred because of inadequate seepage control measures or poor cleanup and preparation of the foundations and abutments. Seepage can lead to soil piping and embankment sloughing or sliding, both of which can lead to dam failure. Soil piping occurs when material is washed out at the base of the downstream face causing a hole to form underneath the dam. This hole is enlarged as more material is washed out by water flow, which increases due to the shorter flow path that gradually develops. Eventually a tunnel or pipe is created within the soil under the dam from the downstream to the upstream face which causes a collapse of the dam embankment. Seepage may be difficult to spot due to vegetation. Probing the soil in suspect areas can help to locate and identify whether seepage is present and the limits of the problem. Differences in vegetation and flowing water on the downstream side of embankments are the two most noticeable signs of seepage.

-Causes of Seepage-

• poor compaction of embankment soils

• poor foundation and abutment preparation

• rodent holes

• rotted tree roots and wood

• open seams

• cracks

• joints in rocks in dam

• coarse gravel or sand in the foundation or abutment

• clogging of coarse drains

• filters or drains with pores so large soil can pass through

• frost action

• shrinkage cracking in the embankment soil

• settlement of embankment soil

• uprooted trees

• earthquakes

• insufficient structural drainage

• trapped groundwater

• Excessive uplift pressures

-Affects of Seepage-

Seepage, if uncontrolled, can erode fine soil material from the downstream slope or foundation and continue moving towards the upstream slope to form a pipe or cavity to the pond or lake often leading to a complete failure of the embankment. This action is known as piping. High velocity flows through the dam embankment can cause progressive, or rapid, erosion and piping of the embankment or foundation soils. If this condition continues unchecked, complete dam failure can result. Saturated soil areas on the embankment slopes, the abutment, or the area at the toe of the dam can slide or slough, resulting in embankment failure. Seepage failures account for approximately forty percent of all embankment failures. The most catastrophic results of reservoir seepage into groundwater occur when saturated rock loses its strength. In such events valley walls can collapse, causing dam failure and disastrous flooding downstream. Excessive seepage can present a safety hazard to the dam and the health and welfare of people and property downstream of the dam. Most failures caused by groundwater and seepage can be classified into one of two categories based on the type of soil movement that is occurring. The failures will typically develop over a relatively long period of time so there will be ample warning if routine inspections are performed. The two categories of failure include those that take place when soil particles migrate to an escape exit and cause piping or erosion failures, and those that are caused by uncontrolled seepage patterns that lead to saturation, internal flooding, excessive uplift, or excessive seepage forces.

-Controlling Seepage-

Regularly scheduled monitoring and inspection is essential to detect seepage and prevent dam failure. Inspections should be made periodically throughout the year. Frequency should be based on hazard classification of the dam. Higher classified dams should be checked more common, compared to those that are lower hazard classified. At a minimum all dams should be visually inspected at least every six months, before a predicted major storm event, during or after severe rainstorms or snowmelts, and inspected weekly after construction is complete and reservoir filling is ongoing, and for at least two months after the reservoir has been filled. Dam inspections performed on a regular basis are the most economical aid a dam owner can use to assure the safety and long life of the structure while reducing liability risks. If seepage is detected on a dam embankment or foundation, it should be closely monitored on a regular basis until it is corrected.

If seepage flow increases or embankment soils are showing signs of instability, corrective action should be taken quickly. Seepage problems at high hazard

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