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-
Ð²Ð‚Ñž poor compaction of embankment soils
Ð²Ð‚Ñž poor foundation and abutment preparation
Ð²Ð‚Ñž rodent holes
Ð²Ð‚Ñž rotted tree roots and wood
Ð²Ð‚Ñž open seams
Ð²Ð‚Ñž joints in rocks in dam
Ð²Ð‚Ñž coarse gravel or sand in the foundation or abutment
Ð²Ð‚Ñž clogging of coarse drains
Ð²Ð‚Ñž filters or drains with pores so large soil can pass through
Ð²Ð‚Ñž frost action
Ð²Ð‚Ñž shrinkage cracking in the embankment soil
Ð²Ð‚Ñž settlement of embankment soil
Ð²Ð‚Ñž uprooted trees
Ð²Ð‚Ñž insufficient structural drainage
Ð²Ð‚Ñž trapped groundwater
Ð²Ð‚Ñž 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.
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 dams need to be corrected immediately. If the problems are neglected and permitted to progress, they may result in loss of life and property downstream of the dam. A qualified geotechnical engineer or dam safety professional should be contacted for inspection and advice for all high hazard dam seepage problems. The type of controls deployed depends on the source, type, and extent of seepage. Flow nets can also be installed as a method of studying the path that the seeping, or moving water follows. Then the correct action can be taken. If excessive water is flowing from soil piping or boils, or if the water is carrying sediment, an engineer or safety professional should be contacted for inspection and recommendations for further action. The reservoir level should be lowered if serious piping or embankment sliding/sloughing is occurring, and the cause of the condition corrected. Sloughing and sliding due to seepage at the toe of the embankment may be corrected by removing the unstable soil and constructing a toe drain with filter out of permeable soil. Permeable soils and materials are commonly used at the toe of dams, to create drains to help control and prevent erosion. By using such materials, it allows the water to pass through without carrying the soil with it. The same thing can be done in locations where water is flowing but piping has not yet occurred.
Seepage, piping, and boils in existing dams may be corrected, or slowed, by intercepting the water before it exits on the down stream side of the dam. Some typical methods of intercepting include impermeable upstream blankets, cutoff trenches in the embankment, grout curtains, relief wells, and toe drains. Impermeable upstream blankets, or liners, are the most effective method, but require complete drawdown of the reservoir. These blankets may consist of low-permeability soil or a synthetic geo-membrane. The blankets may also be deployed on the floor of the reservoir to prevent foundation seepage. All cracks and erosion rills on the embankment should be filled, re-graded, and re-seeded. Borrowing rodents should be eliminated form dams, and any damage created should be repaired by backfilling with a soil or filtered drain.
One approach to preventing failures of these dams from uncontrolled seepage under them is to increase the length of the flow path under the structures by using cutoff walls at the upstream and downstream edges of the dam. Designers of earthen and earth-rock dams adopted the philosophy of increasing the length of the seepage path used for concrete gravity dams. Concrete collars, also called antiseep collars were constructed at regular intervals along conduits through the dams to increase the length of the flow path of water along the outside of the conduit. The theory was that forcing water to take a longer seepage path would dissipate hydraulic forces and reduce the likelihood of piping at the downstream embankment toe. Antiseep collars were often constructed using the same materials used for the conduits. Probably the most common material was formed concrete. Steel, corrugated metal and plastic collars have been used for conduits made of similar materials. Collars were spaced at regular intervals along the conduit within the predicted zone of saturation of the earthfill zone. In the case of central core fills with rockfill shell zones, the collars were usually installed only within the compacted core of the embankment.
Seepage through rock foundations and abutments may be controlled reliably and economically for most dams by drainage measures. Even a shallow line of drain holes is effective for controlling uplift pressures below most gravity dams. This control could be improved, even for very unfavorable geologic conditions, by using deep drain holes.
A homogeneous dam with a height of more than about 6 m to 8 m should have some type of downstream drain. The purpose of a drain is to reduce the pore water pressures in the downstream portion of the dam therefore increasing the stability of the downstream slope against sliding, and control any seepage that exits the downstream portion of the dam and prevent erosion of the downstream slope, also known as piping. The effectiveness of the drain in reducing pore pressures depends on its location and extent. However, piping is controlled by ensuring that the grading of the pervious material from which the drain is constructed meets the filter requirements for the embankment material.
One type of drain is a toe drain. The design of a downstream drainage, or toe drain, system is controlled by the height of the dam, the cost and availability of permeable material, and the permeability of the foundation. For low dams, a simple toe drain can be used successfully. Toe drains have been installed in some of the oldest homogeneous dams in an effort to prevent softening and erosion of the downstream toe. For reservoir depths greater than 15 m, most engineers would place a drainage system further inside the embankment where it will be more effective in reducing pore pressures and controlling seepage. Another type of drain is a horizontal drain blanket. Horizontal drainage blankets are often used for dams of moderate height. Drainage blankets are frequently used over the downstream one-half or one-third of the foundation area. An earth dam embankment tends to be more pervious in the horizontal direction than in the vertical. Occasionally, horizontal layers tend to be much more impervious than the average material constructed into the embankment, so the water will flow horizontally on a relatively impervious layer and discharge on the downstream face despite the horizontal drain. Where this has occurred the downstream slope is prone to slipping and piping. Repairs can be made by installing pervious blankets on the downstream slopes or constructing vertical drains to connect with the horizontal blanket. Such vertical drains are normally composed of sand and gravel.
The affects that seepage has on dams can be major and minor. If proper design is in place then it will not amount to anything more than just a minor thing that can be easily taken care of, but if several factors in design are ignored then many major problems will be encountered down the road.
Davis and Sorenson, Handbook of Applied Hydraulics, 3rd Edition.
McGraw Hill Book Co., New York,