Is this Seepage A Problem? With your host Adam Powers

Round 1

Is this Seepage a Problem? Question 1: Mike M. This Deer Lodge County dam has ponding water and numerous sand boils at the toe of the dam. If you look closely, sand is actively moving (boiling). Is this Seepage a Problem? HINT: The dam is not very high

#1 a. Sand boils often signify high uplift pressures in the foundation. They also can be a sign that internal erosion of the foundation is occurring, a potentially serious problem.

#1 b last However, this seepage is not a serious problem for this Deer Lodge County dam The “boiling” sands appear to be moving only at the surface. Since the dam is not that high, there is not enough driving head from the reservoir to move the finer foundation soils from the foundation and cause backward erosion to initiate. As a precaution, a sand filter was installed, and piezometers are regularly measured (with no warning signs).

Is this Seepage a Problem? Question 2: Brent Z. This Powell County Dam has an over-steepened downstream slope, with ponding water and water loving vegetation at the toe. However, the embankment is mostly dry. Is this Seepage a Problem? HINT: The foundation is glacial till, interbedded clay and silt

Yes, this Seepage is a Problem. A year after construction of a stability berm on the downstream face of the dam, a “blow out” occurred at the edge of the berm. Material was rapidly eroding back towards the dam. Without intervention, failure was imminent. Rapid response and nearby stock piled materials saved the day.

There are a couple of theories on the cause of the blowout: Construction of the new berm concentrated foundation seepage at the edge of the berm The location of the blow out was reported to be where top soil was stockpiled and later removed.

#2 c. last Many older dams were built without foundation preparation. As a result, they often have a layer of organic clay that acts like a cap that can pressurize a foundation. Even a slight penetration of this cap can cause a blow out and backward erosion. Organic Clay Sandy Gravel Always be cautious when doing any kind of construction activity at the toe of the dam.

Is this seepage a problem? Should we be concerned about a blowout? Question 3: Ben F. This Teton County Dam has ponding water, green grass and cattails at the toe. Piezometers show water levels above ground surface when the reservoir is full. However the embankment is dry. Is this seepage a problem? Should we be concerned about a blowout? HINT: The water bearing zone in the foundation is believed to be competent bedrock

Internal Erosion Failure Components For internal erosion and blowout that leads to dam failure there are several factors that must be present: Internal Erosion Failure Components Reservoir Loading: Flaw Exists: Continuation: Progression: Unsuccessful detection and Intervention Dam Breaches Reservoir at or above threshold level Erosion initiates Unfiltered Exit exists Continuous stable roof, evidence of worsening conditions

Reservoir at or above threshold level Erosion initiates (flaw in embankment) Unfiltered exit exists (nothing to stop particle movement) Continuous stable roof

#3 c. last It is uncertain if the seepage is a problem, fortunately, not all of the internal erosion components are present Internal Erosion Failure Components  Piezometers 5’ above ground surface at full pool (High foundation pressure) Reservoir Loading: Flaw Exists: Continuation: Progression Unsuccessful detection and Intervention Dam Breaches x None that are obvious  Unfiltered exit present – no filters at toe ? Unsure if water bearing layer would support continuous “roof”. No evidence of worsening conditions. x Careful monitoring underway to detect developing problems

However, the downstream face looks to be in good condition. Question 4: Tim K. Q #4 This photo shows evidence of erosion near the upstream gate of this Choteau county dam. However, the downstream face looks to be in good condition. Is this caused by seepage and is this seepage a problem?

Yes, this Seepage was a Problem. Seepage that is moving soil into deteriorated outlets can go undetected for years. Failure often occurs suddenly with no warning

#4 b last A new tool is in development to help dam owners rehabilitate their outlet works before failure occurs

STATE CONSULTANTS 2 2

Round 2

Is this Seepage a Problem? Question 5: Ben F. A sinkhole suddenly appeared on the downstream face of this Granite County dam, along with cloudy water from a toe drain Is this Seepage a Problem?

Yes, this Seepage was a Problem BUT…… Not as bad as it could have been When soil above or around a backward erosion pipe is unable to “hold a roof” the following occurs: • backward erosion pipes form but soon collapse, resulting in general movement of the soil above. • If the collapse is upward, it can lead to the development of a near‐vertical cavity (stope) in the embankment Yes, this Seepage was a Problem BUT…… Not as bad as it could have been

#5 b last If the embankment soils would have been able to “hold a roof”, the backward erosion could have made it back to the reservoir and failed the dam So, in general sinkholes on the upstream face are more of a concern than sinkholes on the downstream face.

Is this seepage that caused these sinkholes a problem? Question 6: Tim K. During a low pool inspection of this northern Lewis and Clark County dam, sinkholes were discovered on the upstream face Is this seepage that caused these sinkholes a problem? HINT: The sinkholes were below normal pool level, on the right abutment

#6 a. Yes and No….. Upstream face sinkholes are always a serious concern, however these sinkholes are unique……. The glacial foundation of this dam is full of large boulders. Further investigation revealed that the sinkholes extend vertical deep below the dam

#6 b. last The sinkholes were patched to prevent migration of soils into the foundation. However, over the past 15 years, sinkholes have continually reappeared in the reservoir pool. So YES, this seepage is a problem, as migration of soils is undesirable, but NO, these sinkholes are not believed to be putting the dam at risk. Frequent inspections and seepage monitoring is ongoing at this high hazard dam.

Is this Seepage a Problem? Question 7: Mike M. A seep on the left groin of this Granite county dam was getting progressively worse, with measurable sediment accumulation Is this Seepage a Problem? HINT: The sediment was determined to be coming from infilling in left abutment bedrock

#7a. Yes, this Seepage was a Problem - No doubt, the dam was well on its way to an internal erosion piping failure.

    x x Back to our internal erosion checklist: Internal Erosion Failure Components  Occurs when reservoir above certain level Reservoir Loading: Flaw Exists: Continuation: Progression Unsuccessful detection and Intervention Dam Breaches Abutment / embankment contact may have been improperly prepared during construction   Unfiltered exit present  Seepage and sediment increasing x Careful monitoring and dye tests were key to figuring out the issue and designing a repair x

Successful in using “Upstream Control” (grouting). Seepage and Internal Erosion were completely stopped

Issues – Downstream Control Decided against using “Downstream Control” (Seepage Berm) #7d (last) Issues – Downstream Control High flows (150 cfs) – Difficult to filter out fines, sand size filter needed Sand does not have sufficient permeability to handle large flows Sediment will clog filter – main issue Internal erosion far advanced, concentrated flow Potential to make problem migrate Uncertainty with how far to go with blanket/drainage system, would need to extend outlet works at high cost Berm not needed for stability, no upward gradients

This Madison County Dam has a history of reservoir sinkholes Question 8: Brent Z. This Madison County Dam has a history of reservoir sinkholes The toe drain pipe has some sediment Some odd sand boils appear below the dam Is this Seepage a Problem?

The downstream face of the embankment is dry No, the seepage is not a problem, as long as the reservoir stays relatively low (the reservoir seeps and does not stay full for long). Should the reservoir stay at full pool for an extended period of time, there are some concerns. #8a. It is suspected that the seepage moves deep into the foundation from the reservoir. The downstream face of the embankment is dry The sand boils are well below the toe of the dam

Regarding the sediment in the toe drains: #8b. Regarding the sediment in the toe drains: Back in 1959, during a grouting attempt to stop seepage , the State used dynamite in the grout holes to increase grout intake. This may have damaged the embankment/toe drain, so the sediment may not be reflective of internal erosion. State does not get points because of the use of dynamite, regardless of how good their answer is.

#8c. last DH3 is a key piezometer to measure whenever reservoir gets to full pool. Any change in measured water levels in DH3 is a big warning sign that a problem is developing in the dam. Early grouting efforts may have damaged the dam, problems will likely only show up when dam is at full pool.

STATE CONSULTANTS 3 4

Bonus Round Audience gets to choose contestant

Is the Seepage at this Dam a Problem? Question 9: Audience Choose Contestant During an inspection, white sediment was found at the toe of this Judith Basin County dam. Is the Seepage at this Dam a Problem?

#9 a. Early investigations indicated high foundation pressure exist (piezometers at toe showed above ground water levels) and geology could support internal erosion

High pressure in foundation as evidenced by sand boils (see video)

   ? x To our internal erosion checklist: Internal Erosion Failure Components Internal Erosion Failure Components  When reservoir exceeds a certain, high uplift pressures exist Reservoir Loading: Flaw Exists: Continuation: Progression Unsuccessful detection and Intervention Dam Breaches  Cracks through confining layer, possibly from improperly abandoned boreholes  Unfiltered exit present, no filter at toe ? Bedrock in foundation may limit progression of internal erosion x Detection through regular inspection

Intervention through construction of seepage berm #9 d Intervention through construction of seepage berm

STATE CONSULTANTS 3 5 If audience chooses consultant to answer question

STATE CONSULTANTS 4 4 If audience chooses state to answer question