5.1 Flood Loading Potential Failure Modes
5.1.4 PFM-F3 Breach Near Dam Crest Under High Reservoir Conditions due to
Potential Failure Mode Description
The reservoir pool is at or above the crest of the emergency spillway for a sustained period of time. Seepage develops at the right abutment contact between the dam and the left side of the end Monolith No. 31 of the flood control outlet (FCO) structure (Figure 5.3). Internal erosion of the core initiates and the downstream Zone 2 does not adequately filter
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the Zone 1 fines. A separation (gap) develops between the fill and the wall. It is possible that a small gap may be present there already due to deformations caused by the 1975 earthquake.
Flows increase along the gap, further scouring and enlarging the opening. The crest of the dam caves in to the gap and the dam is initially breached at this point down to the rock elevation of approximately feet. Without intervention to lower the reservoir, flows increase through the high-level breach, widening the gap and the breach.
Preliminary PFM Event Tree:
1. Reservoir is at El. 901 feet or higher for a sustained period.
2. Seepage develops at the contact between the end of the left-most FCO monolith (No.
31) and the fill material at the right abutment near the crest of the dam embankment.
3. Zone 1 core material at the contact begins to erode into Zone 2 materials on the downstream side of the monolith.
4. Zone 2 gradation does not adequately filter the eroding fines
5. Seepage rate increases and a gap is formed at the fill/concrete contact. It is possible that a small gap may already be present due to previous earthquake deformations.
6. Scour erosion widens the gap, the crest of the dam caves in to the gap and the dam is breached down to the rock ) at the location of the base of the monolith.
7. Intervention fails, and the reservoir is not lowered fast enough to stop the flows through the breach.
8. Loss of section of embankment to abutment rock contact ) and uncontrolled release through this section until reservoir level is lowered to approximately El. 885 feet.
The adverse and positive factors related to PFM-F3 are listed on Table 5.3.
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Table 5.3 Adverse and Positive Factors for PFM-F3
PFM-F3: Breach Near Dam Crest Under High Reservoir Conditions due to Erosion of Fill at the Right Abutment Contact with FCO Structure
Adverse Factors Positive Factors
Potential for separation at the concrete wall between the contact of the core and FCO; there is no intervening bedrock ridge between the
embankment and the FCO section, as was previously believed.
Core trench was extended up to the FCO structure, and appears to wrap around the upstream side of the end monolith. The end wall of the monolith was constructed with a batter of 1H:5V which maximizes likelihood of a good connection between the concrete and compacted plastic fill (Zone 1) material. Good design and construction practice was followed.
Under a long period of elevated reservoir, erosion could progress to main embankment.
Low head condition.
Earthquake could exacerbate this condition.
Unusual to have water level this high. Water would not remain at this level long.
Zone 2 gradation is not ideal filter for Zone 1
Exit point is filtered by Zone 2.
Could observe seepage at face of spillway wall and place fill material in this area.
Can discharge through FCO fully to lower reservoir level.
Classification: Category II (voting: 12 for Cat II; 3 for Cat III)
Rationale: This potential failure mode (PFM) is credible; adverse condition of embankment/concrete contact exists and is potentially vulnerable to separation and erosion.
The gap could already have been formed by earthquake deformations. This PFM was not highlighted because of the low likelihood that the reservoir would be held at a sustained high pool elevation needed to initiate the erosion process and the low head conditions. Also, Zone 2 is present to filter the erosion process and although it is not ideal based on modern filter criteria, under the low hydraulic gradients that would be present in this area the imperfect filter may be adequate. The Zone 1 core is broadly graded, with significant particle sizes that are not subject to internal erosion under seepage forces. It is highly unlikely that this condition, if it were to develop, would lead to a breach of the dam, because quick action would be taken to intervene and lower the reservoir as soon as seepage problems in this area were observed. Members of the Core Team who voted for Category III wanted additional clarification and confirmation on the exact configuration of the FCO/dam crest interface, which was briefly covered during the workshop. In preparing this report, some additional information was researched, such as the finding that the end wall of Monolith No. 31 does appear to have been constructed with a batter, as discussed in Section 5.1.1 (5). However, the Core Team members may need additional data or information before the next PFMA update re-visits this PFM.
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Potential Risk Reduction Measures
1. Inspect the FCO/dam contact area at the downstream side immediately following earthquakes, and any time reservoir is at a high water condition.
Other Considerations
1. Determine if the abutment wall at this location is sloped (battered). Get detail of contact/slope of interface at left end of FCO concrete section. [Note: This was done following the workshop, and it appears that the wall was constructed with a batter per Reference 9, Sheet No. 39, Figure 5.4 (b)]
2. Confirm geology drawing which shows fill base at El. 885 ft. [Note: this was done following the workshop and confirmed per Reference 28, Drawing No. IF258, Sheet 2 of 4, Figure 5.4(a)]