A.V. Watkins Dam Failure Incident November 2006 Forensic Investigations and Repair

Transcription

1 A.V. Watkins Dam Failure Incident November 2006 Forensic Investigations and Repair Mark Bliss, P.E. Geotechnical Engineering Group 3 Shlemon Specialty Conference May 17, 2013

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3 Purpose of This Presentation

4 Purpose of This Presentation o Highlight the geologic features at A.V. Watkins Dam and relate these features to their contributing role in the internal erosion, near failure, of the dam.

5 Purpose of This Presentation o Highlight the geologic features at A.V. Watkins Dam and relate these features to their contributing role in the internal erosion, near failure, of the dam. o Show that the estimated annualized probability of this failure mode would have been high (w/ respect to Reclamation guidelines) by a full understanding of the geology at the site and using Reclamation s current state-of-practice risk analysis event tree.

6 Purpose of This Presentation o Highlight the geologic features at A.V. Watkins Dam and relate these features to their contributing role in the internal erosion, near failure, of the dam. o Show that the estimated annualized probability of this failure mode would have been high (w/ respect to Reclamation guidelines) by a full understanding of the geology at the site and using Reclamation s current state-of-practice risk analysis event tree. Event tree:

7 Purpose of This Presentation o Highlight the geologic features at A.V. Watkins Dam and relate these features to their contributing role in the internal erosion, near failure, of the dam. o Show that the estimated annualized probability of this failure mode would have been high (w/ respect to Reclamation guidelines) by a full understanding of the geology at the site and using Reclamation s current state-of-practice risk analysis event tree. Event tree: Reservoir at or above threshold level

8 Purpose of This Presentation o Highlight the geologic features at A.V. Watkins Dam and relate these features to their contributing role in the internal erosion, near failure, of the dam. o Show that the estimated annualized probability of this failure mode would have been high (w/ respect to Reclamation guidelines) by a full understanding of the geology at the site and using Reclamation s current state-of-practice risk analysis event tree. Event tree: Reservoir at or above threshold level Initiation Erosion starts

9 Purpose of This Presentation o Highlight the geologic features at A.V. Watkins Dam and relate these features to their contributing role in the internal erosion, near failure, of the dam. o Show that the estimated annualized probability of this failure mode would have been high (w/ respect to Reclamation guidelines) by a full understanding of the geology at the site and using Reclamation s current state-of-practice risk analysis event tree. Event tree: Reservoir at or above threshold level Initiation Erosion starts Continuation - Unfiltered or inadequately filtered exit exists

10 Purpose of This Presentation o Highlight the geologic features at A.V. Watkins Dam and relate these features to their contributing role in the internal erosion, near failure, of the dam. o Show that the estimated annualized probability of this failure mode would have been high (w/ respect to Reclamation guidelines) by a full understanding of the geology at the site and using Reclamation s current state-of-practice risk analysis event tree. Event tree: Reservoir at or above threshold level Initiation Erosion starts Continuation - Unfiltered or inadequately filtered exit exists Progression Continuous stable roof and/or sidewalls

11 Purpose of This Presentation o Highlight the geologic features at A.V. Watkins Dam and relate these features to their contributing role in the internal erosion, near failure, of the dam. o Show that the estimated annualized probability of this failure mode would have been high (w/ respect to Reclamation guidelines) by a full understanding of the geology at the site and using Reclamation s current state-of-practice risk analysis event tree. Event tree: Reservoir at or above threshold level Initiation Erosion starts Continuation - Unfiltered or inadequately filtered exit exists Progression Continuous stable roof and/or sidewalls Progression Constriction or upstream zone fails to limit flows

12 Purpose of This Presentation o Highlight the geologic features at A.V. Watkins Dam and relate these features to their contributing role in the internal erosion, near failure, of the dam. o Show that the estimated annualized probability of this failure mode would have been high (w/ respect to Reclamation guidelines) by a full understanding of the geology at the site and using Reclamation s current state-of-practice risk analysis event tree. Event tree: Reservoir at or above threshold level Initiation Erosion starts Continuation - Unfiltered or inadequately filtered exit exists Progression Continuous stable roof and/or sidewalls Progression Constriction or upstream zone fails to limit flows Progression No self-healing by upstream zone

13 Purpose of This Presentation o Highlight the geologic features at A.V. Watkins Dam and relate these features to their contributing role in the internal erosion, near failure, of the dam. o Show that the estimated annualized probability of this failure mode would have been high (w/ respect to Reclamation guidelines) by a full understanding of the geology at the site and using Reclamation s current state-of-practice risk analysis event tree. Event tree: Reservoir at or above threshold level Initiation Erosion starts Continuation - Unfiltered or inadequately filtered exit exists Progression Continuous stable roof and/or sidewalls Progression Constriction or upstream zone fails to limit flows Progression No self-healing by upstream zone Intervention fails

14 Purpose of This Presentation o Highlight the geologic features at A.V. Watkins Dam and relate these features to their contributing role in the internal erosion, near failure, of the dam. o Show that the estimated annualized probability of this failure mode would have been high (w/ respect to Reclamation guidelines) by a full understanding of the geology at the site and using Reclamation s current state-of-practice risk analysis event tree. Event tree: Reservoir at or above threshold level Initiation Erosion starts Continuation - Unfiltered or inadequately filtered exit exists Progression Continuous stable roof and/or sidewalls Progression Constriction or upstream zone fails to limit flows Progression No self-healing by upstream zone Intervention fails Dam breaches

15 Willard Bay of Great Salt Lake Sta BACKGROUND Sta Willard Reservoir Wasatch Front Fault Reservoir Constructed by Bureau of Reclamation 1957 to Reclaimed land from Great Salt Lake Dam Length = 14.5 Miles Surface area 10,000 acres Reservoir operated by Weber Basin Water Conservancy District

16 Arthur V. Watkins Dam (Willard Bay Reservoir) Height: 36 feet maximum (El ) Length: 14.5 miles 17,060,000 cubic yards of material Offstream storage facility: 215,000 acre-feet

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18 Wasatch Front General Geology

19 Wasatch Front General Geology Dry bay of Great Salt Lake in historic times Former Glacial Lake Bonneville drained Late Pleistocene ( 14,500 yrs ago). Lake Bonneville deposits hundreds to thousands of feet thick, soft lacustrine clays, silts and sands. A.V. Watkins Dam built on Willard Bay.

20 A.V. Watkins Geologic Units Emb Embankment silty sand, silt, clay and roadbase/fill gravels Qsl Quaternary (Holocene) shoreline deposits consist of interbedded sand, silty sand, silt and clay. Youngest deposit which overlies the Qbs on east and southwest reaches of dam modification. Interbedded with alluvial fan deposits from Wasatcn Front C14 Age ~4K yrs Qbs Quaternary (Late Pleistocene) beach sands consist of silty sand (SM) and poorly graded sand (SP), overlie sandy silt (ML). Hardpan occurs in upper SP-SM layers of Qbs. Directly overlies Qbc in most of mod reach. C14 age = ~ K ybp Qbc Quaternary (Pleistocene) Bonneville Clay Ex Soft clay deposit of Glacial Lake Bonneville Zero blowcount material. Top of clay varies by several feet, so depth of wall varies to meet minimum penetration depth requirement. Pre-dam borings encountered pockets of swamp-gas, presumed to be methane. C14 age = 12.5 >15K ybp Hard-pan Shoreline or near-shore carbonate evaporite deposits which exist as layers in the Qsl related to recent historic levels of the GRL. Consists of sand cemented by calcium carbonate and iron. Generally a few tenths of a foot thick up to 3 feet - Upper hard-pan within 3 feet of ground surface - Lower hard-pan within 7-8 feet of ground surface

21 Hard-pan Fine sand and silt Silty sand and silt Bonneville Clay A.V. Watkins Dam General Cross-Section

22 Geologic/Engineering Challenges at the Dam Site (Given the state-of-practice now) - Low density non-plastic beach sand and silt for 6 miles of proposed foundation (Qsl and Qbs) - Beach sands are highly erodible and uniformly graded - Entire site founded on very soft, sensitive Bonneville Clays (in some areas underlying beach deposits and in other areas, the dike is founded directly on the Bonneville Clay - Bonneville Clay: 1. Unconsolidated lean to fat clay (CL,CH), organic (methane gas) and susceptible to significant settlements when loaded 2. Standard Penetration Test (SPT) blow counts = 0 3. CPT data and vane shear indicate sensitive clays (6-8) and low undrained strength ratios S u /s v = High groundwater table - Artesian water conditions on east side fed by alluvial fans of the Wasatch Front - Qsl and Qbs saturated and potentially liquefiable - Site within ¼ mile of Wasatch Fault - Site is susceptible to tilting downward to the east during a large seismic event leading to seiche wave overtopping

23 Geologic/Engineering Challenges at the Dam Site (Given the state-of-practice now) General Observations oat the time (early 1960s) the state-of-practice was much less evolved in particular: filter design and seismic design oengineers like a challenge othis dam site allows the capture of significant volumes of runoff that previously had to be passed through the upper reservoirs and into the Great Salt Lake onot all factors from previous slide considered in the design oin this case Ignorance was not BLISS!

24 Stages of Construction: 1. Sand & Gravel Dike, then fill 2. Zones 1, 2, & 3 up to elevation Zones 1, 3, and riprap facing to Maximum settlement approx. 18 ft.+ 5. Stage 4 Dam raised to El. 4235

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34 Early role of geology in the embankment performance

35 Early role of geology in the embankment performance Note open channel excavated to help drain Site for dam and agriculture known as South Drain

36 Early role of geology in the embankment performance Note open channel excavated to help drain Site for dam and agriculture known as South Drain

37 Early role of geology in the embankment performance Note open channel excavated to help drain Site for dam and agriculture known as South Drain

38 Early role of geology in the embankment performance Note open channel excavated to help drain Site for dam and agriculture known as South Drain First filling in 1964

39 Early role of geology in the embankment performance Note open channel excavated to help drain Site for dam and agriculture known as South Drain First filling in 1964 Quick conditions noted for thousands of feet at south section

40 Early role of geology in the embankment performance Note open channel excavated to help drain Site for dam and agriculture known as South Drain First filling in 1964 Quick conditions noted for thousands of feet at south section Decision made to install toe drain approx. 4 miles along southern length of dam (unfiltered open-jointed clay tile pipe)

41 Early role of geology in the embankment performance Note open channel excavated to help drain Site for dam and agriculture known as South Drain First filling in 1964 Quick conditions noted for thousands of feet at south section Decision made to install toe drain approx. 4 miles along southern length of dam (unfiltered open-jointed clay tile pipe) Dam filled successfully in 1965

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43 Safety Of Dams Project Overview Three Phases

44 Safety Of Dams Project Overview Three Phases Emergency Response Dam nearly failed during Nov. 13, 2006 incident Completed November 2006

45 Safety Of Dams Project Overview Three Phases Emergency Response Dam nearly failed during Nov. 13, 2006 incident Completed November 2006

46 Safety Of Dams Project Overview Three Phases Emergency Response Dam nearly failed during Nov. 13, 2006 incident Completed November 2006 Phase I Forensic geologic and geotechnical investigations Interim repairs made to embankment/toe drain to allow restricted storage to El (NWS = 4226) Completed May 2007

47 Safety Of Dams Project Overview Three Phases Emergency Response Dam nearly failed during Nov. 13, 2006 incident Completed November 2006 Phase I Forensic geologic and geotechnical investigations Interim repairs made to embankment/toe drain to allow restricted storage to El (NWS = 4226) Completed May 2007

48 Safety Of Dams Project Overview Three Phases Emergency Response Dam nearly failed during Nov. 13, 2006 incident Completed November 2006 Phase I Forensic geologic and geotechnical investigations Interim repairs made to embankment/toe drain to allow restricted storage to El (NWS = 4226) Completed May 2007 Phase II Design geologic investigations performed Permanent repairs made via CB Cutoff Wall and embankment reconstruction Completed in November 2008 (1 yr. ahead of schedule)

49 Safety Of Dams Project Overview Three Phases Emergency Response Dam nearly failed during Nov. 13, 2006 incident Completed November 2006 Phase I Forensic geologic and geotechnical investigations Interim repairs made to embankment/toe drain to allow restricted storage to El (NWS = 4226) Completed May 2007 Phase II Design geologic investigations performed Permanent repairs made via CB Cutoff Wall and embankment reconstruction Completed in November 2008 (1 yr. ahead of schedule)

50 Emergency Response Feed lot operator noticed piping into South Drain on Saturday November 11, 2006 Monday November 13 - he notifies Water District Monday evening November 13 - Foundation piping at AV Watkins Dam (Station ) observed by USBR and Water District staff Sand boils at toe of dam, sand deposits in South Drain, sinkholes, tension cracks and slope failures on downstream slope, transverse crack in crest Emergency response begins November 13 including plans to breach dam into Great Salt Lake if necessary Dam finally stabilized on November 18

51 Brigham City Willard Bay N. Marina I-15 A.V. Watkins Dam To SLC Location of Near Failure Event Incident Area Feed Lot South Drain Willard Canal Willard Bay S. Marina

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53 A.V. Watkins Dam November 2006 Failure Incident

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56 Estimated 400 cubic yards of eroded material in South Drain

57 Estimated 400 cubic yards of eroded material in South Drain

58 Emergency Response A.V. Watkins Dam- Initial Observations

59 Emergency Response A.V. Watkins Dam- Initial Observations Seepage was gpm- coming from 1 large sand boil at the toe and several smaller sand boils a short distance from toe

60 Emergency Response A.V. Watkins Dam- Initial Observations Seepage was gpm- coming from 1 large sand boil at the toe and several smaller sand boils a short distance from toe Small slump on d/s face with crest crackingtransverse and parallel to axis

61 Emergency Response A.V. Watkins Dam- Initial Observations Seepage was gpm- coming from 1 large sand boil at the toe and several smaller sand boils a short distance from toe Small slump on d/s face with crest crackingtransverse and parallel to axis Numerous sinkholes 2-5 in dia. between toe and South Drain- random pattern

62 Emergency Response A.V. Watkins Dam- Initial Observations Seepage was gpm- coming from 1 large sand boil at the toe and several smaller sand boils a short distance from toe Small slump on d/s face with crest crackingtransverse and parallel to axis Numerous sinkholes 2-5 in dia. between toe and South Drain- random pattern Reduction in seepage flow most likely resulted from collapse of erosion path roof and settlement of dam

63 Emergency Response A.V. Watkins Dam- Initial Observations Seepage was gpm- coming from 1 large sand boil at the toe and several smaller sand boils a short distance from toe Small slump on d/s face with crest crackingtransverse and parallel to axis Numerous sinkholes 2-5 in dia. between toe and South Drain- random pattern Reduction in seepage flow most likely resulted from collapse of erosion path roof and settlement of dam No seepage was coming from dam

64 A.V. Watkins Dam- Emergency Response

65 A.V. Watkins Dam- Emergency Response Immediate goal was to stop foundation erosion

66 A.V. Watkins Dam- Emergency Response Immediate goal was to stop foundation erosion Construction equipment and sand/gravel were available nearby

67 A.V. Watkins Dam- Emergency Response Immediate goal was to stop foundation erosion Construction equipment and sand/gravel were available nearby Decision was made within minutes to construct a filter and berm over the leak and surrounding area

68 A.V. Watkins Dam- Emergency Response Immediate goal was to stop foundation erosion Construction equipment and sand/gravel were available nearby Decision was made within minutes to construct a filter and berm over the leak and surrounding area Work was complicated by heavy rain and darkness

69 A.V. Watkins Dam- Emergency Response Immediate goal was to stop foundation erosion Construction equipment and sand/gravel were available nearby Decision was made within minutes to construct a filter and berm over the leak and surrounding area Work was complicated by heavy rain and darkness Declared EAP Response Level 1 but implemented most steps from Response Level 2

70 A.V. Watkins Dam- Emergency Response Immediate goal was to stop foundation erosion Construction equipment and sand/gravel were available nearby Decision was made within minutes to construct a filter and berm over the leak and surrounding area Work was complicated by heavy rain and darkness Declared EAP Response Level 1 but implemented most steps from Response Level 2 Stationed equipment on west dam (LOW hazard section) and instructed operator to breach the dam to allow harmless emptying into Great Salt Lake if conditions became critical at leak

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74 Erosion downstream not halted until upstream sinkholes covered with gravel and pit-run material

75 A.V. Watkins Dam- Forensics

76 A.V. Watkins Dam- Forensics Investigations included geophysics, drilling and test pits at site of leak and expanded from there

77 A.V. Watkins Dam- Forensics Investigations included geophysics, drilling and test pits at site of leak and expanded from there Examined toe drain w/ Video Crawler & trench

78 A.V. Watkins Dam- Forensics Investigations included geophysics, drilling and test pits at site of leak and expanded from there Examined toe drain w/ Video Crawler & trench u/s toe examined for sinkholes or other evidence of seepage entry

79 A.V. Watkins Dam- Forensics Investigations included geophysics, drilling and test pits at site of leak and expanded from there Examined toe drain w/ Video Crawler & trench u/s toe examined for sinkholes or other evidence of seepage entry Test pitting complicated by high water table, flowing sand, etc. Surface observations complicated by snow/ice cover

80 A.V. Watkins Dam- Forensics Investigations included geophysics, drilling and test pits at site of leak and expanded from there Examined toe drain w/ Video Crawler & trench u/s toe examined for sinkholes or other evidence of seepage entry Test pitting complicated by high water table, flowing sand, etc. Surface observations complicated by snow/ice cover Detailed temperature surveys made in South Drain and finger drains looking for seepage entrance points into South Drain

81 Trenches Excavated through Sinkholes and along downstream toe of dam

82 Open collapsed piping conduit in sand above lower hard-pan. Note iron-staining possibly indicates this feature formed over some period of time. photo by R. Pearson

83 Clean, well graded sand deposited in piping conduit after flow choked-off Exposed in sidewall of trench below upper hard-pan. R. Pearson

84 Emergent groundwater under pressure when hard-pan layer broken. photo R. Pearson

85 photo R. Pearson

86 Hard-pan layers form overhang fine piped sand deposited in South Drain. photo R. Pearson

87 Calcium Carbonate cemented sand forms layers of hard-pan Layers conform to historic (~past 1000 yr.) high levels of Great Salt Lake. Photo TSC Petro Lab

88 Failure Mode Conclusion and Geologic Factors Influencing the Failure Mode

89 Failure Mode Conclusion and Geologic Factors Influencing the Failure Mode Reservoir at or above threshold level

90 Failure Mode Conclusion and Geologic Factors Influencing the Failure Mode Reservoir at or above threshold level Yes Reservoir kept as full as possible year round

91 Failure Mode Conclusion and Geologic Factors Influencing the Failure Mode Reservoir at or above threshold level Yes Reservoir kept as full as possible year round Initiation Erosion starts

92 Failure Mode Conclusion and Geologic Factors Influencing the Failure Mode Reservoir at or above threshold level Yes Reservoir kept as full as possible year round Initiation Erosion starts Yes May have been due to animal burrowing and increased seepage gradient and Qls highly erodible at low seepage gradients (0.06)

93 Failure Mode Conclusion and Geologic Factors Influencing the Failure Mode Reservoir at or above threshold level Yes Reservoir kept as full as possible year round Initiation Erosion starts Yes May have been due to animal burrowing and increased seepage gradient and Qls highly erodible at low seepage gradients (0.06) Continuation - Unfiltered or inadequately filtered exit exists

94 Failure Mode Conclusion and Geologic Factors Influencing the Failure Mode Reservoir at or above threshold level Yes Reservoir kept as full as possible year round Initiation Erosion starts Yes May have been due to animal burrowing and increased seepage gradient and Qls highly erodible at low seepage gradients (0.06) Continuation - Unfiltered or inadequately filtered exit exists Yes South drain and open-jointed bell and spigot sewer pipe toe drain

95 Failure Mode Conclusion and Geologic Factors Influencing the Failure Mode Reservoir at or above threshold level Yes Reservoir kept as full as possible year round Initiation Erosion starts Yes May have been due to animal burrowing and increased seepage gradient and Qls highly erodible at low seepage gradients (0.06) Continuation - Unfiltered or inadequately filtered exit exists Yes South drain and open-jointed bell and spigot sewer pipe toe drain Progression Continuous stable roof and/or sidewalls

96 Failure Mode Conclusion and Geologic Factors Influencing the Failure Mode Reservoir at or above threshold level Yes Reservoir kept as full as possible year round Initiation Erosion starts Yes May have been due to animal burrowing and increased seepage gradient and Qls highly erodible at low seepage gradients (0.06) Continuation - Unfiltered or inadequately filtered exit exists Yes South drain and open-jointed bell and spigot sewer pipe toe drain Progression Continuous stable roof and/or sidewalls Yes Relatively continuous hard-pan layer at incident location

97 Failure Mode Conclusion and Geologic Factors Influencing the Failure Mode Reservoir at or above threshold level Yes Reservoir kept as full as possible year round Initiation Erosion starts Yes May have been due to animal burrowing and increased seepage gradient and Qls highly erodible at low seepage gradients (0.06) Continuation - Unfiltered or inadequately filtered exit exists Yes South drain and open-jointed bell and spigot sewer pipe toe drain Progression Continuous stable roof and/or sidewalls Yes Relatively continuous hard-pan layer at incident location Progression Constriction or upstream zone fails to limit flows

98 Failure Mode Conclusion and Geologic Factors Influencing the Failure Mode Reservoir at or above threshold level Yes Reservoir kept as full as possible year round Initiation Erosion starts Yes May have been due to animal burrowing and increased seepage gradient and Qls highly erodible at low seepage gradients (0.06) Continuation - Unfiltered or inadequately filtered exit exists Yes South drain and open-jointed bell and spigot sewer pipe toe drain Progression Continuous stable roof and/or sidewalls Yes Relatively continuous hard-pan layer at incident location Progression Constriction or upstream zone fails to limit flows Yes No u/s zone to limit flows

99 Failure Mode Conclusion and Geologic Factors Influencing the Failure Mode Reservoir at or above threshold level Yes Reservoir kept as full as possible year round Initiation Erosion starts Yes May have been due to animal burrowing and increased seepage gradient and Qls highly erodible at low seepage gradients (0.06) Continuation - Unfiltered or inadequately filtered exit exists Yes South drain and open-jointed bell and spigot sewer pipe toe drain Progression Continuous stable roof and/or sidewalls Yes Relatively continuous hard-pan layer at incident location Progression Constriction or upstream zone fails to limit flows Yes No u/s zone to limit flows Progression No self-healing by upstream zone

100 Failure Mode Conclusion and Geologic Factors Influencing the Failure Mode Reservoir at or above threshold level Yes Reservoir kept as full as possible year round Initiation Erosion starts Yes May have been due to animal burrowing and increased seepage gradient and Qls highly erodible at low seepage gradients (0.06) Continuation - Unfiltered or inadequately filtered exit exists Yes South drain and open-jointed bell and spigot sewer pipe toe drain Progression Continuous stable roof and/or sidewalls Yes Relatively continuous hard-pan layer at incident location Progression Constriction or upstream zone fails to limit flows Yes No u/s zone to limit flows Progression No self-healing by upstream zone Yes No u/s zone to cause self-healing

101 Failure Mode Conclusion and Geologic Factors Influencing the Failure Mode Reservoir at or above threshold level Yes Reservoir kept as full as possible year round Initiation Erosion starts Yes May have been due to animal burrowing and increased seepage gradient and Qls highly erodible at low seepage gradients (0.06) Continuation - Unfiltered or inadequately filtered exit exists Yes South drain and open-jointed bell and spigot sewer pipe toe drain Progression Continuous stable roof and/or sidewalls Yes Relatively continuous hard-pan layer at incident location Progression Constriction or upstream zone fails to limit flows Yes No u/s zone to limit flows Progression No self-healing by upstream zone Yes No u/s zone to cause self-healing Intervention fails

102 Failure Mode Conclusion and Geologic Factors Influencing the Failure Mode Reservoir at or above threshold level Yes Reservoir kept as full as possible year round Initiation Erosion starts Yes May have been due to animal burrowing and increased seepage gradient and Qls highly erodible at low seepage gradients (0.06) Continuation - Unfiltered or inadequately filtered exit exists Yes South drain and open-jointed bell and spigot sewer pipe toe drain Progression Continuous stable roof and/or sidewalls Yes Relatively continuous hard-pan layer at incident location Progression Constriction or upstream zone fails to limit flows Yes No u/s zone to limit flows Progression No self-healing by upstream zone Yes No u/s zone to cause self-healing Intervention fails NO Intervention was successful

103 A.V. Watkins Dam - Failure Mode

104 A.V. Watkins Dam - Failure Mode

105 Geologic Factors Influencing the Failure Mode Lacustrine deposits of highly erodible, low density, fine sand and silt in the foundation Fine sand and silt were very uniform.low probability of self filtering within coarser zone Unfiltered exit at South Drain (man-made) Presence of continuous hard-pan layers capable of forming a roof for a backwards erosion piping conduit Low density foundation soils capable of large settlements

106 A.V. Watkins Dam Interim Modifications for Restricted Storage District needed to store as much water as possible in order to support community needs (3 feet of water is worth approximately $50 million) Interim construction measures include: Enlarging the upstream berm to remove reservoir from the incident area Installing a modern toe drain and monitoring system within the incident area Allow for restricted filling to 4214 under 24-hour monitoring (4226 is top of normal pool)

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109 A.V. Watkins Dam Dam Repair for Full Operations Permanent Repair allows for facility to be put back into full service by reducing risk of static piping failure below Reclamation SOD guidelines Reconstruct embankment in incident area Stop / cut off / intercept seepage and piping paths Corrective Action-Design Investigations SPT, Test Pits and CPT over 6 mile reach

110 A.V. Watkins Dam Geologic Factors Impacting Design Considerations

111 A.V. Watkins Dam Geologic Factors Impacting Design Considerations Erodible silts and fine sands

112 A.V. Watkins Dam Geologic Factors Impacting Design Considerations Erodible silts and fine sands - Reduce seepage gradients at d/s toe and South Drain

113 A.V. Watkins Dam Geologic Factors Impacting Design Considerations Erodible silts and fine sands - Reduce seepage gradients at d/s toe and South Drain - Provide filtered exits for all locations (toe, South Drain, and area in between)

114 A.V. Watkins Dam Geologic Factors Impacting Design Considerations Erodible silts and fine sands - Reduce seepage gradients at d/s toe and South Drain - Provide filtered exits for all locations (toe, South Drain, and area in between) - Preferred alternative should address the possibility of existing voids from undiscovered piping erosion

115 A.V. Watkins Dam Geologic Factors Impacting Design Considerations Erodible silts and fine sands - Reduce seepage gradients at d/s toe and South Drain - Provide filtered exits for all locations (toe, South Drain, and area in between) - Preferred alternative should address the possibility of existing voids from undiscovered piping erosion - Dewatering very difficult due to low permeability

116 A.V. Watkins Dam Geologic Factors Impacting Design Considerations Erodible silts and fine sands - Reduce seepage gradients at d/s toe and South Drain - Provide filtered exits for all locations (toe, South Drain, and area in between) - Preferred alternative should address the possibility of existing voids from undiscovered piping erosion - Dewatering very difficult due to low permeability

117 A.V. Watkins Dam Geologic Factors Impacting Design Considerations Erodible silts and fine sands - Reduce seepage gradients at d/s toe and South Drain - Provide filtered exits for all locations (toe, South Drain, and area in between) - Preferred alternative should address the possibility of existing voids from undiscovered piping erosion - Dewatering very difficult due to low permeability Hard Pan Layers

118 A.V. Watkins Dam Geologic Factors Impacting Design Considerations Erodible silts and fine sands - Reduce seepage gradients at d/s toe and South Drain - Provide filtered exits for all locations (toe, South Drain, and area in between) - Preferred alternative should address the possibility of existing voids from undiscovered piping erosion - Dewatering very difficult due to low permeability Hard Pan Layers - Break up hard pan if possible

119 A.V. Watkins Dam Geologic Factors Impacting Design Considerations Erodible silts and fine sands - Reduce seepage gradients at d/s toe and South Drain - Provide filtered exits for all locations (toe, South Drain, and area in between) - Preferred alternative should address the possibility of existing voids from undiscovered piping erosion - Dewatering very difficult due to low permeability Hard Pan Layers - Break up hard pan if possible - Provide Hydraulic pressure relief below lower hard pan layer

120 A.V. Watkins Dam Geologic Factors Impacting Design Considerations Erodible silts and fine sands - Reduce seepage gradients at d/s toe and South Drain - Provide filtered exits for all locations (toe, South Drain, and area in between) - Preferred alternative should address the possibility of existing voids from undiscovered piping erosion - Dewatering very difficult due to low permeability Hard Pan Layers - Break up hard pan if possible - Provide Hydraulic pressure relief below lower hard pan layer

121 A.V. Watkins Dam Geologic Factors Impacting Design Considerations Erodible silts and fine sands - Reduce seepage gradients at d/s toe and South Drain - Provide filtered exits for all locations (toe, South Drain, and area in between) - Preferred alternative should address the possibility of existing voids from undiscovered piping erosion - Dewatering very difficult due to low permeability Hard Pan Layers - Break up hard pan if possible - Provide Hydraulic pressure relief below lower hard pan layer Concerns for soft Bonneville Clay in the foundation

122 A.V. Watkins Dam Geologic Factors Impacting Design Considerations Erodible silts and fine sands - Reduce seepage gradients at d/s toe and South Drain - Provide filtered exits for all locations (toe, South Drain, and area in between) - Preferred alternative should address the possibility of existing voids from undiscovered piping erosion - Dewatering very difficult due to low permeability Hard Pan Layers - Break up hard pan if possible - Provide Hydraulic pressure relief below lower hard pan layer Concerns for soft Bonneville Clay in the foundation - Minimize additional fill that could lead to settlement (especially

123 A.V. Watkins Dam - Alternatives

124 A.V. Watkins Dam - Alternatives Design Options:

125 A.V. Watkins Dam - Alternatives Design Options: 1. U/S Cutoff Wall

126 A.V. Watkins Dam - Alternatives Design Options: 1. U/S Cutoff Wall 2. Cutoff Wall through Crest

127 A.V. Watkins Dam - Alternatives Design Options: 1. U/S Cutoff Wall 2. Cutoff Wall through Crest 3. Filter / drainage blanket at D/S toe

128 A.V. Watkins Dam - Alternatives Design Options: 1. U/S Cutoff Wall 2. Cutoff Wall through Crest 3. Filter / drainage blanket at D/S toe 4. Filter intercept trench/keyway between toe and South Drain

129 A.V. Watkins Dam - Alternatives Design Options: 1. U/S Cutoff Wall 2. Cutoff Wall through Crest 3. Filter / drainage blanket at D/S toe 4. Filter intercept trench/keyway between toe and South Drain 5. Combinations of above alternatives

130 A.V. Watkins Dam - Alternatives Design Options: 1. U/S Cutoff Wall 2. Cutoff Wall through Crest 3. Filter / drainage blanket at D/S toe 4. Filter intercept trench/keyway between toe and South Drain 5. Combinations of above alternatives

131 A.V. Watkins Dam - Alternatives Design Options: 1. U/S Cutoff Wall 2. Cutoff Wall through Crest 3. Filter / drainage blanket at D/S toe 4. Filter intercept trench/keyway between toe and South Drain 5. Combinations of above alternatives

132 A.V. Watkins Dam - Alternatives Design Options: 1. U/S Cutoff Wall 2. Cutoff Wall through Crest 3. Filter / drainage blanket at D/S toe 4. Filter intercept trench/keyway between toe and South Drain 5. Combinations of above alternatives

133 A.V. Watkins Dam - Alternatives Design Options: 1. U/S Cutoff Wall 2. Cutoff Wall through Crest 3. Filter / drainage blanket at D/S toe 4. Filter intercept trench/keyway between toe and South Drain 5. Combinations of above alternatives

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135 4K yrs 15K yr

136 Detailed geologic investigations to define limits of cutoff wall (drill holes, CPT, SPT) 4K yrs 15K yr

137 Detailed geologic investigations to define limits of cutoff wall (drill holes, CPT, SPT) 4K yrs 15K yr

138 Detailed geologic investigations to define limits of cutoff wall (drill holes, CPT, SPT) 4K yrs 15K yr East end of CB cutoff wall Sta

139 Detailed geologic investigations to define limits of cutoff wall (drill holes, CPT, SPT) 4K yrs 15K yr East end of CB cutoff wall Sta

140 Location Map

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142 Cement-Bentonite Cutoff Wall Contract Awarded May 28, 2008 Geo-Solutions, Inc. (Slurry Production) New Kensington, PA Nordic Industries Marysville, CA $17.4 Million

143 July 2008

144 Baker tank contained 12,000 gallons of water, Super-sack of bentonite = 3,000 lbs. 1 tank water for every 2 bags bentonite.

145 Bentonite Slurry Mix (by weight of water): 6% Sodium Montmorillonite Bentonite API 13A section 9 Baker tank contained 12,000 gallons of water, Super-sack of bentonite = 3,000 lbs. 1 tank water for every 2 bags bentonite.

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147 Cement-Bentonite Slurry Mix (by weight of water): 6% Bentonite 18% Cement (Type V) Admixtures (as per manuf. Recommendations)

148 Lignosulfonate added continuously to cement-bentonite as a plasticizer and set retarder A highly anionic polymer used to deflocculate clay-based muds. Lignosulfonate is a byproduct of the sulfite method for manufacturing paper from wood pulp. Sometimes it is called sulfonated lignin. Lignosulfonate is a complex mixture of small- to moderate-sized polymeric compounds with sulfonate groups attached to the molecule.

149 Lignosulfonate added continuously to cement-bentonite as a plasticizer and set retarder A highly anionic polymer used to deflocculate clay-based muds. Lignosulfonate is a byproduct of the sulfite method for manufacturing paper from wood pulp. Sometimes it is called sulfonated lignin. Lignosulfonate is a complex mixture of small- to moderate-sized polymeric compounds with sulfonate groups attached to the molecule.

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151 At full daily production (2 shifts): 350 tons cement 115 tons bentonite 340,000 gallons water CB slurry supply rate = 1 to 2.2 cubic yards / min.

152 Komatsu PC1250 Excavator with custom built boom, automatic lubrication system, and oversized hydraulic cylinders Curbside weight = 265,000 lbs

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158 Cut 5 feet (minimum) into CB from previous day in order to prevent windows in wall

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161 Qbs/Qbc interface

162 Qbs/Qbc interface

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165 Cost per day up to $200,000 Cost per week over $1 Million

166 Typical production (double shifts): 320 to 480 linear feet per day (16,000 to 22,000 ft 2 per day) Cost per day up to $200,000 Cost per week over $1 Million

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169 November 21, 2008

170 Wall completed! 6 miles long 1.57 Million ft 2 Permeability (28 day) = 5.8 x 10-6 cm/sec Strength (28 day) = 12 psi November 21, 2008

171 Section at Incident Area

172 Bio-Polymer Slurry 550 gallons Water 25 lbs Natural Guar 2 Quarts Soda Ash 1/3 Quart Synthetic Guar Recirculate in tank until MFV and ph stabilizes

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181 Incident area reconstruction

182 Reservoir May 8, 2009 Elevation: (1.5 feet from full)

183 A.V. Watkins Dam - Summary Geologic factors made all the difference in the development of the failure mode: Erodible foundation soils (erodible under very small seepage gradients) Unfiltered seepage exit Roof-forming unit (hardpan) Geologic Factors are Critical in determination of the Limits of the SOD Modification C-B Cutoff Wall Understanding geology key to defining limits of Qbs deposit to define length of modification Understanding geology key to defining depth to Qbc deposit to determine depth for cutoff Understanding geology key to developing specifications to define hard-pan layers and difficulty of excavation, dewatering.

184 Questions?