The Quail Creek Dike Failure

Transcription

1 The Quail Creek Dike Failure (Lessons Not Learned) AEG Shlemon Specialty Conference Dam Foundation Failures and Incidents Denver, CO May 2013 Douglas D. Boyer, PE, CEG US Army Corps of Engineers Risk Management Center

2 Acknowledgement Mr. Larry Von Thun Retired Bureau of Reclamation Served as a member of the independent panel for the failure of the dike

3 Before The Quail Creek Reservoir was formed by Quail Creek Dam and Quail Creek Dike near the town of Saint George in South Western Utah near the Nevada border. This picturesque, arid, country basked in the beauty of this reservoir that stored water from Quail Creek just before it flowed into the Virgin River.

4 and After But on New Years Eve 1988 that beauty became a beast as the dike began to fail and the resulting breach outflow striped the ground bare for miles causing millions of dollars in damage. The failure date is given as Jan 1, 1989 as the breach occurred just after midnight on New Years Eve. The estimates were that the breach effects extended 89 miles downstream and that there was over $12 million in damages 1.

5 LESSONS not LEARNED This presentation is titled Lessons not Learned - in order to highlight the proximity of this Dike s design and construction in terms of both time and location to Teton Dam. The design and construction of this dike took place in the period 7-9 years after the failure of Teton Dam. The lessons relative to internal erosion, foundation preparation and foundation inspection garnered from the Teton Dam failure were well known by that time. Utah is a neighboring state to Idaho and the failure of Teton Dam and the lessons from its failure should have been ever present in their minds. But these basic lessons were not learned and failure occurred because they were not heeded. The good news was that there was no life loss, despite the complications to warning resulting from the New Year being rung in juxtaposed with sirens and other warnings going out. The other fortunate thing from a technical evaluation standpoint was that unlike so many dam failures all the evidence was not washed away with the breach. This is not because the breach was partial. As can be seen here the breach area was stripped bare to the foundation.

6 Dike Cross Section However, the failure mechanism development process was being replicated and was preserved in the adjoining potions of the dam. Further the instrumentation at the site recorded the hydrogeologic process going on in the dam and foundation during the grouting remediation and provided the necessary information to gain insight on the failure causes.

7 First Filling The potential for trouble at the site was evident upon the first filling of the reservoir. Water came pouring from the foundation as can be seen here. In a unwitting and unintentional manner, as we shall see later, the efforts to reduce this flow ultimately contributed to the failure of the dike. But we will start at the beginning with the design, move into construction and then finally into the performance, watching along the way for the lessons from Teton Dam that were not heeded.

8 St. George, Utah

9 Quail Creek Reservoir

10 Geology Sandstone with interbedded gypsiferous siltstones, gypsum, and dolomite (Moenkopi Fm) Bedding dips 5 to 25 degrees (into left abutment) Highly fractured with some open joints Soft to hard beds results in hogback appearance As indicated on this slide the rock at the foundation level at this site was primarily sandstone with interbedded gypsiferous siltstones, gypsum, and dolomite. The bedding dipped 5 to 25 degrees (into the left abutment). The rock was quite weathered and highly fractured with some very open joints. This description is a great simplification of the complexity of the geology on both the regional scale and the local scale.

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14 Bedrock Characteristics This slide of the upper portion of the foundation within the breach erosion zone gives insight as to the condition of the weathered bedrock that formed the dam foundation.

15 Dike Statistics Height = 78 ft Crest length = 1,980 ft Crest width = 20 ft Approximately 40,000 acre-ft of storage Constructed 1984 Reservoir began filling in 1985 The dike was almost 80 feet high at its maximum section and nearly 2000 feet long. In retrospect calling this structure a dike was not appropriate, as that designation may have resulted in the designer, the inspectors, the regulators, and the contractor considering it less important and thus apply less in the way of defensive design, give less attention to the inspection and take shortcuts or make allowances that made the dam less safe. When the structure was rebuilt it was called Quail Creek South Dam.

16 Cross section This slide shows the design cross section for the dike. (go over each of the zones) The failure mechanism of piping by internal erosion was definitely recognized and understood by the time of the failure of Teton Dam in However, the means to recognize when a piping and seepage erosion failure may be possible and how to defend against its occurrence was not everywhere understood and recognized by practicing engineers and dam regulators. That situation changed considerably after the failure of Teton Dam. The lesson from that failure resulted in: Piping becoming a commonly recognized threat to dam safety by all. Greater attention to filters. The recognition of, importance of, need for and use of filters to protect the impervious core of dams became the practice. Foundation treatment to ensure that material could not be piped through joints or other discontinuities in the foundation and foundation inspections and approval for placement by a geologist and the design engine became the norm. Further, the piping mechanism, although previously understood and discussed in theory by educators and leading geotechnical engineering consultants it now became de-mystified and was able to be described recognized - understood and defended against in very practical terms. For piping by backward erosion leading to dam failure / loss of reservoir to occur, the following three simply stated conditions had to exist: There had to be a flow path from the reservoir to an unfiltered free exit. The erodibility of the material at that free exit point of the flow path combined with the seepage force (gradient) of the flow emerging at that exit point had to be great enough to remove and carry away a soil particle in order to begin the process of backward erosion. The soil being piped (or something directly above the soil) had to support a pipe or opening in the soil so that the erosion could progress back to the reservoir. Although these steps have now been extended and developed in piping / seepage erosion event tress to cover a number of subtleties and variations in piping / seepage erosion mechanisms the basic piping process described above, after the Teton Dam failure should have been commonly understood by dam designers and to allow piping to be defended against in one or more ways. This design cross section illustrates that right from the get go in design these piping lessons while generally recognized were not fully understood and defended against. So although the potential failure mode of piping through the dam was protected against, the failure mode of piping at the dam foundation contact through the cutoff trench was not. This latter potential piping failure mode is evident in the desig cross section. The Zone I (Sandy Silt and Silty Sand) was erodible and could support a pipe. The foundation key trench extended through open, highly permeable highly weathered rock. The trench was protecte on the bottom with a layer of Zone II which was a very good, plastic, non-erodible, quite impermeable clay. This clay was called the purple clay. However, the sides of the cutoff trench were neither filtered nor protected with the clay. Thus, a flow path from the reservoir into the Zone III, into the weathered rock and then through the key trench (Zone 1) with a free exit for Zone I into the weathered rock and / or into the Zone III. This would allow for tunnel formation at the dam foundation contact leading to seepage erosion, expansion of the tunnel and collapse of the dam. This mode was very similar to the actual failure mode f the dam and had other causative actions not taken place, which will be explained in detail in this presentation, this design flaw could have ultimately resulted in the failure of the dike. Now let s move to the geology of the foundation for the dam and its preparation.

17 Foundation Preparation Provisions 1-foot of foundation stripping 10-foot-deep cutoff trench through weathered rock 200-foot-long triple-line grout curtain through left abutment sandstone No dental concrete or slush grouting required except for sandstone area This slide lists the work called for relative to foundation preparation: 1-foot of foundation stripping (indicative of this arid environment and lack of vegetation and soil development) A 10-foot-deep cutoff trench through the weathered rock (the intent was not to reach un-weathered rock) just to get down a ways to what might be considered better un-weathered rock on which to place the fill. Provide a 200-foot-long triple-line grout curtain through left abutment sandstone (thought to have more open jointing based on observation of the sandstone outcrops) No dental concrete or slush grouting required except for sandstone area

18 Dam Excavation This slide shows how the foundation looked when excavation was completed. The foundation either had been approved for fill placement or was ready for approval and then fill placement. The foundation rock was not washed off and cleaned for inspection. It is reasonable to consider that the weathered rock (as seen in the previous picture) had been broken, consolidated and filled in to form an acceptable looking base on which to place earthfill. When in reality a geologic Pandora s box that had never been examined by trenching lay under the surface (it was revealed to a degree upon first filling). The lesson not well followed at Quail Creek Dike that was widely adopted after Teton Dam Failure of performing foundation approvals before fill placement, had been given short shrift here by virtue of inadequate foundation preparation and cleanup for proper inspection. Likely the consideration that this was just a dike played a role in the apparent nonchalance in the lack of serious investigation of the foundation and in the lack of preparations for approving the foundation.

19 This schematic shows what the 10 foot of excavation requirement may have actually looked like when it was carried out. The contractor, concerned about the rolling topography in profile would have tended to cut out less in the valleys with rock more susceptible to weathering and relatively more in the more erosion resistant bedrock highs in order to smooth the bedrock profile. Although this portrayal of the probable excavation as well as the above discussions on foundation cleanup and approval was not documented via measurements or records, this characterization described was made by the Investigation Board 3 after discussions with representatives of the dam owner, the dam designer and construction inspector and the contractor.

20 Now we come to the most astonishing aspect of this project with regard to the lack of understanding and guarding against piping and internal erosion. What was allowed as a convenience to the contractor during construction actually invited a piping failure to occur. The gross error in judgment made can only indirectly be called a lesson not learned from the Teton Dam failure because what was done here was not done at Teton nor anywhere else to my knowledge. As we have just seen the topographic profile consisted of a series of small hills and valleys rather than a relatively uniform decent to the valley floor that is commonly experienced. Because of these elevation irregularities along the profile, the contractor requested permission to fill the local valleys in the right side and central portion of the dam with the Zone 1 material in order to create a smoother foundation profile on which to begin placement of the various material zones in a continuous manner - cross valley as is the general practice. This slide shows this fill being placed in the intermediate valleys. The left abutment can be seen in the background. The leveling course as placed created an artificial foundation as shown in the next slide.

21 The dam design cross section in these intermediate valley sections now contained an: unprotected, unfiltered Zone 1 layer extending from upstream to downstream consisting of erodible sandy silt and silty sand with enough plasticity to hold a pipe.

22 Dike Remnant Investigations The evidence of the vulnerability of this zone to piping was readily exposed when the remaining dam on either side of the breach was excavated and trenched, as shown in this shot. The zones of the dam can readily be made out. The trenches in the slide running both parallel and transverse to the axis were within the artificial foundation and were replete with piping channels, as shown in the next photo.

23 Piping Channel in Zone 1 FOUNDATION? This oval shaped opening about 10 inches long and 2-3 inches high in the bottom portion of the picture is a shot of a typical piping channel. Some were large and some were smaller. The forensic excavation was made on either side of the breach. When the downstream face of the remnant on the left side was peeled back and exposed the face of the Zone 1 foundation there were several dozen of these channels visible all at once. Getting to see these pipes and make a close examination of them was an incredible experience after having studied, discussed and preached on their potential for development on a number of dams for several years. It is safe to say that what we were able to witness in the Quail Creek Dike forensic investigation was remarkable and instructive. It allowed strong visual verification and unequivocal documentation of the backward erosion piping process that we formulate in failure mode descriptions. Further, it allowed a definitive finding on the failure mechanism at Quail Creek Dike as opposed to the multiple failure mode scenarios and theories where the direct evidence is often washed away. Multiple theories on failure often result from the forensic evaluation of a dam failure, the evidence we saw plus the instrumentation results that we will now discuss allowed a very solid case to be made on all aspects of the failure mode.

24 Dam Performance Seepage, Piezometric Response, and Remediation Activities The performance of the dam is best evaluated by examining the interrelationship of the leakage through the dam s foundation, the attempts to reduce or shut that leakage off by grouting and the resulting response in the piezometers in the dam and foundation. It may be noted that avoidance of water loss was important for Washington County who were the dam owners.

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26 About 5 cfs Leakage as noted previously, foundation leakage commenced during first filling with a large portion of the captured portion of the leakage emerging from the sandstone foundation of the left abutment. The foundation was in general quite porous as we saw in the earlier photograph and thus a considerable amount of water could have been entering the foundation rock, drop in elevation and flow downstream without being visible at the surface (not emerging until it reached the Virgin River).

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30 This graph shows the measured seepage versus reservoir level during the first season of filling ( ). By the time the reservoir level had reached about 80-85% of normal maximum the dike seepage had reached 7cfs, which was excessive, well beyond expectations. Reservoir filling stopped (probably mid April 1986 and drawdown and foundation grouting commenced around mid May 1986). In addition to the seepage flowing out of the sandstone there were a few localized gusher locations just beyond the downstream toe. Due to this toe seepage two actions were taken both in direct conflict with piping lessons learned following the Teton Dam failure. The first was that a trench about 5 feet deep was dug along the toe and filled with sewer rock. This rock was clean (no fines) sized from about 1 ½ inches to 3 inches in diameter points, This trench drained to the valley thalweg. This trench provided a free exit for material to be piped into and not only that, the flow into the exit was hidden from view. It was ultimately dirty water bubbling out of the top of the sewer rock that alerted the Washington County Director that a failure was underway. Also, horizontal drains were installed at the toe of the dam towards the upstream to capture and control the foundation seepage. These drains were 4 ½ inch diameter pipes and were not filtered in any way.

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36 The time line plot showing reservoir level and grouting period shows that most of the grouting took place with a nearly full reservoir and the evidence of grout in the rock well downstream showed up in the path scoured by the breach flow as shown in this slide. This was to be expected. What was feared but not necessarily expected was the evidence of grout throughout the dam remnant. The foundation grouting that had been done by drilling through the dam had found pathways back into the dam, fractured the dam and filled seams in the dam with grout nearly to the crest of the dam.

37 Grout in the Rock Downstream and in the Dam The time line plot showing reservoir level and grouting period shows that most of the grouting took place with a nearly full reservoir and the evidence of grout in the rock well downstream showed up in the path scoured by the breach flow as shown in this slide. Grout travel in the foundation towards the downstream would be expected as it would be pushed downstream by the seepage water. What might have been feared but not necessarily expected was the evidence of grout throughout the dam remnant. The foundation grouting that had been done by drilling through the dam had found pathways back into the dam, fractured the dam and filled seams in the dam with grout nearly to the crest of the dam.

38 Throughout this entire time period of filling and refilling and then reservoir held nearly full, the dam itself except for the upstream shell apparently remained high and dry. The 10 foot thick purple clay layer was quite impervious, tight enough such that there was probably little progression of the phreatic surface through it in the two years that the reservoir was held up. Remnant excavation showed no sign that any water ever entered the dam itself. All the seepage was through the foundation until the grouting in the foundation forced the phreatic surface in the foundation up into the artificial zone 1 foundation. The higher pressures in this material created gradients which allowed piping initiation. This process is illustrated in the following slides of the piezometric response in the foundation.

39 One piezometer (Piez. 18) was about 15 feet into the foundation rock at the toe and another (Piez. 16) was at the same elevation (2900 ft) and located about 40 feet downstream of the axis. A third (Piez. 17) was also located 40 feet downstream of the axis but at an elevation about 5 feet above the cutoff trench. The pressures in Piezometer 17, although it may have been located in just a bit of foundation bedrock rock at station 7+50 can be considered representative of the pressures (at elevation 2925 ft.) that would have been at the contact of the artificial Zone 1 and would likely have penetrated several feet within it.

40 This is illustrated in the typical cross section through the area of breach. Note the location of elevation 2925 with respect to the Zone 1 leveling fill.

41 This slide clearly shows the pattern of the development and change of pressures in the foundation rock and the artificial foundation zone as a function of the foundation grouting and the onset piping. Because of local variations in foundation conditions, amount of leakage and grouting effects and due to timing pressures could have been higher or lower in adjacent areas, however th general pattern captured by these piezometers illustrates a response that makes sense. The timing of the effects also likely varied as different areas were grouted at different times. The 2 nd grouting began to take effect in this reach in October of 1987 and the pressures in the foundation rock near the axis (Piez. 16) gradually rose for a year due to the grouting, gaining about 40 feet in head a substantial amount. The pressures at or in the Zone 1 at the foundation contact also rose but to a somewhat lesser degree because of its higher elevation (gaining 20 feet in head between June and August once the actual water level had reached elevation 2925 ). This increase in head obviously established a high gradient between this piezometric location and the toe. This is directly demonstrated by the response of piezometer 18 at the toe (recall it is under 15 feet of rock ) which shows a continual rise in pressure during the entire period up to the point of failure with the gradient between Piezometer 16 and 18 reaching a maximum of over 40 feet of head loss over a distance of 100 feet in July of The gradient between Piezometer 17 and the toe (elevation 2913 with 0 pressure head) would also have been comparably high. The head in Piez. 17 reaching a high of 2942 feet in July gives head differential of 29 feet over a distance of about 120 feet. Considering that there is a free exit in the 5 foot deep toe trench results in another 5 feet of head differential. A gradient / head lo of 34 feet over 120 feet in this silty sand, sandy silt material certainly seems great enough to initiate piping and of course we know it did. The piezometric response in the piezometers also illustrates a commonly preached insight. That is the potential development of and the initiation of piping may be indicated by either rising piezometric levels or falling levels. The response of Piezometers 16, 17 and 18 show that piping channels had probably developed in earnest back to near the axis by August of At that time the pipes provided pressure relief and the pressures at the axis began to drop while increase flow cause the pressures at Piez 18. to continue to rise. To me this was an remarkable instrumentation portrayal and documentation of a concept that is discussed in principal but rarely able to be seen.

42 This slide shows this gradient change with time in cross section. Another lesson not learned at Quail Creek Dike was that instrumentation can tell us what is going on and for an embankment dam it needs to be actively read and interpreted by personnel familiar with piping and seepage dam failure mode mechanisms.

43 December 30, 1988 Seepage measurements taken Visual observations made All is well

44 December 31, 1988

45 December 31, :00 am observation all is well

46 December 31, :00 am observation all is well 10:00 am observation all is well

47 December 31, :00 am observation all is well 10:00 am observation all is well 10:30 am observation New seepage area 200 to 300 gpm Brown-reddish coloring in water

48 Decisions Made Build a graded filter over the seepage area Channelize seepage into downstream flume for monitoring Monitor seepage Call design engineer

49 12:00 pm Seepage steady Beginning to move small amount of sand and some gravel

50 1:30 pm Filter constructed Seepage still about gpm No movement of materials through filter Slight discoloration of seepage Monitor for changes

51 4:30 pm Slight increase in seepage Extend filter to capture growing/migrating seepage area Continue monitoring

52 8:20 pm Seepage increasing significantly Not enough material on-site Sound of concern

53 Decisions Made Call design engineer Attempted to contact state personnel Contact County Emergency Management Contact contractor and material supplier

54 8:30 pm More and larger equipment on-site More material delivered to site

55 10:00 pm Equipment and materials not making progress Seepage continues to increase significantly

56 10:30 pm Equipment and personnel removed from area due to concern for safety and impending failure

57 Decisions Made Contact County Emergency Management Contact state personnel Contact police Close downstream highway Begin downstream evacuation in inundation area

58 11:00 pm Embankment starts to fail

59 Midnight Embankment breaches

60 Damage No loss of life Significant damage to property and livestock Roads and bridges washed out Some homes with minimal inundation Utility lines washed out

61 Quail Creek Dike Piping Failure Around mid-day on Dec. 31 reddish brown seepage water could be seen in and then began emerging from the toe trench and at new seepage locations. Although attempts to control and contain the increasing seepage with filter material were made, by 8:30 dam failure appeared imminent and warnings went out. As water emerged and erosion of the toe occurred the shell of the dam began collapsing into the eroded area and that material was carried away by the ever increasing discharge from the tunnel under the dam.

62 What Happened?

63 Failure Mode Based on the evidence gathered and the witnessing of the failure development this failure mode was formulated. High leakage / seepage flow occurred through the foundation of the dam. This flow entered upstream and dropped in elevation as it proceeded under the da leaving the dam cross section above the cutoff essentially unaffected. (It is conceivable that this leakage could have stabilized with time but it is also possible th it could have eroded out a high capacity tunnel through the foundation that would resulted in loss of the reservoir.) A free exit toe trench was dug at the toe of the dam and filled with coarse rock, increasing the gradient and providing a free exit for escape of soil material. This toe trench exacerbated the free exit condition that already existed. The potential failure mode existed without it and it is likely that failure would have resulted even if the toe trench had not been built. However, the likelihood of intervention and avoidance of failure would have increased without it. Grouting the foundation forced the water level up to the Zone 1 leveling material and the resulting high pressures provided a gradient through this mate that was great enough to initiate backward erosion piping in the Zone 1. Backward erosion piping continued through the Zone 1leveling material until it reached the reservoir. It appears that it took on the order of 6-9 months for pipes to go from the toe to the reservoir through this material. Seepage erosion along the pipe in the Zone 1 leveling material once it reached the reservoir rapidly expanded the tunnel and increased the discharge. upstream downstream continuity of this fill material and its relative erodibility allowed this development to occur very rapidly and preclude intervention. The dam collapsed into the eroded toe and the high discharge was able to carry away the collapsing material until a through breach occurred.

64 Lessons Not Learned Need to understand the concepts behind and piping and seepage erosion failure modes Need for appreciation of geologic conditions and their impact on the seepage design features Need for good Foundation cleanup prior to foundation approval Need for foundation filters or other foundation treatment at critical contacts of embankment and foundation Lessons not learned -- Need to understand the concepts behind and piping and seepage erosion failure modes Need for appreciation of geologic conditions and their impact on the seepage design features Need for good Foundation cleanup prior to foundation approval Need for foundation filters or other foundation treatment at critical contacts of embankment and foundation

65 Remnant cross section for illustration purposes if needed while discussing lessons

66 Lessons Not Learned Do not use grout to treat a potential piping problem Do not use drain trenches at the toe of the dam without providing appropriate filtering capacity Read, Review and Interpret Instrumentation Need to not relax criteria and diligence on the design and construction of a structure on the basis of designation (dike versus dam) Lessons not learned Do not use grout to treat a potential piping problem Do not use drain trenches at the toe of the dam without providing appropriate filtering capacity Read, Review and Interpret Instrumentation Need to not relax criteria and diligence on the design and construction of a structure on the basis of its designation (dike versus dam)

67 References Carlson, D.D. and Meyer, D.F., Flood on the Virgin River, January 1989, in Utah, Arizona, and Nevada Von Thun, J.L., The Quail Creek Dike Failure, Annual Lecture Series No 10 Foundations of Dams, New Orleans, LA, 1990 Investigation of the Cause of the Failure of Quail Creek Dike, Report of Independent Review Team (Robert James, J. Lawrence Von Thun, Alan O Neil, Richard Catanach), 1989 Final Design Report Quail Creek Dam and Dike; Rollins, Brown and Gunnell, Inc; August, 1983

68 Thank You