REPORT TO US. DEPARTMENT OF THE INTERIOR AND STATE OF IDAHO FAILURE OF TETON DAM INDEPENDENT PANEL TO REVIEW CAUSE OF TETON DAM FAILURE DECEMBER 1976

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1 REPORT TO US. DEPARTMENT OF THE INTERIOR AND STATE OF IDAHO ON FAILURE OF TETON DAM BY INDEPENDENT PANEL TO REVIEW CAUSE OF TETON DAM FAILURE DECEMBER 1976

2 SUMMARY AND CONCLUSIONS The Independent Panel to Review Cause of Teton Dam Failure has completed its task, as charged by the Secretary of the United States Department of the Interior and the Governor of the State of Idaho in letters from Secretary Kleppe, dated June 11, 23, and 30, The Panel submits its report herewith. These pages present a summary and conclusions. Teton Dam failed on June 5, 1976, when the reservoir was at El , 3.3 ft below the spillway sill. Although downstream warnings are believed to have been timely, deaths of 14 persons and property damage estimated variously from 400 million to one billion dollars have been attributed to the failure. Construction of Teton Dam was authorized on September 7, 1964, by Public Law The dam is situated on the Teton River, three miles northeast of Newdale, Idaho. Prior to 1963, the proposed dam was known as Fremont Dam. Teton Dam and its reservoir were principal features of the Teton Basin Project, a multipurpose project embodying flood control, power generation, and supplemental irrigation water supply. The S dam was a central-core zoned earthfill structure, with a height of 305 ft above the riverbed and 405 ft above the lowest point in the foundation. Provisions for seepage control included a key trench in the foundation rock above El and a cutoff trench to foundation rock below that elevation. A grout curtain extended below these trenches. investigations of site possibilities for a dam on the Teton River commenced as early as 1904 and continued at various times until bids for construction of Teton Dam were invited on July 22, A construction contract was awarded on December 13, The embankment was topped out November 26, Filling of the reservoir commenced October 3, 1975, and continued until the failure on June 5, The Panel's approach to its assignment has been to: (1) obtain, analyze and evaluate all relevant information which could be obtained in document form from the United States^Bureau of Reclamation, the United States Geological Survey, the c / -, / (- / construction contractor, and any other available and knowledgeable source regarding the regional and ** site geology, pre-siting investigations, siting decisions, pre-design investigations, design, contract specifications and drawings; construction practices, progress and inspections; in-progress changes, it any; pre-failure operation; mechanism of failure, including sworn eyewitness accounts; and actions of respective authorities during and immediately following failure; (2) supplement the documentary information by such further inquiry, including public hearings, as became necessary; (3) make (a) detailed studies of the post-failure condition of the dam, its auxiliary structures and its foundation, by inspection, dissection and subsurface drilling; (b) special tests on foundation materials; (c) detailed geologic maps and joint surveys; (d) tests of remnant materials; (e) detailed stress analyses; (0 studies of photographs for comparison of post-failure conditions with pre-construction and construction conditions; (g) measurements of post-failure geodetic positions of surface and subsurface points, as determined before failure and before filling of the reservoir; (4) contract with various organizations for special studies required by the Panel;

3 (5) evaluate relevant data in order to sort out those of greatest significance in determining cause; (6) complete a report of the results of the foregoing activities prior to January 1,1977. Hie approach was initiated by telegrams, dated June 11 and June 14, 1976, to the Director, Engineering and Construction, U.S. Bureau of Reclamation, Denver, and by setting the Panel's first working session and its inspection of conditions at the site for the week of June 28-July 2, The telegrams requested information concerning (l)site geology in plan and sections with any test results on foundation materials; (2) site exploration with detail of drill logs, exploration trenches, borrow materials and tests; (3) grout records in detail showing non-average takes by location and depths, the patterns used and records of any interconnections; (4) foundation preparation showing both before and after conditions; (5) design memoranda for embankment, spillway, diversion structures and outlets; (6) basic drawings and technical specifications; (7) any outside report regarding the site or designs; (8) construction history of borrow pits, material preparation placement, progress, inspection, in-place tests; (9) seepage measurements or observations; (10) eyewitness accounts on progress of failure; (11) hydrology of the site; (12) seismicity of the site; (13) drain designs and drainage observations; (14) post-failure changes in spillway or auxiliary outlet structures; (15) any changes in precise level or horizontal control survey points; (16) changes in topography up and downstream; (17) photos of the foundation as approved at the start of embankment placement, particularly in the key trench and the cutoff trench; (18) record of any seeps or springs in the cutoff and core contact area; and (19) records of cofferdam seepage and pumpage from the foundation area. Prior to the Panel's convening for its first session, the Department of the Interior had recorded sworn testimony of 37 eyewitness observers of pre-failure and during-failure conditions, of whom 14 were Bureau of Reclamation staff and employees, 13 were employees of the construction contractor, and 10 were from the general public. In parallel with these eyewitness accounts, there became available several excellent photographic sequences in still and later in motion picture form. In order to supplement these eyewitness accounts with any available observations of failure-related, but pre-failure conditions, a public call was issued, and two public hearings were held in Idaho Falls on July 21, During its first working session, the need for professional staff and technical and administrative support was recognized. To fill this need, the services of Robert B. Jansen, as Executive Director, were secured through the cooperation of the Governors of Idaho and California. Also, the services of Clifford Cortright, Staff Engineer, and Larry James, Staff Geologist, and Frank Sherman, also a staff geologist, were secured within a few days of Mr. Jansen's appointment. Through the excellent cooperation of the Office of the Secretary, Department of the Interior, supporting properties, services, technicians and administrative assistance have been made available to the Panel through various bureaus of the Department. Because of the importance of determining existing embankment and foundation conditions, the Panel early addressed the Director, Design and Construction, USBR, Denver, requesting specific work on the right abutment to permit detailed examination of the remnant there, and excavation to uncover both the auxiliary outlet tunnel for internal inspection and the site of the large, lower spring observed early on June 5, Response was prompt, and on July 16, 1976, the Bureau of Reclamation awarded its Contract No. DC-7232 to Gibbons and Reed, Salt Lake City, to cover the required work. Actual dissection of the right remnant of the dam started July 26, This excavation proceeded expeditiously, in five-foot vertical increments, to El. 5200, with trenching in each incremental level to allow taking of samples as VI

4 well as inspection of the core remnant for any evidence of water channeling, or cracking, and of the manner in which the key trench was excavated, sealed, filled and compacted. The Bureau's response to the Panel's request for records, data and descriptions was also prompt. A large volume of information was furnished. Many of these records have been supplemented by others furnished to the Panel's staff at the site in response to oral and written requests. Further information was sought on the manner in which the grout curtains were closed and in which the core was built into the key trench. This information was desired both from the Bureau of Reclamation as designers and construction engineers of the dam, and of the contractor, who implemented that construction. Accordingly, a questionnaire was directed concurrently to the Director, Design and Construction, USBR, and to the Chief Executive Office of Morrison-Knudsen, as the sponsoring member of the constructing contractor, Morrison-Knudsen-Kiewit. The USBR response was quite complete. The contractor's response is in two parts. One is from the prime contractor per se, and the other is from the grouting sub-contractor, McCabe Bros., Inc. The prime contractor's answer was rather general. Staff investigations started immediately upon appointment of the various staff members. Their efforts have been interested, diligent, competent, and tireless. They have greatly expedited the completion of the Panel's task and the compilation of its report. The Panel met in Denver on June 28 and 29 to organize and initiate its inquiry through information presentations by the Bureau of Reclamation. A site inspection was made on June 30. Information meetings were held with the Bureau engineers at the site during the following day. Working sessions were continued in Denver on July 2. The Panel met again in working session in Idaho Falls August 3 through 5, again October 5 through 7, November 1 through 3, and December 7 through 10. Between its working sessions, individual Panel members worked with the staff, and independently on assignments from the Panel. Frequent individual visits were made to the site exploratory work. Careful study was made of all eyewitness accounts of their observations prior to the breach. All available photographs of the failure events were studied and arranged in chronologic sequence. All available relevant documentary records have been reviewed for significant content. Continuous professional examination was conducted of all trenching in the right abutment embankment remnant. Detailed mapping of the bedrock joints and fractures in and adjacent to the right abutment key trench was conducted between Stas and Laboratory testing of undisturbed samples of Zone 1 (core) material was carried out. Subsurface water loss tests were conducted at many locations near the centerline right abutment grout curtain. Surface ponding tests were conducted at the key trench invert at prominent joints crossing the invert. Hydraulic fracturing tests were made in drill holes in the left abutment core remnant. Analytic studies were made to assess the stress conditions on sections of the embankment and key trench in the zone of failure. * The Panel's conclusions are summarized below: 1. The records show that the pre-design site selection and geological studies were appropriate and extensive. The pilot grouting program carried out in 1969 forecast the difficulties to be experienced in construction of the final grout curtain. 2. The design followed USBR practices, developed over a period of many years from experience I with other Bureau projects, but without sufficient consideration of the effects of differing and > unusually difficult geological conditions at the Teton Damsite. Every embankment can be said to > vii *

5 have its own personality requiring individual design consideration and construction treatment. Treatment of such individualities produces most of the continuing advances in dam design and construction technology. 3. The volcanic rocks at the Teton Damsite are highly permeable and moderately to intensely jointed. Water was therefore free to move with almost equal ease in most directions, except locally where the joints had been effectively grouted. Thus during reservoir filling, water was able to move rapidly to the foundation of the dam. Open joints existed in the upstream and downstream faces of the right abutment key trench, providing potential conduits for ingress or egress of water. 4. The wind-deposited nonplastic to slightly plastic clayey silts used for the core and key trench fill are highly erodible. The Panel considers that the use of this material adjacent to the heavily jointed rock of the abutment was a major factor contributing to the failure. 5. Construction of the project was carried out by competent contractors under formal contracts administered in accord with well-accepted practices. Controversy between the contractors and Bureau of Reclamation officials which might have affected the quality of the work seems not to have occurred. Construction activities conformed to the actual design in all significant aspects except scheduling. 6. One construction condition which affected the Bureau's ability to control the rate of filling of the reservoir was the delay that occurred in completion of the river outlet works. However, the Panel believes that the conditions which caused the piping and consequent failure of the dam were not materially affected by the fact that the reservoir was filled at a more rapid rate than had been originally planned. A slower rate of filling would have delayed the failure but, in the judgment of the Panel, a similar failure would have occurred at some later date. 7. The records show that great effort was devoted to constructing a grout curtain of high quality, and the Panel considers that the resulting curtain was not inferior to many that have been considered acceptable on other projects. Nevertheless, the Panel's on-site tests and other field investigations showed that the rock immediately under the grout cap, at least in the vicinity of Stas to 15+00, was not adequately sealed, and that additional unsealed openings may have existed at depth in the same locality. The leakage beneath the grout cap was capable of initiating piping in the key trench fill, leading to the formation of an erosion tunnel across the base of the fill. The Panel considers that too much was expected of the grout curtain, and that the design should have provided measures to render the inevitable leakage harmless. 8. The geometry of the key trenches, with their steep sides, was influential in causing transverse arching that reduced the stresses in the fill near the base of the trenches and favored the development ot cracks that would open channels through the erodible fill. Arching in the longitudinal direction, due to irregularities in the base of the key trenches, and arching adjacent to minor irregularities and overhangs, undoubtedly added to the reduction of stress. 9. Stress calculations by the finite element method indicated that, at the base of the key trench near Stas and 15+00, the arching was great enough that the water pressure could have exceeded the sum of the lateral stresses in the impervious fill and the tensile strength of the fill material. Tims, cracking by hydraulic fracturing was a theoretical possibility and may have led to flow of water in the base of the key trench between Stas and 15+00, and erosion of the key trench fill. Vlll

6 10. Close examination of the interior of the auxiliary outlet tunnel showed no distress of any kind such as would be expected had the right abutment, through which the tunnel passes, been subjected to significant settlement or other structural change. Geodetic resurveys showed only minor surface movements as a result of reservoir filling and emptying. Accordingly, differential movements of the foundation are not considered to have contributed to the failure. 11. The Panel found no evidence that seismicity was a factor in failure of the dam. 12. The dam and its foundations were not instrumented sufficiently to enable the Project Construction Engineer and his forces to be informed fully of the changing conditions in the embankment and its abutments. 13. Following its first working session, the Panel reported that it then seemed apparent that the failure resulted from piping, a process by which embankment material is eroded internally and transported by water flowing through some channel in the embankment section. That conclusion remains valid. The Panel's investigations since that time have been directed particularly to determining the most probable manner in which such piping erosion started. The Panel believes that two mechanisms are suspect. Either could have worked alone or both could have worked together. One is the flow of water against the highly erodible and unprotected key trench filling, through joints in the unsealed rock immediately beneath the grout cap near Sta and the consequent development of an erosion tunnel across the base of the key trench fill. The other is cracking caused by differential strains or hydraulic fracturing of the core material filling the key trench. This cracking would also result in channels through the key trench fill which would permit rapid internal erosion. In either case, leakage occurring through the key trench ultimately initiated further erosion along the downstream contact of the core and the abutment rock. Since the core material was both easily erodible and strong, any erosion channels in the core, along the contact with the rock, readily developed into large tunnels or pipes before becoming visible along the downstream parts of the dam. It should be noted that this description of the failure mechanism does not provide a final answer to the specific cause of failure of Teton Dam. Clearly many aspects of the site and the embankment design contributed to the failure, but because the failed section was carried away by the flood waters, it will probably never be possible to resolve whether the primary cause of leakage in the vicinity of Sta was due to imperfect grouting of the rock below the grout cap, or cracking in the key trench fill, or possibly both. There is evidence to support both points of view. Nevertheless, while the specific cause may be impossible to establish, the narrowing of the possibilities to these two aspects of design and construction is likely to serve as an important but tragic lesson in the design and construction of future projects of this type. 14. The fundamental cause of failure may be regarded as a combination of geological factors and design decisions that, taken together, permitted the failure to develop. The principal geologic factors were (1) the numerous open joints in the abutment rocks, and (2) the scarcity of more suitable materials for the impervious zone of the dam than the highly erodible and brittle windblown soils. The design decisions included among others (1) complete dependence for seepage control on u combination of deep key trenches filled with windblown soils and a grout curtain; (2) selection of a geometrical configuration for the key trench that encouraged arching, cracking and hydraulic fracturing in the brittle and erodible backfill; (3) reliance on special compaction of the impervious IX

7 materials as the only protection against piping and erosion of the material along and into the open joints, except some of the widest joints on the face of the abutments downstream of the key trench where concrete infilling was used; and (4) inadequate provisions for collection and safe discharge of seepage or leakage which inevitably would occur through the foundation rock and cutoff systems. The difficult conditions of the site called for basing the design on the most unfavorable assumptions compatible with the geologic conditions concerning the behavior of the water and its possible effect on the embankment. Instead of placing so much dependence on the key trenches and grout curtain, measures should have been developed to render harmless whatever water did pass, irrespective of the reasons. In final summary, under difficult conditions that called for the best judgment and experience of the engineering profession, an unfortunate choice of design measures together with less than conventional precautions was taken to ensure the adequate functioning of the Teton Dam, and these circumstances ultimately ted to its failure.