Managing Our Water Retention Systems

Transcription

1 Managing Our Water Retention Systems 29th Annual USSD Conference Nashville, Tennessee, April 20-24, 2009 Hosted by Corps of Engineers

2 On the Cover Wolf Creek Dam is on the Cumberland River in South Central Kentucky near Jamestown, Kentucky. It provides flood control, hydropower, recreation, water supply, and water quality benefits for the Cumberland River system. Construction began in 1941 and was interrupted by WWII from 1943 to The reservoir was impounded in December The 5,736 foot-long dam is a combination earthfill and concrete gravity section. U.S. Highway 127 crosses the top of the dam. Vision U.S. Society on Dams To be the nation's leading organization of professionals dedicated to advancing the role of dams for the benefit of society. Mission USSD is dedicated to: Advancing the knowledge of dam engineering, construction, planning, operation, performance, rehabilitation, decommissioning, maintenance, security and safety; Fostering dam technology for socially, environmentally and financially sustainable water resources systems; Providing public awareness of the role of dams in the management of the nation's water resources; Enhancing practices to meet current and future challenges on dams; and Representing the United States as an active member of the International Commission on Large Dams (ICOLD). The information contained in this publication regarding commercial projects or firms may not be used for advertising or promotional purposes and may not be construed as an endorsement of any product or from by the United States Society on Dams. USSD accepts no responsibility for the statements made or the opinions expressed in this publication. Copyright 2009 U.S. Society on Dams Printed in the United States of America Library of Congress Control Number: ISBN U.S. Society on Dams 1616 Seventeenth Street, #483 Denver, CO Telephone: Fax: stephens@ussdams.org Internet:

3 SCOUR UPSTREAM OF DAM PIERS Amanda Sutter, PE 1 Edward Demsky, PE 2 Kenneth Koller 3 Michael Navin, PE, PhD 4 ABSTRACT Scour has occurred immediately upstream of several dam piers of Lock and Dam 24, Lock and Dam 25, and Melvin Price Locks and Dam on the Mississippi River. The scour has eroded the upstream protection and shortened the seepage path which could lead to piping of the sand foundation of the pile founded structures. Voids were found under the dam piers upstream of the cutoff wall at Lock and Dam 24 and Lock and Dam 25, which have a portion of the foundation piles upstream of the cutoff wall. At Melvin Price Locks and Dam, the scour was below the bottom of the concrete. Rock was placed at Melvin Price Locks and Dam to fill the holes. A Phase 1 Dam Safety Modification Report has been written for Lock and Dam 25 to address the scour problem. The results show those piers which have voids to have a low reliability of satisfactory performance. Consequences of unsatisfactory behavior were determined and alternatives were developed. The alternatives were analyzed to determine the economic impacts of doing nothing and taking corrective measures. Complete and immediate correction by placement of additional stone for the upstream scour protection is economically justified and is necessary to provide a reliably operating dam. A similar study is planned to be conducted for Lock and Dam 24. INTRODUCTION Scour has occurred immediately upstream of several dam piers of Lock and Dam 24, Lock and Dam 25, and Melvin Price Locks and Dam on the Mississippi River. This type of scour is typical of the scour patterns seen upstream of bridge piers. This problem was first detected at Melvin Price Locks and Dam. The other two dams were then surveyed to determine whether or not this was a systemic problem. All three dams showed similar results. The scour has eroded the upstream protection and shortened the seepage path which could lead to piping of the sand foundation of the pile founded structures. Voids were found under the dam piers upstream of the cutoff wall at Lock and Dam 24 and Lock and Dam 25, which have a portion of the foundation piles upstream of the cutoff wall. At Melvin Price Locks and Dam, the scour was below the bottom of the concrete. Sheet pile was exposed at all three dams. Rock was placed at Melvin Price Locks and 1 Civil Engineer, St. Louis District, United States Army Corps of Engineers, 1222 Spruce Street, St. Louis, MO 63103, Amanda.J.Sutter@usace.army.mil 2 Civil Engineer, St. Louis District, United States Army Corps of Engineers, 1222 Spruce Street, St. Louis, MO 63103, Edward.C.Demsky@usace.army.mil 3 Civil Engineer, St. Louis District, United States Army Corps of Engineers, 1222 Spruce Street, St. Louis, MO 63103, Kenneth.R.Koller@usace.army.mil 4 Civil Engineer, St. Louis District, United States Army Corps of Engineers, 1222 Spruce Street, St. Louis, MO 63103, Michael.P.Navin@usace.army.mil Scour Upstream of Dam Piers 227

4 Dam to fill the scour holes and provide protection. A Phase 1 Dam Safety Modification Report has been written for Lock and Dam 25 to address the scour problem. A similar study is planned to be conducted for Lock and Dam 24. MELVIN PRICE LOCKS AND DAM The scour problems were initially discovered based upon a routine hydrographic survey of Melvin Price Locks and Dam. The surveys were performed by Corps of Engineers personnel using multi-beam sonar equipment: a RTK (Real Time Kinematic) GPS-aided Inertial Positioning and Orientation System. The survey equipment had accuracy within 2 inches in the horizontal direction and 6 inches or less in the vertical direction. The primary purpose of the investigation was to determine if any erosion had occurred to the concrete stilling basin and baffle blocks. No erosion of the paving downstream of the stilling basin was detected. Since the project was at an open-river condition, the survey boat was able to maneuver through the open gate bays. This combined with the use of multi-beam sonar (as opposed to the single beam sonar previously employed) allowed data to be collected closer to the upstream face of the dam than was ever collected previously. Melvin Price dam originally had upstream scour protection consisting of 20 inches of 400 pound top size riprap over 6 inches of bedding material for a distance 64 feet upstream of the dam. Large scour holes varying in depth from 7 to 23 feet were detected upstream, immediately against the dam structure at Piers 2, 5, 6, 7, 9, 10 and 11. The results of the survey were plotted and show elevation color contours of the area downstream of the dam gates. See Figure 1. Scour Upstream Baffle Blocks Stilling Basin Figure 1. Overview of hydrographic survey results for Melvin Price Locks and Dam. The deepest scour hole, 23-feet deep, exposed seven feet of the sheet pile cutoff wall under the dam to the river water. Any holes in the sheet piling or separated interlock joints would allow water pressures and flows to directly enter the protected sand foundation. This is an unacceptable condition. If upstream pool water were to get through the steel sheet piling wall it would increase the uplift pressure under the dam and potentially erode away the foundation sand beneath the dam piers, causing loss of lateral support to the foundation piling and excessive movement of the dam piers. In a worst Pier L o c k Downstream 228 Managing Our Water Retention Systems

5 case scenario, this would ultimately lead to failure of the dam. All instrumentation (piezometers, inclinometers, trilateration surveys, and sounding holes) continue to read within the expected range. This indicates that the sheet pile cutoff walls appear to be intact and functioning properly and that no voids exist beneath the dam. Divers from the Corps Engineering Research and Development Center (ERDC) were contacted to take acoustic pictures of the exposed sheet piling of Piers 2 and 5 to further verify sheet pile integrity. The camera inspection showed the sheet pile interlocks to be intact (Figure 2), which was also confirmed by the diver touching the interlocks. Upstream Face of Concrete Pier Sheet Pile Cutoff Figure 2. Acoustic photograph of sheet pile cutoff wall upstream of Pier 5. The maximum scour was 7 feet below the top of the sheet piling. If the scour reaches 20 feet below the base of the pier, the internal soil pressures would rupture the sheet piling causing a cascading failure of the dam monolith. If scour were allowed to continue to develop, dam failure would result under all scenarios. Therefore, the area upstream of the piers was repaired to restore the original cross-sections upstream of the dam. The bottom portion of the deep scour holes were filled with Illinois State Highway specification CA7 course aggregate or crushed stone bedding material. This is to fill the lower part of the scour hole to minimize the potential for damage to the steel sheet piling. The next four feet of the scour holes were filled with graded stone C, with 400 pound top size stone. The upper ten feet of the scour holes shall be filled with graded stone A, with 5000 pound top size stone. Proper placement techniques were used so as not to damage the steel sheet piling and the dam piers. All scour holes had at least 12 inches of CA7 course aggregate or crushed stone bedding material and 36 inches of graded stone C under the larger size stone. This is to keep the sand from being sucked up through the large stone which would then collapse into the hole. Another survey was taken in July 2008 to verify that the repair was intact. Instead, the survey reported that some of the scour reappeared, but not to the original extent. No sheet pile was exposed. The new scour was repaired in November 2008 using a new rock gradation, consisting of 5,000 pound rip rap placed on top of the scour. No smaller stone Scour Upstream of Dam Piers 229

6 was required due to the shallower scour depths and the quantity and gradation of stone from the first repair still remaining in the hole. One of the stilling basins was unwatered around this time. The stilling basins at this project do not have end sills to trap any rock which passes through the gates. However, a manhole cover was missing within the stilling basin and some of the larger stone was found within the manhole. This gives some clues as to the failure mechanism. A model of Mel Price will be constructed and tested by the Corps Engineering, Research, and Design Center (ERDC) to determine the mode of failure. LOCK AND DAM 25 Lock and Dam 25 was surveyed during open river conditions using the multi-beam sonar equipment to determine if scour had also occurred upstream of that dam. Original construction of Dam 25 upstream scour protection consisted of 33 inches of derrick stone (5000 pound top size) over 12 inches of riprap on top of 6 inches of stone or gravel on a lumber mattress for a distance of 20 to 24 feet upstream of the dam. Large upstream scour was found, which was much larger than that found at the Melvin Price Dam. See Figure 3. The deepest scour found immediately upstream of Dam No. 25 was 11 feet beneath the bottom of the dam pier/sill base near Pier 16 at Gatebay 15 exposing 11 feet of the sheet pile cutoff wall. However the deepest scour found directly adjacent to pier base 16 would expose 7 feet of the upstream sheet pile cutoff wall beneath and adjacent to dam pier 16. Figure 3. Close up view of hydrographic survey results near Pier 13 at Lock and Dam 25. The dam piers are founded on timber friction piles 30 feet in length. Unfortunately, approximately 15 percent of the timber piles are located upstream of the sheet pile cutoff wall. See Figure 4. Some of the 30 feet long timber piles therefore have as much as 7 feet of the supporting foundation material removed by the upstream scour. 230 Managing Our Water Retention Systems

7 Flow Figure 4. Construction photo of pile foundation May Inspection and acoustic photos performed by a diver of some of the piers confirmed that the timber piles were intact with no apparent damage and that no openings were found in the sheet pile cutoff. See Figure 5 for one of the acoustic photos and Figure 6 for a sketch to clarify the condition. Nearly all piers have one piezometer and two piers have multiple piezometers. All of the piezometers show levels close to tailwater, indicating no piping under the dam. Timber Pile Sheet Piles El 409 El 403 Timber Piles Flow Timber piles not shown. El 392 Debris Figures 5. Acoustic photo of timber piles. Sheet pile cutoff EL 373 Figure 6. Sketch of timber piles. A reliability model was created to determine the probability that the sheet pile wall could be overstressed due to scour upstream of the dam. Reliability analysis was performed in the model using the Taylor Series Method as described in USACE ETL , Introduction to Probability and Reliability Methods for Use in Geotechnical Engineering. Low, medium and high levels of consequences have been developed for the without project condition and include the physical damage, the emergency repair cost El 359 Scour Upstream of Dam Piers 231

8 to restore performance, and the duration of any repairs that affect lock closure. Scour is generally progressive and becomes worse over time, but is highly random as sand is alternatively removed and redeposited upstream of the dam. The reliability model showed that for four feet of scour the probability of unsatisfactory performance is 6.8%. This unsatisfactory performance would consist of movement of at least one pier. The mean value of four feet used in this analysis is based on a diver inspection where the diver was able to crawl beneath the dam and physically measure scour at the sheet pile. The probability of unsatisfactory performance would be much larger than the calculated 6.8% if the deeper scour measured elsewhere along the face of the dam would extend under the dam to the sheet pile at one of the other pier locations. There was eleven feet of scour recorded at Gatebay 15. The scour condition is critical. In addition to the scour immediately upstream of the dam, more general scour conditions have developed further upstream in the vicinity of the dam on each side of the river. See Figure 7. The color plot of the survey shows the large areas further upstream of the dam that have also scoured. Figure 7. Note the blue areas on both sides of the river channel which indicate scour. Low, medium and high levels of consequences have been developed, with loss of piers or dam due to foundation scour being the primary failure concern. The results show those piers which have voids to have a low reliability of satisfactory performance. Consequences of unsatisfactory behavior were determined and alternatives were developed. The alternatives were analyzed to determine the economic impacts of doing nothing and taking corrective measures. Complete and immediate correction by placement of additional stone for the upstream scour protection is economically justified 232 Managing Our Water Retention Systems

9 and is necessary to provide a reliably operating dam. Efforts are currently underway to secure funding to correct the scour problems at Lock and Dam 25. LOCK AND DAM 24 Similar to Lock and Dam 25, Lock and Dam 24 upstream scour protection originally consisted of 33 inches of derrick stone (5000 pound top size) over 12 inches of riprap on top of 6 inches of stone or gravel on a lumber mattress for a distance of 20 to 24 feet upstream of the dam. A similar scour condition exists at Dam No. 24 with the maximum depth of scour directly adjacent to Pier 4 being at Elevation 414 feet NGVD, four feet beneath the bottom of the pier base exposing 4 feet of the upstream sheet pile cutoff wall and the timber piling upstream of the cutoff wall. Since 12 of the 72 piles under the tainter gate piers (equal to 17 percent) are upstream of the sheet pile cutoff, it is likely that some of these 30-foot long friction piles could have 4 feet of the support foundation material removed by the upstream scour. Exposed timber friction piles reduce the ability of the pile foundation to carry the loading on the dam piers and will result in excess pier movement and settlement. For Pier 4, the loss of 4 feet of foundation material would affect the lateral movement of the piles more than the vertical bearing capacity since the piles under this pier are end-bearing on rock. Scour occurred immediately upstream of the dam structure to a lesser degree at Piers 5 through 11. A study similar to the one conducted at Dam 25 is planned for Dam 24 as soon as funding is available. This study will determine the best course of action for repairs. CONCLUSION None of the dams had adequate upstream scour protection to prevent severe scour from occurring at these structures. The protective rock was not of an adequate size and thickness and did not extend the proper distance from the dam to protect the dam from the serious scour that has occurred. Exposed unprotected upstream sheet pile cutoff walls can lead to serious underseepage conditions developing that can cause foundation piping. Exposed timber friction piles reduce the ability of the pile foundation to carry the loading on the dam piers potentially resulting in excess pier movement and settlement. When piles are exposed over time, they can be damaged by forces developed when passing ice through the dam. Any of these conditions can cause the dam to become inoperable or lead to dam failure resulting in hundreds of millions of dollars of navigation loss and associated repair costs. The scour problem developed because the designers of these navigation dams did not realize that the constriction they placed in the river (locks, dam piers, sills, and overflow dikes) would greatly increase the river velocities upstream of the dam resulting in large scour forces acting on unprotected areas and undersized scour protection. The Corps of Engineers St. Louis District is concerned that inadequate upstream scour protection for navigation dams on the upper Mississippi River is a systematic dam safety problem since Scour Upstream of Dam Piers 233

10 most of the navigation dams were designed during the same time period and were of similar design. Inadequate upstream scour protection could exist on other navigation systems throughout the country. 234 Managing Our Water Retention Systems