Dams and Extreme Events Reducing Risk of Aging Infrastructure under Extreme Loading Conditions

Transcription

1 Dams and Extreme Events Reducing Risk of Aging Infrastructure under Extreme Loading Conditions 34th Annual USSD Conference San Francisco, California, April 7-11, 2014 Hosted by San Francisco Public Utilities Commission

2 On the Cover Aerial view of the Calaveras Dam Replacement Project taken on January 27, The San Francisco Public Utilities Commission is building a new earth and rock fill dam immediately downstream of the existing dam. The replacement Calaveras Dam will have a structural height of 220 feet. Upon completion, the Calaveras Reservoir will be restored to its historical storage capacity of 96,850 acre-feet or 31 billion gallons of water. The project is the largest project of the Water System Improvement Program to repair, replace and seismically upgrade key components of the Hetch Hetchy Regional Water System, providing water to 2.6 million customers. 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 2014 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 EFFECTIVE SURVEILLANCE AND MONITORING ALLOWS A PHASED APPROACH TO STABILZATION Gerald Robblee, P.E., G.E. 1 Andy Baxter, P.G., P.E. 2 Robert Cannon, P.G. 3 Jesus Gomez, PhD, P.E. 4 Adam J. Monroe, P.E. 5 6 ABSTRACT The right side tailrace retaining wall and abutment slope above the retaining wall at the Hodenpyl Plant in Manistee Michigan was closely monitored with geotechnical instrumentation and deformation surveys during design investigations and subsequent stabilization of the wall and tailrace slope. The effectiveness of the monitoring allowed the investigations and stabilization to be performed in phases. The subsurface conditions consisted of high-plasticity, highly over-consolidated clay with OCR values greater than 10. The clay was underlain by an artesian aquifer, which created an upward gradient within the clay. Analyses include both conventional limit state analysis and 2D- and 3Dfinite element analyses to model ground behavior. Final stabilization measures include a significant excavation and construction of a 26-ft high permanent soil nail wall to unload the failure surface as well as installing 165-ft long, Single Bore Multiple Anchor (SBMA) tiebacks to provide additional restraint to the failure mass. The surveillance and monitoring program included inclinometers, piezometers, survey monitoring points, and annual lift-off tests of tiebacks installed as an initial measure to slow the rate of slope movement. The instrumentation data was used to monitor the slope to evaluate if emergency measures needed to be put in place prior to completing final design of the stabilization measures. Post construction monitoring has demonstrated that the landslide mass/slope has been stabilized and the additional restraint provided by the new SBMA tiebacks has resulted in a reduction in total deformation and shear strain in the shear zone identified in the clay deposit. INTRODUCTION The Hodenpyl Hydroelectric Plant was constructed in 1925 near Mesick, Michigan. The construction included excavating the right bank of the Manistee River to allow a powerhouse and tailrace channel to be constructed. The tailrace channel had retaining walls that varied in height from 25 feet to 45 feet high constructed on both the sides of 1 Schnabel Engineering, 11A Oak Branch, Greensboro, NC 27407, grobblee@schnabel-eng.com; 2 Schnabel Engineering, 1380 Wilmington Pike, Suite 100, West Chester, PA 19382, abaxter@schnabeleng.com 3 Schnabel Engineering, 11A Oak Branch, Greensboro, NC 27407, rcannon@schnabel-eng.com 4 Schnabel Engineering, 1380 Wilmington Pike, Suite 100, West Chester, PA 19382, jgomez@schnabeleng.com 5 Consumers Energy, Hydro Operations Department, 330 Chestnut Street, Cadillac, MI 49601, ADAM.MONROE@cmsenergy.com 6 Schnabel personnel offer engineering services in Michigan through a management agreement with AG&E, Inc. Effective Surveillance and Monitoring 1589

4 the tailrace channel that extended 157 feet downstream from the powerhouse. A concrete slab was placed on the river floor for scour protection during powerhouse releases. The concrete slab extended from the Powerhouse about 57feet downstream. An electric switchyard and substation are located at the top of the right abutment slope. The project area is shown on Figure 1. Scale 1 =40 40 Substation Bldg E1 E2 E3 E4 Dam Crest East Tailrace Wall Deformation Monitoring Points E1 thru E4 Concrete Apron Flow Figure 1. Project Location Map. In 1996, the tailrace walls were rehabilitated by placing a steel sheet pile in front of the concrete wall, filling the annular space between the sheet piles and the retaining walls with concrete and installing tiebacks. The length of the tiebacks was appropriate to restrain a typical Rankine wedge type failure. Deformation monitoring performed after the 1996 rehabilitation indicated the right retaining wall (East Wall) was creeping toward the river. A plot of the wall movement data from June 1997 through June 2000 is shown in Figure Dams and Extreme Events

5 1.80 Movement of the East Downstream Retaining Wall Cap Since 6/23/97 Deformation Towards River (Inches) E-1 E-2 E-3 E Dec Dec Dec Dec Dec-00 Figure 2. Post 1996 Rehabilitation Wall Movement Data June 1997 through June In 2000, the Owner (Consumers Energy) engaged a new consultant to investigate the cause of the East Wall deformation. Investigations in 2000 identified a deep clay deposit underlain by an artesian sandy aquifer. Slope inclinometer casings and vibrating wire piezometers were installed soils behind the East Wall in 2001 to allow the monitoring of abutment and tailrace channel slope deformations. Installation of geotechnical instrumentation allowed the Owner to monitor the abutment slope and East Wall as part of the facility s overall Dam Surveillance and Monitoring Program and study the cause of the observed deformations. Since the 2001 investigations, several phases of explorations, analysis and design, and stabilization were implemented. The general sequence of activities is listed below and details for each sequence are provided later in this paper. Analysis and Stabilization Timeline 1996: Rehabilitate tailrace walls and install short tiebacks through 2000: Perform deformation monitoring by traditional survey methods. 2000: Hire new Consultant and perform field investigations, continue deformation monitoring. 2001: Install inclinometers and piezometers and perform slope stability analyses. Identify deep seated shear failure in clay as likely cause of observed deformation 2001 thru 2002: Monitor slope and wall deformations with traditional survey methods and inclinometers. 2002: Design initial stabilization measures for observed deep shear failure in clay. Thirteen Single Bore Multiple Anchors tiebacks (SBMA tiebacks) are installed. Effective Surveillance and Monitoring 1591

6 2003 through 2008: Continue monitoring slope deformations including traditional survey, inclinometers, piezometers, and annual lift-off tests on SBMA tiebacks. 2004: Install sheet piles downstream of concrete retaining wall to address surficial stability issues : Install replacement inclinometer due to magnitude of measured movement and develop additional stabilization concepts to increase factor of safety for known failure mass and address potential deeper failures. Perform 2D and 3D finite element modeling to develop better understanding of ground behavior in abutment : Design a phased approach to final stabilization and submit for FERC review. Phased approach includes unloading the abutment failure mass (Phase 1) and installing deep SBMA tiebacks (Phase 2). 2009: Based on observed movements and measured tieback loads, perform Phase 1 of final stabilization. 2010: Perform Phase 2 of final stabilization through 2013: Perform post stabilization surveillance and monitoring. INITIAL ANALYSIS AND STABILIZATION (2000 THRU 2002) The initial work performed in 2000 included drilling 13 test borings and installing piezometers to develop an understanding of the geologic conditions within the tailrace area. Drilling started at the top of the abutment slope to reduce the potential for encountering high artesian pressures. The initial borings at the top of the slope indicated an upper sand deposit underlain by a clay stratum over a sand and gravel deposit. The upper sand layer appeared to be an unconfined aquifer, the clay layer appeared to function as a confining layer and the lower sand and gravel deposit was a confined aquifer. Pore pressures measured on the lower sand and gravel layer indicated that confined aquifer was artesian and the total head measured in the deep sand and gravel aquifer was up to about 20 feet above normal river flow elevations. A generalized soil profile and section through the East Wall are shown in Figure Dams and Extreme Events

7 Inclinometer Casing Figure 3. Soil Profile at East Wall In 2001, two slope inclinometer casings (INC-A and INC-B) were installed about 10 feet behind the East Wall. Deformation surveys performed in INC-A and INC-B indicated the well-defined shear zones existed in the deep clay stratum. Figure 4 shows a plot of cumulative displacement versus depth for INC-A. Over a 7 month period, about ½ inch of deformation was measured in a 2-ft-thick shear zone. The observed thickness of the shear zone could be a result of the stiffness of the inclinometer casing and the vertical distance between inclinometer probe readings (2 feet). The actual thickness of the shear zone could be less than 2 feet. Effective Surveillance and Monitoring 1593

8 Shear Zone Figure 4. Cumulative Displacement vs Depth (Dec2001 thru June 2002). Slope stability analyses performed in 2001suggested that a deep seated shear failure in the clay was the likely cause of the observed deformations. The deformation monitoring data collected in the inclinometers supported that conclusion. The survey data also indicated that deformation of the wall was greater at the downstream end of the wall. Based on the results of the slope stability analyses and the observed rate of deformation, the Owner decided to implement a slope stabilization program. The work was planned to be designed in late 2001 with bidding and construction being performed in The 2002 slope stabilization work consisted of installing 13 single bore multiple anchor (SBMA) tiebacks through the retaining wall a few feet above the river water level. Each SBMA tieback included four strands, each with its own bond zone and having the free length of each strand being progressively longer so as to stagger the bond zones for the strands in the drilled hole. Figure 5 shows a photo of the drilling of the SBMA tiebacks using a duplex drilling system where the drill cutting were flushed from the hole with an internal flush of water. It was anticipated that the tiebacks would provide sufficient restraint to improve stability and slow the rate of movement. It was also anticipated that the stabilization may not be sufficient to completely halt deformations and additional stabilization measures may be needed in the future. Therefore, the stabilization design included surveillance and 1594 Dams and Extreme Events

9 monitoring during construction and post construction surveillance and monitoring including annual lift-off testing of the SBMA tiebacks. Figure 5. Drilling of 2002 SBMA Tiebacks. DEFORMATION BEFORE SBMA DRILLING ~ 0.7 INCHES IN 8 MONTHS DEFORMATION AFTER DRILLING ~0.1 INCHES IN 4 MONTHS ~0.2 INCH DEFPORMATION DURING SBMA INSTALATION Figure 6. Deformation in Shear Zone During 2002 (Jan 2002 thru Jan 2003). A cofferdam was constructed in the river during the 2002 SBMA tiebacks installations. The impacts of removing a stabilizing water force from the wall and potential ground Effective Surveillance and Monitoring 1595

10 disturbance during drilling and grouting of the tiebacks resulted in an increase in the deformation rate during construction. The observed deformation rate decreased after installing and prestressing the 2002 SBMAs. Figure 6 shows a plot of lateral deformation vs time during 2002 at the observed shear zone deformations measured in the inclinometer casing at the downstream end of the wall. An increase in deformation rate and can be observed during the SBMA drilling and grouting followed by a several months of lower deformation rates. INC-B was destroyed during the 2002 SBMA tieback installation. Deformation Surveys and Inclinometer Surveys The surveillance and monitoring of the East Wall and abutment slope consisted of monthly deformation surveys of the East Wall, monthly inclinometer surveys, and lift off testing of the SBMA tiebacks. Items that affected the surveillance and monitoring from 2002 through 2008 included: Severe winter conditions sometimes made taking monthly readings very difficult. A slope stabilization project downstream of the East Wall was implemented to stabilize the upper surficial sandy soils in the tailrace slope downstream of the concrete East Wall. The stabilization program included driving steel sheet piles downstream of the concrete East Wall, and terracing the slope with segmental block walls. Two additional inclinometer casings (INC-C and INC-D) were installed in December 2002 that and were located beyond the top of the tailrace slope. Deformations measured in the downstream inclinometer (INC-A) were concentrated in a two-foot thick zone and exceeding 1.5 inches by 2005, therefore a replacement inclinometer casing (INC-E) was installed in In 2007, inclinometers where installed at the top of the tailrace slope near the substation (INC-F) and near the upstream end of the East Wall near the installation of INC-B that was destroyed in The post 2002 SBMA installation surveillance and monitoring of the East Wall indicated that after few months of slow movement, the observed deformation rate increased but was slower than the observed deformation rate before the 2002 SMBA tiebacks were installed. The surveys performed in INC-C and INC-D that were located beyond the top of the tailrace slope showed no deformation. The lack of observed deformation suggested the failure mass being monitored did not extend much past the top of the slope. Figure 7 is a plot of deformation versus time for Inclinometer A from prior to the installation of the 2002 SBMA tiebacks through Figure 8 show a plot of deformation survey data for the East Wall from 2002 through Review of the data shown in Figures 7 and 8 resulted in the following conclusions and concerns: A 3 to 6 month period of slow to no movement was observed after completion of the 2002 SBMA tiebacks Dams and Extreme Events

11 Observable deformation resumed in 2003 The rate of deformation for the East Wall and slope decreased after the stabilization measures for the surficial sandy soils downstream of the East Wall was performed. The rate of deformation of the East Wall and slope appeared to increase in late 2007 Given the observed increase in deformation rate in late 2007 and 2008, would the spring 2009 SBMA lift-off tests indicate that the load in at least some of the 2002 SBMA tiebacks stands were approaching their maximum allowable load SBMA Tiebacks Installed Aug 31, Downstream Stabilization Slower deformation rate observed in upstream inclinometer after work completed Figure 7. Deformation vs Time Plot for INC-A, June 2002 thru Dec Movement of Tailrace Retaining Wall near Water Line Since 2002 SBMA Tiebacks Installed Hand drawn approximate trend lines r e iv R e th ) rd s a e w h c to t (In n e m e v o M E-A10 E-B10 E-C10 E-D n a -J n a -J n a -J c e Ḏ 1 3 Figure 8. Survey Deformation Data for East Wall 2002 through c e Ḏ n a -J n a -J c e Ḏ 1 3 Effective Surveillance and Monitoring 1597

12 Lift-Off Testing of 2002 SBMA Tiebacks (2004 through 2008) As part of the surveillance and monitoring plan for the East Wall, the Owner had lift-off testing of the 2002 SBMA tiebacks performed in 2004, 2005, 2007 and Lift-off testing was also performed in the spring of 2009, in 2010 before the installation of additional SBMA tiebacks, and in 2010 after installing and prestessing the additional SBMA tiebacks. The results of the 2009 and 2010 lift-off testing will be discussed later in this paper. Due to each strand of the SBMA tiebacks having progressively longer free lengths, the lift off tests were performed on each strand using a mono-strand jack. Each tieback strand was stressed until wedge lift-off was achieved as measured with a feeler gauge. Details on the tieback lift-off testing procedures can be found in the technical paper authored by Bruce, M.E. et al (Bruce 2007). Photographs of lift off testing in progress are shown in Figure 9. Figure 9. Lift-Off Test Jacking of 2002 SBMA Tiebacks. The results of the 2004 through 2008 lift-off testing indicated that tieback load was increasing. When the magnitudes of the measured load increases were compared to the measured deformations and the expected increases in strand load due to an assumed elastic elongation equal to the measured deformation, the measured stand load generally was similar to the expected increase in load if anchor head movement was only due to the elastic deformation. Therefore, it was concluded observed wall deformation resulted in increased tieback strand load without slippage of the bond zones (geotechnical capacity intact). A plot of total tieback load for each of the SBA tiebacks are shown in Figure 10 for the lift-off testing performed though Dams and Extreme Events

13 Figure 10. Lift off Test Results (2004 through 2008). ADDITONAL INVESTIGATIONS AND ANALYSES (2005 THRU 2007) Explorations and Lab Testing In 2005 and 2007 additional explorations were performed and geotechnical instrumentation was installed to provide data for additional analysis of the tailrace slope and subsequent final design of stabilization measures. Additional explorations and instrumentation included: Installing INC-E as a replacement for INC-A in 2005 Performing lab testing including, 6 unconfined compression tests, 3 unconsolidated undrained (UU) triaxial tests, 2 consolidated undrained (CU ) triaxial compression tests with pore pressure measurements, 2 direct shear tests, and 1 ring shear test Installing INC-F at the top of the slope near the substation Installing INC-G as a replacement for INC-B Performing 4 CU tests, 7 one dimensional consolidation tests, and 4 ring shear tests The results of the field investigations indicated that the clay stratum consisted of an upper lean clay layer and a lower fat clay layer. As previously discussed, an upper sandy layer is present above the clay and acts as an unconfined aquifer. Below the clay stratum is a sand and gravel deposit that acts as a confined aquifer. Effective Surveillance and Monitoring 1599

14 Seepage and Stability Analyses Analyses performed in 2005 included: Revaluation of soils strengths and potential 3D effects Seepage analyses (GeoStudio SEEP/W) to model the pore pressure distributions within the upper and lower aquifers and the through the lean and fat clay layers. Limit equilibrium slope stability analyses (GeoStudio SLOPE/W) were performed to model the stability of the East Wall and slope and the potential effectiveness of additional stabilization measures. The criteria for evaluating the effectiveness of additional stabilization measures were: Increase the actor of safety for the presumed failure mass to at least 1.5 Provide sufficient stabilization force that slope deformations are not observed Consider the potential for future deeper failure surfaces being developed and provide sufficient stabilization such that their factors of safety are 1.5 or higher. Potential stabilization measures evaluated included: Unloading of the slope by excavating the slope Installing additional tiebacks Depressurization of the clay deposit and thereby increasing its strength after consolidation by a pumped well system or a vacuum well system The three stabilization methods considered did not provide the desired stability for both the presumed failure mass and potential deeper failure surfaces. Therefore a combination of unloading the tailrace slope and installing a 3 rd row of tiebacks were expected to be needed to achieve the stabilization goals. Finite Element Analysis To develop a better understanding of the ground behavior and the downstream limits for future stabilization, both 2D and 3D finite element models were developed. Staged analyses using PLAXIS were performed. For soils elements that reached two percent strain, the soil strength was lowered to a residual strength value. The analysis stages included the original construction of the dam and powerhouse, the 1996 East Wall rehabilitation, and the installation of the 2002 SBMA tiebacks. Potential future stabilization measures were included in the finite element analyses. The 2D finite element model included consolidation stages to allow pore pressure distributions to be evaluated at selected times. The analyses stages analyzed in the 2D finite element model are summarized in Table Dams and Extreme Events

15 Table 1. Construction Sequence in 2-D Analysis. CONSTRUCTION SEQUENCE IN ANALYSIS 1. Initial stress state prior to excavation in Excavate and install retaining wall. 3. Consolidate to 1996, prior to installation of tiebacks. 4. Install 1996 tiebacks on retaining wall. 5. Consolidate to Install 2002 tiebacks on retaining wall. 7. Consolidate to Perform upslope cut with support of excavation. 9. Install 3rd tiebacks on retaining wall. 10. Consolidate to The 3D finite element model construction sequence is summarized in Table 2. A shorter analysis sequence was used due to concerns about model complexity and reliability of the results if the 2002 and 2004 construction activities were modeled. Table 2. Construction Sequence in 3D Analysis. CONSTRUCTION SEQUENCE IN ANALYSIS 1. Initial stress state prior to excavation in Excavate and install retaining wall. 3. Consolidate to Change clay clusters with shear strain exceeding 4% from Short Term to Long Term properties. 5. Consolidate to Install 1996 tiebacks. 7. Consolidate to The 2D finite element model results showed that strain concentrations due to the slow dissipation of excess negative pore pressures caused by the excavation of the abutment in 1925 and slow creep of the clay deposit could be expected to cause concentrated zones of strain in the clay. Figure 11 shows the deformation pattern predicted by the 2D finite element analyses after installing the 1996 tiebacks with the location of the concentrated shear zone observed in INC-A. Effective Surveillance and Monitoring 1601

16 Figure 12 shows a slice through the 3D finite element at the location of INC-A with deformation contours shown. Similar stain concentrations were observed in the 3D finite element model results. The results of the 3D finite element model also suggested that the factor of safety, or level of stability, decreases in the downstream direction. Figure 11. Deformation Contour Plot for 1996 from 2D Finite Element Analysis. Figure 12. 2D Slice from 3D Finite Element Model- Deformation Contours at INC-A Dams and Extreme Events

17 DESIGN OF FINAL STABILIZATION (2007 THRU 2008) The final stabilization measures included unloading the failure mass in the vicinity of the slope behind the East Wall and installing new long tiebacks to provide additional restraint to the slope. The unloading was designed to primarily provide increased stability to the presumed failure mass. A factor of safety above 1.5 was desired for the presumed failure mass due the potential for ground disturbance during drilling and grouting of new tiebacks to disturb the clay soils. To provide a factor of safety of at least 1.5 for a potential deep failure surface, a combination of the unloading and the additional row of tiebacks was needed. The results of the investigation and analyses performed from 2005 through 2007 indicated that the fat cay layer had experience strain softening for at least some portion of the fat clay along the presumed failure surface. The upstream end of the failure mass is also fully restrained d and has not experienced as much strain as the downstream portion of the East Wall. These types of 3D effects can t be explicitly considered in 2D analyses. To account for some soils being subject to strain softening and some 3D effects, a 2D limit equilibrium model was used to back-calculate an average mobilized strength along the presumed failure surface. The post 2002 SBMA tieback configuration was assumed in the mobilized shear strength back calculation along with the residual strength friction angle of the fat clay. An apparent cohesion was used in the model to account for 3D effects assuming the factor of safety for the post 2002 SBMA tieback configuration was 1.0. The results of the limit equilibrium analysis used to estimate the value of mobilized strength on the presumed failure mass are shown in Figure 13. FS~1.0 Figure 13. Back Calculation of Mobilized Shear Strength Post 2002 SBMA Tiebacks. Effective Surveillance and Monitoring 1603

18 The average mobilized strength of the fat clay along the presumed failure surface corresponded to a friction angle of 15 0 and a cohesion of 370 psf. The above strength properties were used to evaluate the increase in stability of the presumed failure mass due to the planned unloading of the slope. The calculated factor of safety for the presumed failure mass after the unloading was increased from 1.0 to 2.1 due to the unloading. The results are shown in Figure 14. FS~2.1 Figure 14. Factor of Safety of Presumed Failure Mass After Unloading. The potential for new and deeper failure surface to develop was evaluated. The fat clay soils at depth and behind the presumed failure mass had not experienced measurable shear strain. Therefore residual strengths for the fat clay layer were not used in the analyses for potential deep failure surfaces. The authors however believed that friction angles based on peak shear strengths were not warranted as well. A friction angle of 24 0 was selected as a fully-softened shear strength combined with a cohesion intercept of 370 psf. The limit equilibrium model for deep failure surfaces included the planned soil nails for the stabilization of the vertical face from the unloading and it also included 165-ft long tiebacks with 105-ft long free lengths. The results of the limit equilibrium analyses of a potential deep failure surface are shown on Figure 15. The calculated factor of safety for a future deep failure surface was 1.6. The lateral extent of the stabilization was selected based on the desire to protect hydroelectric plant facilities located at the top of the slope. Therefore, the unloading and new tiebacks were installed along a 120-foot long section of the tailrace slope (~ 40 feet downstream of East Wall) to provide protection for the East Wall and an electrical switchyard and substation building located at the top of the tailrace slope. The tiebacks were designed as 6-strand SBMA tiebacks with free lengths of at least 100 feet, a total bond zone length of at least 60 feet (10 feet per anchor), and a capacity of 200 kips Dams and Extreme Events

19 FS~1.6 Figure 15. Factor of Safety for Potential Deep Failure Surface. IMPLEMENTATION OF FINAL STABILIZATION (2009 THRU 2010) The Phase 1 excavation extended five feet below the top of the East Wall and about 48 feet behind the East Wall. A soil nail wall was installed to retain the face of the excavation next to the existing substation. Phase 2 of the slope stabilization included installing twenty 6-strand, Class 1 corrosion protected SBMA tiebacks through the retaining wall and anchored behind a potential deep seated failure plane. A photograph of the final condition is shown in Figure 16. The soil nail wall facing varies in height up to 26 ft. The upper rows of the soil nails ranged in length from 20 to 50 ft. and were double corrosion protected. The bottom row of soil nails were 125 ft. long to provide additional resistance to deep seated failures. The design considered a factor of safety of 2.0 for bond with confirmation through verification and proof load testing. The soil nail wall was designed assuming that full water pressure develops behind the wall, in addition to potential ice lensing in the lean clay adjacent to the soil nail wall face. Drainage panels were placed behind the shotcrete, and perforated PVC pipe drains were installed in predrilled holes through the excavation face. Figure 17 shows a photograph of the soil nail wall during construction in Phase 2 stabilization was performed in A photograph of the performance and proof load test setup is shown in Figure 18. A photograph of tieback installation within the cofferdam is shown in Figure 19. Effective Surveillance and Monitoring 1605

20 Figure 16. View of the Tailrace Channel, East Wall, and Soil Nail Wall Figure 17. Soil Nail Wall During Construction Dams and Extreme Events