CECW-CE DEPARTMENT OF THE ARMY ETL 1110-2-575 U.S. Army Corps of Engineers Technical Letter Washington, DC 20314-1000 1110-2-575 1 September 2011 Engineering and Design EVALUATION OF I-WALLS Distribution Restriction Statement Approved for public release; distribution is unlimited. DEPARTMENT OF THE ARMY ETL 1110-2-575 U.S. Army Corps of Engineers CECW-CE Washington, DC 20314-1000 Technical Letter 1 September 2011 No. 1110-2-575 EXPIRES 1 June 2016 Engineering and Design EVALUATION OF I-WALLS 1. Purpose. This Engineer Technical Letter (ETL) provides updated technical criteria and guidance for evaluation of existing I-walls. General guidance for performing I-wall evaluation is provided along with detailed updates to existing guidance that focus on three I-wall performance items: the flood-side gap that was discovered at I-walls in New Orleans after Hurricane Katrina; the rotational stability failure mode found to be the critical failure mode for most I-walls evaluated nationwide in the Phase II evaluation; and criteria for consideration of deflections. This guidance is for I-walls designed to provide flood risk reduction from inland flooding, not from coastal storm surges with significant wave and vessel impact loads. I-walls in coastal areas shall be evaluated in consultation with the Headquarters, U.S. Army Corps of Engineers. 2. Applicability. This ETL applies to all USACE commands having civil works responsibilities. This ETL applies to levee systems that are USACE operated and maintained and levee systems that are federally authorized and locally operated and maintained. 3. Distribution Statement. Approved for public release, distribution is unlimited. 4. References. References are at Appendix A. 5. Background. Investigations of the hurricane risk reduction systems in Louisiana identified possible deficiencies in the guidance used to design I-walls. The design deficiencies centered on the phenomenon of a gap formed by flood loading between sheet pile and the soil on the flooded side of the wall. This gap contributed to several breaches of I-walls in New Orleans prior to their overtopping related to global stability or seepage. The U.S. Army Corps of Engineers (USACE) therefore issued guidance regarding these deficiencies (Headquarters, U.S. Army Corps of Engineers (HQUSACE), 2006a). The compilation of data and site inspections required in this guidance was considered Phase I of a multiphase approach to evaluating existing I-walls under USACE jurisdiction throughout the United States. Phase II Interim Guidance was then prepared and disseminated (Headquarters, U.S. Army Corps of Engineers (HQUSACE), 2006b) to help Districts evaluate and identify projects that may be at risk of poor performance. This effort resulted in the identification of over 50 projects with potential performance concerns. Most of the projects not meeting the criteria of the Phase II guidance failed to meet factors of safety for rotational stability or the check of a minimum ratio of 2.5 for depth of penetration to wall height. 6. Discussion. This ETL provides guidance for detailed evaluation of I-walls and was developed under Phase III of the I-walls evaluation program. It resulted from research of I-wall ETL 1110-2-575 1 Sep 11 behavior and failure modes that expanded on the knowledge gained from the performance of the New Orleans Hurricane and Storm Damage Risk Reduction System (HSDRRS). Additional numeric model studies were performed on wall sections and soil types representing typical projects from around the United States. Analysis criteria were developed from the research and from past full-scale load tests. Risk and reliability were used to evaluate the research data and to determine factors of safety and deflections that provide appropriate performance. 7. Action. The guidance in Appendix B shall be used for the evaluation of existing !-walls. FOR THE COMMANDER: 4 Appendixes: JAMES C. DALTON, P.E., SES As listed in Table of Contents Chief, Engineering and Construction Division Directorate of Civil Works 2 DEPARTMENT OF THE ARMY ETL 1110-2-575 U.S. Army Corps of Engineers CECW-CE Washington, DC 20314-1000 Technical Letter No. 1110-2-575 1 September 2011 EXPIRES 1 June 2016 Engineering and Design EVALUATION OF I-WALLS TABLE OF CONTENTS Subject Paragraph Page APPENDIX A REFERENCES .......................................................................................................................... A-1 APPENDIX B GUIDANCE FOR EVALUATION OF I-WALLS Objective ............................................................................................................. B-1 B-1 Requirements for Evaluation ............................................................................... B-2 B-1 Geotechnical Information for Evaluation ............................................................ B-3 B-5 Stability and Seepage Failure Modes for Evaluation ........................................... B-4 B-6 Evaluation Performance Criteria .......................................................................... B-5 B-12 Additional Evaluation .......................................................................................... B-6 B-20 APPENDIX C STABILITY ANALYSIS OF I-WALLS CONTAINING GAPS BETWEEN THE I-WALL AND BACKFILL SOILS Organization of Appendix.................................................................................... C-1 C-1 Source of the Analysis Procedure Reported in this Appendix ............................. C-2 C-1 Introduction .......................................................................................................... C-3 C-1 How Gaps Form in Uniform Cohesive Soils ....................................................... C-4 C-3 Determining the Depth of the Gap ....................................................................... C-5 C-3 Modeling Gaps in Slope Stability Calculations ................................................... C-6 C-9 Summary .............................................................................................................. C-7 C-13 Example 1: Full-Depth Gap Analysis of I-wall in Uniform Soil Conditions .......................................................................... C-8 C-14 Example 2: Partial-Depth Gap Analysis of I-wall in Uniform Soil Conditions .......................................................................... C-9 C-15 Example 3: Partial-Depth Gap Analysis of I-wall in Layered Cohesive Soils ........................................................................... C-10 C-17 Example 4: Partial-Depth Gap Analysis of I-wall E-1 ETL 1110-2-575 1 Sep 11 Penetrating a Sand Layer ............................................................................. C-11 C-20 APPENDIX D COMPUTATION OF ROTATIONAL FACTOR OF SAFETY FOR I-WALLS ON LEVEES General .................................................................................................................D-1 D-1 Basic Procedure ...................................................................................................D-2 D-1 Example Section ..................................................................................................D-3 D-2 Computation of Correct Passive Pressure Forces ................................................D-4 D-3 CWALSHT Analyses Using the Corrected Passive Pressure ..............................D-5 D-3 FLAC Factor of Safety Analyses .........................................................................D-6 D-7 Summary of Rotational Factors of Safety ............................................................D-7 D-7 APPENDIX E I-WALLS EVALUATION FLOWCHART ............................................................................... E-1 GLOSSARY .................................................................................................................... Glossary-1 List of Figures Page Figure B-1 Potential crack propagation below flood-side gap assumed for seepage analyses .............................................................................................B-9 Figure B-2 Rotational factors of safety versus displacement at ground surface for I-walls ..............................................................................................B-14 Figure B-3 Computed displacement for varied tip depths, St. Paul I-wall ..........................B-17 Figure B-4 Relationship between factor of safety and deflection ........................................B-18 Figure B-5 Definition of water height for Table B-2 ...........................................................B-19 Figure C-1 Gap on the canal side of the Michoud Canal I-wall in New Orleans after Hurricane Katrina ........................................................................................C-2 Figure C-2 Gap formation for I-wall for uniform embankment and foundation soil properties .......................................................................................................C-4 Figure C-3 I-wall in layered cohesive soils ............................................................................C-6 Figure C-4 Pressure distributions against sheet-pile I-wall of Figure C-3 .............................C-6 Figure C-5 Situation for I-wall penetrating sand layer showing gap depth and resultant forces acting on sheet-pile wall .............................................................C-8 Figure C-6 Pressure distributions with sheet pile penetrating into a sand layer ....................C-8 Figure C-7 Modeling a full-depth gap in a slope stability analysis......................................C-10 Figure C-8 Illustration of “soil removal” method for partial gap conditions for slope stability analysis ..................................................................................C-11 Figure C-9 Water pressure and earth pressure applied to an I-wall for a partial gap analysis in cohesionless soil.............................................................C-13 Figure C-10 Cross-section for Example 1 ..............................................................................C-14 Figure C-11 Results of noncircular search using soil removal technique ..............................C-15 Figure C-12 Cross-section for Example 2 ..............................................................................C-16 Figure C-13 Results of partial gap analysis for uniform soil conditions using soil removal technique .............................................................................C-17 ii ETL 1110-2-575 1 Sep 11 Figure C-14 Cross-section for Example 3 ..............................................................................C-18 Figure C-15 Horizontal active earth pressure and water pressure profiles for determination of gap depth ..........................................................................C-19 Figure C-16 Results of noncircular search for partial gap analysis using soil removal technique .......................................................................................C-21 Figure C-17 Cross-section for Example 4 ..............................................................................C-21 Figure C-18 Pore-water pressure and effective active earth pressures acting on the sheet-pile wall ..............................................................................C-23 Figure C-19 Results of slope stability analysis ......................................................................C-24 Figure D-1 Analysis section .................................................................................................. D-2 Figure D-2 Slope/W model used to find the passive resistance FS for an actuating load............................................................................................ D-3 Figure D-3 Slope/W passive resistance for a fixed initial point of the slip surface at el -11.2 ............................................................................... D-4 Figure D-4 Input for the CWALSHT of the example section with water at el 10 ................ D-5 Figure D-5 Uniform pressure from the ground surface to el -11.2........................................ D-5 List of Tables Page Table B-1 Minimum Factors of Safety ..................................................................................B-15 Table B-2 Maximum Water Heights for Deformation Control .............................................B-19 Table C-1 Summary of Horizontal Active Earth Pressure and Water Pressure Calculations for Determination of the Gap Depth for Example Problem 3 ..........C-19 Table D-1 Summary of FLAC and CWALSHT Factors of Safety ......................................... D-7 iii ETL 1110-2-575 1 Sep 11 APPENDIX A REFERENCES EM 1110-2-1901 Seepage Analysis and Control for Dams EM 1110-2-1902 Slope Stability EM 1110-2-1913 Design and Construction of Levees EM 1110-2-2504 Design of Sheet Pile Walls EM 1110-2-2906 Design of Pile Foundations Brandon et al. 2008 Brandon, T. L., S. G. Wright, and J. M. Duncan. 2008. “Analysis of the Stability of I-walls with Gaps between the I-wall and the Levee Fill.” Journal of Geotechnical and Geoenvironmental Engineering 134(5): 692-700. Duncan et al. 2008 Duncan, J. M., T. L. Brandon, S. G. Wright, and N. Vroman. 2008. “Stability of I-Walls in New Orleans during Hurricane Katrina.” Journal of Geotechnical and Geoenvironmental Engineering 134(5): 681-691. GEO-SLOPE International 2007 GEO-SLOPE International. 2007. “SLOPE/W, Slope Stability Analysis Program.” http://www.geo-slope.com/ Headquarters, U.S. Army Corps of Engineers 2006a Headquarters, U.S. Army Corps of Engineers. May 23, 2006. “Implementation of Findings from the Interagency Performance Evaluation Task Force on I-wall Type Floodwalls.” Memorandum for Major Subordinate Commands. Washington, DC. Headquarters, U.S. Army Corps of Engineers 2006b Headquarters, U.S. Army Corps of Engineers. October 13, 2006. “Implementation of Findings from the Interagency Performance Evaluation Task Force on I-wall Type Floodwalls (second letter).” Memorandum for Major Subordinate Commands. Washington, DC. A-1 ETL 1110-2-575 1 Sep 11 IPET 2007 Interagency Performance Evaluation Task Force. 2007. “The Performance--Levees and Floodwalls.” Interagency Performance Evaluation of the New Orleans and Southeast Louisiana Hurricane Protection System, Final Report. Volume V. Washington DC. Slide 2005 Rocscience, Inc. 2005. “Slide v5.0 – 2D limit equilibrium slope stability analysis.” Toronto, Ontario. U.S. Army Engineer Research and Development Center 2007 U.S. Army Engineer Research and Development Center. 2007. “Design/Analysis of Sheet-Pile Walls by Classical Methods.” Computer-Aided Structural Engineering Project CASE/GCASE Program CWALSHT, Program X0031. http://case.wes.army.mil/ProgramsMain.aspx Wright 1999 Wright, Stephen G. 1999. “UTEXAS4, A Computer Program for Slope Stability Calculations.” Austin, TX: Shinoak Software. A-2 ETL 1110-2-575 1 Sep 11 APPENDIX B GUIDANCE FOR EVALUATION OF I-WALLS B-1. Objective. a. General. Evaluation of I-walls shall be performed by carefully considering all potential failure modes. This appendix provides criteria for evaluation of existing I-walls, including a detailed description of evaluation of stability and seepage. An I-wall is defined as a slender cantilever wall, deeply embedded in the ground or in an embankment. The wall rotates when loaded and is thereby stabilized by reactive lateral earth pressures. A description of performance requirements for the evaluation is provided that includes performance modes, basic criteria, and requirements for supporting analysis information. The failure modes to be evaluated are described and defined, as are the methods and tools to be used to analyze them. Safety factors to compare to limit equilibrium analyses are included. Guidance and design methodology for the flood-side gap between the sheet pile and soil are given. b. Limitations. (1) This document addresses stability, seepage, and deformation only. It does not contain new guidance for evaluation of the strength or serviceability of structural components. (2) The guidance is for I-walls intended to provide flood risk reduction for inland flooding, not for I-walls in coastal storm surge risk reduction projects where waves are a significant portion of the load and vessel impact is possible. B-2. Requirements for Evaluation. a. General Evaluation Performance for I-walls. (1) It is essential that the evaluation of the I-walls and their ancillary features critical to the operation of a levee system ensures the integrity to the top of the levee. The stability and operational adequacy of an I-wall during a storm event are of paramount consideration during inspection, assessment, and evaluation. The I-wall shall have sufficient robustness to survive without incurring the type of damage to the system that would impact its ability to prevent catastrophic flooding and maintain interior drainage operations. (2) Wall deformation under load is another requirement for evaluation. For more frequent events, it is desirable for I-walls to have little or no permanent deformation after loading although for unlikely events some amount of permanent deformation may be allowed. Requirements for deformation control are provided in this ETL through the use of maximum water elevations likely to result in acceptable deformation for different soil types and frequency of flood events. b. Engineering performance guidance for I-walls. I-walls will generally be evaluated using traditional limit-state type analyses, for which requirements and factors of safety are B-1