ENGINEERING BULLETIN EB 20-004 Title: HDM CHAPTER 9: SECTION 9.4 RETAINING WALLS AND REINFORCED SOIL SLOPES Approved: /s/ Robert L. Sack 1/10/2020 Robert L. Sack, P.E. Date Deputy Chief Engineer (Research) Expires one year after issue unless replaced sooner ADMINISTRATIVE INFORMATION: • This Engineering Bulletin (EB) is effective upon signature. • This EB does not supersede any other issuance. • Disposition of issued materials: The information transmitted by this issuance will be incorporated into the Highway Design Manual Chapter 9 Soils, Walls, and Foundations. PURPOSE: The purpose of this EB is to announce the availability of the revision to the Highway Design Manual Chapter 9: Section 9.4 Retaining Walls and Reinforced Soil Slopes. TECHNICAL INFORMATION: • A revised Geotechnical Engineering Manual (GEM-16) Mechanically Stabilized Earth System Inspection Manual is being issued concurrently via EB 20-003. • A revised Standard Specification Section 554 Fill Type Retaining Walls is being issued concurrently via EI 20-001. • The revisions to Section 9.4 Retaining Walls and Reinforced Soil Slopes includes the following: 1. Section 9.4.2.1 Externally Stabilized Cut Structures C. – added information regarding Braced Walls. 2. Section 9.4.2.3 Internally Stabilized Fill Structures C. – differentiated between a GRSS Slope and a GRSS Wall. 3. Section 9.4.2.3 C.2. GRSS Slopes – included a warning regarding the susceptibility to erosion. 4. Section 9.4.2.3 C.3. Vegetated Face Vertical GRSS – included additional information regarding geocells. 5. Section 9.4.5 Wall Design & Detailing – added a summary of the major design concerns with a cut type retaining wall in A. Cut Type Retaining Walls. 6. Section 9.4.5 Wall Design & Detailing – identified the specific design requirement manual in A1. Sheeting, Soldier Pile & Lagging Wall, or Anchored Walls. 7. Section 9.4.5 Wall Design & Detailing – identified the specific design requirement manual in A2. Soil Nail Walls. 8. Section 9.4.5 Wall Design & Detailing – added a summary of the major design concerns with a fill type retaining wall in B. Fill Type Retaining Walls. 9. Section 9.4.5 Wall Design & Detailing – identified the specific design requirements in B1. Cast-in-place/Precast Cantilever/Gravity Walls. 10. Section 9.4.5 Wall Design & Detailing – identified the specific design requirements in B2. Proprietary Retaining Wall Systems. 11. Section 9.4.5 Wall Design & Detailing – Expanded on the designer’s responsibilities in B2. Proprietary Retaining Wall Systems. 12. Section 9.4.5 Wall Design & Detailing – Added guidance regarding the assumed fill wall volume to address constructability issues in B2b. Fill Type Retaining Wall Volume. EB 20-004 Page 2 of 2 13. Section 9.4.5 Wall Design & Detailing – Added guidance and figures from the FAQ’s regarding jersey barrier installation in B2c. Fill Type Retaining Walls Supporting a Shoulder and Requiring a Jersey Barrier. 14. Section 9.4.5 Wall Design & Detailing – Added guidance and figures from the FAQ’s regarding subsurface installations conflicting with the fill type retaining wall volume in B2d. Addressing Conflicts with the Fill Type Retaining Wall Volume and Surface/Subsurface Installations. 15. Section 9.4.5 Wall Design & Detailing – Added guidance regarding the choices in the Approved List and highway work permits in B2e. The Exception – Highway Work Permits. 16. Section 9.4.5 Wall Design & Detailing – identified the specific design requirement manual in B4. Geosynthetically Reinforced Soil System (GRSS). TRANSMITTED MATERIALS: Revision to Section 9.4 Retaining Walls and Reinforced Soil Slopes BACKGROUND: Retaining walls and reinforced soil slopes are used in areas where free-standing (natural) earth slopes are undesirable, usually because of space restrictions. These walls have, in the past, normally been poured, reinforced concrete (gravity or cantilever), timber, steel, or precast concrete cribbing, stone-filled wire-basket gabions, timber or steel sheeting, or steel soldier pile and lagging walls, all of which provide external support to the retained soil mass. Several innovations in types of retaining walls have become available, including using inherent characteristics of retained or reinforced soil as part of the support system. Inadequate drainage of the backfill material can result in unsatisfactory long-term performance of the retaining wall system. A subsurface drainage system is typically installed simultaneously with the erection of the wall to ensure a continuous, uninterrupted system to serve to prevent the accumulation of destabilizing water pressure on the wall. CONTACT: Questions or comments regarding this issuance should be directed to Randall J. Romer, P.E., of the Geotechnical Engineering Bureau at (518) 457-4714, or via e-mail at [email protected]. SOILS, WALLS, AND FOUNDATIONS 9-75 9.4 RETAINING WALLS AND REINFORCED SOIL SLOPES Retaining walls and reinforced soil slopes are used in areas where free-standing (natural) earth slopes are undesirable, usually because of space restrictions. These walls have, in the past, normally been poured reinforced concrete (gravity or cantilever), timber, steel, or precast concrete cribbing, stone-filled wire-basket gabions, timber or steel sheeting, or steel soldier pile and lagging walls, all of which provide external support to the retained soil mass. Several innovations in types of retaining walls have become available, including using inherent characteristics of retained or reinforced soil as part of the support system. Inadequate drainage of the backfill material can result in unsatisfactory long-term performance of the retaining wall system. A subsurface drainage system is typically installed simultaneously with the erection of the wall to ensure a continuous, uninterrupted system to serve to prevent the accumulation of destabilizing water pressure on the wall. Regional Designers should consult with the Regional Geotechnical Engineer during the retaining wall design phase (see Section 9.4.4 Wall Selection Process). Good, long term performance of any wall is dependent on the use of well-compacted, good quality backfill. It is not possible to adequately compact backfill in below-freezing temperatures, unless special material that does not require water for compaction (i.e. crushed stone) is used. 9.4.1 Definitions There are three categories of support systems based on their intended functional life: permanent, temporary, and interim. 1. Permanent: A permanent system provides a structural support function for the life of the facility. 2. Temporary: A temporary system is designed to provide structural support during construction, and is removed when construction is complete. 3. Interim: An interim system is identical to a temporary system in function, except it remains in place (although it no longer provides a structural function) because its removal would be detrimental to the finished work. The classification of retaining wall systems is based on the basic geotechnical mechanism used to resist lateral loads and the construction method used for the installation of the wall. The following are definitions used to classify retaining wall systems: 1. Externally Stabilized Structures: Externally stabilized structures rely on the integrity of wall elements (with or without braces, struts, walers and/or tiebacks or anchors) to both resist lateral loads and also prevent raveling or erosion of the retained soil. 2. Internally Stabilized Structures: Internally stabilized structures rely on friction developed between closely-spaced reinforcing elements and the backfill to resist lateral soil pressure. A separate, non-structural element (facing, erosion control mat and/or vegetation) is attached to prevent raveling or erosion of the reinforced soil. 3. Fill Type Retaining Walls: Retaining structures constructed from the base of the wall to the top (i.e. “bottom-up” construction). 4. Cut Type Retaining Walls: Retaining structures constructed from the top of the wall to the base (i.e. “top-down” construction). An overview of the classification of retaining wall systems is provided in Table 9-6. The table provides a breakdown of available retaining wall systems, its associated method of construction, means of stability, design requirements and constraints (e.g. typical height range, maximum wall height). 1/10/20 §9.4 9-76 SOILS, WALLS, AND FOUNDATIONS Table 9-6 Classification of Retaining Wall Systems(14m) Wall Wall Wall Construction Wall Design Constraints Class Type Type1 Group Typical Height Range: Designed & detailed Sheeting Walls Cut Wall Cantilever 6 ft. to 15 ft. in contract. Maximum Wall Height= 15 ft. Soldier Pile & Designed & detailed Typical Height Range: Cut Wall Cantilever Lagging Walls in contract. 6 ft. to 15 ft. Externally Deadman Designed & detailed Stabilized Anchors in contract. Cut Anchored or Detailed in contract. Typical Height Range: 15 ft. to Structures Braced Walls Grouted Designed by 65 ft. (Sheeting or Cut Wall Tiebacks Contractor’s Design Soldier Pile & Consultant. Lagging Walls) Designed & detailed Braced Walls in contract. Precast Designed & detailed Typical Height Range: 6 ft. to Primarily Fill Cantilever in contract. 30 ft. Wall. May be Wall Cantilever Wall installed as a CIP Typical Height Range: 6 ft. to Designed & detailed Cut wall. Cantilevered 30 ft. in contract. Wall Maximum Wall Height = 30 ft. Typical Height Range: 6 ft. to Designed & detailed Gabion 20 ft. in contract. Externally Maximum Wall Height = 20 ft. Stabilized Primarily Fill Fill Wall.