Table Of Contents

Best Practices for Segmental Retaining Wall Design
The intent of this document is to communicate the best practices for design of Segmental Retaining Walls (SRW) as determined by Allan Block Corporation based on 30 plus years of research, design and field experience.
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Part 2 - Best Practice Considerations

Chapter 1.0 Design Guidelines and Pre-Construction Considerations
  • 1.1 Meeting with Owner
  • 1.2 Determining when Engineering is Required
  • 1.3 Existing and Proposed Utilities
  • 1.4 Wall Layout, height and geometry
  • 1.5 Geotechnical Report Considerations
  • 1.6 Understanding sites soils
  • 1.7 Site Visit
  • 1.8 Temporary Load Considerations
  • 1.9 Scope of Responsibility and Design Methodologies
  • 1.10 Minimum Design Safety Factors
  • 1.11 Coherent Gravity Mass and Connection Strength Considerations
  • 1.12 Contractor Requirements
  • 1.13 Manufactured Product Specifications
  • 1.14 Freeze Thaw Durability
  • 1.15 Pre-Construction Meeting
  • 1.16 Visiting the Site During Construction
  • 1.17 Construction Drawings
  • 1.18 For the Bidding Process
  • 1.19 Quality Control, Quality Assurance
   

Chapter 2.0 Typical Wall Construction
  • 2.1 Inspection of Materials
  • 2.2 Allowable Foundation Soils
  • 2.3 Allowable Infill Soils
  • 2.4 Wall Rock Guidelines
  • 2.5 Soil Parameter Verification
  • 2.6 Typical Wall Embedment
  • 2.7 Base Trench Requirements
  • 2.8 Base Trench Considerations
  • 2.9 Minimum Grid Lengths
  • 2.10 Initial Grid Location
  • 2.11 Maximum Grid Spacing
  • 2.12 Minimum Wall Facing Depth
  • 2.13 Capping the Wall
   

Chapter 3.0 Water Management - Typical
  • 3.1 Identifying Potential Water Sources
  • 3.2 Blanket and Chimney Drains
  • 3.3 Venting of Drain Pipes
  • 3.4 Above Grade Water Management
   

Chapter 4.0 Water Management - Alternate Drain
  • 4.1 Alternate Drain Locations
  • 4.2 Heel Drain Recommendations
   

Chapter 5.0 Water Application
  • 5.1 Below Grade Water Management
  • 5.2 Water Application Construction
   

Chapter 6.0 Soil and Compaction
  • 6.1 Understanding sites soils
  • 6.2 Allowable Foundation Soils
  • 6.3 Allowable Infill Soils
  • 6.4 Wall Rock Guidelines
  • 6.5 Soil Parameter Verification
  • 6.6 Inspection and Testing Recommendations
  • 6.7 Compaction Requirements at the Face of Wall
  • 6.8 Maximum Compaction Lift Spacing
  • 6.9 Compaction Requirements for Backfill Soil
  • 6.10 Testing Location and Frequencies
  • 6.11 Water Management During Construction
  • 6.12 Wall Step Ups in Base Course
  • 6.13 Stair Considerations
   

Chapter 7.0 Geogrid Reinforcement Requirements, Corner and Radius Design Practices
  • 7.1 Geogrid Reinforcement Requirements and Certification
  • 7.2 Proper Grid Orientation
  • 7.3 Wall Rock Design for Corners and Curved Walls
   

Chapter 8.0 Tall Walls Considerations
  • 8.1 Tall Wall Definition
  • 8.2 Variable Rock Thickness at Face
  • 8.3 Compaction and Soil Considerations
  • 8.4 Increased Forces in Lower Portion of Walls
  • 8.5 Global Stability of Tall Walls
  • 8.6 Internal Compound Stability Calculations
  • 8.7 Minimum Wall Facing Depth
   

Chapter 9.0 Global Stability - General
  • 9.1 Wall Embedment with Toe Slope
  • 9.2 When to Analyze for Global Stability
  • 9.3 Increasing Global Stability Options
  • 9.4 Effect of Groundwater on Global Stability
   

Chapter 10.0 Global Stability - Terraced
  • 10.1 Terraced Wall Considerations
  • 10.2 Upper Wall Influence - Surcharge
  • 10.3 Height and Grading
  • 10.4 Grid Considerations
  • 10.5 Compaction and Testing
  • 10.6 Toe and Heel Drain
  • 10.7 Global Stability
  • 10.8 Tall Wall Terraces
   

Chapter 11.0 Seismic Considerations
  • 11.1 Recommendations Associated with Seismic Loading
  • 11.2 Slope Above Seismically Loaded Walls
  • 11.3 Mononobe-Okabe Slope Above Limitations
  • 11.4 Alternate Design Approach – Trial Wedge Method
   

Chapter 12.0 Above Wall Considerations
  • 12.1 Minimum Grid Lengths at the Top of the Wall
  • 12.2 Fences and Railings
  • 12.3 Slopes Above the Wall
  • 12.4 Stability of Slopes Above
  • 12.5 Compaction Requirements for Slopes Above
  • 12.6 Reinforcing Slopes Above Walls
  • 12.7 Plantings
   

Allan Block Resources
Allan Block Spec Book
AB Engineering Manual
AB Commercial Manual
Seismic Testing Book
   
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Chapter 9: Global Stability - General

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9.1   Wall embedment depth should be determined by the wall design engineer based on typical industry standard and specific site requirements.

  1. Walls with slopes below must have additional buried course for stability. For walls with slopes below, local ordinances commonly reference to bury enough blocks to have a 5 - 7 ft (1.5 - 2.1 m) level bench in front of the wall, and then the 1 in (2.5 cm) of depth per 1.0 ft (30 cm) of wall height rule is used beyond that point, Figure 9-1. After the wall is complete, the level area can be backfilled to continue the appearance of a continuous slope.
  2. In general when a slope below the wall is present, there is need for a global stability analysis to confirm the overall stability of the site.
Wall Embedment with Toe Slope

Figure 9-1: Toe Slope Embedment


9.2   If a global stability analysis is not included in the wall designer's scope of work, establish that when a global stability analysis is required by the wall designer, using their best engineering judgment, then the owner will contract with a geotechnical engineer for overall site stability. Global stability analysis should be run for a variety of conditions including:

  1. For those walls with slopes below or above.
  2. Walls built in sites with clays, silts, poorly graded sands, expansive clays and/or soils with a PI greater than 20 and or a LL greater than 40.
    1. The infill soil used must meet or exceed the designed friction angle and description noted on the design cross sections, and must be free of debris and consist of one of the following inorganic USCS soil types: GP, GW, SW, SP, GP-GM or SP-SM meeting the following gradation as determined in accordance with ASTM D422.
  3. For walls where more than 50% of the Internal Compound Stability (ICS) slip arcs fall at the back of the wall design envelope when ICS is run in AB Walls.
Sieve Size Percent Passing
1 in (24 mm) 100-75
No. 4 (4.75 mm) 100-20
No. 40 (0.425 mm) 0-60
No. 200(0.075 mm) 0-35
Global Stability analysis detail

Figure 9-2: Design Envelope ICS

We have identified a Design Envelope for the wall designer that defines the extents of an ICS evaluation within that envelope. An ICS analysis does not take into account a slope below the wall, as the bottom slip arc is located at the base of the bottom block. Therefore it will not provide guidance for what happens below the wall. One easy to identify guide for additional global evaluation may be found by reviewing where the ICS slip arcs with the minimum factors of safety originate. When a concentration of ICS slip arcs originate at the back of the Design Envelope, a complete global stability analysis should be conducted, See Figure 9-2.

9.3   For walls with global stability concerns, it is common to increase the stability by:

  1. Increasing the length of the geogrid layers to force the minimum slip arcs deeper into the hillside.
  2. Increasing the depth of buried block to force the minimum slip arcs deeper into the hillside.
  3. Increasing the strength of the geosynthetic reinforcement layers to force the minimum slip arcs deeper into the hillside.
  4. Increasing the friction angle of the infill soil will increase the soil’s shear resistance which will increase stability.
  5. If the cause for the global stability problems is weak foundations soils, it may be necessary for the geotechnical engineer on the project to do foundation improvements before the retaining walls are built.

9.4   Whenever the groundwater is within 0.66H of the bottom of the wall, the global stability of the SRW system should be analyzed to assure that the adequate factor of safety exists for deep failures that pass behind the geosynthetic reinforcement and for compound failures that pass partially through the reinforced soil mass and partially behind the reinforced soil mass.