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 12: Above Wall Considerations

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Above Wall Construction Details Including Fences and Slopes Using Allan Block Best Practices

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12.1   Minimum grid lengths.

  1. Top layers of grid should be lengthened to 90% of wall height or an additional 3 ft (0.9 m), whichever is greater, in seismically active areas or with any type of loading above the wall including slopes and surcharges. In the event that an interference may exist with the top layer based on other structures in the reinforced zone it is acceptable to move this extended layer to a lower grid elevation at the top of the structure.
  2. The portion of wall above the top layer of geogrid should be reviewed as a gravity wall taking into account all soils and surcharge parameters.

12.2   Details for fencing and other potential lateral loads from objects located above or within the reinforced soil mass.

  1. Impact structures or fence posts should be positioned a minimum of 3 ft (0.9 m) from the back of top course to allow a properly designed load for local overturning.
  2. If fence posts must be placed within 3 ft (0.9 m) of the back of the wall facing, the designer must consider the localized top of wall overturning force into their design.
  3. Common lateral loads from railings are 50 lb/lf (0.74 kN/m) along the top of the railing or 200 lb (2.92 kN/m) point load at the top of the post or both depending on the local requirements.
  4. Common post footings are formed by using construction tubes placed at desired on-center locations during construction. If posts are placed after construction is complete, holes must be hand dug as using a power auger will cause severe damage to the geogrid layers.
  5. If the post design calls for an engineered product to solve localized overturning forces, a product such as the Sleeve-IT System can be specified. Sleeve-IT is a Stratagrid product designed for use in segmental retaining walls to resist top of wall overturning forces from fences and railing. For more information on design capacities and installation, contact Stratagrid at www.geogrid.com.

12.3   Details for placement and compaction of a slope above the wall structure.

  1. Proper lift and compaction recommendation should be followed from the geotechnical recommendations. See Chapter 6.0 Soils and Compaction.

12.4    Stability of slopes above the wall.

  1. There are limits to the steepness of a slopes above a wall that must be considered when designing any soil structure.
    1. In static designs, the maximum unreinforced slope above any wall cannot exceed the friction angle of the soil used to reconstruct the slope.
    2. In seismic designs, the maximum unreinforced slope above any wall cannot exceed the friction angle of the soil used to reconstruct the slope minus the seismic inertial angle. The seismic inertial angle is based on the horizontal and vertical seismic coefficients ( kh and kv ) determined by the soils engineer for that specific project site.
    If the desired slope above exceeds either of the two limits above, the designer must analyze the slope above in a global stability program and provide slope reinforcement as required.
  2. For any wall having a slope above greater than 3:1, and/or any slope with poor soils or walls with seismic requirements, it is recommended that the designer call for the slope to be reconstructed with stabilizing geogrid layers. These layers typically match the standard grid lengths in the wall along with their spacing.

12.5   Slopes above the wall must be compacted following the proper lift and compaction recommendation from the geotechnical recommendations. See Chapter 6.0, Soils and Compaction.

12.6   Reinforcing the slope above the wall may be necessary to provide adequate stability to the wall system. Geogrid lengths typically matching the standard grid lengths in the wall along with their spacing are commonly used in walls with steep slopes above, greater than 3:1, and/or any slopes with poor soils or walls with seismic requirements.

12.7   Details for plantings located above the wall structure.

  1. In general, plantings in the reinforced zone are acceptable and may in fact enhance the reinforced soil mass provided:
    1. Engineer should review with the owner the planting plans from the landscape architect to verify size of root balls and if any geogrid layers will be disrupted.
    2. Language can be placed in the project specifications.
    3. Augers should never be used within the reinforced grid zone.
    4. Consider the effect of excess irrigation. See Water Management, Chapter 3.0.