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Best Practices for Segmental Retaining Walls Best Practices


The intent of this material is to communicate the best practices for design of Segmental Retaining Walls (SRW) as determined by Allan Block Corporation based on 25 plus years of research, design and field experience. This is not meant to be a final authority as each project has its own set of unique situations.

The local engineer of record must use their best engineering judgment to account for those situations that present themselves and provide a safe and efficient design for the customer. At no time does the contractor or local building official have the authority to override the approved plans and specifications provided from the local engineer of record.

It is the recommendation of Allan Block Corporation that the local engineer of record work for and be paid by the project owner. It has been determined that the local engineer of record should be the Project Site Civil Engineer as they are best suited to take responsibility for the design, and how it affects the site, whether they do the design in-house or use an outside consultant to do the design for the project.

The Project Site Civil Engineer has control of several of the overall aspects of the project and therefore is most able to properly handle the integration and communication required to ensure the performance of the wall complies with the needs of the site. For wall design applications that are outside of the experience level of the Project Site Civil Engineer, a wall designer with the appropriate knowledge and experience should be contracted with by the Project Site Civil Engineer. It is recommended that the wall contractor not be responsible for securing the engineering.

last updated: 1/14/2022

Chapter 2: Typical Retaining Wall Construction

typical wall construction

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Click on the topics below to view more information on the best practices for Allan Block segmental retaining wall design for residental and commercial applications.

Design Guidelines Item: The term, "owner" refers to the property owner or their designated representative.

2.1    Inspection of Materials

2.1   Contractors must inspect all materials upon delivery to the site to assure proper material has been received. Contractor must protect all materials from damage or contamination prior to use within the wall (ASTM C1372). Utilize Allan Block Product Standards in the Allan Block Spec Book.

  1. No substitutions in geogrid or SRW units should be allowed
  2. Any changes in specified materials should be evaluated and approved by the SRW Designer and may require redesign.

2.2    Allowable Foundation Soils

2.2    Allowable soil to be used below the wall structure face.

  1. Geotechnical report should include parameters and recommendations for sub-soils.
  2. If poor soils are encountered during construction, consult with owner and geotech for removal and replacement recommendation.
  3. Utilize Section 3.1 Foundation Soils in the Allan Block Spec Book.
    1. Foundation soils to be inspected by the on-site soils engineer to ensure they meet or exceed design soil parameters.
    2. Over excavated areas shall be filled with compactable material approved by the on-site soils engineer.

2.3    Allowable Infill Soils

2.3    Allowable soil to be used in the reinforced mass.

  1. Best soil is wall rock or select/structural fill with less than 10% fines, to the limits of the geogrid lengths.
  2. If site soils are allowed by the geotechnical engineer; on site soils engineer to verify they meet the minimum requirements set forth in the soils report, and they are in compliance with the soil properties used in the design process.
  3. Utilize Section 1 Part 2.3 Infill Soils in the Allan Block Spec Book.
    1. Unsuitable soils for backfill (heavy clays or organic soils) shall not be used.
    2. Poorly graded sands, expansive clays and/or soils with a PI greater than 20 or a LL greater than 40 should not be used in wall construction.
    3. Fine grained cohesive soils with a friction angle less than 31 degrees with a PI ranging between 6 and 20 and LL from 30 to 40, may be used in wall construction, but additional backfilling, compaction and water management efforts, such as blanket drains and chimney drains are required.
    4. Soils with a PI of less than 6 and LL less than 30 are generally considered granular and can be used as infill material.
  4. The suggested gradation requirements for the reinforce (infill) soils in SRW’s are:
Sieve Size Percent Passing
25 mm 100-75
No. 4 (4.75 mm) 100-20
No. 40 (0.425 mm) 0-60
No. 200 (0.075 mm) 0-35

2.4    Wall Rock Guidelines

2.4    Wall rock column size and material used.

  1. The wall rock column is typically 30 cm deep directly behind the wall facing and consists of material summarized in Section 1 Part 2.2 in the Allan Block Spec Book.
    1. Material must be well-graded compactible aggregate, 0.6 cm – 3.8 cm with no more than 10% passing the #200 sieve. (ASTM D422)
  2. At the top of the wall, above the wall rock column it is common to place a horizontal layer of landscape fabric to protect the wall rock from being infiltrated by the plantable soil placed to finish the wall.

2.5    Soil Parameter Verification

2.5   Soil parameter verification.

  1. On-site soils engineer to verify and document that soils meet those specified in the soils report and wall design.
  2. Minimum infill soils to meet requirements outlined in Section 1 Part 2.3 in the Allan Block Spec Book: USCS Soil types (GP, GW, SW, SP, GP-GM and SP-SM) are ideal select/structural fill, with PI less than 6 and LL less than 30, but if soils with friction angle less than 31 degrees are to be used (PI of less than 20 and LL of less than 40) special installation and drainage details and specifications are required, see Chapter 1, Section 1.6 and Chapter 6 for more information.

2.6    Typical Wall Embedment

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

  1. A commonly used embedment depth calculation for walls with level ground below is 2.5 cm of depth per 30 cm of wall height with a typical minimum of 15 cm for commercial projects.
  2. For walls with slopes above or below see Chapter 9 Global Stability.

2.7    Base Trench Requirements

2.7    Base trench verification and compaction requirements.

  1. Typical depth of the trench is based on a minimum 15 cm deep compacted wall rock base and buried block depth equal to 2.5 cm of depth per 30 cm of wall height, with a typical minimum of 15 cm for commercial projects.
  2. Design methodology used may require deeper wall trenches as would slopes below.
  3. Base trench to be compacted to the level specified in the geotechnical report and inspected by the on-site soils engineer prior to any base material being placed.

2.8    Base Trench Considerations

2.8    Imported base material, size of base, compaction requirements of base and location of the base course unit on the base material.

  1. Typical base material used is consistent with wall rock. For more information on wall rock see Chapter 2, Section 2.4.
  2. 15 cm deep by 60 cm wide (or 30 cm wider than the block depth) is the typical minimum base size. A larger base may be required for poor foundation soils in order to meet minimum bearing factors of safety.
  3. Poor soils should be removed and replaced, unless the engineer provides guidance based on the application.
  4. Wall unit is typically centered on the base.
  5. For walls with a deeper than typical minimum sized base, it is common to add a continuous longitudinal layer of geogrid, 7.5 cm to 15 cm above the bottom of the trench. This geogrid provides additional stability to the compacted base and helps to span lesser quality soils.

2.9    Minimum Grid Lengths

2.9    Minimum grid lengths.

  1. 60% of total wall height measured from the top of the base to the top of the top wall block.

    Example – 3 m wall, 1.8 m minimum long grids. Grid lengths are measured from the face of the wall.

2.10    Initial Grid Location

2.10    Typical location of initial layer of grid.

  1. The first layer of geogrid should be placed on top of the base course of block and every other course from that point
    (Figure 2-1).
Grid Location
Figure 2-1: Grid Location

2.11    Maximum Grid Spacing

2.11    Maximum suggested grid spacing.

  1. 40 cm maximum spacing provides the best overall performance of the reinforced structure.

2.12    Capping the Wall

2.12    For walls using the manufacturer’s optional capping system or approved equal:

  1. Capping system shall be secured in place using a high quality flexible exterior grade masonry sealant, see Figure 2-2.
Cap Adhesive
Figure 2-2: Cap Adhesive