Chapter 2: Typical Retaining Wall Construction


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updated: 6/30/2016

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.

Typical Retaining Wall Drawing
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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
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

2.4    Wall Rock Guidelines

2.4    Wall rock column size and material used.

  1. The wall rock column is typically 12 in (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, 0.25 - 1.5 in, (0.6 – 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 topsoil placed to finish the wall. Topsoil is defined as low permeable soils ranging from 8 - 12 in. (20 – 30 cm) to minimize infiltration of surface water into the reinforced mass.
  3. When designing curved and radius wall segments the designer should provide details to the installer for wall rock placement.
    1. For inside curved and inside cornered walls, the minimum wall rock specified should follow Section 6.4 (paragraph a) and Section 8.2 for Tall Wall applications.
    2. For outside curved and outside cornered walls, additional depth of wall rock should be specified to promote greater stability in these areas. Additional depth of wall rock is dependent on the total height of the wall and should be a minimum of H/2 as detailed in Drawing No. 6 in the front of this document.

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 1 in (2.5 cm) of depth per foot (30 cm) of wall height with a typical minimum of 6 in (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 6 in s5 cm) deep compacted wall rock base and buried block depth equal to 1 in (2.5 cm) of depth per foot (30 cm) of wall height, with a typical minimum of 6 in (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. 6 in (15 cm) deep by 24 in (60 cm) wide (or 12 in (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 deeper facing units such as AB Fieldstone with the Long Anchoring unit (LAU), the typical base width would be 36 in (90 cm).
  6. For walls with a deeper than typical minimum sized base, it is common to add a continuous longitudinal layer of geogrid, 3 in (7.5 cm) to 6 in (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 – 10 ft (3 m) wall, 6 ft (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 7-1).

2.11    Maximum Grid Spacing

2.11    Maximum suggested grid spacing.

  1. 16 in (40 cm) maximum spacing provides the best overall performance of the reinforced structure.

2.12   Minimum Wall Facing Depth

2.12    Minimum Wall Facing Depth.

  1. The recommendations contained within this guide, including those for geogrid spacing and length as discussed in 2.9 – 2.11 above, are based on the use of a facing unit that has an average (front to back) depth of at least 10 in. (25 cm). The 10 in. (25 cm) minimum depth of facing unit is recommended because SRW units of this depth or greater have a proven performance record while in service over many decades. This recommendation also recognizes that as the unit depth decreases, so does the stability of the facing, particularly overturning resistance during construction or while in service.
  2. Facing units having a minimum depth of less than 10 in. (25 cm) should be limited to walls not exceeding 10 ft (3 m) in total height.

2.13    Capping the Wall

2.13    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 2-1.

Cap Adhesive
Figure 2-1: Cap Adhesive