Chapter 6: Soil & Compaction


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updated: 1/14/2022

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.

Soil and Compaction Drawing
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6.1   Allowable Foundation Soils

6.1    Understanding the site soils as well as the soils used in the infill zone is essential to understanding how the retaining wall will

function. One of the economic advantages to an SRW system is that site soils can usually be used in the infill zone provided they are of a certain quality and surface and groundwater conditions at the site are controlled by recommendations given in Chapter 3, Chapter 4, and Chapter 5.

While cohesionless, free graining materials (less than 10% fines and or PI less than 6 and LL less than 30) are preferred, soils with low plastic fines (i.e. SC with PI less than 20 and LL less than 40) may be used for lower height SRW construction provided the following additional design criteria are implemented:

  1. Proper internal drainage is installed including wall rock in and behind the facing and blanket and chimney drains to keep the infill mass dry, see Chapter 3, Chapter 4, and Chapter 5.
  2. In areas where frost heaves are possible, only soils with low to moderate frost heave potential shall be utilized. Verify parameters with geotech. Expanding the depth of wall rock behind the facing can help reduce the effects of frost heaves. See Chapter 6, Section 6.4 for information on the wall rock column.
  3. The cohesive shear strength parameter (c), for the reinforced fill, is ignored for internal and external stability analysis. Cohesion values are allowed in the foundation and global stability analysis. However, it is recommended that no more than 10% of the tested/reported values should be used due to the unpredictability of cohesive soils.
  4. The final design is checked by a qualified geotechnical engineer to ensure that the use of cohesive soils does not result in unacceptable time-dependent movement of the SRW system.
  5. High plastic or organic soils including MH, CH, OH, OL and PT are not recommended for any segmental retaining wall construction as their use can cause excessive settling over time and or excessive internal stress to build up causing internal lateral forces to occur. See Chapter 8 for more information on tall walls.

6.2   Understanding Site Soils

6.2   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.

6.3   Allowable Infill Soils

6.3   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. 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 of 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. Some poorly graded sands may require additional fabric to prevent migration.
  4. The suggested gradation requirements for the reinforced (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

6.4   Wall Rock Guidelines

6.4    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.

6.5   Soil Parameter Verification

6.5    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 6, Section 6.1.

6.6   Inspection and Testing Recommendations

6.6    Establish inspection and testing requirements in advance of providing a design. The professional in charge of this work must be retained by the owner.

  1. Size and scope of project will require different levels of testing and inspection.
  2. An independent testing firm should be hired by the owner to provide services.
  3. Independent firm to keep inspection log and provide written reports at predetermined intervals to the owner.
  4. Testing frequency should be set to establish a proper compaction protocol to consistently achieve the minimum compaction requirements set by the design requirements. If full time inspection and testing at 8 in (20 cm) lifts is not provided, then the following testing frequency should be followed:
    1. One test for every 8 in (20 cm) of vertical fill placed and compacted, for every 25 lineal ft (7.6 m) of retaining wall length, starting on the first course of block.
    2. Vary compaction test locations to cover the entire area of reinforced zone; including the area compacted by the hand-operated compaction equipment.
    3. Once protocol is deemed acceptable, testing can be conducted randomly at locations and frequencies determined by the on-site soils engineer.
  5. Greater attention to compaction and compaction testing should be placed on the foundation soils below the upper terraces and in transition areas where the wall splits from one wall into two. If the soils are not properly compacted in these areas, settlement can occur over time that could cause aesthetic concerns.
  6. Slopes above the wall must be compacted and checked in a similar manner.

6.7   Compaction Requirements at the Face of Wall

Compaction path first course

Figure 6-1: Compaction Path - First Course

Compaction Path - Second Course and Above

Figure 6-2: Compaction Path - Second Course and Above

6.7    Compaction requirements on wall rock and facing unit.

  1. Hand operated plate compactor to be used behind the base course within the 3 ft (0.9 m) wide consolidation zone with a minimum of two passes, Figure 6-1.
  2. Compaction of second course and above will begin by running the plate compactor directly on top of the block facing and then compacting in parallel paths from the wall face until the entire consolidation zone has been compacted, Figure 6-2.
  3. Utilize Section 1 Part 3.4 E in the Allan Block Spec Book.
    1. Final compaction requirements in the consolidation zone shall be established by the engineer of record, typically 95% of Standard Proctor.
    2. A minimum of two passes of the plate compactor are required with maximum lifts of 8 in. (20 cm).
    3. Expansive or fine-grained soils may require additional compaction passes and/or specific compaction equipment such as a sheepsfoot roller.
    4. Maximum lifts of 4 in. (10 cm) may be required to achieve adequate compaction within the consolidation zone.
    5. Employ methods using lightweight compaction equipment that will not disrupt the stability, alignment or batter of the wall.
    6. Install each subsequent course in like manner. Repeat procedure to the extent of wall height.

6.8   Maximum Compaction Lift Spacing

6.8    Block and infill soil placed in 8 in (20 cm) lifts maximum.

  1. Industry Standard from Allan Block Corporation, CMHA, AASHTO, FHWA, etc.

6.9   Compaction Requirements for backfill Soil

6.9    Compaction requirements for all soils in areas in, above and behind the reinforced mass.

  1. Typical minimum requirements are: 95% of maximum Standard Proctor dry density (ASTM D698) with a moisture content control of +1% to -3% of optimum. Section 13.5.3, NCMA TR 127 (Design Manual for segmental retaining walls).

6.10   Testing Locations and Frequencies

6.10   Compaction testing locations and frequencies. Documentation of on-site soil testing.

  1. See Section 6.6 above.

6.11   Water Management During Construction

6.11   Water Management During Construction.

  1. Utilize Section 3 Part 1.1 Surface Drainage in the Allan Block Spec Book.
    1. At the end of each day’s construction and at final completion, grade the backfill to avoid water accumulation behind the wall or in the reinforced zone.

6.12   Wall Step Ups in Base Course

retaining wall Step Up Detail

Figure 6-3: Step Up Detail

6.12   For walls with step ups in the base course, extra care should be given to properly compact the base material at the end of each course to add greater stability to the next course, Figure 6-3.

6.13   Stair Considerations

Step Up Detail with retaining wall stairways

Figure 6-4: Stair Detail

6.13   For a walls with steps built into the wall, additional compaction requirements and wall rock should be included into the project specs,
Figure 6-4.

  1. A minimum of 6 in (15 cm) of wall rock base material shall be installed beneath each tread block, treating each course as a wall base course.
  2. Minimum compaction requirements for standard wall construction shall be followed. See Section 6.7 above.