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Specifications for Modular Segmental Retaining Walls   PDF  Print this Document

The following specifications provide Allan Block Corporation's typical requirements and recommendations. At the engineer of record's discretion these specifications may be revised to accommodate site specific design requirements.

Section 1

Part 1: General

1.1 Work includes furnishing and installing modular concrete block retaining wall units to the lines and grades designated on the construction drawings and as specified herein.

1.2 Applicable Sections of Related Work

Section 2: Geogrid Wall Reinforcement

1.3 Reference Standards

  1. ASTM C1372 Standard Specification for Segmental Retaining Wall Units.
  2. ASTM C1262 Evaluating the Freeze thaw Durability of Manufactured CMU’s and Related concrete Units
  3. ASTM D698 Moisture Density Relationship for Soils, Standard Method
  4. ASTM D422 Gradation of Soils
  5. ASTM C140 Sample and Testing concrete Masonry Units

1.4 Delivery, Storage, and Handling

  1. Contractor shall check the materials upon delivery to assure proper material has been received.
  2. Contractor shall prevent excessive mud, cementitious material, and like construction debris from coming in contact with the materials.
  3. Contractor shall protect the materials from damage. Damaged material shall not be incorporated in the project (ASTM C1372).

1.5 Contractor Requirements

Contractors shall be trained and certified by local manufacturer or equivalent accredited organization.

  1. Allan Block and NCMA have certification programs that are accredited. Identify when advanced certification levels are appropriate based on complexity and criticality of project application.
  2. Contractors shall provide a list of projects they have completed.

Part 2: Materials

AB retaining wall block Approximate Dimensions

Standard AB Unit - Approximate Dimensions

2.1 Modular Wall Units

  1. Wall units shall be Allan Block Retaining Wall units as produced by a licensed manufacturer.
  2. Wall units shall have minimum 28 day compressive strength of 20.7 MPa in accordance with ASTM C1372. The concrete units shall have adequate freeze-thaw protection with an average absorption rate in accordance with ASTM C1372 or an average absorption rate of 120 kg/m3 for northern climates and 160 kg/m3 for southern climates.
  3. Exterior dimensions shall be uniform and consistent. Maximum dimensional deviations on the height of any two units shall be 3 mm.
  4. Wall units shall provide a minimum of 555 kg total weight per square meter of wall face area (555 kg/m2). Hollow cores to be filled with wall rock and compacted by using plate compactor on top of wall units (see section 3.4). Unit weight of wall rock in cores may be less than 100% depending on compacted base.
  5. Exterior face shall be textured. Color as specified by owner.
  6. Freeze Thaw Durability: Like all concrete products, dry-cast concrete SRW units are susceptible to freeze-thaw degradation with exposure to de-icing salts and cold temperature. This is a concern in northern tier states that use deicing salts. Based on good performance experience by several agencies, ASTM C1372, Standard Specification for Segmental Retaining Wall Units should be used as a model, except that the compressive strength for the units should be increased to a minimum of 28 - 40 MPa unless local requirements dictate higher levels. Also, maximum water absorption should be reduced and requirements for freeze-thaw testing increased.
    • Require a current passing ASTM C1262 or equivalent governing standard or public authority, test report from material supplier in northern or cold weather climates.
    • See the Best Practices for SRW Design document for detailed information on freeze thaw durability testing criteria and regional temperature and exposure severity figures and tables to define the appropriate zone and requirements for the project.

2.2 Wall Rock

  1. Material must be well-graded compactable aggregate, 6 mm - 38 mm with no more than 10% passing the #200 sieve. (ASTM D422)
  2. Material behind and within the blocks may be the same material.

2.3 Infill Soil

  1. Infill material shall be site excavated soils when approved by the on-site soils engineer unless otherwise specified in the drawings. Unsuitable soils for backfill (heavy clays or organic soils) shall not be used in the reinforced soil mass. Fine grained cohesive soils with friction angle (φ) 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 are required. Poorly graded sands, expansive clays and/or soils with a plasticity index (PI) greater than 20 or a liquid limit (LL) greater than 40 should not be used in wall construction.
  2. 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.

Sieve Size Percent Passing
25 mm 100 – 75
4.75 mm 100 – 20
0.425 mm 0 - 60
0.075 mm 0 - 35

  1. Where additional fill is required, contractor shall submit sample and specifications to the wall design engineer or the onsite soils engineer for approval and the approving engineer must certify that the soils proposed for use has properties meeting or exceeding original design standards.

Part 3: Wall Construction

3.1 Excavation

  1. Contractor shall excavate to the lines and grades shown on the construction drawings. Contractor shall use caution not to over-excavate beyond the lines shown, or to disturb the base elevations beyond those shown.
  2. Contractor shall verify locations of existing structures and utilities prior to excavation. Contractor shall ensure all surrounding structures are protected from the effects of wall excavation.

3.2 Foundation Soil Preparation

  1. Foundation soil shall be defined as any soils located beneath a wall.
  2. Foundation soil shall be excavated as dimensioned on the plans and compacted to a minimum of 95% of Standard Proctor (ASTM D698) prior to placement of the base material.
  3. Foundation soil shall be examined by the on-site soils engineer to ensure that the actual foundation soil strength meets or exceeds assumed design strength. Soil not meeting the required strength shall be removed and replaced with acceptable material.

3.3 Base

  1. The base material shall be the same as the Wall Rock material (Section 2.2) or a low permeable granular material.
  2. Base material shall be placed as shown on the construction drawing. Top of base shall be located to allow bottom wall units to be buried to proper depths as per wall heights and specifications.
  3. Base material shall be installed on undisturbed native soils or suitable replacement fills compacted to a minimum of 95% Standard Proctor (ASTM D698).
  4. Base shall be compacted at 95% Standard Proctor (ASTM D698) to provide a level hard surface on which to place the first course of blocks. The base shall be constructed to ensure proper wall embedment and the final elevation shown on the plans. Well-graded sand can be used to smooth the top 13 mm on the base material.
  5. Base material shall be a 100 mm minimum depth for walls under 1.2 m and a 150 mm minimum depth for walls over 1.2 m.

3.4 Unit Installation

gravity retaining wall consolidation zone

Consolidation Zone - Gravity Wall

  1. Install units in accordance with the manufacturer’s instructions and recommendations for the specific concrete retaining wall unit, and as specified herein.
  2. Ensure that units are in full contact with base. Proper care shall be taken to develop straight lines and smooth curves on base course as per wall layout.
  3. Fill all cores and cavities and a minimum of 300 mm behind the base course with wall rock. Use infill soils behind the wall rock and approved soils in front of the base course to firmly lock in place. Check again for level and alignment. Use a plate compactor to consolidate the area behind the base course. All excess material shall be swept from top of units.
  4. Install next course of wall units on top of base course. Position blocks to be offset from seams of blocks below. Perfect "running bond" is not essential, but a 75 mm minimum offset is recommended. Check each block for proper alignment and level. Fill all cavities in and around wall units and to a minimum of 300 mm depth behind block with wall rock. Wall rock and infill soil placed in uniform lifts not exceeding 8 in. (200 mm). Compaction requirements for all soils in areas in, around and behind the reinforced mass shall be compacted to 95% of maximum Standard Proctor dry density (ASTM D698) with a moisture content control of +1% to -3% of optimum.
  5. For taller wall applications, structural fill should be specified for a minimum bottom 1/3 to 1/2 of the reinforced fill. If structural fill is not utilized in the reinforced mass, the depth of wall rock behind the block should be increased. See the Best Practices for SRW Design document.
  6. The consolidation zone shall be defined as 0.9 m behind the wall. Compaction within the consolidation zone shall be accomplished by using a hand operated plate compactor and shall begin by running the plate compactor directly on the block and then compacting in parallel paths from the wall face until the entire consolidation zone has been compacted. A minimum of two passes of the plate compactor are required with maximum lifts of 200 mm. Expansive or fine-grained soils may require additional compaction passes and/or specific compaction equipment such as a sheepsfoot roller. Maximum lifts of 100 mm may be required to achieve adequate compaction within the consolidation zone. Employ methods using lightweight compaction equipment that will not disrupt the stability or batter of the wall. Final compaction requirements in the consolidation zone shall be established by the engineer of record.
  7. Install each subsequent course in like manner. Repeat procedure to the extent of wall height.
  8. As with any construction work, some deviation from construction drawing alignments will occur. Variability in construction of SRWs is approximately equal to that of cast-in-place concrete retaining walls. As opposed to cast-in-place concrete walls, alignment of SRWs can be simply corrected or modified during construction. Based upon examination of numerous completed SRWs, the following recommended minimum tolerances can be achieved with good construction techniques.

Vertical Control - ±32 mm max. over 3 m distance

Horizontal Location Control - straight lines ±32 mm over 3 m distance.

Rotation - from established plan wall batter: ±2.0°

3.5 Additional Construction Notes

Leveling the Base Course

Level Base Course

  1. When one wall branches into two terraced walls, it is important to note that the soil behind the lower wall is also the foundation soil beneath the upper wall. This soil shall be compacted to a minimum of 95% of Standard Proctor (ASTM D698) prior to placement of the base material. Achieving proper compaction in the soil beneath an upper terrace prevents settlement and deformation of the upper wall. One way is to replace the soil with wall rock and compact in 200 mm lifts. When using on-site soils, compact in maximum lifts of 100 mm or as required to achieve specified compaction.
  2. Vertical filter fabric use is not suggested for use with cohesive soils. Clogging of such fabric creates unacceptable hydrostatic pressures in soil reinforced structures. When filtration is deemed necessary in cohesive soils, use a three dimensional filtration system of clean sand or filtration aggregate. Vertical filter fabric may be used to separate wall rock zone from fine grained, sandy infill soils if the design engineer deems it necessary based on potential water migration from above or below grade, through the reinforced zone into the wall rock on the project. Horizontal filter fabric should be placed above the wall rock column to prevent soils from above migrating into the wall rock column.
  3. Embankment protection fabric is used to stabilize rip rap and foundation soils in water applications and to separate infill materials from the retained soils. This fabric should permit the passage of fines to preclude clogging of the material. Embankment protection fabric shall be a high strength polypropylene monofilament material designed to meet or exceed typical Corps of Engineers plastic filter fabric specifications (CW-02215); stabilized against ultraviolet (UV) degradation and typically exceeding the values in Table 1, page 7 of the AB Spec Book.
  4. Water management is of extreme concern during and after construction. Steps must be taken to ensure that drain pipes are properly installed and vented to daylight or connected to an underground drainage system and a grading plan has been developed that routes water away from the retaining wall location. Site water management is required both during construction of the wall and after completion of construction.

Consult the Allan Block Engineering Department for additional details at