Chapter 1: Design Guidelines & Pre-Construction Considerations

Print this Document

Download Document

updated: 5/29/2018

Print this Chapter

Print Chapter

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.

1.1    Meeting with Owner

1.1    Meeting with owner to fully understand the basic needs for the site.

  1. What is the general use of the site? Know if the site will be used for commercial, residential, public, etc.
  2. Knowing the site use will often dictate design loading, design methodology and level of effort required to complete the project so proper bidding can be provided.
  3. Identify miscellaneous items such as drop structures, light standards, building foundations and property lines that may encroach into the geogrid zone.
  4. Engineer should determine if as-built documents will be required at the end of the project. If so, provisions should be set in place to document changes during construction.

1.2    Determining when Engineering is Required

1.2    Determining when engineering is required.

  1. Local codes and municipalities will have minimum height requirements set for walls that require engineering. However, height is not the only factor that should be considered. Engineering should be required for walls of any height that have any of the special considerations within this manual such as but not limited to the following: poor soils, multiple terrace arrangements, steep slopes above or below, high seismic loading, roadway surcharges, etc.

1.3    Existing and Proposed Utilities

1.3    Make sure the site plans call out locations of all existing and proposed utilities.

  1. Have a process in place to verify location of existing utilities.
  2. Avoid placing utilities, especially storm sewer, sanitary sewer, water, landscape irrigation, and gas lines within the reinforced zone of segmental retaining walls. If no other alternatives exist, provisions should be set in place for future maintenance.

1.4    Wall Layout, Height and Geometry

1.4    Determine wall layout, wall heights, conditions above and below the wall, as well as live and dead loads (locations and levels).

  1. Wall layout should be approved by the owner.
  2. Utilize the Wall Design Checklist in the Allan Block Spec Book.

1.5    Geotechnical Report Considerations

1.5    Obtain a thorough geotechnical report in the area where the wall will be located.

  1. Geotechnical report should be available to all parties doing design work and should be an all-encompassing document covering all aspects of the site such as the following:
    1. Soil strength – preferably the tested friction angle of the on-site soils
    2. Clear description of on-site soils
    3. Gradation of soils
    4. Groundwater conditions
    5. Settlement
    6. Soil unit weights
    7. Plasticity Index (PI) and Liquid Limit (LL)
    8. Site specific seismic coefficients
    9. Global stability recommendations
  2. The geotechnical investigation and report should be paid for by the owner.
  3. If no other guidance has been provided, a geotechnical investigation should include soil borings with sampling and logs at an interval of not more than 100 ft (30 m) along the centerline of the proposed retaining wall(s) and at 150 ft (46 m) along the back of the reinforced soil zone.

1.6    Understanding Sites Soils

1.6   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 draining 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.

1.7    Site Visit

1.7    Visit the site to ensure that the site plans adequately capture the important details of the site.

  1. Site drainage
  2. Surface water – lakes, rivers, ponds or detention basins, etc.
  3. Slopes above or below.
  4. Make determination that site soils appear to be similar to what is indicated in the geotechnical report.
  5. Proposed locations of structures, roadways and other surcharges.

1.8    Temporary Load Considerations

1.8    Consider temporary construction loads and future snow and/or storage loads.

  1. Will construction loading govern design over final loading conditions?
  2. Snow loads are not only vertical but snowplows can provide lateral loading.
  3. Consider reinforced barriers to prevent the lateral loads.
  4. Magnitude of load will range depending on condition.

1.9    Scope of Responsibility and Design Methodologies

1.9    Establish scope of responsibility and required design methodology of project with owner including seismic design requirements.

  1. The owner should understand the limits of design responsibility. The SRW Engineer Suggested Roles as provided in Section 3, Roles and Responsibilities in the National Concrete Masonry Association (NCMA) Design Manual are as follows:
    1. Design of SRW for structural stability including external stability (sliding and overturning), internal stability, internal compound stability and facial stability.
    2. Determination of the maximum unreinforced height of SRW.
    3. Design of geogrid layout for taller walls requiring soil reinforcement.
    4. Determination of minimum embedment of wall (except in the case of scour depth or erosion control issues, which should be determined by site civil engineer).
    5. Specification and/or approval of wall unit, geogrid reinforcement, drainage material within wall structure, and reinforced soil properties.
    6. Determination of what structures can or cannot be placed within reinforced soil zone and wall face, and detailing for SRWs to accommodate acceptable structures.
    7. Under the direction of geotechnical engineer, assist in the coordination of slope stability evaluation around and through the SRW and the design of the geogrid in reinforced SRWs to address slope stability in vicinity of SRW, as needed.
    8. If contracted to and notified, construction observation of the overall wall structure installation and review of SRW material submittals (generally on a time and materials basis, separate from the wall design contract).
  2. Unless other arrangements have been made between the owner and the wall design engineer, the wall design engineer is responsible for the area in and around the wall known as the Design Envelope – Figure 1-1.

    Design Envelope
    Figure 1-1: Design Envelope

    This Design Envelope is defined as follows: The horizontal distance, measured from the toe of the wall, is the greater of twice the height of the wall (2H) or, the height of the projection from the tail of the reinforcement layers to the surface (He) plus a distance equal to the length of the reinforcement (L). The vertical height is the height of the wall facing, measured from the top of the base to the top of the top facing unit (H).
  3. Agree upon the design method to be used, Allan Block, NCMA, AASHTO or FHWA. For information about the Coulomb Design Methodology see the Allan Block Spec Book. A complete step by step description can be found in the AB Engineering Manual.

1.10    Minimum Design Safety Factors

1.10    Retaining wall design calculations should follow good engineering judgment and or standard design methodologies such as Allan Block, NCMA, AASHTO or FHWA. All methodologies should follow standard minimum design safety factors.

  1. Sliding: Minimum Safety Factor > 1.5
  2. Overturning: Minimum Safety Factor > 2.0
  3. Bearing: Safety Factor > 2.0
  4. Grid Overstress: Minimum Safety Factor > 1.5
  5. Pullout from Soil: Minimum Safety Factor > 1.5
  6. Pullout from Block: Minimum Safety Factor > 1.5
  7. Internal Compound Stability: Minimum Safety Factor > 1.3
  8. Global: Minimum Safety Factor > 1.3
  9. Seismic: Minimum Safety Factor > 75% of static or 1.1 minimum

For a thorough discussion and design methodology of each, see the Allan Block Engineering Manual.

1.11    Coherent Gravity Mass & Connection Strength Considerations

1.11    All standard design methodologies listed in Section 1.9c use a coherent gravity mass design theory for stability calculations.

A coherent gravity mass is made up of the wall facing and layers of geogrid reinforcement placed horizontally through properly compacted soils. The geogrid length, strength and horizontal spacing between layers are key to calculating the internal stability of the mass. Much independent testing has been performed on the mass as a whole and on the individual parts of the system. None more so than the connection between the geogrid and the wall facing. More information on the actual connection test method can be found in ASTM D 6638. It has been determined that walls constructed using 60% minimum lengths of grid and a two course (16 in (40 cm)) maximum vertical spacing builds a high quality reinforced mass that easily distribute the calculated internal pressures. Testing has also proven that actual internal forces within a properly constructed reinforced mass are much less than the theoretical design loads based on active earth pressure calculations. Internal Compound Stability (ICS) calculations provide an analysis from a global stability perspective within the design envelope defined in Section 1.9b.

1.12    Contractor Requirements

1.12    Require that Contractors must 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. Require contractors to provide a list of projects they have completed.

1.13    Manufactured Product Specifications

1.13    Manufactured product specifications and associated test results.

  1. Call for block requirements and testing per ASTM C 1372, Table 1
  2. Specify allowable height deviations between adjacent units. To provide best quality units for construction reasons, a height tolerance between adjacent units should not exceed 0.125 inches (3mm) for walls over 10 feet (3.0 m) in height.
  3. Identify when freeze thaw resistance requirements are appropriate per ASTM C 1262.
  4. Geogrids should have “Mill Reports” or other proper documentation, See Chapter 7 for more information.
  5. Block producer’s QA/QC manual
Strength and Absorption Requirements (ASTM)

Minimum Required Net Average Compressive Strength, psi (MPa)

Average of 3 units
3000 (20.7)
Individual Unit
2500 (17.2)

Maximum Water Absorption Requirements, lb/ft³ (kg/m³)

Light Weight
18 (288)
Medium Weight
15 (240)
Normal Weight
13 (208)

Weight Classification Oven-Dry Density of Concrete lb/ft³ (kg/m³)

Light Weight
Less than 105 (1680)
Medium Weight
105 (1680) to less than 125 (2000)
Normal Weight
125 (2000) or more
Table 1: ASTM Requirements

1.14    Freeze-Thaw Durability

1.14   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 or countries that use deicing salts. Based on good performance experience by several agencies, ASTM C1372, or equivalent governing standard or public authority, Standard Specification for Segmental Retaining Wall Units should be used as a model, except that, to increase durability, the compressive strength for the units should be increased to a minimum of 4,000 psi (28 MPa) unless local requirements dictate higher levels. Also, maximum water absorption should be reduced and requirements for freeze-thaw testing increased.

  1. Require a current passing ASTM C 1262 or equivalent governing standard or public authority, test report from material supplier in northern or cold weather climates. See Figure 1-2 for Freeze Thaw Durability Criteria and Regional Map to define the appropriate zone and requirements for the project.
  2. Avoid using SRW products for steps or walkways where de-icing salts will be used. Use SRW as the base material with a concrete or stone slab step cover.
  3. Where runoff may flow over or on to the wall, provide a collection basin and either pipe the water around the wall or provide an extended shoot where the saline water does not flow down the wall.
  4. Where de-icing chemicals land on a SRW retaining wall, consider a more durable capping unit. Durability concerns occur where there are saturated conditions in repeated freezing and thawing conditions.
  5. In areas where SRW’s are exposed to repeated exposure from snow removal equipment, consider sealants or water repelling chemicals periodically applied to the walls (silane, siloxane compounds).
  6. Units selected for a project should be tested for compressive strength, density, and absorption no less than annually. The tested compressive strength and density should equal or exceed the values reported at the project initiation and the absorption should be less than or equal to the absorption previously reported. If these conditions are met, the durability of the units delivered to the project would be expected to be equivalent to or better than the units previously evaluated for durability through ASTM C1262 testing. If the compressive strength is more than 5% less, the density is more than 2% less, or the absorption is more than 5% greater (relative) than that was previously reported, than additional units should be sampled and tested for freeze/thaw durability (either in water or saline, as appropriate) and verified in compliance with the project specifications.

Figure 1-3:Freeze Thaw Durability
Figure 1-2: Freeze Thaw Durability Criteria

Freeze-Thaw Durability Criteria

Exposure/Temperature Conditions

Minimum Compressive Strength3

Minimum Freeze-Thaw Durability Criteria

Negligible Per design specs No freeze-thaw testing required
Where units will NOT be exposed to De-Icing Salts:
28 MPa (4000 psi)(min) Less than 1% weight loss after 100 cycles for 5 of 5 specimens OR less than 1.5% weight loss after 150 cycles for 4 of 5 specimens. Tested using ASTM C1262 in tap water
Where units will be exposed to De-Icing Salts:
Moderate2 28 MPa (4000 psi)(min) Less than 1% weight loss after 20 cycles for 5 of 5 specimens OR less than 1.5% weight loss after 30 cycles for 4 of 5 specimens. Tested using ASTM C1262 in 3% saline solution
Severe2 40 MP ⊂ (5800 psi) Less than 1% weight loss after 40 cycles for 5 of 5 specimens OR less than 1.5% weight loss after 50 cycles for 4 of 5 specimens. Tested using ASTM C1262 in 3% saline solution
1 Exposure to de-icing salts is unlikely or unplanned, but may include occasional exposure. Average de-icing salt contact would be one or less exposure per year during the design life of the SRW.
2 Exposure to de-icing salts is likely or expected.
3 Compressive strength is a historical minimum guide to what has been found to provide units that pass weight loss requirements when tested to ASTM C1262. Actual Minimum Compressive Strength should be established based on results when testing to ASTM C 1262 and found to be in compliance with minimum requirements of ASTM C 1372, from the plant supplying the product.

1.15    Pre-Construction Meeting

1.15    Requirement for a pre-construction meeting with all parties associated with the project to approve design and best practice requirements.

  1. A pre-construction meeting should be held with all parties attending so the entire construction plan can be discussed from start to finish and any issues can be worked out with a plan in place to move forward successfully.
  2. All parties are all of those that will be working on or around the wall site including but not limited to the owner, architect, site civil, geotech, wall designer, general contractor, excavation contractor, wall builder, Allan Block supplier representative, railing or fence installer, local utility representative, inspectors (private and government as appropriate) etc.
  3. Language can be placed in the project specifications requiring a pre-construction meeting.
  4. Reference the AB Construction and Inspection Checklist in the AB Spec Book as a good agenda for the meeting.
  5. Meeting topics shall include, but not be limited to: contractor qualifications, schedule and phasing of wall construction and inspection, coordination with other on-site construction activities, responsibilities of parties, and source, quality, and acceptance of materials.

1.16    Visiting the Site During Construction

1.16    Include in the scope of the project three additional site visits.

  1. The first at the beginning of the project to reinforce the need for the contractor to comply with the specifications in the approved design and to answer any questions.
  2. The second visit would be a random site inspection to ensure that the work methods are in agreement with the approved plans and to answer any questions.
  3. The third visit to be at the conclusion of the project to verify that the details above and below the wall structure have been completed as required. Additional inspections are available at the request of the owner based on a preset rate schedule. A written summary will be provided to the owner at the conclusion of the project for each visit made to the site.

1.17    Construction Drawings

1.17    A typical construction drawing submittal should include but is not limited to the following items (Size and complexity of project may lessen or increase this list):

  1. Front wall profile view which depicts the location of geogrid reinforcement, elevation of top and bottom of wall and finished grade at bottom and top of wall.
  2. Wall station data with corresponding elevation for exposed grade at top and bottom of wall (i.e. profile data).
  3. Panel section markers depicting locations of design sections.
  4. Design sections showing:
    1. Geogrid locations, type and length
    2. Thickness of wall rock material and infill depth
    3. Any surcharges
    4. Any required drain locations
    5. Block type
    6. Base size
  5. Wall plan view depicting proper wall orientation should be provided. However, layout of walls should be done using the site plans provided by the owner based on survey information.
  6. Any required specialized or standard details that will provide guidance to the contractor, such as design specifications, form work above, at the ends, or in front of the wall structure.
  7. Detailed information and assumed on-site soil conditions, reinforced fill requirements, soil and compaction testing, site visits from the local engineer of record, and documentation requirements before, during, and after construction.
  8. Contractor training and certification requirements.
  9. Expected water conditions above and below grade.

1.18    For the Bidding Process

1.18    For the bidding process, the owner is recommended to provide the following to ensure accurate and comparable bids between contractors.

  1. The owner should provide a complete design for all walls, unless the project is a negotiated design/build contract on the front end. If design/build is used, the owner should ensure that a complete design is submitted and construction verification is validated by an engineering firm working directly for the owner.
  2. A complete bid package including items listed in Section 1.17, a complete geotechnical report including items listed in Section 1.5.
  3. Any other information or details about the site and final requirements that are pertinent to the completion of the project.

1.19    Quality Control, Quality Assurance

1.19    Provide detailed requirements for onsite inspection to ensure that specifications are complied with during construction. The testing and inspection firm needs to evaluate conditions prior to construction and then to document the construction process. This includes, but is not limited to:

  1. Confirmation that the foundation soils are suitable for support of the structure.
  2. Confirmation that the retained soils are as defined in the wall design.
  3. Verify the reinforced soils comply with the material specified in the wall design.
  4. Confirm reinforcement material is that specified in the wall design.
  5. Verify the wall unit is that specified in the wall design.
  6. Document the construction process, verifying the wall is constructed in general accordance to the plans and specification.
  7. Perform the required soil testing in order to evaluate/validate that the soil placed meets the specified gradation, shear strengths, weight, Atterberg limits and compaction requirements.
  8. A detailed construction log should be used with sign offs at incremental levels by the responsible parties.