Frequently Asked Questions about Fill Type Retaining Walls

The fill type classification refers to the construction method used for the installation of a wall. Fill type retaining walls are retaining structures constructed from the base of the wall to the top (i.e. “bottom-up” construction). Fill type retaining walls are further classified according to the basic mechanism of lateral load support. These classifications include internally stabilized fill structures and externally stabilized fill structures.

The following are some frequently asked questions (FAQ’s) regarding EI 10-031 Section 554 Fill Type Retaining Walls.

1. Why did we change from specifying a particular wall type to a generic specification?

The initial reason for the change from specifying an individual wall system to a generic specification which incorporates multiple alternative wall systems is because it is the policy of both the Federal government and the State to not use proprietary specifications due to the fact that is places an unnecessary limit on the number of competitors, which is essentially unfair and can potentially lead to higher prices.

EI 08-037 revised Section 554 Internally Stabilized Fill Structures, incorporating some special specifications into the standards. EI 08-020 revised Section 632 Precast Modular Walls, emulating the procedures defined in Section 554 for approving a wall system. Both sections refer to the Approved List for acceptable proprietary retaining wall systems and outline a submittal process for approving the Shop Drawings of a particular job specific wall system design. However, these were separate Approved Lists impeding competition. In addition, a third separate Approved List contained additional proprietary retaining wall systems (segmental block retaining walls), further hindering competition. In many cases, multiple types of wall would function equally well, and the designer would choose a particular type or system based on personal preference. As a result, VE proposals and substitutions were a common occurrence during the construction phase.

The latest revision to Section 554 Fill Type Retaining Walls outlined in EI 10-031 takes the next logical step by combining the two sections into one. Section 554 Fill Type Retaining Walls includes all proprietary fill type retaining walls (both internally stabilized fill structures and externally stabilized fill structures) which will be bid based on the height of wall and introduces a broader, more expansive Approved List by combining the existing three into one.

2. What is a proprietary retaining wall system?

Proprietary retaining wall systems are vendor designed retaining wall systems that are protected by specific patents that make them unique from other retaining wall systems. Generally, this is a product, specification, or process identified in the plans or specifications as a "brand" or trade name.

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Frequently Asked Questions about Fill Type Retaining Walls

3. If it is the policy of both the Federal government and the State to not use proprietary specifications, how can the State tap into the vast variety of proprietary retaining wall systems?

The FHWA may participate in the costs of a proprietary product under the following circumstances:

  1. Competitive bidding, provided under 23 CFR 635.411(a)(1)

a. The proprietary product is obtained through competitive bidding with other suitable proprietary and non-proprietary products from multiple manufacturers. Where both proprietary and non-proprietary products are available, the STA or LPA must compose specifications that allow the contractor to choose amongst as many acceptable products and technologies as possible. If the specification lists specific products, it must list all or at least a reasonable number of products, and must include the words "or equal" to ensure the broadest range of choice.

b. A competitively bid performance-based warranty specification is permitted, if it does not limit product selection to a single source. The warranty specification must clearly describe all potential products that are acceptable for use at the time of project advertisement

2.A certification by the STA or LPA, as provided in 23 CFR 634.411(a)(2), which the specified proprietary product is either:

a. Necessary for synchronization with existing facilities; or

b. A unique product for which there is no suitable alternative.

  1. A proprietary item is to be used for research or for a distinctive type of construction on relatively short sections of road on an experimental basis as provided in 23 CFR 635.411(a)(3).
  2. If there are a number of acceptable materials or products, the STA or LPA may require a specific material or product when the Division Administrator approves of its use as being in the public interest as provided in 23 CFR 635.411(c). See Question and Answer #4 above.

The regulation 23 CFR 635.411 may be viewed at

EI 10-031 revises Section 554 Fill Type Retaining Walls by combining the two Sections into one and developing an expanded Approved List for all proprietary fill type retaining walls (both internally stabilized fill structures and externally stabilized fill structures) to be in conformance with #1 - Competitive bidding, provided under 23 CFR 635.411(a)(1), identifying as many acceptable products as possible.

4. What is the intent of the new Section 554 Fill Type Retaining Walls?

The intent of this new Standard Section is to: (1) realize a more competitively bid product and (2) have all the systems on the Approved List suitable for the site conditions and available to bid, unless otherwise spelled-out in Special Notes concerning potential design concerns (i.e. not to eliminate any walls but to enumerate design concerns such that they may be addressed by the Contractors Consultant in the submittal of the final design of the wall).

By merging the specifications, we are developing broader, expanded Approved List for all proprietary fill type retaining walls (both internally stabilized fill structures and externally stabilized fill structures).

  • This is in conformance with #1 - Competitive bidding, provided under 23 CFR 635.411(a)(1), identifying as many acceptable products as possible.
  • It separates the cost of the wall function from the cost of the wall aesthetics.
  • It eliminates bias in selection of wall type. Under the current system, designers may specify a certain type of wall for project after project, and never find out if other systems provide an advantage, such as greater speed and ease of construction, at reduced cost.
  • It eliminates the State losing potential cost savings to VE proposals. Under the old system, the designer only chose one system type, locking the other types out unless they were submitted as VE proposals. This new system allows more competition between types and brands of fill-type retaining wall. This encourages innovation and should result in reduced costs to the State.
  • It eliminates the problem of the designer sometimes specifying proprietary wall systems, and then needing a spec and a “back-justification”.

5. Are all fill wall systems equal?

There are advantages and disadvantages to each individual wall system. However, it would be presumptuous to assume that all advantages and disadvantages are known for each wall system simply based on past experience. The specific capabilities and recent developments and innovations in any wall system are best known by the individual Designer-Suppliers, who should have a say in whether or not their system is appropriate for the site conditions identified in the contract documents. In aspects where there are design concerns identified by the Department, individual Designer-Suppliers may see an avenue to modify current materials/members/connections etc. and/or incorporate/marry other products into their own to address the concerns while developing their product.

Under the old system, different wall types were paid for under various items – some included backfill, while some paid for the backfill material separately. In order to standardize the existing dissimilar specifications for all of the different wall systems, the new specification had to be normalized. The new specification was normalized to be equated against one measure – area of wall face. To combine all wall types into one specification, the normalized measure of volume includes all work for the installation of a fill wall in the volume extending behind the wall a distance of 70% the height of wall.

The Department will provide a volume for the installation of any fill type retaining wall and allow the wall system Designer-Suppliers to incorporate their systems into the project at-hand, knowing the constraints as laid-out in the contract documents or opt not to bid. We are giving the choice to all the wall systems on the Approved List rather than initially rejecting and pruning the list of available systems out of preferences and/or potential ignorance of capabilities.

6.It appears that the approving authority is different for prefabricated wall systems (PWS), depending on whether they are wet-cast or dry-cast. To illustrate,refer to the submittal process identified in §554-3.01 B.3.ii. Here, it states that a PWS consisting of solid face units shall be approved by the Engineer. However, PWS consisting of solid face units manufactured via the wet-casting process shall be submitted to the Materials Bureau. How do I know if the solid face unit is a wet-cast or dry-cast product, so I can get the right person to approve it?

Dry-cast units in unreinforced applications are small units which cannot achieve great heights, as identified through the maximum unreinforced height restriction in the Master Approved List. The Department has reviewed these systems and placed the maximum unreinforced height restriction to aid the Engineer in the approval process. The Engineer may review the site specific layout drawing with respect to the field conditions and design parameters and maximum unreinforced height restriction in the Approved list to determine approval.

If reinforcement is added to the backfill to attain greater heights, the PWS changes categories to a mechanically stabilized wall system (MSWS), which can be built upwards of 80’ high. These wall systems are more complex in their design. Therefore, the Office of Technical Services will be involved in the submittal review and approval.

As for wet-cast products, these are bigger units, allow infill, have rebar, etc. Therefore, as with any precast product, the Office of Technical Services will be involved in the submittal review and approval.

In order to determine if the solid face unit is a wet-cast or dry-cast product, follow the wall system link from the Master Approved List. If the link sends you to the Precast Concrete Wall Unit list (§704-06), it is a wet-cast product. If the link sends you to the Drycast Concrete Wall Unit list (§704-07), it is a dry-cast product.

7. How do I handle specialized wall designs such as walls requiring lightweight backfill material?

The generic pay items are for common wall installations. This is a Standard Specification, to be used in a majority of instances where fill type retaining walls are needed. Special notes on the plans should be used to describe specific requirements, information and constraints. However, it is recognized that there will be a limited number of special cases where site-specific designs and exceptions to the general rule will be necessary. In these instances, special specifications will be required and need to be justified. However, it is not to say that one has to jump to the conclusion that a specific, proprietary wall is necessary. Given the example of walls requiring lightweight backfill material, a generic special specification could be developed to continue competition between fill type retaining wall Designer-Suppliers while requiring lightweight material (i.e. some wall systems may introduce geofoam while others may incorporate Elastizell).

8. Will I know which wall system the Contractor will utilize?

During the design phase, the Department cannot assume a specific wall system which will be utilized by the Contractor. The Designer shall outline (1) the location, (2) a general layout of the proposed wall for bidding purposes,(3) the location of all planned and anticipated utilities within the backfill area of the wall, and (4) all Special Notesto address potential design concerns into the contract documents. The details and notes are to provide sufficient information for a Contractor to bid the wall and for the Contractors Consultant to perform an engineering analysis and final detailing of the wall.

Although the Designer cannot assume a specific wall system, the Approved List will identify the (1) maximum unreinforced height, (2) if reinforcement applications are available, and (3) if abutment support applications are acceptable. Based on this information, the Designer can identify which various prefabricated wall systems (PWS), mechanically stabilized wall systems (MSWS), and mechanically stabilized earth systems (MSES) will be applicable considering the projects constraints.

If aesthetics are not a concern, there are standard default treatments in the specification. However, if aesthetics are a concern, the Regional Designer will add, in addition to the fill type retaining wall item, an item for the aesthetic treatment for the face units. The Designer needs to vividly describe the desired aesthetic treatment using industry-standard descriptions for color and texture, and provide special notes, and special details. The Regional Landscape Architect and the Landscape Architecture Bureau are available to help develop these descriptions.

Designers need to become familiar with the available categories (textured surface, exposed aggregate finish, architectural pattern) for presentation to communities. They need to keep in mind that the Contractor will ultimately choose the specific wall system. Therefore, while negotiating a “look”, the Designer should avoid promises regarding the specific type of wall and concentrate on the finish or treatment and the possible variations (joints, unit size, etc.) between wall types.

9. Except for the face limits & aesthetics, the Contractor is given almost complete control of the wall system choice. Are we expecting Contractors to be wall experts?

We are not. During the design phase, as with any retaining wall design, the Department will investigate the subsurface conditions, analyze global stability and incorporate site-specific details to the wall volume. The Designer shall outline the (1) location, (2) a general layout of the proposed wall for bidding purposes,and (3) all Special Notesto address potential design concerns into the contract documents. The Special Notes are developed from the same thought process previously used in selecting a particular wall type. In the past, wall systems were rejected from further consideration and the list of available systems was pruned based on personal preferences and/or potential ignorance of capabilities. The Special Notes should not eliminate wall systems but rather enumerate the Departments design concerns. Given the opportunity and an understanding of the concerns, the wall system Designer-Supplier may augment the wall design to fit the wall system in the designed volume, given the project constraints, or opt not to bid.

Please not that this is no different from how the Department used to specify a mechanically stabilized earth system (MSES). The Contractor did not have to perform a detailed design before bidding. All a Manufacturer/Supplier needs to know is the quantity (elevation), layout and any restrictions. Detailed design doesn’t need to be done immediately.

10. What are the changes in design responsibilities?

Regional Geotechnical Engineer (RGE):This does not change the basic role of the RGE. The RGE is still responsible for determining the effect of the site and subsurface conditions on the wall selection process. The only change is that instead of recommending a single wall type based on those conditions, requirements andlimitations, he describes those conditions on the plans.

Regional Landscape Architect (RLA):This puts more of an onus on the RLA. They will need to vividly describe the desired aesthetic treatment on the plans using generic, industry-standard terms.

Regional Designers/ Project Managers: Designers need to become familiar with the available categories (textured surface, exposed aggregate finish, architectural pattern) for presentations to communities. They need to keep in mind that the Contractor will ultimately choose the specific wall. Therefore, while negotiating a “look” with affected communities and stakeholders, the PM should avoid promises regarding a specific type of wall and concentrate on the finish or treatment and the possible variations (joints, unit size, etc.) between wall types.

11. Some wall Designer-Suppliers incorporate CIP concrete within the top unit to enhance its capability to withstand impact loads. How should walls that are located directly below the shoulder, and require a jersey barrier, be detailed?

These items should be addressed separately.

The designer should identify the wall volume and, if the wall is located directly below a shoulder and a jersey barrier is required, the detail should identify a jersey barrier – moment slab directly on top of the wall (with appropriate details & pay items – see Figure 1, 2, and 3). This way, all walls are appropriate for the application at hand.