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Section 3: Design Considerations

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General Design

Design and analyze walls following accepted geotechnical engineering industry standards. In analyses, use earth pressures that follow governing sections of the current edition of the AASHTO Standard Specifications for Highway Bridges. For load conditions or walls that are not specifically covered by AASHTO, refer to the TxDOT web page for recommendations.

The project engineer must ensure that the retaining wall system is appropriate for its location. Check walls to ensure minimum factors of safety are met for all potential modes of failure. These include sliding, overturning, bearing pressure, and global stability. Consult governing wall standard sheets for assumptions and minimum factors of safety for various modes of failure. The minimum global factor of safety is set at 1.3 for conditions where the designer has adequate soils laboratory and field testing data on which to base the analysis. Make the global safety factor 1.5 or greater where the data obtained for the design and analysis is based primarily on strength correlations or walls that support abutment, buildings, critical utilities, or for other installations with a low tolerance for failure. If a TxDOT retaining wall standard is used for the wall design, it is the designer's responsibility to validate the strength values shown on the retaining wall standard used. If the actual soil conditions show a strength weaker than that shown on the governing standard the designer must determine what modifications, if any, are necessary to the standard and if any ground improvements are necessary to ensure wall performance.

Avoid perching walls on slopes. When walls must be placed on slopes, conduct both short- and long-term stability analyses using appropriate soil strengths, geometry, and loading conditions (live load surcharge, hydrostatic, etc.).

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Design Criteria for Specific Wall Types

Spread Footing Walls. The engineer who selects this type of wall for inclusion in the plans is responsible for overall (global) stability of the wall. Ensure that the actual wall geometry and loading conditions apply to the standard drawing selected. Ensure that interruptions to the stem or footing steel by utilities or curved sections of walls do not compromise the design and performance of the wall. Ensure that skewed abutment ends do not pose conflicts with the footprint of the wall. Provide guidance or structural details when deviations from the wall standard drawings are warranted. Standard drawings provide a choice between high pressure (H) and low pressure (L) footings: selection of the appropriate standard drawing is a function of the loading, geometry, and allowable soil pressures. Standard drawings are developed based on the design parameters for foundation and retained soils of a cohesion of zero, a friction angle of 30 degrees for the retained and foundation soil, and a unit weight of 120 pcf for each. Give special consideration to walls subject to inundation. Considerations include drainage and draw-down stability analysis. Standard specification Item 423 governs the design and construction of this wall type.

MSE Walls. The engineer who selects this type of wall for inclusion in the plans is responsible for overall (global) stability and for providing information to complete the RW (MSE) DD sheet. MSE wall suppliers are responsible for internal stability of the walls and for ensuring that external stability, as defined on the RW (MSE) standard, is met. The RW (MSE) standard drawing is available, utilizing the following design parameters:

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  • Retained soil — a cohesion of zero and a unit weight of 125 lbs.
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  • Foundation soil — a cohesion of zero and a unit weight of 125 lbs.
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  • Select fill - a cohesion of zero, a friction angle of 34 degrees, and a unit weight of 105 pcf or 125 pcf depending on the stability analysis being conducted. Refer to the RW(MSE) standard for additional information.
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  • The friction angle of both the foundation soil and the retained soil must be defined by the wall designer and input on the TxDOT RW(MSE) DD sheet. Default minimum earth reinforcement is set at 8 ft. or 70 percent of the wall height, whichever is greater. The wall designer is responsible for ensuring that the minimum earth reinforcement length selected on the RW(MSE)DD sheet satisfies the factor of safety requirement with the defined friction angle of the foundation soil and the retained soil. To ensure proper performance of the wall in place, evaluate project-specific requirements for wall backfill type, wall embedment, wall drainage, conflicts within the wall reinforced zone, and other considerations as necessary. Give special consideration to walls that are subject to inundation. Type BS backfill is the default backfill for permanent walls. Type DS backfill must be specified for walls that are subject to inundation. Analyze walls subject to inundation for 3 ft. of draw-down. Refer to the RW(MSE)DD standard for guidance on the draw down design condition. Walls to be placed in front of bridge abutments should have a 1.5-ft. minimum and 3-ft. desirable clearance from back of wall panel to face of abutment cap to facilitate wall construction. Standard specification Item 423 governs the design and construction of this wall type.

Concrete Block Walls. The engineer who selects this type of wall for inclusion in the plans is responsible for overall (global) stability of the wall. Concrete block wall suppliers are responsible for internal stability of the walls and for ensuring that external stability, as defined on the RW (CB) standard, is met. The RW (CB) standard drawing is available utilizing the following design parameters:

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  • Retained soil — a cohesion of zero, a friction angle of 30 degrees, and a unit weight of 120 lbs.
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  • Foundation soil — a cohesion of zero, a friction angle of 30 degrees, and a unit weight of 120 lbs.
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  • Select fill — a cohesion of zero, a friction angle of 34 degrees, and a unit weight of 120 lbs.
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  • If the site condition soil properties differ from those indicated above then the RW(CB) standard needs to be modified to reflect the actual site soil properties.

Concrete block walls may be classified as either structural or landscape walls. The minimum strap length varies depending on the wall function. Minimum earth reinforcement lengths are 6-ft. for walls designated as landscape walls, and 8-ft. otherwise. To ensure proper performance of the wall in place, evaluate project-specific requirements for wall backfill type, wall embedment, wall drainage, conflicts within the wall reinforced zone, and other considerations as necessary. Type BS backfill is the default for permanent walls. Give special consideration to walls that are subject to inundation. Specify Type DS backfill, and analyze these walls for 3 ft. of draw-down. The maximum particle size of the select backfill is limited to ¾" for nonmetallic reinforcements. Consult the standard drawing for guidance on wall definition. Standard specification Item 423 governs the design and construction of this wall type.

Tied-Back Walls. The prestressed ground anchors (tie backs) are nearly horizontal elements that are drilled, grouted, and stressed in place. Determine tied-back loads and soldier pile bending moments from the apparent earth pressure diagrams. Fill and live load surcharges are included in the pressure diagram. Determine loads and moments by the tributary area method. The minimum tie-back length is 25 ft. This length is composed of a minimum 15-ft. debonded length and a minimum 10-ft. bonded length. The ultimate length of tie-back is determined by the wall contractor. Anchor loads and soil conditions may warrant tied-back anchors on the order of 60 to 70 ft. long. The anchors are then stressed to the load specified in the construction drawings. Consider the distance the tie backs will project behind the wall and any potential conflicts with subsurface obstructions or right of way limitation. Ensure that tie backs have a minimum 6-in. clear cover from any obstructions. Obtain permanent easements for tie backs that cross the right-of-way line. Consider equipment accessibility due to horizontal and vertical clearance restrictions. Standard specification Item 423 governs the construction of this wall type and is supported by special specifications Prestressed Ground Anchors and Prefabricated Soil Drainage Mats.

Soil Nailed Walls. Soil nails are nearly horizontal elements that are drilled and grouted in place. Walls are typically designed using a limit state equilibrium program such as Goldnail, SNAP-2 or Snail-Z. Consider the distance the nails will project behind the wall and any potential conflicts with subsurface obstruction or right of way limitation.

For permanent walls, use the following minimum criteria:

Standard specification Item 423 Retaining Walls and Item 410 Soil Nail Anchor govern construction of this wall type and are supported by the special specification Prefabricated Soil Drainage Mat.

Ensure that nails have a minimum 6-in. clear cover from any obstructions. Obtain permanent easements for nails that cross the right-of-way line. The top of the wall should be no more than 2 ft. above existing grade to ensure constructability of the soil nail wall; special design considerations are required when this distance is exceeded. Nail spacing depends on project-specific site and loading conditions. A 3-ft. to 4.5-ft. vertical spacing and a 3.0-ft. to 4.5-ft. horizontal spacing is typical. Soil strengths used in the design of soil nail walls are typically determined from correlations of strength to Texas Cone Penetration values conducted through the embankment to be nailed. Use ultimate strengths in the analysis. Design walls considering the proposed wall geometry and loading. Limit head strength to avoid a bad design. Unrealistic or high head strength results in shorter nails and causes the lowest nails to carry a disproportionate amount of load. In practice, head strength is the variable manipulated to achieve a reasonable distribution of nail forces and is the capacity of the nail anchorage in the fascia. Manipulate head strength until the nails in the upper half of the wall carry at least half of the total load. This distribution may not be possible for very tall walls, walls with near-infinite back slopes or layered soil systems. For these cases, increase the nail lengths to engage the upper portion of the failure surface to develop a better load distribution. Final verification on design should include a global check using the analysis mode of the design program used or an independent slope-stability program that is capable of modeling soil nail anchors.

Consider equipment accessibility due to horizontal and vertical clearance restrictions.

Rock Nailed Walls. Rock nails are nearly horizontal elements that are drilled and grouted in place. Rock nailed walls are based on an empirical design approach. Maximum nail spacings are set at 5 ft. vertically and 5 ft. horizontally. Because this is an empirical design, confirm that site conditions are conducive to this type of design. Rock nail walls are used in materials classified as rock and have TCP values of 4 in. or less per hundred blows. Consider rock nail walls for rock with TCP values less than 6 in./100 blows and more than 4 in./100 blows on a case-by-case basis. Evaluate shale for applicability of this wall type because of its tendency to revert to its parent material. Consider the dip, bedding thickness, Rock Quality Designator, percent recovery, joint spacing, and joint pattern of the rock formation. Nail lengths may be adjusted to ensure that nailed rock mass is inherently stable in the primary modes of failure (sliding and overturning).

For permanent walls, use the following minimum criteria:

Standard specification Item 423 Retaining Walls and Item 411 Rock Nail Anchors govern construction of this wall type and are supported by the special specification Prefabricated Soil Drainage Mat.

Consider the distance the rock nails will project behind the wall and any potential conflicts with subsurface obstructions or right of way limitations. Ensure that nails have a minimum 6-in. clear cover from any obstructions. Obtain permanent easements for nails that cross the right-of-way line. The top of wall should be no more than 2 ft. above existing grade to ensure constructability of the rock nail wall; special design considerations are required when this distance is exceeded. Consider equipment accessibility due to horizontal and vertical clearance restrictions.

Drilled Shaft Walls. Drilled shafts are vertical elements that are drilled and concreted in place. They vary in size, diameter, and spacing depending on soil conditions, loading, and wall geometry. Derive wall loading using a Coulomb analysis. Soil information necessary for design includes friction angle, cohesion, and unit weight. Determine soil strengths below the proposed ground line at face of wall from correlations of strength to Texas Cone Penetration values. Use ultimate strengths in the analysis. The following soil strength reductions can be used in design:

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  • Reduction based on close shaft spacing (see the following figure)
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  • Reduction of surface soil strength based on expected swelling/softening of the soil

Ultimate Load Ratio vs. Clear Spacing/Drilled Shaft Diameter for Various Soil types (click in image to see full-size image) Anchor: #NNTBEWBQgrtop

Figure 6-1. Ultimate Load Ratio vs. Clear Spacing/Drilled Shaft Diameter for Various Soil types

Rock is typically modeled as very stiff clay with a very high cohesion. Design the walls iteratively varying length of shaft for successive runs. Make a plot of shaft embedment versus top of shaft deflection to determine when additional embedment does not result in a reduced deflection. The minimum embedment length that results in no additional top of shaft deflection is defined as the depth to fixity. Typically, a final length of shaft is taken as 133% of the embedded length of shaft to fixity. Maximum tolerable top of shaft deflection is set at 1% of the wall height. The maximum steel percentage is 2.5% to 3%. Minimum clear spacing between adjacent shafts is set at 1 ft. Design wall fascia to account for the maximum earth pressure at the bottom of the wall. The load applied to the fascia should be applied through the window between the shafts assuming simple supports at the centerline of the shafts. The Contractor is responsible to ensure that face stability is maintained between shafts throughout construction. This should be addressed by a note in the plans. Consider equipment accessibility due to horizontal and vertical clearance restrictions. Standard specification Item 416 Drilled Shafts and Item 423 Retaining Walls govern construction of this wall type and are supported by special specification Prefabricated Soil Drainage Mat.

Temporary MSE Wall. The engineer who selects this type of wall for inclusion in the plans is responsible for overall (global) stability of the wall. Temporary MSE wall suppliers are responsible for internal stability of the walls and for ensuring that external stability, as defined on the RW (TEW) standard, is met. The RW (TEW) standard drawings are available based on the following design parameters:

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  • Retained soil — a cohesion of zero, a friction angle of 30 degrees, and a unit weight of 120 pcf.
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  • Foundation soil — a cohesion of zero, a friction angle of 30 degrees, and a unit weight of 120 pcf.
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  • Select fill — a cohesion of zero, a friction angle of 30 degrees, and a unit weight of 120 pcf.
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  • If the site condition soil properties differ from those indicated above, then the RW(TEW) standard will need to be modified to reflect the actual site soil properties.

Minimum earth reinforcement length is set at 6 ft. To ensure proper performance of the wall in place, evaluate project-specific requirements for wall backfill type, wall embedment, wall drainage, conflicts within the wall reinforced zone, and other considerations as necessary. Give special consideration to walls that are subject to inundation. Type C backfill is the default backfill for temporary walls. Specify Type D backfill for walls that are subject to inundation. Analyze walls subject to inundation for 3 ft. of draw-down. Backfill the 2-ft. zone immediately behind the facing with clean coarse rock or cement-stabilized backfill. A designer who prefers to use coarse rock or cement-stabilized backfill must state this in the plan documents. If a temporary MSE wall will be in service for longer than 3 years, the designer must state this in the plan documents to ensure that the wall supplier provides a design with an adequate service life. Temporary MSE walls placed adjacent to permanent MSE walls must be detailed with earth reinforcement that will prevent corrosion of the permanent earth reinforcements due to contact of dissimilar metals. This may be accomplished by providing galvanized or synthetic earth reinforcements for the temporary MSE walls.

Standard specification Items 403 and 423 govern construction of this wall type.

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