Structures Design Guidelines | July 2005
3 - Substructure and Retaining Walls |

## 3.12 Retaining Wall Design (07/05)

(optional for projects let before July 1, 2006)

### 3.12.1 General

A. Use Chapter 30, Plans Preparation Manual for retaining wall plans preparation and administrative requirements in conjunction with the design requirements of this Section. Refer to SDG Chapter 1 for the retaining wall concrete class
and reinforcing steel cover
requirements.

B. Rankine earth pressure may be used in lieu of lateral earth loads on walls developed from Coulomb earth pressure. If Rankine earth pressure is used, the resultant lateral earth load can be assumed to be located at the centroid of the earth pressure diagram.

C. During the design process, review wall locations for conflicts with existing or proposed utilities located beneath proposed reinforced fill wall volume. Analyze for settlement effects, maintenance repair access, etc.

D. Do not place utilities in the soil-reinforced zone behind Mechanically Stabilized Earth (MSE) or tie-back walls.

Commentary:

Utilities placed in the reinforced zone cannot be maintained because excavation in this zone will compromise the structural integrity of these wall types. It is also preferable to keep drainage systems outside the limits of the reinforced soil volume when possible. Leaking pipes could wash out and destroy the structural integrity of the wall.

### 3.12.2 Mechanically Stabilized Earth Walls

Commentary:

FHWA Publication No. FHWA-NHI-00-043, “Mechanically Stabilized Earth Walls and Reinforced Soil Slopes Design & Construction Guidelines" contains background information on the initial development of MSE wall design and is still referenced by LRFD Article 11.10.1 as design guidelines for geometrically complex MSE walls.

A. Minimum Service Life [11.5.1]

1.) Design permanent walls for a service life of 75 years, except those supporting abutments or on spread footings. Design these walls for a service life of 100 years.

2.) Design temporary walls for a service life of three years or a period of time not less than the contract time, whichever is greater.

B. Concrete Leveling Course

1.) All permanent walls will have a non-structural concrete leveling course as a minimum.

2.) The entire bottom of the wall panel will have bearing on the concrete leveling course.

C. Bin Walls [11.10.1]

1.) When two walls intersect forming an internal angle of less than 70 degrees, design the nose section as a bin wall. Submit calculations for this special design with the plans for review and approval.

2.) Design structural connections between wall facings within the nose section to create an at-rest bin effect without eliminating flexibility of the wall facings to allow toleration of differential settlements.

3.) For wall facings without continuous vertical open joints, such as square or rectangular panels, design the nose section to settle differentially from the remainder of the structure with a slip joint. Facing panel overlap, interlock or rigid connection across vertical joints is not permitted.

4.) Design soil reinforcements to restrain the nose section by connecting directly to each of the facing elements in the nose section. Run soil reinforcement into the backfill of the main reinforced soil volume to a plane at least 3 feet beyond the Rankine failure surface (See Figure 3-1).

5.) Design of facing connections, pullout and strength of reinforcing elements and obstructions must conform to the general requirements of the wall design.

D. Minimum Length of Soil Reinforcement [11.10.2.1]

Delete the requirements for minimum soil reinforcement lengths in LRFD Article [C11.10.2.1] and substitute the following:

Soil reinforcement lengths, “L”, measured from the back of the facing element, must not be less than the greater of the following:

Walls in Front of Abutments on Piling

L = 8 feet

L = 0.7 H1, ( H1 = mechanical height of wall, in feet. H1 is defined as the height measured to the point where the potential failure plane (line of maximum tension) intersects the ground surface. )

L = length in feet, required for external stability design.

Walls In Front of Abutments on Spread Footings

L = 22 feet

L = 0.6 ( H1 + d ) + 6.5 feet, ( d = fill height above wall )

L = 0.7 H1

Commentary:

As a rule of thumb, an MSE Wall with reinforcement lengths equal to 70% of mechanical height, the anticipated factored bearing pressure (quniform) can be estimated to be about 200% of the overburden weight of soil and surcharge. It may be necessary to increase the reinforcement length for external stability to assure that the factored bearing pressure does not exceeds the factored bearing resistance (qr) of the foundation soil at this location.

E. Minimum Front Face Wall Embedment [11.10.2.2]

1.) Consider scour and bearing capacity when determining front face embedment depth.

2.) Consult the District Drainage and Geotechnical Engineers to determine the elevation of the top of leveling pad.

3.) In addition to the requirements for minimum front face embedment in LRFD Article [11.10.2.2], the minimum front face embedment for permanent walls must comply with both a minimum of 24 inches to the top of the leveling pad and Figure 3-2. Also, consider normal construction practices.

F. Facing [11.10.2.3]

1.) The typical panel size must be square and not exceed 30 square feet in area (5 feet by 5 feet, nominal).

2.) The typical non-square (i.e., diamond shaped, not rectangular) panel size must not exceed 40 square feet in area.

3.) Special panels (top out, etc.) must not exceed 50 square feet in area.

4.) Full-height facing panels must not exceed 8 feet in height.

5.) SDO will consider use of larger panels on a case-by-case basis. The reinforcing steel concrete cover must comply with SDG Table 1.2 .

G. External Stability [11.10.5]

1.) The reinforced backfill soil parameters for analysis are:

a. Sand Backfill (Statewide except Dade and Monroe Counties)

b. Moist Unit Weight: 105 lbs per cubic foot

c. Friction Angle: 30 degrees

2.) Limerock Backfill (Dade and Monroe Counties only)

a. Moist Unit Weight: 115 lbs per cubic foot

b. Friction Angle: 34 degrees.

3.) In addition to the three conditions shown in LRFD, horizontal back slope, infinite slope and broken back slope, Figure 3-3 illustrates a broken back slope condition with a traffic surcharge. If a traffic surcharge is present and located within 0.5H1 of the back of the reinforced soil volume, then it must be included in the analysis.

4.) The Geotechnical Engineer of Record for the project is responsible for designing the reinforcement lengths for the external conditions shown in Figure 3-4 and any other conditions that are appropriate for the site.

H. Apparent Coefficient of Friction [11.10.6.3] The coefficient of friction (f*) need not be reduced for properly placed and compacted, saturated backfill.

I. Soil Reinforcement [11.10.6.4]

1.) Delete the corrosion rates specified in LRFD Article [11.10.6.4.2a] and substitute the following requirements:

The following corrosion rates for metallic reinforcement apply to non-corrosive (Slightly or Moderately Aggressive)
environments only:

a.) Zinc (first 2 years) | 15 microns/year
b.) Zinc (subsequent years to depletion) | 4 microns/year
c.) Carbon Steel (after depletion of zinc) | 12 microns/year
d.) Carbon Steel (75 to 100 years) | 7 microns/year

2.) Do not use Metallic reinforcement in Extremely Aggressive Environments without approval from the State Structures Design Engineer.

3.) Epoxy coated reinforcement as specified in LRFD Commentary [C11.10.6.4.2a] is not permitted. Passive metal soil reinforcement (i.e., stainless steel, aluminum alloys, etc.), is permitted only with written SDO approval.

4.) Geosynthetic reinforcements (LRFD 11.10.6.4.2b) must comply with Chapter 31 of the Plans Preparation Manual, Volume I.. Reinforcement properties will be the same as those for geosynthetic reinforced soil slopes as shown on Design Standards Index No. 501.

5.) For geosynthetic reinforcement, supplement LRFD Table 11.10.6.4.3b-1 with the following:

Total Reduction Factor, RF = 7.0 for critical temporary wall applications with non-aggressive soils and polymers meeting the requirements listed in Table 11.10.6.4.2b-1.

6.) For permanent wall systems using welded wire soil reinforcement, the minimum wire size in both the longitudinal and transverse directions shall be W10 for walls with a 75 year service life and W11 for walls with a 100 year service life.

7.) Do not design soil reinforcement to be skewed more than 15 degrees from a position normal to the wall panel unless necessary and clearly detailed for acute corners. In these instances, follow the pre-approved bin wall details shown in the QPL Vendor Drawings.

Commentary:

There are times when the 15 degree criteria cannot be met due to vertical obstructions such as piling, drainage structures or bridge obstructions with angles. In these cases, clearly detail the soil reinforcement skew details in the Shop Drawings.

8.) Do not design soil reinforcement to be skewed more than 15 degrees from a horizontal position in elevation view to clear horizontal obstructions.

9.) Soil reinforcement must not be attached to piling, and abutment piles must not be attached to any retaining wall system.

J. Reinforcement/Facing Connection [11.10.6.4.4]

Design the soil reinforcement to facing panel connection to assure full contact of the connection elements. The connection must be able to be inspected visibly during construction.

Commentary:

Normally mesh and bar mats are connected to the facing panel by a pin passing through loops at the end of the reinforcement and loops inserted into the panels. If these loops are not aligned, then some reinforcement will not be in contact with the pins causing the remaining reinforcement to be unevenly stressed and/or over stressed. If the quality of this connection cannot be assured through pullout testing and quality assurance during installation, then the strength of the soil reinforcement and its connections should be reduced accordingly.

K. End Bents on Piling behind MSE Walls

1.) All end bent piles must be plumb.

2.) The minimum clear distance shall be a minimum of 2’-0” for the following:

a.) Between the front face of the end bent cap or footing and the back face of wall panel.

b.) Between the face of piling and leveling pad. (The 2’-0” dimension is based on the use of 18-inch piles. Whenever possible for larger piles, increase the clear distance between the wall and pile such that no soil reinforcement is skewed more than 15 degrees).

3.) Soil reinforcement to resist the overturning produced by the earth load, friction, and temperature must be attached to end bents, unless the long-term settlement exceeds 4 inches. In this case, the reinforcement must not be attached to the end bent and a special wall behind the backwall must be designed to accommodate the earth load.

L. End Bents on Spread Footings behind MSE Walls:

1.) The spread footing must be sized so that the factored bearing pressure does not exceed 6000 psf.

2.) The footing must be a minimum of 1’-0” behind the facing panel.

3.) The minimum distance between the centerline of bearing on the end bent and the back of the facing panel must be 4’-0”.