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

## 3.13 Retaining Wall Design

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. Show the retaining wall concrete class on the plans.

### 3.13.1 Earth Loads for Wall Design

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

### 3.13.2 Corrosion Rates [11.9.8.1]

A. Delete the corrosion rates specified in LRFD Article [11.9.8.1] and substitute the following requirements:

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

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

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

### 3.13.3 Soil Reinforcement

A. Design soil reinforcement and connections for permanent walls with a design life of 75 years. Design walls supporting abutments on spread footings with a design life of 100 years.

B. Design soil reinforcement for temporary walls with a design life of not less than the contract time of the project or three years whichever is greater.

C. Maximum stress in the steel soil reinforcement is 0.55F y for steel straps, and 0.48F y for welded wire mats or grids. Specify ASTM A82, Grade 65 ksi steel on the plans, however use 60 ksi in the design.

D. Analyze vertical stresses at each reinforcement level for local equilibrium of all forces acting at that level. Include the effects of any eccentricity.

E. For retaining wall systems that utilize steel soil reinforcement, specify galvanized steel soil reinforcement for permanent walls. Steel soil reinforcement for temporary walls may be galvanized or uncoated (black) steel.01/05

F. Epoxy coated reinforcement as specified in LRFD [11.9.8.1] is not permitted. Passive metal soil reinforcement (i.e., stainless steel, aluminum alloys, etc.) is permitted only with written SDO approval.

G. Geosynthetic Geogrids must comply with Chapter 31 of the Plans Preparation Manual, Volume I.

H. Do not design soil reinforcement skewed more than 15 degrees from a position normal to the wall panel except in cases where there is an acute corner requiring a bin wall. In these instances, follow the pre-approved bin wall details.

I. When the 15 degree criteria cannot be met due to obstructions such as piling or drainage structures, bridge obstructions with angles, and soil reinforcement per the pre-approved details.

J. In addition to LRFD Article [11.9.5.1.4], soil reinforcement lengths, “L”, measured from the back of the facing element, must not be less than the greater of the following:

MSE Walls and Walls in Front of Abutments on Piling [11.9.5.1.4]

L = 8 feet minimum length.

L = 0.7 H1, H1 = mechanical height of wall, in feet

(See Figures 3-16 and 3-17)

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

MSE Walls In Front of Abutments on Spread Footings [11.9.7]

L = 22 feet

L = 0.6 H1 + 6 feet

L = 0.7 H1

Commentary:

As a rule of thumb, for an MSE structure with reinforcement lengths equal to 70% of mechanical height, the anticipated maximum calculated bearing pressure can be anticipated to be about 135% of the overburden weight of soil and surcharge. It may be necessary to increase the reinforcement length for external stability to assure that the allowable bearing pressure specified equals or exceeds this value.

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

L. The horizontal soil reinforcement connection force of 85% of the maximum calculated force as stated in LRFD Article [11.5.1.2] must be modified to a minimum of 100% T max, where:

Tmax = Ta (maximum stress in soil reinforcement)

M. Creep reduction factors in geosynthetic soil reinforcement must be included in seismic designs.

N. In addition to LRFD Article [11.9.4.3], use a soil resistance factor of 0.67 for overturning.

O. For Polymeric Reinforcements in Soils, supplement LRFD Article [11.9.5.1.3] with the following:

For geosynthetics, T a must not exceed the following values:

a.) T a = 19% T ult for permanent walls and critical temporary walls.

b.)T a = 29% T ult for temporary walls.

### 3.13.4 External Stability

A. The required factors of safety for the external stability are:

1.) Overturning: | 2.0
2.) Sliding: | 1.5
3.) Internal Pullout: | 1.5 (Allowable deformation ¾-inch)
4.) Bearing Capacity: | 2.5
5.) Overall Stability: | 1.5

B. The reinforced backfill soil parameters for the analyses are:

1.) Sand fill (Statewide except Dade and Monroe)

a. Moist Unit Weight: 105 lbs per cubic foot.

b. Friction Angle: 30 degrees.

2.) Limerock fill (Dade and Monroe counties only)

a. Moist Unit Weight: 115 lbs per cubic foot.

b. Friction Angle: 34 degrees.

C. For steel reinforcements:

1.) Bars and Straps:

a. 0.55 F y' at end of design life.

b. 0.55F u at net section of bolted connection

2.) Grids and bar mats: 0.48 F y

D. Maximum pullout factors:

1.) Ribbed strips: f* max = 1.5

2.) Grids and bar mats: N pmax = 30

E. The properties for geogrids will be the same as the geogrids used for steepened slopes as shown on Index 501 of the Design Standards.

F. H1, the mechanical height, of an MSE structure must be taken as the height measured to the point where the potential failure plane (line of maximum tension) intersects the ground surface, as shown in Figure 3-16 and 3-17 and can be calculated using the equation provided in Figure 3-16. The methodology for this analysis is included in FHWA’s Mechanically Stabilized Earth Walls and Reinforced Soil Slopes Design and Construction Guideline. In addition to the two conditions shown in AASHTO, horizontal back slope and infinite sloping backfill, Figures 3-16 and 3-17 illustrate broken back slope conditions with and without traffic surcharge. If the break in the slope behind the wall facing is located within two times the height of the wall (2H), then the broken back slope design must apply. If a traffic surcharge is present and is located within 0.5H of the back of the reinforced soil mass, then it must be included in the analysis.

G. The Geotechnical Engineer of Record for the project is responsible for designing the strap lengths for the external conditions shown in Figure 3-1 and any other conditions that are appropriate for the site.

### 3.13.5 Acute corners less than 75 degrees (MSE Walls)

A. When two walls intersect forming an internal angle of less than 75 degrees, design the nose section as a bin wall. Submit the calculations for this special design with the plans for review and approval.

B. Design structural connections between the wall elements within the nose section to create an at-rest bin effect without eliminating the flexibility of the facing to tolerate differential settlements. The design for connections to facing elements and allowable stresses in the connecting members must conform to the requirements for the wall design in general.

C. Design the nose section to settle differentially from the remainder of the structure with a slip joint. Facing panel overlap or interlock or rigid connection across that joint is not permitted.

D. Design soil reinforcements to restrain the nose section by connecting directly to each of the facing elements in the nose section and running back into the backfill of the main reinforced soil mass at least to a plane 10 feet beyond a Rankine failure surface.

E. Design for facing connections, obstructions, pullout and allowable stresses in the reinforcing elements must conform to the requirements of the wall design in general (See Figure 3-18).

### 3.13.6 Minimum Wall embedment in Soil

Consider scour and bearing capacity when determining wall embedment depth. Consult the District Drainage and Geotechnical Engineers to determine the elevation of the top of leveling pad, which must also conform to a minimum embedment to the top of leveling pad of 24-inches. Normal construction practices should be considered.

### 3.13.7 Apparent Coefficient of Friction

The coefficient of friction (f*) need not be reduced for properly placed and compacted, saturated backfill.

### 3.13.8 Connections (MSE Walls)

Design the soil reinforcement to facing panel connection to assure full contact of the connection elements. 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 then the allowable stress in the soil reinforcement and it's connections should be reduced accordingly.

### 3.13.9 End Bents behind MSE Walls

A. End Bents on Piling:

1.) All end bent piles must be plumb.

2.) The front face of the end bent cap or footing must be a minimum distance of 24-inches behind the front face of the wall. (24-inch minimum clear distance between face of piling and leveling pad). This dimension is based upon the use of 18-inch piles. For larger piles, the clear distance between the wall and pile must be increased such that no strap 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. If the long-term settlement exceeds 4-inches, the reinforcement must not be attached to the end bent. In this event, a special wall behind the backwall must be designed to accommodate the earth load.

B. End Bents on Spread Footings:

1.) The spread footing must be sized so that the bearing pressure does not exceed 3000 psf.

2.) The footing must be a minimum 12-inches 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 48-inches.

### 3.13.10 Facing Panels (MSE Walls) [11.9.5.1.6]

The basic (typical) panel size must not exceed 40 square feet in area, and special panels (top out, etc.) must not exceed 50 square feet in area. Full-height facing panels must not exceed 8 feet in height. 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
.

### 3.13.11 Utilities

A. 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.

B. Do not place utilities in the soil-reinforced zone behind 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. In addition, leaking pipes could wash out and destroy the structural integrity.

### 3.13.12 Minimum Front Face Embedment 01/05

The minimum front face embedment for permanent retaining wall systems must comply with Figure 3-19.