A. Consider site, economics, aesthetics, maintenance and constructability when determining the appropriate wall type.
B. Partial height walls are not permitted. Walls founded at or near natural ground level should extend to the top of embankment. The embankment should not slope down to the top of the wall as this condition presents safety and maintenance issues. Walls founded in new embankment are subject to bearing, rotation, settlement, erosion and maintenance issues and should be avoided. The embankment in the outer portion is very difficult if not impossible to compact.
C. Walls subject to large short-term settlements during construction should be carefully analyzed for construction problems. Large short-term settlement is defined according to wall type; for C.I.P. walls the settlement may be as little as 1-inch and as much as 4-inches for MSE walls. Settlements in both longitudinal and transverse directions must be considered. The EOR for the project is responsible for determining the wall type to be used.
D. Walls requiring a two-faced wall system should be included in the plans along with any special soil improvement techniques and instrumentation and monitoring program. Show, on the plans, soil improvement techniques for consolidating foundation soil before wall construction.
E. The following wall types are being used or considered for use by FDOT.
A. CIP walls are normally used in either a cut or fill situation. These walls are sensitive to foundation problems. The foundation soil must be capable of withstanding the design bearing pressure and must exhibit very little differential settlement. Verify that during the life of the structure the bearing capacity of the soil will not be diminished (i.e., french drains in close proximity to the walls) thus requiring pile-supported walls. This type of wall has an advantage over MSE walls because they can be built with conventional construction methods even in Extremely Aggressive Environments. Another advantage over MSE walls is on cut/widening projects where the area behind the wall is not sufficient for soil reinforcement (See Figures 3-2 and 3-3).
B. The relative cost of CIP walls is greater than MSE walls when the site and environment are appropriate for each wall type. This is assuming the area of wall is greater than 1000 square feet and greater than 10 feet in height.
A. Pile-supported walls are utilized when the foundation soil is not capable of supporting the retaining wall and associated dead and live loads on a spread footing.
B. Pile-supported retaining walls are extremely expensive compared to CIP cantilevered and MSE walls and are only appropriate when foundation soil conditions are not conducive for CIP or MSE walls. Pile supported walls are appropriate for cut or fill sites. Temporary sheeting may be required in cut sites. (See Figures 3-4 and 3-5)
A. MSE walls are not a cure-all for poor foundation soil and are not appropriate for all sites. MSE walls are very adaptable to both cut and fill conditions and will tolerate a greater degree of differential settlement than CIP walls. Because of their adaptability, MSE walls are being used almost exclusively. The design of MSE walls with metallic reinforcement, however, is sensitive to the electrochemical properties of the back fill material and to the possibility of a change in the properties of the back fill materials due to submergence in water classified as Extremely Aggressive or from heavy fertilization. Consider using geosynthetic reinforcement in areas where the water is classified as an Extremely Aggressive Environment, when the 100-year flood can infiltrate the back fill, and when the wall is within 12 feet of the Mean High Water (MHW).
B. MSE walls are generally the most economical of all wall types when the area of retaining wall is greater than 1000 square feet, and the wall is greater than 10 feet in height. (See Figs. 3-6 and 3-7)
A. Precast counterfort walls are applicable in cut or fill locations. Their advantage is in cut locations such as removing front slopes under existing bridges and in certain widening applications where sheet piling would be required to stabilize excavation for earth reinforcements for MSE walls. In addition, their speed of construction is advantageous in congested areas where maintenance of traffic is a problem. This type of wall is also applicable in areas where the back fill is or can become classified as extremely corrosive.
B. This type of wall is generally not as economical as MSE walls but is competitive with CIP walls (See Figure 3-8) and may offer aesthetic and constructability advantages.
A. Steel sheet pile walls are applicable for use in permanent locations (i.e., bulkheads) but their more common use is for temporary use (i.e., phase construction). Generally steel sheet pile walls can be designed as cantilevered walls up to approximately 15 feet in height. Steel sheet pile walls over 15 feet are tied back with either prestressed soil anchors, soil nails, or dead men.
B. Steel sheet pile walls are relatively expensive initially and require periodic maintenance (i.e. painting, cathodic protection. This type of wall should only be used if there are no other more economical alternates (See Figure 3-9).
A. Concrete sheet piles are primarily used as bulkheads in either fresh or saltwater. Rock in close proximity to the ground surface is a concern with this type of wall as they are normally installed by jetting. Concrete sheet piles when used as bulkheads are normally tied back with dead men.
B. This type of wall is relatively costly and should only be used when less costly alternates are not appropriate or when the environment is appropriate (See Figure 3-10).
Wire faced walls are applicable in temporary situations, phase construction where large amounts of settlement are anticipated or where surcharges are required to accelerate settlement. This type of wall is a form of MSE wall. The soil reinforcement may be either steel or geogrid. Verify that proposed systems have been pre-approved. Pre-approved companies have their standard details in the Roadway and Traffic Design Standards or on the Structures Design Office’s Internet Homepage (See Figure 3-11).
The Department is allowing the use of soil nails. The relative cost of this type wall has not been determined because Contractors have yet to bid this alternate (See Figure 3-12).
This type of wall is applicable in bulkheads and retaining walls where the environment is Extremely Aggressive and/or rock is relatively close to the ground surface. The cost of this type of wall is very competitive with concrete sheet pile walls (See Figure 3-13).
Modular blocks consist of dry cast, unreinforced blocks, which are sometimes used as a gravity wall and sometimes used as a wall facing for a MSE variation normally utilizing a geogrid for soil reinforcement. The Department is considering systems of this type for low-height, non-structural applications only.
Geogrids are presently approved for use in temporary and select (Extremely Aggressive Environments) permanent walls only. Their use in permanent walls is being reviewed. Some of the concerns about geogrids are the long term stress-strain characteristics of polymers, hydrolysis of polyesters, environmental stress cracking, and brittle rupture of polyolefins (high density polyethylene, polyethelene and polypropylene) and appropriate factors to ensure an allowable stress condition at the end of the structure's service life.
As more highways are widened, problems have been encountered at existing grade separation structures. The existing front slope at the existing bridge must be removed to accommodate a new lane and a retaining wall must be built under the bridge. Several methods have been used to remove the existing front slope and maintain the stability of the remaining soil. One method is to excavate slots or pits in the existing fill to accommodate soldier beams. The soil is then excavated and timber lagging is placed horizontally between the vertical soldier beams. The soldier beams are tied-back by the use of prestressed soil anchors. This procedure will maintain the soil while the permanent wall is built. The permanent wall should be designed to accept all appropriate soil, dead and live loads. The temporary lagging must not contribute to the strength of the permanent wall (See Figure 3-3).
Hybrids are basically gravity systems with some MSE characteristics. The FDOT approved system has a concrete stem that extends into the fill a sufficient length to satisfy the external stability requirement for the wall. However, due to its rigid stem, the system is sensitive to both longitudinal and transverse settlement. This system can be used in cut and/or fill applications (See Figure 3-15)