Formerly, Staff Bridge Engineer
Colorado Department of Transportation
Denver, Colorado
S. C. (Trever) Wang, Ph. D., P. E.
Senior Engineer
Colorado Department of Transportation
Denver, Colorado
Retaining walls are an important element in highway construction. They are most frequently constructed in the highway environment to retain a mass of earth. They are also used to enable the highway designer to establish grade lines for roadways at differing elevations when such roadways are in close proximity to one another and are to be constructed within limited rights-of-way, as is generally the case in densely populated urban locations.
Until 1972, when the first Reinforced Earth wall in the United States was built in California, retaining walls utilized in highway construction were usually plain gravity or reinforced concrete walls. Now, the use of mechanically stabilized earth (MSE) walls has become widespread in construction throughout the United States. Because early walls included metal strap reinforcement as the primary mechanism for stabilizing the soil, corrosion of the reinforcement and lack of long-term durability were a major impediment to immediate acceptance. Currently, the utilization of metal reinforcing requires the addition of sacrificial galvanizing materials selected to ensure the design life of the structure.
More recent earth reinforcement systems utilize geosynthetic materials, which are deemed inert to attack by deicing salts used on the highways. Salts are a primary inducer of corrosion in metal reinforcement. Long-term creep characteristics of geosynthetic reinforcements, however, must be carefully considered. There are also increasingly more specialty-type walls, such as the soil nail type for both temporary and permanent wall locations, especially for slope stabilization where slope materials are appropriate for nailing. Each of these wall types is discussed briefly in this chapter.
The material in this chapter is drawn from many sources, including personal experience, but primarily from the following sources: (1) Section 5, “Retaining Walls,” American Association of State Highway and Transportation Officials (AASHTO), Standard Specifications for Highway Bridges, 17th ed., 2002; and (2) Subsection 5 of the
Colorado Bridge Design Manual, “Earth Retaining Wall Design Requirements,” Colorado Department of Transportation. A list of references is given at the end of the chapter.
Retaining walls can also be designed by the Load and Resistance Factor Design (LRFD) method as given by AASHTO in LRFD Bridge Design Specifications. This is a method of proportioning structural elements by applying factors to both the loads (load factors) and the nominal strengths (resistance factors). The specified factors are based on the mathematical theory of reliability and a statistical knowledge of load and material characteristics. The load factors are multipliers (typically greater than 1.0) that take account of the variability of different types of loads, such as earth loads and live loads. Resistance factors (typically 1.0 or lower) account for inaccuracies in theory and variation of properties. Although AASHTO’s goal is to use LRFD for all new construction, the traditional methods are currently the choice of most retaining wall designers and, hence, are the focus of this chapter.
