FHWA Apparent Pressures in Deep Excavations
FHWA Apparent Pressure Diagrams
FHWA apparent earth pressure diagrams are prescribed by the Federal Highway Administration for design purposes of vertical "flexible" retaining walls for transportation projects. FHWA recommends a number of diagrams that depend on soil type, and number of brace supports. In general, FHWA recommends using trapezoidal earth pressure diagrams that derive from active earth pressures times a factor that ranges from 1.1 to 1.3. For undrained clays a multiplication factor of 0.2 to 0.4 is recommended. One of the oversights of FHWA is that the apparent earth pressure diagrams reduce to zero at the excavation base. Applying zero earth pressure at subrade can result in undersizing of the required wall embedment. As engineers we should recognize the limitations inhereted with apparent earth pressure diagrams. For this reason, FHWA apparent earth pressure diagrams should be used as preliminary tools that can complement more comprehensive non-linear solutions.
Figure 1 below shows the basic FHWA recommendations for apparent earth pressure diagrams. Figure 2 shows an apparent earth pressure diagram as calculated by our DeepEX software program.
Figure 1: Basic recommendations by FHWA
Figure 2: FHWA Apparent Pressures Diagrams - DeepEX Software
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