FHWA Recommended Apparent Earth Pressure Diagram for Soft to Medium Clays
Temporary and permanent anchored walls may be constructed in soft to medium clays (i.e. Ns>4) if a competent layer of forming the anchor bond zone is within reasonable depth below the excavation. Permanently anchored walls are seldom used where soft clay extends significantly below the excavation base.
For soft to medium clays and for deep excavations (and undrained conditions), the Terzaghi-Peck diagram has been used to evaluate apparent earth pressures for design of temporary walls in soft to medium clays. For this diagram apparent soil pressures are computed with a “coefficient”:
Where m is an empirical factor that accounts for potential base instability effects in deep excavations is soft clays. When the excavation is underlain by deep soft clay and Ns exceeds 6, m is set to 0.4. Otherwise, m is taken as 1.0 (Peck, 1969). Using the Terzaghi and Peck diagram with m=0.4 in cases where Ns>6 may result in an underestimation of loads on the wall and is therefore not conservative. In this case, the software uses Henkel’s equation as outlined in the following section.
An important realization is that when Ns>6 then the excavation base essentially undergoes basal failure as the basal stability safety factor is smaller than 1.0. In this case, significant soil movements should be expected below the excavation that are not captured by conventional limit equilibrium analyses and may not be included in the beam-on-elastoplastic simulation (Nonlinear).
The software in the case of a single soil layer will use this equation if Ns>4 and Ns<=6.
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