Modifications to stiff clay and FHWA diagrams
Over the years various researchers and engineers have proposed numerous apparent lateral earth pressure diagrams for braced excavations. Unfortunately, most lateral apparent pressure diagrams have been taken out of context or misused. Historically, apparent lateral earth pressure diagrams have been developed from measured brace reactions. However, apparent earth pressure diagrams are often arbitrarily used to also calculate bending moments in the wall.
In excavations supporting stiff clays, many researchers have observed that the lower braces carried smaller loads. This has misled engineers to extrapolate the apparent lateral earth pressure to zero at subgrade. In this respect, many apparent lateral earth pressure diagrams carry within them a historical unconservative oversight in the fact that the lateral earth pressure at subgrade was never directly or indirectly measured. Konstantakos (2015) has proven that the zero apparent lateral earth pressure at the subgrade level assumption may be unconservative. This historical oversight, can lead to severe underestimation of the required wall embedment length and of the experienced wall bending moments.
If larger displacements can be tolerated or drained conditions are experienced the apparent earth pressure diagrams must not, at a minimum, drop below the theoretical active pressure, unless soil arching is carefully evaluated. Alternatively, in these cases, for fast calculations or estimates, an engineer can increase the apparent earth pressure from 50% at midway between the lowest support level and the subgrade to the full theoretical apparent pressure or the active pressure limit at the subgrade level. As always, these equations represent a simplification of complex conditions.
If tighter deformation control is required or when fully undrained conditions are to be expected, then the virtual reaction at the subgrade level has to take into account increased lateral earth pressures that can even reach close to fifty percent of the total vertical stress at the subgrade level. The initial state of stress has to be taken into consideration as overconsolidated soil strata will tend to induce larger lateral earth stresses on the retaining walls. In such critical cases, a design engineer must always compliment apparent earth pressure diagram calculations with more advanced and well substantiated analysis methods.
The above modifications can be applied within the software by double clicking on the driving earth pressure button when the FHWA or Peck method is selected.
Figure: Minimum lateral pressure option for FHWA and Peck apparent pressure diagrams (check box).
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Figure: Proposed modifications to stiff clay and FHWA apparent lateral earth pressure diagrams (Konstantakos 2010).
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