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FHWA Loading for Stratified Soil Profiles

FHWA Loading for Stratified Soil Profiles

The apparent earth pressure diagrams described above were developed for reasonably homogeneous soil profiles and may therefore be difficult to adapt for use in designing walls in stratified soil deposits. A method based on redistributing calculated active earth pressures may be used for stratified soil profiles. This method should not be used for soil profiles in which the critical potential failure surface extends below the base of the excavation or where surcharge loading is irregular. This method is summarized as follows:

  • Evaluate the active earth pressure acting over the excavation height and evaluate the total load imposed by these active earth pressures using conventional analysis methods for evaluating the active earth pressure diagram assuming full mobilization of soil shear strength. For an irregular ground surface the software will perform a trial wedge stability analysis to evaluate the total active thrust.

  • The total calculated load is increased by a factor, typically taken as 1.3. A larger value may be used where strict deformation control is desired.

  • Distribute the factored total force into an apparent pressure diagram using the trapezoidal distribution.

Where potential failure surfaces are deep-seated, limit equilibrium methods using slope stability may be used to calculate earth pressure loadings.

The Terzaghi and Peck (1967) diagrams did not account for the development of soil failure below the bottom of the excavation. Observations and finite element studies have demonstrated that soil failure below the excavation bottom can lead to very large movements for temporary retaining walls in soft clays. For Ns>6, relative large areas of retained soil near the excavation base are expected to yield significantly as the excavation progresses resulting in large movements below the excavation, increased loads on the exposed portion of the wall, and potential instability of the excavation base. In this case, Henkel (1971) developed an equation to directly obtain KA for obtaining the maximum pressure ordinate for soft to medium clays apparent earth pressure diagrams (this equation is applied when FHWA diagrams are used and the program examines if Ns>6):


Where m=1 according to Henkel (1971). The total load is then taken as:


Figure: Henkel’s mechanism of base failure

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The Figure below shows values of KA calculated using Henkel’s method for various d/H ratios. For results in this figure Su = Sub. This figure indicates that for 4<Ns<6, the Terzaghi and Peck envelope with m=0.4 is overly conservative relative to Henkel. Also, for Ns<5.14 the Henkel equation is not valid and apparent earth pressures calculated using m=1.0 in the Terzaghi and Peck envelope are unrealistically low. For the range 4<Ns<5.14, a constant value of Ka=0.22 should be used to evaluate the maximum pressure ordinate for the soft to medium clay apparent earth pressure envelope. At the transition between stiff-hard clays to soft-medium clays, i.e. Ns= 4, the total load using the soft to medium apparent earth pressure diagram with Ka= 0.22 is 0.193 H2 resulting in a maximum pressure p=0.26 H.  Use of Ka= 0.22, according to FHWA, represent a rational transition value for these cases.

Henkel’s method is limited to cases where the clays soils on the retained side of the excavation and below the excavation can each be reasonably characterized using a constant value for undrained shear strength. Where a more detailed shear strength profile is required, limit equilibrium methods may be used to evaluate the earth pressure loadings on the wall described in section 5.7.3 of the FHWA manual (not performed within the software).


Figure: Comparison of apparent lateral earth pressure coefficients with basal stability index (FHWA 2004).

For clays the stability number is defined as:

Stability number

Please note that software uses the effective vertical stress at subgrade to find an equivalent soil unit weight, Water pressures are added separately depending on water condition assumptions. This is slightly different from the approach recommended by FHWA, however, after personal communication with the late Dr. Peck, has confirmed that users of apparent earth pressures should use the effective stress at subgrade and add water pressures separately.

By ignoring the water table, or by using custom water pressures, the exact same numerical solution as with the original FHWA method can be obtained.

Training Video: Lateral Earth Coefficients and Soil Pressure Methods in DeepEX




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