Recommendations for Assessing Bending Moments for Stiff Wall Systems

Abstract

Empirical Apparent Earth Pressure (AEP) diagrams, such as those proposed by Terzaghi and Peck (1967), are regularly used for assessing support loads for propped or anchored retaining structures. This approach has been used successfully on many flexible wall projects over the years in the United States. Initially developed for assessing support loads, these diagrams are now commonly being used to assess bending moments and shear forces developed in the retaining structures by application of the AEP diagrams to a beam simply supported at the position of each of the struts or tiebacks (Sabatini et al., 1999). While this approach may achieve satisfactory results for relatively flexible wall systems, many authors have recommended more sophisticated analysis methods for stiff wall systems. This paper presents the results of comparisons of the empirical AEP diagram with a beam on rigid supports (RIGID) analysis to beam on spring (Winkler) and numerical analysis methods for the assessment of structural forces in deep multi-propped excavations. Comparisons are provided for a stiff slurry diaphragm wall system and a flexible soldier pile and lagging wall system. For the stiff slurry diaphragm wall with tiebacks, the RIGID approach was found to be highly non-conservative for estimating bending moments in the retaining structure. The reason for this is shown to relate to the staged excavation, tieback installation sequence adopted, and locked-in deformation of the ground below the excavation level, which are not considered in the RIGID analysis. The Winkler and numerical analyses, however, both consider these effects and result in global deflection of the wall which in turn leads to large bending moments. The analyses show that the magnitude of these deformation-induced bending moments far exceed those resulting from the distributed load between rigid supports. For the flexible soldier pile with lagging wall, the RIGID analysis still estimated smaller bending moments than the Winkler or numerical approach but by a much smaller degree of difference than the stiff diaphragm walls. In view of the limitations associated with using AEP diagrams with the RIGID analysis for stiff wall systems, it is recommended that a more sophisticated analysis method be the minimum requirement for assessing structural forces in these walls.