The Long-Term Performance of a Drilled Shaft Retaining Wall in an Expansive Clay Page: 144
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possibility of large in situ lateral stresses caused by overconsolidation was considered. Values for
K0 of 2 to 3 have been measured in these highly plastic, overconsolidated clays in Central Texas
(e.g., Smith et al. 2009). These larger in situ lateral stresses increase the absolute displacement
(Figure 6.7), but the total calculated displacement is still less than one-half the measured
displacement. The possibility of the soil mass stiffness being less during unloading than loading
(as it was measured in both the laboratory UU tests and in the SASW tests) was also considered.
Based on trial and error, a profile of stiffness with E/su equal to 180 above the excavation depth,
where the unloading is most significant, and E/su equal to 400 below the excavation depth
produces a calculated displacement profile that is very similar to the measured one (Figure 6.7).
This stiffness reduction in unloading is consistent with observations in other stiff fissured clays,
such as the London Clay (e.g., Cripps and Taylor, 1981).
The global displacements of the soil (Figure 6.8) and the shear strains in the soil (Figure
6.9) show how the soil is essentially carrying the wall with it in response to the stress relief from
the excavation. The displacements in the soil extend well below the depth of the wall (Figure
6.8). To illustrate this behavior further, a sensitivity study was conducted by lengthening the
drilled shafts in the FEM analysis. These calculated results predict that even if the shafts had
been 50 feet long, embedded below the excavation more than twice the depth of the excavation,
the deflected shape would have been similar to the 35-foot long shafts and the deflection at the
top of the wall would have been only about 0.1 inches smaller (Figure 6.10).144
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Brown, Andrew C.; Dellinger, Gregory F.; El-Mohtar, Chadi; Zornberg, Jorge G. & Gilbert, Robert B. The Long-Term Performance of a Drilled Shaft Retaining Wall in an Expansive Clay, report, October 2013; Austin, Texas. (https://texashistory.unt.edu/ark:/67531/metapth639712/m1/168/?q=%222009%22: accessed July 16, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu; crediting UNT Libraries Government Documents Department.