Clay behavior depends on the rate of loading (or unloading). When “fast” stress changes take place then clay behavior is typically modeled as “Undrained” while slow stress changes or long term conditions are typically modeled as “Drained”.
In the software, the default behavior of a clay type soil is set as “Undrained”. However, the final Drained/Undrained analysis mode can be defined in the software for each design section/each construction stage. The software offers the following Drained/Undrained Analysis Options:
- Drained: All clays are modeled as drained. In this mode, conventional analysis methods use the effective cohesion c’ and the effective friction angle φ’ to determine the appropriate lateral earth pressures. For clays, the NONLINEAR analysis automatically determines the effective cohesion from the stress state history and from the peak and constant volume shearing friction angles (φpeak’ and φcv’ respectively) and it does not use the defined c’ in the soils tab.
- Undrained: All clays are modeled as undrained. In this mode, conventional analysis methods use the Undrained Shear Strength Su and assume an effective friction angle φ’=0o to determine the appropriate lateral earth pressures. For clays, the NONLINEAR analysis automatically determines the Undrained Shear Strength from the stress state history of the clay element and from the peak and constant volume shearing friction angles (φpeak’ and φcv’ respectively), but limits the upper Su to the value in the soils tab.
- Undrained for only initially undrained clays: Only clays whose initial behavior is set to undrained (Soils form) are modeled as undrained as described in item b) above. All other clays are modeled as drained.
In contrast to the non-linear analysis, conventional analyses do not generate excess pore pressures during undrained conditions for clays.
Image: Wall shear and horizontal soil stresses in DeepEX - Undrained Clay
Image: Wall shear and horizontal soil stresses in DeepEX - Drained Clay
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