ABSTRACT
Effective (not total) stresses affect soil shear strength and volume change characteristics of geomaterials. It is misleading to think that all “sands” behave “drained” and all “clays” behave “undrained” when loaded. These effects are important. Examples include: dilative sands: “undrained” strength can be a factor 3 higher than the “drained” strength, contractant sand can have little or no strength if rapidly loaded “undrained” andlong-term foundation settlement on clay is a “drained” process. Soil response depends on soil permeability (coefficient of consolidation), soil drainage path length (foundation geometry) and rate of load application (rapid, slow). No soil in the ground has a membrane around it. Hence partial drainage is always a possibility in all soil types. Even if in-situ tests show drained conditions, due to size effects, the soil may respond undrained or partially drained in the case of large(r) foundations.
