The 'shear strength' of soil measured in an 'undrained triaxial test' (UU test) is:
Choose the correct answer
Dependent on effective confining pressure
Constant Su = (σ1-σ3)/2 regardless of cell pressure for saturated clay (φu=0 — effective stress unchanged during shearing)
Only for dry sands
Sum of c and tanφ times confining pressure
Correct Answer
B. Constant Su = (σ1-σ3)/2 regardless of cell pressure for saturated clay (φu=0 — effective stress unchanged during shearing)
Why is this the answer?
UU triaxial test: no drainage permitted during cell pressure application or shear. For saturated clay: increase in cell pressure = increase in pore pressure → effective stress unchanged → failure envelope is horizontal (φu = 0). Su = (σ1-σ3)/2 = constant regardless of cell pressure. Results: total stress envelope only. Used for: rapid loading problems (embankment stability at end of construction in soft clay).
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