UCLA

This thesis was undertaken in order to expand upon the study conducted at UCLA by Mortezaie (2012) and Mortezaie and Vucetic (2012) to show that the phenomenon of stiffness increase in saturated sands with the buildup of cyclic pore water pressure, ∆uN, at small cyclic shear strain amplitude, ϒc, as discovered by them is universal. Results of six new single-stage and multi-stage cyclic strain-controlled tests conducted in the NGI type simple shear device and four cyclic strain-controlled and two stress-controlled tests from a previous investigation are analyzed. These tests were conducted at conditions that have not been scrutinized yet, such as different vertical consolidation stresses, σ’vc, and different sequences of ϒc. Amplitude ϒc varied between 0.005% and 0.08% except at the end of two tests when it reached 0.16%.

The study confirmed that below ϒc=0.1% the stiffness index, δ=GSN/GS1, where GSN is the secant shear modulus at cycle N, increases with N in spite of the continuous buildup of ∆uN and then decreases, and that before GSN drops back to GS1 pressure ∆uN may reach 40% of σ’vc. Current liquefaction models assume that δ must go down if ∆uN goes up so the corresponding software can be improved if the above phenomenon is incorporated. The variation of damping ratio, λ, with N was also analyzed and was found consistent with previous findings that λ always decreases with N.