Cyclic secant shear modulus versus pore water pressure in sands at small cyclic strains: Fid-Bau Portal

Cyclic secant shear modulus versus pore water pressure in sands at small cyclic strains

Vucetic, Mladen / Mortezaie, Ahmadreza

Abstract Cyclic strain-controlled behavior of fully saturated sands in undrained condition is analyzed at small cyclic shear strain amplitudes, γ c, around the threshold shear strain for cyclic pore water pressure buildup, γ tp≈0.01%. The cyclic triaxial and simple shear test results obtained in the past by different researchers and the results of new cyclic simple shear tests reveal that: (i) at very small γ c below γ tp where there is no buildup of cyclic pore water pressure, Δu N, with the number of cycles, N, the cyclic secant shear modulus, G SN, initially increases with N for 10–20% of its initial value G S1 and then levels off or just slightly decreases, (ii) at small γ c between γ tp≈0.01% and 0.10–0.15%, Δu N continuously increases with N while the modulus G SN first increases for up to 10% of G S1 and then gradually decreases, and (iii) at γ c larger than approximately 0.15%, relatively large Δu N develops with N while the modulus G SN constantly and significantly decreases. This means that at γ c between γ tp and 0.10–0.15% the sand stiffness initially increases with N in spite of the reduction of effective stresses caused by the cyclic pore water pressures buildup. In this range of γ c, the pore water pressure Δu N can reach up to 40% of the initial effective confining stress before G SN drops below G S1. The microstructural mechanisms believed to be responsible for such a complex behavior are discussed. It is suggested that during cyclic loading the changes at mineral-to-mineral junctions of grain contacts can cause soil stiffening while, at the same time, the buildup of cyclic pore water pressure causes the softening.

Highlights • Behavior of fully saturated sand at small cyclic strains, γ c, is analyzed. • Focus is on γ c around threshold for cyclic pore water pressure γ tp≈0.01%. • Change of secant shear modulus, G SN, and cyclic pore pressure, Δu N, are correlated. • At γ tp<γ c<0.10% G SN increases and then decreases, while Δu N always increases. • Consequently, at γ tp<γ c<0.10% stiffness goes up while effective stress goes down.