Sand response to a large number of loading cycles under zero-lateral-strain conditions: evolution of void ratio and small-strain stiffness

Geotechnical structures often experience a large number of repetitive loading cycles. This research examines the quasi-static mechanical response of sands subjected to repetitive loads under zero-lateral-strain boundary conditions. The experimental study uses an automatic repetitive loading frame operated with pneumatic pistons. Both vertical deformation and shear wave velocity are continuously monitored during 10 000 repetitive loading cycles. The void ratio evolves towards the terminal void ratio e(T) as the number of load cycles increases. The terminal void ratio e(T) is a function of the initial void ratio e(0) and the stress amplitude ratio Delta sigma/sigma(0). The number of cycles N* required to reach half of the final volume contraction ranges from N* -> 1 for densely packed sands (e(0)-> e(min)) to N -> 10(3) for loosely packed sands (e(0)-> e(max)). As the soil approaches terminal density at a large number of cycles, peak-to-peak strains are dominated by elastic deformations, and the minute plastic strains that remain in every cycle reflect local and sequential contact events. The shear wave velocity increases during cyclic loading with data suggesting a gradual increase in the coefficient of earth pressure K-0 during repetitive loading. Changes in shear wave velocity track the evolution of the constrained modulus M; in fact, the constrained modulus can be estimated from the shear wave velocity to compute soil deformation in a given cycle. A simple procedure is suggested to estimate the potential settlement a layer may experience when subjected to repetitive mechanical loads.