The seismic performance of many nuclear power plant (NPP) structures depends on the cyclic shear stress-shear strain-volumetric strain behavior of dense, compacted coarse-grained soils used to support their foundations. However, when these deposits are thick, even relatively small volumetric strains can result in nontrivial settlements that can impact NPP structures. Here, we describe the results from dynamic centrifuge tests performed on thick layers (prototype thicknesses of 10.25 and 20.5 m) of saturated dense (relative density, Dr∼95%), Ottawa sand. Models were excited using unidirectional (1D) and bidirectional (2D) broadband motions (Arias intensities ranged from 0.1 to 5 m/s). Centrifuge test results indicate minor differences between 1D and 2D response spectra. In contrast, 2D shaking in dense sands caused increases in porewater pressure (PWP) generation and volumetric strains (εv) of approximately 200% compared to 1D shaking. These increases in PWP and εv were considerably larger than observed by others for loose sands.