This paper describes an investigation into the body wave velocity and stiffness characteristics of the foundation soils at the site formerly considered for Sizewell C Nuclear Power Station. Results from both in situ and laboratory tests are presented. In situ measurements comprised shear and compression wave velocities using cross-hole and down-hole techniques, including the velocity of horizontally propagating, horizontally polarized shear waves. Laboratory tests included torsional shear tests on hollow cylindrical samples, triaxial compression and extension tests with local measurements of strain, and resonant column tests. Sample quality and the effects of destructuring are evaluated. The anisotropy of the wave velocity and of the stiffness characteristics is highlighted. L'article decrit des recherches sur la vitesse de propagation des ondes de volume et les caracteristiques de rigidite des sols d'assise de l'emplacement propose de la centrale nucleaire Sizewell C. Il presente les resultats des essais...