Determination of the small-strain stiffness of hard soils by means of bender elements and accelerometers

Direct determination of seismic wave velocities in the laboratory is becoming common practice worldwide, given its great potential in the definition of the stiffness at very small strains. One of the techniques for seismic wave measurement makes use of piezoelectric transducers, such as bender elements (BEs). However, some limitations remain to the applicability of this technique, namely for stiff geomaterials, such as compacted soils, naturally or artificially cemented soils and soft or weak rocks. For this purpose, two accelerometers have been used in conjunction with BEs. In the present paper, this combined test setup implemented on a stress-path triaxial chamber will be detailed. An application study will be presented for a hard soil, prepared by laboratory compaction and tested in triaxial compression at different isotropic stress levels. The equipments, procedures and interpretation analyses will also be described. The advantages of this setup are twofold: i) the interpretation of the acceleration measurements is straightforward, since the signals are of the same nature; ii) these measurements can be used to verify the bender element signals, and thus minimize the subjectivity of the interpretation of BE results. Additionally, the accelerometers can be used autonomously wherever the interpretation of BEs becomes too complex. The results of this research enabled to validate the interpretation methods used for bender element testing. Moreover, this combined setup of transducers provided a simple yet powerful tool for eliminating the subjectivity inherent to BE testing, enabling reliable measurements of small-strain stiffness for a wide range of materials. Détermination directe des vitesses des ondes sismiques dans le laboratoire est devenue une pratique courante dans le monde entier, compte tenu de son grand potentiel dans la définition de la rigidité à petites déformations. Une des techniques pour la mesure des ondes sismiques utilise des transducteurs piézoélectriques, comme des bender elements (BEs). Toutefois, certaines limites restent à l'applicabilité de cette technique, pour les géomatériaux raide, comme les sols compactés, les sols cimentés et des roches tendres ou faibles. A cet effet, deux accéléromètres ont été utilisés en conjonction avec des BEs. Dans cet article, cette installation d'essai combiné mise en oeuvre sur une chambre triaxial de stress-path sera détaillée. Une étude d'application sera présentée pour un sol dur, préparé par comp