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In triaxial tests, it is of great importance to accurately measure the radial strain of a specimen. However, devices for radial strain measurement are relatively fewer than the ones for vertical strain measurement. In this study, a new measurement device based on the fiber Bragg grating (FBG) was developed to determine the small and large radial strains of a soil specimen. The new device includes two yokes and two compass-type mechanisms, which are mounted on diametrically opposite sides of the specimen. Metal strips bonded with FBG sensors were chosen as sensing elements and installed at two ends of two legs of a compass-type mechanism so that the small deformation of the specimen could be amplified by a constant ratio to a more detectable value with a higher signal-noise ratio. Considering the temperature sensitivity of FBG and the real working environment, the new device was carefully calibrated underwater. Validation tests on an intact triaxial specimen demonstrated that the new device can work reliably and accurately under both static and cyclic loadings. Key words: triaxial test, radial strain, small strain measurement, cyclic, fiber Bragg grating (FBG). Dans les essais triaxiaux, il est tres important de mesurer avec precision la deformation radiale d'un echantillon. Cependant, les dispositifs de mesure de la deformation radiale sont relativement moins nombreux que ceux de mesure de la deformation verticale. Dans cette etude, un nouveau dispositif de mesure base sur le reseau de Bragg a fibres (FBG) a ete developpe pour determiner les petites et grandes deformations radiales d'un echantillon de sol. Le nouveau dispositif comprend deux jougs et deux mecanismes de type compas, qui sont montes sur des cotes diametralement opposes de l'echantillon. Des bandes de metal collees avec des capteurs FBG ont ete choisies comme elements de detection et installees aux deux extremites des deux jambes d'un mecanisme de type boussole afin que la petite deformation du specimen puisse etre amplifiee par un rapport constant a une valeur plus detectable avec un rapport signal-bruit plus eleve. Compte tenu de la sensibilite a la temperature du FBG et de l'environnement de travail reel, le nouveau dispositif a ete soigneusement calibre sous l'eau. Des tests de validation sur un echantillon triaxial intact ont demontre que le nouveau dispositif peut fonctionner de maniere fiable et precise sous des charges statiques et cycliques. [Traduit par la Redaction] Mots-cles: test triaxial, deformation radiale, mesure de la petite deformation, cyclique, reseau de Bragg a fibres (FBG)., Introduction In conventional triaxial tests, it is generally acknowledged that determining the small-strain stiffness of a soil specimen accurately has been hindered by its seating and bedding errors as well [...]