Upgrading equipment and procedures for stress path triaxial testing of coarse grained materials

Advanced laboratory testing is often the only way to gain an insight into the mechanical behaviour of soils in the whole range from small strains to failure, which in some cases is required for the engineering modeling of complex structures. When such structures are rockfill dams, the use of a large apparatus in-laboratory is virtually unavoidabie because of particle size, even though testing becomes far more difficult than usual. This paper presents problems arising from the upgrading of an existing large triaxial cell in order to provide fully automated stress-path control. Because of the large dimensions of the apparatus, implementation of both stress rate and strain rate controls posed a number of unusual problems. Even though the control had to be carefully tuned in order to consider strain-softening behaviour, the adopted procedure proved successful. The new cell produced good results for different kinds of stress paths. with satisfactory control in all stages.