Consequences of slenderness and boundary conditions in triaxial testing on the reliability of design parameters

The traditional way of designing foundations is based on a single representative strength and deformation parameters. Often the strength properties are derived from triaxial testing. A triaxial test using samples with slenderness ratio H/D = 1 and smooth ends is found to be more reliable by several studies in the literature. Despite of that, the majority of triaxial tests are conducted with the traditional H/D = 2 ratio and rough ends. There are consequences in choosing to use the set-up with H/D=1 and smooth ends or the traditional one. This paper shows that the settlements of a rigid footing can be up to six times higher when the design uses deformation properties of a triaxial test run with H/D = 1 and smooth ends. This is due to a much lower secant stiffness than the one resulting from test with H/D = 2. Depending on the interpretation and way of conducting the test, the bearing capacity of a strip footing calculated by using properties from the H/D = 2 ratio and rough ends set-up can be almost two times greater than the one returned by the set-up with H/D=1 and smooth ends.