The boundary conditions in direct simple shear tests

Abstract

More than the half of the Netherlands is under the high level of sea and rivers. Therefore, evaluating the safety of dykes is primordial. A specific interest is given to peat dykes safety which suffer of a lack of knowledge which manifested recently by some peat dykes failures (Van Baars 2005). The behaviour of peat is also of interest in others countries, for assessing peat slope stability for instance (Long and Jennings 2006). Due to its high anisotropy and fibrosity, peat cannot be tested with any device in the laboratory. The direct simple shear test is routinely used since it can mimic several in situ conditions and provides conservative results for peat dyke stability evaluation. Furthermore, it does not show the inconvenience of triaxial testing with peat (Landva 1980). Larger samples than for usual testing are desirable to investigate the effect of fibres on tests results. The Direct simple shear testing devices remain imperfect since it is unable to provide additional shear stresses on the sides of the specimen. As consequence, non uniformities develop on all the faces of the specimen, in particular compression in the obtuse corners and tension in the acute corners. In practice, thin samples are used (height over diameter around 0,2 to 0,3) to limit the nonhomogeneities to the sides and leave the major part of the sample in an homogenous state of stress. Testing peat at low vertical stress, remains a challenge and necessities the development of adapted devices (Boylan and Long 2009). A series of tests has been performed on a wood and sedge peat with the Geonor device in order to compare the effect of two boundary conditions on tests results. The first one is a classical reinforced membrane (Bjerrum and Landva 1966) and the second is an unreinforced membrane enclosed in a stack of rings. The vertical stresses applied during the tests varied between 10kPa and 120kPa. The results show small differences when the Mohr Coulomb parameters are determined. The comparison is limited considering the variability of the material tested. A more accurate calibration of the stack of rings would be desirable. Some improvements are needed on the actual apparatus to test peat at low vertical stress. Removing the membrane between the soil and the rings would give more accuracy in the results. A direct simple shear prototype has been developed in order to test larger samples (with height over diameter ratio of 0.5) at low vertical stress. The effect of two innovative rough boundaries on the stress-strain homogeneity of the sample has been investigated. The sidewalls of the device are transparent and make possible a visual assessment of the deformation of the sample. The Particle Image Velocimetry analysis is also considered to assess the shear strain homogeneity in the sample. The results show an improving shear strain homogeneity and reduced tension forces in the acute corners. Slippage is also observed between the top cap and the sample and the normal load could not be measured. Further research is needed to validate the utility of this prototype. Stress-strain curves obtained from the three boundaries should be compared to quantify the improvement of rough boundaries. A finite element analysis of the prototype boundaries has been performed with two models (Mohr Coulomb and Soft Soil Creep model). The boundaries considered were perfectly rough at the top and bottom and perfectly smooth at the sides. The presence of strips and even more the presence of vanes increase the stress – strain homogeneity inside the sample with both models. Reliable stress – strain curves as measured in classical devices could not be obtained with such boundaries. Interfaces should be preferred to model more realistic conditions.