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This paper presents a series of cyclic 2D direct shear tests on sand-rough material interfaces under constant normal load (CNL) and constant normal stiffness (CNS) conditions. The aim of these tests is to describe the behavior of the soil-pile contact subjected to a large number of cycles due to environmental or anthropic loadings. These cycles (typically 10 or less due to an early rupture) are small (10, 20 and 40 kPa in terms of shear stress). A new interpretation of the direct shear tests is proposed. The sample of soil is schematically composed of a sheared interface and of a buffer under oedometric load. The problem of sand leakage between the shear box and the rough plate, classical phenomenon in this type of test, is focused. The effect of initial density, position of 'center of cycles' in stress plane (mean cyclic variables) and cyclic amplitude is investigated. The cycles are defined by the initial mean cyclic normal stress, the level of initial mean cyclic stress ratio and the normalized cyclic amplitude. Under CNL condition, either dilation or contraction is exhibited, in agreement with the characteristic state developed by Luong (International symposium on soils under cyclic and transient loading, Swansea, 7-11 January, pp 315-324, 1980). The influence of a prescribed normal stiffness is especially considered. It can be highlighted that CNS cyclic paths are always contractive. This contraction results in a drop of mean cyclic normal stress often called degradation of friction. [ABSTRACT FROM AUTHOR]

This paper deals with a laboratory and in situ study on the improvement of lateral friction along inclusions embedded in fine grained soils. Several soil-inclusion interfaces are considered and characterised by direct shear at constant normal stress and at constant normal stiffness, between silt and concrete/silica sand/silica sand + geotextile/silica sand + metallic grid. The performances of metric inclusions arc measured by pull-out tests, compared and simply modelled. Taking advantage of the dilatancy of sand, properly managed, is highlighted. [ABSTRACT FROM AUTHOR]