A laboratory study has been made of the direct shearing of concrete-sandstone interfaces under conditions of constant normal stiffness. The results demonstrate that the peak and residual strength envelopes (stress ratio versus normal stress) are independent of the test path.
Une etude de laboratoire a ete faître sur le cisaillement direct de la surface separante beton-grès sous la condition de rigidite normale constante. Les resultats montrent que les enveloppes de la resistance maximale et de la resistance residuelle (rapport des contraintes vs la contrainte normale) sont independentes du chemin d'essai.
Eine Laboruntersuchung der direkten Scherung von Beton-Sandstein Zwischenflachen unter konstant normalen Steifheitbedingungen wurde durchgefuehrt. Die Resultate zeigen, dass die maximalen und die residuellen Widerstande (Spannungsverhaltnis verglichen mit der Normalspannung) unabhangig sind von der Spannungstestrichtung.
The mechanical behaviour of interfaces between concrete and rock can have an important influence on the response of certain foundations systems to applied loading. A particular example occurs when a concrete pile, formed in a rock socket, is used to support structural loads. Resistance to the loading will be provided by the development of shear stresses at the cylindrical interface along the length of the shaft. Some resistance will also be provided in end bearing at the base of the pile, but, unless the pile is extremely short or the magnitude of the loading is extremely high, most of the applied loading will be carried in side shear. Laboratory studies of the shear behaviour of concrete-rock interfaces are important, not only because they provide basic data essential in the design of rock-socketed piles, but also because they provide insight into the fundamental behaviour. Conventionally, the testing of interfaces has been carried out in the direct shear apparatus with constant normal stress applied across the shear interface. This type of testing provides a reasonable model for cases in practice where no constraint is placed upon the normal displacements accompanying the shearing, e.g. when rock blocks slide freely under gravity. However, in cases such as a concrete pile contained within a rock socket, the stress normal to the plane of shearing can be far from constant. When this type of pile is loaded axially, the pile shaft will displace vertically and at large enough loads (often within the working load range for the pile) relative displacements (slip) will occur between the shaft and the surrounding rock. If the socket containing the pile has a rough surface, then the relative displacement will be accompanied by some dilation at the interface. Because the surrounding rock mass tends to restrain this dilation, the normal compressive stresses acting on the side of the pile will not remain constant but will increase. This phenomenon has been measured and described previously, e.g. by Johnston (1977) and Williams (1980), and it has been shown that, to sufficient accuracy, the normal stiffness of the surrounding rock mass is approximately constant. Hence, it is reasonable to expect that direct shearing under conditions of constant normal stiffness will provide a more realistic laboratory model of the shaft behaviour of concrete piles socketed into rock, than would direct shear testing under conditions of constant normal stress. Several direct shear devices, capable of applying a variety of constraints on the normal mode of deformation, have already been described in the literature, e.g. Lam and Johnston (1982), Desai et al (1985), Natau et al (1979), and some data are available for interfaces of concrete and artificial mudstone (Johnston and Lam, 1984). This paper describes a new constant normal stiffness, direct shear device capable of applying static and cyclic shear loading to a sample containing one of a variety of possible interfaces. The sample may contain a specially prepared interface such as a concrete-sandstone interface of arbitrary roughness and bonding, it may be formed of two blocks from either side of an artificially prepared or a natural discontinuity (e.g. a joint or a bedding plane), or it may be an intact specimen of rock or cohesive soil which does not contain a pre-existing discontinuity plane. When used in a servo-controlled testing machine the device is capable of applying either load or displacement controlled static or cyclic shear loading. The cyclic shear loading may be either one-way or two-way in nature. The device has been used to study the behaviour of concrete-sandstone interfaces with reference to pile sockets, and a comprehensive set of results is presented in this paper.
As suggested above, if dilation accompanies shearing at the interface between a pile and the surrounding rock formation, then to a first approximation the stiffness of the formation with respect to the normal displacement can be regarded as constant.