Pure elastic stiffness of sand represented by response envelopes derived from cyclic triaxial tests with local strain measurements

Acta Geotechnica - The elastic stiffness of a fine sand at small to moderate strains ( $$\varepsilon \le 2 \times 10^{-4}$$ ) has been studied based on cyclic triaxial tests on cube-shaped samples...     

Shear banding in geomaterials under extensional plane strain conditions: Physical and analytical model

The shear band (SB) spacing phenomenon in geomaterials is addressed in this paper under plane strain extensional conditions. On the basis of sand box observations, a method is proposed for the prediction of the spacing between shear bands based on the so-called “bookshelf” deformation mechanism. The proposed method is based on the assumption that the material is softening inside the shear bands whereas outside the SB it undergoes elastic unloading. The underlying assumption for the prediction of SB spacing is that the material reaches the lowest possible energy rate. An analytical expression for the determination of the shear band spacing is presented taking into account the plane strain extensional deformation of the specimen. The friction due to the normal off-plane shear acting on the boundaries is then taken into account.     

Centrifuge model tests on sand specimen under extensional load

The appearance of shear banding in granular materials has been investigated intensively during the last decades and is still of ongoing importance in terms of understanding the stress–strain behaviour of the material, the localization phenomena and the interaction between soil and structure. Only less attention has been paid to the occurrence of systems of shear bands although such systems can be identified in geotechnical structures as well as in geological formations. In this paper we present results of experiments on sand specimens under extensional load in natural gravity as well as in increased gravity in the centrifuge where the influence of the stress level on the geometry of a shear band pattern, specified by the spacing of the shear bands and the angle between failure surfaces and minor stress direction, has been investigated. X‐ray technique has been used to visualize the failure zones inside the specimen, an optical measurement system called Digital Image Correlation has been applied to identify and observe the appearing deformation mechanism on the sides of the specimens in natural gravity as well as during the flight in the centrifuge. It can be shown that the geometry of the shear band pattern is sparsely influenced by the change of the stress level. Copyright © 2004 John Wiley & Sons, Ltd.     

Experimental evidence of a unique flow rule of non-cohesive soils under high-cyclic loading

The presented results of cyclic triaxial tests on sand demonstrate that the cumulative effects due to small cycles obey a kind of flow rule. It mainly depends on the average stress ratio about which the cycles are performed. This so-called “cyclic flow rule” is unique and can be well approximated by flow rules for monotonic loading. Amongst others it is shown that the cyclic flow rule is only moderately influenced by the average mean pressure, by the strain loop (span, shape, polarization), the void ratio, the loading frequency, the static preloading and the grain size distribution curve. A slight increase of the compactive portion of the flow rule with increasing residual strain (due to the previous cycles) was observed. These experimental findings prove that the cyclic flow rule is an essential and indispensable concept in explicit (N-type) accumulation models.     

Diffraction effects between foundations due to incident rayleigh waves

The dynamic behaviour of a finite number of rigid, adjacent foundations on the surface of a linear-elastic, isotropic and homogeneous halfspace due to a far-field excitation of the Rayleigh type is the subject of the present work. The dynamic behaviour of this system differs from that of a single foundation subjected to the same excitation due to the existence of the natural coupling between adjacent foundations caused by the wave propagation through the underlying soil. For the determination of the diffraction forces acting on the foundations an analytical procedure is followed. The stresses at the interfaces between the foundations and subsoil are approximated by series expansions of orthogonal polynomials. It is interesting to notice that, apart from the loads appearing in the direction of incidence, additional loads perpendicular to the given incidence direction act on the foundations due to scattered waves. Their intensity depends on the excitation frequency and the distance between the foundations. The method followed here can be applied for the determination of the dynamic loads acting on fixed foundations in the case of a seismic excitation of relatively long duration.     

Liapunov instability of the hypoplastic model for soils

The hypoplastic constitutive model is known for its numerous application to the problems of soil mechanics and also for its excessive ratcheting. The paper shows that this deficiency can be interpreted as Liapunov instability in dynamic case. This is demonstrated with a simple one-dimensional swinger. Infinitesimally small as well as finite oscillations are analytically examined and the variability of stiffness upon a single cycle is considered. Several methods to circumvent the problem of ratcheting are discussed: implementation of ‘elastic’ range by means of so-called intergranular strain, usage of overlay modelling with parallel coupling of hypoplastic materials as originally proposed by Valanis in his endochronic theory and a special coupling of hypoplastic models based on comparison of partial stiffnesses.     

Strain accumulation in sand due to cyclic loading: Drained cyclic tests with triaxial extension

This paper presents results of numerous drained cyclic tests with triaxial extension. The influence of the strain amplitude, the average stress and the number of load cycles on the accumulation rate was studied. A simple cyclic flow rule was observed. Most findings confirm a previous study with cyclic triaxial compression tests. The test results serve as the basis of an explicit accumulation model.