Directional response of a reconstituted fine‐grained soil—Part I: experimental investigation

This paper discusses the results of a large experimental program designed to investigate in a systematic manner the main features of the incremental response of fine‐grained soils. The results are obtained from triaxial stress probing experiments carried out on a French silty clay (Beaucaire Marl). All the tests have been performed on reconstituted specimens, normally consolidated to an initial state which is either isotropic or anisotropic. In the interpretation of the experimental results, extensive use is made of the concept of strain response envelope. The response envelopes obtained for different stress increment magnitudes are remarkably consistent with each other and indicate an inelastic and irreversible material response, i.e. a strong dependence on the stress increment direction, also at relatively small strain levels. A companion paper (Int. J. Numer. Anal. Meth. Geomech., this issue, 2006) assesses the performance of some advanced constitutive models in reproducing the behaviour of reconstituted Beaucaire Marl as observed in this experimental program. Copyright © 2006 John Wiley & Sons, Ltd.     

A Laboratory Experimental Study of the Hydromechanical Behavior of Boom Clay

This paper reports some results of a large experimental program on Boom Clay conducted in Grenoble in the framework of the European project SELFRAC. The program included isotropic compression up to relatively high stress, drained triaxial compression tests at different cell pressures, as well as permeability measurements under isotropic and deviatoric stress. Local measurement of axial and radial displacements allowed the detection of strain localization during deviatoric loading. The permeability of Boom Clay is found to depend on the mean effective stress. The response of Boom Clay during deviatoric loading appears to be strongly affected by the swelling experienced during the isotropic stage preceding triaxial compression. The rate of swelling decreases with isotropic stress. The longer the swelling before shear, more the response under shear becomes ductile and the lower the initial stiffness. Permeability depends on the mean effective stress and it is found to decrease of about two orders of magnitude when the mean stress increases from 1 to 32 MPa. Permeability during shear loading is essentially constant and does not seem to be affected by strain localization. These results are complemented by a few observations obtained using X-ray microtomography in the framework of the more recent European project TIMODAZ. These findings illustrate the impact of pre-existing inclusions and fissures on specimen deformation upon deviatoric loading in the laboratory.     

A constitutive model for granular materials with grain crushing and its application to a pyroclastic soil

A constitutive model for granular materials is developed within the framework of strain–hardening elastoplasticity, aiming at describing some of the macroscopic effects of the degradation processes associated with grain crushing. The central assumption of the paper is that, upon loading, the frictional properties of the material are modified as a consequence of the changes in grain size distribution. The effects of these irreversible microscopic processes are described macroscopically as accumulated plastic strain. Plastic strain drives the evolution of internal variables which model phenomenologically the changes of mechanical properties induced by grain crushing by controlling the geometry of the yield locus and the direction of plastic flow. An application of the model to Pozzolana Nera is presented. The stress–dilatancy relationship observed for this material is used as a guidance for the formulation of hardening laws. One of the salient features of the proposed model is its capability of reproducing the stress–dilatancy behaviour observed in Pozzolana Nera, for which the minimum value of dilatancy always follows the maximum stress ratio experienced by the material. Copyright © 2002 John Wiley & Sons, Ltd.     

Structural features and mechanical behaviour of a pyroclastic weak rock

The results of an extensive programme of laboratory testing on intact and reconstituted samples of a pyroclastic weak rock from the volcanic complex of the Colli Albani (Central Italy) are presented. The deposit is known as Pozzolana Nera and may be assimilated to a bonded coarse grained material. The nature of bonds and the micro‐structural features were examined by means of diffractometry, optical and electron microscopy. As bonds are made of the same constituents of grains and aggregates of grains, bond deterioration and particles breakage upon loading are indistinguishable features of the mechanical behaviour. The testing programme consisted mainly of one‐dimensional and drained and undrained triaxial compression tests in a wide range of confining pressures up to 58 MPa. As confining stress increases, the mechanical behaviour of the material changes from brittle and dilatant to ductile and contractant; for both brittle and ductile behaviour failure is associated with the formation of shear surfaces separating the sample in several parts at the end of test. The experimental stress–dilatancy relationships are compared with the classical stress–dilatancy theories for a purely frictional material and for a material with friction and cohesion between particles. The analysis of the data indicates that peak strength results from the interplay between degradation of inter‐particle bonds, increasing friction between particles and increasing rate of dilation. Copyright © 2001 John Wiley & Son, Ltd.     

Micromechanically inspired investigation of cemented granular materials: part II— from experiments to modelling and back

Acta Geotechnica - This paper presents an experimental and analytical/numerical study of the mechanics of cemented granular materials (CGMs). This study incorporates both in situ X-ray tomography...     

A glimpse into rapid freezing processes in clay with x-ray tomography

Acta Geotechnica - This short paper proposes time-series x-ray tomography as a valuable tool for quantifying freezing processes in a remoulded clay. As an example, three simple closed system...     

A landslide in stiff,intact clay

A landslide in a stiff clay formation, interrupting the excavation of a tunnel for a major railway in Sicily, is reported. Limit equilibrium and FEM undrained and drained analyses of the slope before tunnel excavation agree in showing that the slope was stable with a relatively high factor of safety and the critical slip surface is located well above the tunnel. The undrained stability of the tunnel checked both via FEM and via standard analytical solutions for face stability is also verified. The FEM analyses of the slope have been repeated considering the excavation of the tunnel in undrained and drained conditions. The advancement of the tunnel face is simulated in a plane strain analysis by the Panet method. In undrained conditions, the system keeps stable. In drained conditions with a stress release factor of 50 %, the slope is on the verge of failing with a very low safety factor, while with a stress release factor of 80 %, it fails following a complex mechanism that matches the observed failure surface. These findings are discussed and some conclusions attempted.     

A general formulation of hypoplasticity

A general formulation of the theory of hypoplasticity is given, under the assumption that Cauchy stress is the only state variable. Such a formulation allows to discuss the relation between the so‐called out‐of‐axis and in‐axis moduli. It is proved that, in general, the out‐of‐axis moduli are independent from the in‐axis moduli, which allows the former to be calibrated making use of experimental shear band data, as it is done in CLoE‐hypoplasticity. The implicit assumptions made in K‐hypoplasticity are detailed for two particular models of this family. Copyright © 2004 John Wiley & Sons, Ltd.     

Micromechanically inspired investigation of cemented granular materials: part I—from X-ray micro tomography to measurable model variables

Acta Geotechnica - Cemented granular materials are abundant in nature and are often artificially produced. Their macroscopic behaviour is driven by small-scale material processes, which are...