Fabric Of Sand Research Articles

Liquefaction and Fabric of Sand

It is shown for simple shearing that the liquefaction and densification potentials of a cohesionless sample of sand do not necessarily decrease because of preliquefaction or because of large amplitude (drained) prestraining. In fact, in simple shearing, the resistance to liquefaction (undrained) or densification (drained) of a preliquefied sample actually increases, as a result of the concomitant densification, if the preliquefaction is terminated at zero residual shear strain, whereas this resistance is reduced considerably, if the preliquefaction is terminated at zero residual shear stress. The inherent anisotropy associated with dry pluviating and moist tamping sample preparation techniques is shown to affect the sample response to consolidation and to simple cyclic shearing. It is found that, within each cycle of simple shearing, the induced anisotropy is essentially wiped out in the neighborhood of the zero shear strain (but not at zero shear stress), and the anisotropy that remains at this state basically stems from the sample preparation technique. With the aid of the dilatancy‐fabric relation proposed by Nemat‐Nasser for simple shearing, the volumetric strain of the sample is estimated using a simple distribution function for illustration purposes, and the results are compared with the experimental data.

Microscopic Deformation Mechanism of Granular Material in Simple Shear

The deformation of granular material was simulated by an assembly of cylinders (made -of photoelastic material) packed at random in a two-dimensional simple shear apparatus. From the detailed observations on the change of granular fabric due to shear, and from the analyses of interparticle contact force among grains, we obtained some experimental facts which must be taken into account in proposing a granular model of sand. With the increase of shear stress in the simple shear test and direct shear test, the normals Ni to the contact planes tend to concentrate toward the maximum principal stress axis. This tendency of the concentration of Ni is not determined by the magnitude of applied shear displacement but by the intensity of mobilized stress ratio τ/σN. The preferred direction of Ni gradually rotates with the rotation of principal stress axes during shear deformation. The sliding in microscopic scale at contact is, at any one instant, confined at some preferred contacts. According to the microscopic study of granular fabric of sand deformed in the direct shear apparatus, the similar characters are also observed.

SEDIMENTSTRUKTUREN AUS DEN NIEDERRHEINISCHEN BRAUNKOHLENSCHICHTEN DES MIOZÄNS

SUMMARYSedimentary structures in the Miocene fluviatile deposits of the Lower Rhine Basin, lying immediately beneath the lignite formations have been studied at Tagebau Frechen near Cologne. The structures investigated include slump structures, load casts, clay breccia, cross‐bedding, sand fabric and orientation of driftwood.The slump structures are composed of alternating laminae of clay and silt and have resulted from sub‐aquatic sliding of a sedimentary sheet over a surface of erosion or of a foreset bed of 10–20° inclination. The slumping has caused folding or brecciation of the strata involved.The base of a sand bed is difficult to obtain as the sediment is weakly consolidated. Load casts, therefore, have been studied exclusively in their internal structure. It is considered that the load casts of smaller dimensions have suffered a diminution of relief through settlement after the process of load casting had ceased.Two types of clay breccia have been recognised, namely those which have undergone erosion and transportation by water and those which have resulted from slumping.In an exposure, tabular‐ and trough‐type of cross‐bedding alternate with each other in geological succession. From these examples sand fabrics have been studied. Both in tabular‐ and trough‐type of cross‐bedding the apparent long axes show two maxima. In the former case, these maxima are disposed to each other at an acute angle, in the latter case, however, they are perpendicular to each other. Imbrication, after tilting the Sf‐to original position, is down‐current in tabular cross‐bedding. In trough type, on the contrary, it may be upcurrent or the long axes of the grains dip symmetrically in up‐ and down‐current direction.