Ultrasound Impulse Propagation in Sand Specimen Under Load

Abstract

Analysis of the pattern of stress chains in dense and loose sands and velocity of ultrasound impulse propagation in these loaded sands clarify the mechanisms by which granular materials, such as loose and dense sand respond to external loads. Experiments are carried out in pure marine quartz sand in three axial test cameras with acoustic control. Simultaneously was made triaxial tests and ultrasound sounding of sand specimens. Are used triaxial devises LIIZT (Russian)‐step‐by‐step loading is real used and GDS Geotechnical Digital System (UK)—loading with defined velocity of deformation is used. Sounding of marine sand prove existence of Force chains in granular materials under load. Force chains instability and connection with dilatancy of the granular material is observed. Do of dilatancy, the loaded force chains in dense sand collapse at deformations with less density, the loaded force chains in friable sand collapse at deformations with heightened density. At the micro level, dilatancy can be explained by repacking of particles from dense condition in a less dense condition, dilatancy coefficient >0, and by repacking from loose condition in more dense, dilatancy coefficient <0. The sign of dilatancy depends on ‐density of sandy soil. The density at which dilatancy changes its sign is referred to as critical density. The well‐known in geophysics phenomena of Vp lowering in places of earth crust some time before an earthquake is well modelled in sands: under the load step immediately before destruction in dense sand the decrease in Vp is observed.