The Effects Of Saturation And Sample-preparing Method On Stress-strain Characteristics And Shear Band Of Fine Sand

Triaxial consolidation draining shear test of unsaturated soil can’t be done straightly on the conventional triaxial apparatus, because of the requirement of samples being saturated. The successfully application of digital image measuring system to triaxial test overcomes the requirement of samples being saturated when measuring volumetric strain and makes non-contact measurement come true. So the new system can be used to the measurement of unsaturated soil. Moreover, a big advantage of the digital image measuring system is that strain fields which reflect every time of the local deformations of samples during shearing can be obtained.With a new generation of the whole surface digital image measuring system measuring deformation, a series of consolidation drained shear tests of different saturations were down with Fujian standard sand passed through 0.5 mm standard screen. The samples were made respectively with dry and met methods. The dense degrees of the tests were 30% and 60%, and the confine pressures were respectively 100,200 and 300kPa. And the initial sampling water content of samples made with wet method was controlled at w=10%. With the results of the tests, the accuracy and superiority of the whole surface digital image measuring system was analyzed. The characteristics of isotropic consolidation of fine sand and the influence of sample-preparing method and saturation on characteristics of deformation during the isotropic consolidation of fine sand were studied. And with the shearing process, axial strain-time curves, radial strain-time curves and volumetric strain-axial strain curves were obtained, the influence of different sample-preparing methods and saturations studied. At last, with the strain fields got by the whole surface digital image measuring system, the failure modes of samples, the occurrence of strain localization and the formation and development of shear band were analyzed.The results of the tests show that the whole surface digital image measuring system can be used straightly to the deformation measurement of unsaturated soil and the result is accurate and reliable. Saturation hardly has any influence on radial strain and volumetric strain in the process of isotropic consolidation. Under the condition of different kinds of saturations, shear dilatancy occurs when samples are made with dry method and when the dense degree of samples made with wet method is 30%, shear dilatancy happened after shear shrinkage, and the influence of sample-preparing method on the peaks of stress-strain, axial strain, radial strain and volumetric strain of fine sand samples under different kinds of confining pressure is significant during the shear process. There are three failure modes of unsaturated fine sand which are one single shear band, two crossed shear bands and bulging damage. One single shear band usually occurs in samples made with dry method, and bulging damage happens in samples made with wet method when the dense degree is 30%, and one single shear band may happen when the dense degree is 60% with high confining pressure and low saturation. The formation and development of shear band are hardly influenced by saturation but the time when shear band occurs is influenced by saturation. The axial strain of local area of the sample within the shear band changed obviously with the developing of shear band while the area out of the shear band insignificantly. And the whole surface digital image measuring system is of great importance for the study of width and inclination angle of shear band.