Development of the method for reducing the frictional restraint at the upper and lower ends of soil specimens by means of torsional ultra sonic wave excitation

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One of the most frequently used laboratory tests is a triaxial test, which assumes a cylindrical soil specimen bounded by upper and lower cylindrical plates, called "cap" and "pedestal", and soil specimens are subjected to compression. In order to acquire the deformation and strength characteristics of soils in an ideal manner, the soil specimens need to deform uniformly. However, they are subjected to the effects of frictional restraint from upper and lower ends. To make these effects as small as possible, the ratio of height to diameter of specimens is conventionally made 2. However, due to high costs of acquiring undisturbed soil samplesin the field, it is considered desirable to inccrease the number of soil samples by reducing the height of specimensThe first year of research was dedicated to examining if the ultra sonic wave excitation can be useful in reducing the frictional restraint of upper and lower ends of soil specimens for triaxial tests, which have been found to be useful … More in lessening friction in industrial applications. The generators for ultra sonic wave were housed in the cap and pedestal, and multiple series of triaxial compression tests were carried out, in which the height of specimens were varied. From the test results on Toyoura sand, it was found that the prevention of the development of frictional restraint was not achieved from the following reasons. (1) When the specimens are isotropically consolidated and are subjected to ultra sonic wave excitation, a large amount of volume contraction occurs, and formation of soil fabrics is disturbed. (2) When the specimens are axially loaded and are subjected to ultra sonic wave excitation, rapid reductions in shear stress occur again and again, probably due to volume contraction, which is induced rapidly in manner similar to above. From the test results on Fujinomori clay, the same curious phenomena as those of Toyoura sand were not observed. However, the frictional restraint was not completely removedThe last year of research was dedicated to examining the influence of ultra sonic wave excitation on the volume contraction of granular materials during consolidation and axial compression. The volume contraction during consolidation is found to become up to a few percent of volumetric strain, although the amplitude of ultra sonic wave is as small as 1 to 4 μm. When the ultra sonic wave excitation is applied to the soil specimens, which developed negative pore pressures, the reduction in shear stress occurs, which in turn causes the reduction in negative pore pressures, and even causes the development of positive pore pressures. Therefore, the volume contraction occurs upon drainage of pore water after the application of ultra sonic wave excitation Less