Measurement of shear modulus of soils in triaxial test using bender elements
The deformation modulus of ground materials at small strain level, which is necessary for analysis of the dynamic (cyclic) load such as seismic response analysis, traffic and machine loads, is determined by laboratory or in-situ tests. As is generally known, the deformation modulus of ground materials has a strong non-linear characteristic. The shear modulus decreases and the damping ratio increases with the increase of the strain level. As laboratory testing methods to measure these nonlinearities, there are a static loading method, where the deformation modulus is calculated from the stress-strain relationships using a monotonic or a cyclic triaxial and torsional shear tests, a vibration method, where the modulus is estimated from the resonant frequency or free vibration characteristics in a resonant column test, and a wave propagation method, where velocity of the wave under small strain is measured in a ultrasonic pulse test or a bender element test.
The shear modulus is usually calculated from the Young's modulus and Poisson’s ratio obtained from a triaxial test with precise small strain measurement. However, there are problems in precise measurement of the Poisson’s ratio using triaxial tests although the measurement precision of the Young's modulus is high. Then, the bender element method, which is recently spreading around Europe, was adopted in the study. The bender element was install to an exiting triaxial test and the shear modulus was evaluated from the shear wave velocity. It is necessary to establish the test method because there is no standard in bender element method.
Therefore identification method of propagation time, frequency of input wave and removal method of noise including received wave were examined to establish reliable testing method. The sample used in the tests was Japanese standard sand, Toyoura sand. The tests ware performed using the samples, which were adjusted the void ratio under drying and saturation conditions.
The results obtained from the study are shown as follows:
・ As a result of having examined the effect of frequency of input wave on received wave pattern, the frequency of the input wave is appropriate from 10 to 20 kHz for the triaxial specimen of around 12 cm in height.
・ The noise suppression process was very effective to reduce a noise of received wave. Using this method the reading accuracy of arrival time of wave was improved.
・ As the void ratio became small, shear wave velocity and the shear modulus were increased in each examination condition. The obtained values were at the same level as findings of the past.
Next, it is necessary to examine the precision of the direct measurement of Poisson’s ratio using both the triaxial test with precise small strain measurement and the bender element test, and to examine the applicability of the bender element test to the cohesive soils.
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