EFFECT OF FREQUENCY CONTENT ON DYNAMIC SOIL BEHAVIOR OF SILTY SOILS USING IMPROVED TRANSFER FUNCTION METHODS

摘要

To simulate the impulse type loading used in the field, and to ensure that the various extrapolation techniques of different field data are correct, the effect of variation in the frequency content of random loading on the dynamic soil properties of silty soils, namely, damping and shear moduli, was studied. The main objective of this study was to compare the shear moduli and damping values of silty soils at various frequency contents under random vibration conditions, evaluated by the conventional estimator of the transfer function with those properties evaluated by the improved estimators. To accomplish this objective, random torsional vibrations at various cut off frequencies were applied to silty specimens in a resonant column device. Confining pressures in the range of 40 KPa to 220 KPa were considered. The liquid limit and plastic limit were approximately 49% and 26%, respectively. In this study, improved estimators of the transfer function (H2, H3, H4), in addition to the conventional estimator (H1), were used to evaluate the dynamic soil properties and to study the effect of frequency content on shear moduli and damping of soils. The frequency contents ranged from 50 to 10,0000 Hz. The findings of this study indicated that as the cut-off frequency increased, the damping values decreased. The variation in the method of transfer function measurement influences the relationship between the damping values obtained by the various estimators. The results also imply that the damping values, particularly at high frequency contents, may have to be corrected to obtain values that are more representative of actual field conditions during earthquakes. On the other hand, the frequency content did not significantly influence the shear moduli using any of the transfer function estimators. This is explained by the fact the shear modulus is a function of natural frequency, and natural frequencies are similar using any of the transfer function estimators. The results also indicate that shear moduli from routine soil dynamics testing can be directly applied to the field conditions.