Investigating the Interrelationships Between Shear-Wave Velocity and Particle Size of a Sandy Soil

Under small strains (ε≤10−3%), the shear-wave velocity (Vs) and its resultant maximum shear modulus (Gmax) are important parameters in geotechnical engineering calculations and soil dynamics analyses. At present, the shear wave velocity of sand is typically determined using measurement and theoretical analysis methods. The measurement methods include in-situ and laboratory tests. In-situ tests are commonly conducted using a borehole method or a surface wave dispersion analysis method. Laboratory tests include bender element tests, resonant column tests, ultrasonic tests, and dynamic triaxial tests. In this regard, the evaluation of the influences of soil particle size on the dynamic behaviour of soils during wave propagation has been an important issue in geotechnical engineering. Heretofore, the effects of particle size on shear-wave velocities in soils have been examined using various experimental techniques. Most of this research was carried out over a limited range of particle sizes, and the results indicated various effects of particle size on shear-wave velocity: there has been no comprehensive and unambiguous outcome describing the influences of particle size on shear-wave velocity in soils. This research focused on the influences of particle size on shear-wave parameters in a particular type of sandy soil. A digitally controlled triaxial testing machine equipped with bender elements was used. A significant advantage of bender element test is that it can be incorporated in standard soil mechanics apparatuses such as triaxial and oedometer devices, and the approaches for data interpretation are relatively simple. This research aims to experimentally examine the effects of a wider range of particle sizes on shear-wave velocity and other shear-wave parameters, transmitted in dry sandy soils, using a bender element apparatus embedded in a triaxial testing machine under confining pressures of 50-500 kpa. In this research, the sandy soil was initially categorized into 10 different groups using ASTM standard sieves, and all triaxial samples were prepared with an identical void ratio. The void ratio plays a vital role in the determination of the maximum shear modulus of soil. For all ranges of particle size, the maximum and minimum void ratios were determined, in order to provide an acceptable level of comparison among the results, all samples were prepared with a single void ratio of 0.80. In this study, homogeneously identical samples were assumed as a prerequisite for all experiments. Therefore, it was necessary to take practical measures to ensure this crucial prerequisite in all specimens. In this regard, various experimental methods may be used to achieve a desirable void ratio, including the wet and dry tamping method, dry pouring technique, and water precipitation methods. In this study, the dry tamping method was carried out to prepare similar specimens with an identical void ratio. To measure the shear-wave travel time, the frequencies between 5 and 12 kHz were used. The significant results obtained in this study were as follows. 1) With reference to different methods of determining the shear-wave travel time, the results of this research showed that the cross-correlation and peak-to-peak methods gave the most reasonable values of the shear-wave velocity. 2) The outcomes revealed that, in a particular soil sample, as the excitation frequency increases, the received signals possess significant amounts of higher frequency components, and surprisingly, these signals are similar in shape. 3) Particle size influences the shape of the received signals, such that the frequency content of received signals in both fine and coarse grained soils are quite similar, but medium-sized soils increased with increasing confining pressure. 5) The results showed that the increasing size of soil grains leads to increased shear-wave velocity in a particular range of particle sizes, and decreased shear-wave velocity in the other range. 6) Although the effects of particle size on shear-wave velocity were the main subject of this study, it seems that this factor alone cannot dominate, and other factors must also be considered, such as the type and shape of particles and the surface roughness.