The use of ultrasonic testing to determine pulse velocities and small-strain elastic constants for rocks has been standardized in American Society for Testing and Materials (ASTM) standard D2845-95. However, the use of ultrasonic testing to determine pulse velocities and small-strain elastic constants of soils is less common, as soils have higher damping characteristics which result in measurement difficulty. The signal transmitted through soil is weak and very noisy. As a result, the signal must be properly processed to provide a reliable estimate of the wave travel time. In this paper, an ultrasonic test system consisting of compression and shear wave transducers, a pulser, and a data-acquisition system is evaluated for measurement of both compression and shear wave velocities. Among the specimens tested were fully saturated and unsaturated soil specimens. The effects of acoustic coupling and signal processing on the transmitted pulse were investigated. The strain levels associated with the determination of the wave velocities were also measured. Furthermore, a method for determining attenuation characteristics for soil specimens from the frequency spectra is suggested.Key words: laboratory, compression wave, shear wave, velocity, attenuation, ultrasonic.

There is very little work done on characterization of soil stiffness and damping ratio with strain for residual soils in Singapore. This is because routine geotechnical design does not usually incorporate the non-linearity of soil stiffness and damping ratio is only needed for dynamic problems. However, the use of large strain (>1%) soil stiffness to compute ground movement is over conservative as the mobilized strain due to construction loads and working loads is usually in the range of 0.01 % to 1%. Furthermore, the demand for seismic analysis is increasing due to ground tremors felt in Singapore arising from earthquakes in Indonesia. Thus, the characterization of soil stiffness and damping ratio with respect to strain is important. To characterize the Singapore residual soils' stiffness-strain and damping ratio-strain relationships, pulse• transmission tests (bender element tests and ultrasonic tests), cyclic simple• shear and triaxial compression tests for shear strain ranging from 0.0005% to 5% were performed. Inaddition,theeffects of soil parameters such as void ratio, confining pressure, degree of saturation and loading conditions such as frequency and number of loading cycles on soil stiffness and damping ratio are investigated. Both compacted and undisturbed residual soils were tested. However, most of the work concentrated on compacted residual soils because Singapore residual soils are highly heterogeneous and soil characteristics such as void ratio, density and degree,of saturation cannot be systematically studied. Anisotropy of soil was not considered in the study. A few tests on undisturbed. residual soils were performed for comparison with the compacted residual soils. The pulse transmission tests (bender element tests and ultrasonic tests) were evaluated with different materials to gain a• better understanding of the teclmique. The study shows that the waveform, magnitude and frequency of the applied voltage affect the receiver signal for bender element tests. The travel time should be based on first arrival time. Acoustic coupling is essential to obtain reliable wave velocities for ultrasonic test. The L/D and L/λ w ratios affect the measurement of compression and shear wave velocities in bender element tests and ultrasonic tests.