Sample disturbance from shear wave velocity measurements

Effective techniques are currently available to obtain undisturbed samples of cohesive soils. However, little advance has been made in the procurement of undisturbed samples of cohesionless soils such as sands, silty sands, and clayey sands. In the area of earthquake design and liquefaction, researchers and practitioners are becoming increasingly aware of the importance of obtaining high-quality undisturbed samples of cohesionless soils. In situ ground-freezing techniques can be used to obtain undisturbed samples of cohesionless soils. However, there is still concern regarding the possibility of disturbance during the freezing and thawing of the samples. Shear wave velocity is a direct measurement of the stiffness of the soil skeleton at small strains (<10−4%). Hence, shear wave velocity can be a sensitive measurement to detect changes in void ratio and soil structure due to freezing and thawing. A laboratory study has been performed to evaluate the use of shear wave velocity measurements to detect sample disturbance due to freezing and thawing of cohesionless soils. Samples prepared with different amounts and type of fines were frozen using uniaxial freezing techniques and subsequently thawed. Shear wave velocity measurements were made before and after freezing and thawing of the reconstituted samples. The measured shear wave velocities were unchanged for samples that did not heave (undisturbed) during the freeze–thaw cycle. Samples that heaved (disturbed) showed an associated change in shear wave velocity. Hence, measurements of shear wave velocities in situ and in the laboratory have the potential to identify sample disturbance in granular soils. Key words : in situ, sampling, freezing, disturbance, shear wave velocity.