On the relationship between soil strength and wave velocities of sandy loess subjected to freeze-thaw cycling

The impact of freeze-thaw cycling on freshly exposed loess is a major concern in design and construction of embankments or earth dams in the Loess Plateau in China. Due to complex nature of loess as compared with the conventional soils such as sand or clay, characterizing the strength of loess to account for this impact is challenging, in particular in the field conditions. In previous research a systematic approach in this regard is still lacking. This paper presents an investigation into the problem through well-controlled experiments using a pair of bender/extender elements. The shear wave (S-wave) velocities and the compression wave (P-wave) velocities were both measured on loess specimens subjected to freeze-thaw cycling. Under otherwise similar conditions, the unconfined compression tests were carried out to evaluate the soil strength of original loess and those with different fractions of add-in quartz sands. Several influence factors including the numbers of freeze and thaw cycles, the sand content and the water content were analyzed in a collective manner to characterize the changes of the unconfined compressive strength (UCS) of loess and the associated wave velocities. It was found that as the number of freeze-thaw cycles increases, the wave velocities of loess decrease. A clear difference in the magnitude of wave velocity was observed in the original loess samples due to the freeze-thaw cycling, whereas the discrepancies were less evident for the loess with higher sand contents. Based on the test results, exponential relationships were established respectively for the S wave and P wave velocities against the UCS values of the tested materials. Discussions were highlighted on whether the wave velocity can be used as an indicator to evaluate the UCS value of loess.