Study of localization in sandy soil by shear wave logging

Abstract

Strain localization phenomenon in soils has been one of the main research topics in geotechnical engineering for more than 50 years ago. Though investigations by theoretical and numerical studies can expose some of the strain localization characteristics but are impeded because of the complexity inside that localized zones. On the other hand, the experimental studies can obviously investigate that particular behavior in more details. Both conventional and new apparatuses, i.e. Gamma-ray, X-ray Computed Tomography and Digital Image Analysis, have been adopted in the strain localization analyses. However, those techniques require technical proficiency and experience in the installation, operation and result interpretation. The bender element test nowadays is becoming quite popular in geotechnical engineering because of its simplicity, relatively low cost and nondestructive test. A pair of bender elements has been extensively used to measure shear wave velocity for the determination of small strain shear modulus inside the soil sample. This shear wave propagation throughout the soil specimen influences in the same way by several of the same factors, e.g. void ratio and state of stress, compared to localization mechanism. A series of compression triaxial tests implemented by bender element installation was performed to investigate the occurrence of strain localization in sand of various types, i.e. local and Silica sand. The results showed that this shear wave propagation technique can describe to some extent some characteristics of localization mechanism. The shear wave velocity tends to decrease from its maximum value at a certain strain level, i.e. 0.5 - 3% of global axial strain, depending on the state of stress as well as the initial packing condition of the sample. This diminution of shear wave velocity is the initiation of soil non-uniformity deformation or the onset of strain localization inside the sand specimen.