Influence of specimen nonuniformity and end restraint conditions on drained triaxial compression test results in sand

Abstract

Drained triaxial compression tests on sand are common in geotechnical engineering due to their application in determining strength and critical state properties, and calibration of constitutive models. However, questions around reliability of test results due to strain localization and specimen non-uniformity have sparked debate around the adequacy of various specimen preparation techniques and configuration of top cap and bottom pedestal, among other variables. A rigorously calibrated and validated 3-D finite element model using an advanced constitutive model, Norsand, was used to address some of these concerns. The results generally confirmed that common specimen preparation techniques, moist tamping, and (air or water) pluviation, if done properly, can reasonably estimate the properties of a perfectly uniform specimen with the same density. Unsurprisingly, it was also confirmed that the post-peak response of dense specimens is by far the most sensitive part of sand behaviour characterized by these tests. It was suggested that for two-to-one height-to-diameter ratio specimens end platen lubrication does not necessarily improve test results and can cause concentration of localized deformations near the platens. A fixed cap appeared to enforce a better boundary condition than a free-to-rotate one by preventing cap rotation driven by the formation of a single failure plane.

Use of Linear Extrapolation to Estimate Critical State Void Ratio from Drained Triaxial Shear Tests on Dense Cohesionless Soil

“…In this situation, it becomes necessary to estimate the values of e cs , p cs , and M from measured data that are free of interference from strain localization into shear bands. This interference could be minimized by utilizing the testing devices and procedures recommended by [21,22].…”

Assessing continuum plasticity postulates with grain stress and local strain measurements in triaxially compressed sand

“…Experimental constraints also required the use of less than 2,000 particles per sample, suggesting the possibility of finite size and boundary effects. However, boundary and size effects in triaxial tests are still the subject of study and debate in geomechanics ( 62 – 66 ), and we are careful to offer explanations for differences between the responses of the small samples used here and larger samples found in the literature. Despite the limitations of our experiments, the data and analysis serve a critical need for numerical and theoretical researchers developing constitutive models and studying the critical state behavior of soils and rocks.…”