Development and Evaluation of Preconditioning Protocols for Sand Specimens in Constant-Volume Cyclic Direct Simple Shear Tests

Abstract

Textured platens are often used to improve the transfer of shear stresses from the platens to the soil specimen during direct simple shear (DSS) tests. However, constant-volume DSS tests when textured platens are used can be affected by inadequate engagement of the soil at the platen-soil interface, leading to large reductions in the vertical stress at the start of shearing.The application of a preconditioning sequence involving small-strain drained cycles prior to constant-volume shearing can improve engagement at the platen-soil interface, but when excessively implemented, it can also have adverse effects on the measured soil behavior (e.g., strength, stiffness). A series of constant-volume cyclic DSS tests preceded by different preconditioning sequences was performed to evaluate the effect of preconditioning on the engagement of sand specimens at the platen-soil interface and the stress-strain response of these specimens. Results showed that textured platens that are properly engaged with sand specimens can reduce slippage at the platen-soil interface. This engagement can be achieved by applying a limited number of small-strain drained cycles at a low vertical stress while still obtaining representative soil behavior during the subsequent equivalent undrained constantvolume cyclic loading. Although the preconditioning protocol presented herein is specific to the testing equipment and materials considered, similar procedures may be adopted to develop preconditioning protocols for other soils, platens, and testing devices.

Mechanisms of shear strain accumulation in laboratory experiments on sands exhibiting cyclic mobility behavior

“…This pre-conditioning stage consisted of 50 drained cycles applied under a vertical stress of 100 kPa for tests with a s 0 vo of 100 or 400 kPa, and 115 drained cycles at a vertical stress of 25 kPa for tests with a s 0 vo of 50 kPa. Both pre-conditioning sequences were found to lead to similar results in terms of stress-strain response and liquefaction resistance (Humire et al 2022). Following pre-conditioning, the vertical stress was raised to the target initial consolidation stress if the target value was larger than the vertical stress during pre-conditioning.…”

“…Saturation was ensured by monitoring air bubbles in the outgoing water as described in Parra Bastidas (2016). Following saturation, all samples were subjected to a series of preconditioning strain-controlled drained cycles with an amplitude of 0.01 mm (about 0.045% shear strain) to ensure engagement of the textured top platen with the specimen (Humire et al 2022). This pre-conditioning stage consisted of 50 drained cycles applied under a vertical stress of 100 kPa for tests with a s 0 vo of 100 or 400 kPa, and 115 drained cycles at a vertical stress of 25 kPa for tests with a s 0 vo of 50 kPa.…”