Measurement of Small-Strain Shear Moduli of Partially Saturated Sand During Infiltration in a Geotechnical Centrifuge

Abstract

This paper describes the use of bender elements to measure changes in small strain shear modulus, G max , of sand layers due to the change in degree of saturation during centrifuge tests. The goal of the measurements is to verify that steady-state infiltration is an appropriate technique to control the effective stress in centrifuge physical modeling of partially saturated sands. Specifically, the suitability of infiltration is assessed by checking if the measured values of G max of partially saturated sand layers follow a similar trend to dry and saturated sand layers when the effective stress is defined from the suction and degree of saturation profiles during steady-state infiltration. Three pairs of bender elements were installed at different depths in a container of Ottawa sand, and the shear wave velocities of the sand were measured during steady-state infiltration into the sand layer. The applied infiltration rate was varied to obtain different uniform distributions of degrees of saturation with depth. Consistent with results from suction-controlled resonant column tests performed on the same sand, the values of G max measured from the bender element tests varied nonlinearly with degree of saturation with a peak value at a degree of saturation between 0.3 and 0.4. When interpreted in terms of mean effective stress, the values of G max from the bender element tests on partially saturated sands followed a unique trend consistent with measurements for dry and saturated sands.

Cyclic Direct Simple Shear Test to Measure Strain-Dependent Dynamic Properties of Unsaturated Sand

“…However, such relations need to be verified using experimental data before their application. For example, Ghayoomi and McCartney (2011) combined the results from bender element and resonant column tests on dry, saturated, and unsaturated sands and proposed a consistent formula for small-strain shear modulus, regardless of the degree of saturation given the modified effective stress equation. However, recent work by Ghayoomi et al (2017) indicated that a modified effective stress-based formula may still not fully capture the suction dependency of soil dynamic properties.…”

“…A custom built direct simple shear (DSS) system developed at the University of New Hampshire was originally designed and created to perform cyclic shear or other dynamic displacement-controlled tests (Miller 1994;Dunstan 1998). This system has been recently renovated and re-operated by upgrading the motion controller and measurement sensors (Le 2016).…”

“…These studies denoted a higher shear modulus and lower damping in partially saturated soils due to the presence of inter-particle suction stresses that increase the soil stiffness. Results by Qian et al (1991) and Ghayoomi and McCartney (2011) indicated a general increase in small-strain shear modulus of sands up to a peak value before the residual water content. However, a continuous rise of modulus was reported by Khosravi and McCartney (2011) as suction increases in silty material.…”

“…In recent years, theoretical and experimental advancements in understanding the behavior of unsaturated soils have revealed the importance of the degree of saturation on the shear modulus and damping in geomaterial. Small-strain moduli (0.0001 %-0.01 %) of various soils under different degrees of saturation using the resonant column and bender element tests have been extensively investigated and several empirical relations were proposed (Wu et al 1984;Qian et al 1991;Marinho et al 1995;Mancuso et al 2002;Mendoza et al 2005;Ng et al 2009;Khosravi et al 2010;Khosravi and McCartney 2011;Ghayoomi and McCartney 2011;Kumar and Madhusudhan 2012;Hoyos et al 2015). These studies denoted a higher shear modulus and lower damping in partially saturated soils due to the presence of inter-particle suction stresses that increase the soil stiffness.…”

“…This paper focuses on the modification, calibration, and implementation of a custom built direct simple shear apparatus (Miller 1994;Dunstan 1998) in order to control suction and study the effects of degree of saturation and applied shear strain on the shear modulus and damping. Background theory on dynamic properties and unsaturated soil mechanics are briefly discussed, followed by the apparatus description, modifications to the machine, experimental procedures, and data reduction methods.…”