Effect of Cyclic Preshear on Strength and Stiffness Properties of Sand in Drained Triaxial Testing-Paper

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The study examined how cyclic preshearing affects the strength and stiffness of sand in drained triaxial tests. Researchers tested 26 specimens of medium-dense/dense and very dense Leighton Buzzard sand, consolidated at 50 or 200 kPa. Specimens received 400 drained loading cycles at cyclic stress ratios (CSR) of 0.07–0.40, or no preshear, before monotonic shearing to failure.<br /><br />Test repeatability was generally good: maximum shear modulus (Gmax) measurements varied by about 4.4%, and peak friction angles by less than one degree across comparable tests. Preshearing produced no significant change in peak strength, and its average reduction in Gmax of about 3% was within the range of measurement and test variability.<br /><br />The main effect was on stiffness at small and intermediate strains. Preshear increased stiffness by approximately 50–200% at axial strains below 0.1% and changed the stiffness-degradation curve. Higher CSR generally increased the strain at which stiffness degradation began, by roughly a factor of four, and maintained higher stiffness to larger strains. Low CSR did not improve small-strain stiffness in every test.<br /><br />Because preshearing changed stiffness without significantly changing density or peak strength, the authors attribute the effect to changes in soil fabric. They conclude that preshear conditions should be selected to represent the expected in-situ soil structure. This is important for offshore foundation design, where strain-dependent stiffness influences deformation, fatigue, and turbine natural-frequency assessments.

Keywords

cyclic preshearing

sand stiffness

drained triaxial tests

Leighton Buzzard sand

cyclic stress ratio

peak shear strength

small-strain stiffness

soil fabric

offshore foundation design

stiffness degradation