Repeatability of GMAX Bender Element Measurements on Triaxialand Resonant Column Sand Specimens-Paper

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The study evaluates the repeatability of bender-element (BE) measurements of small-strain shear modulus (Gmax) in clean sand, investigates sources of measurement uncertainty, and compares BE results with resonant-column (RC) measurements. The work is motivated by the growing importance of reliable soil stiffness estimates in offshore wind turbine foundation design.<br /><br />Researchers tested reconstituted sand at relative densities of 58% and 80% in triaxial and resonant-column apparatuses, measuring Gmax at three consolidation stress levels. Repeat tests used physically different specimens prepared under nominally identical conditions.<br /><br />Across most conditions, the variation in BE-derived Gmax was 3.6–11.2%; it reached 18.9% for the looser sand at the lowest stress. Overall, repeatability was considered good. Gmax increased with stress, and the effect of density depended on stress: at low and intermediate stresses, the density-related change could be similar in size to specimen-to-specimen variability. Gmax measurements varied more than peak shear strength measurements, indicating that stiffness is more sensitive to small differences between specimens.<br /><br />Signal quality was identified as a key source of uncertainty. A frequency sweep helps locate the optimal excitation frequency, where traces are clearer and easier to interpret. Traces recorded far from this frequency can be complex and may lead to incorrect travel-time estimates and greater scatter. When signals were clear, peak-to-peak, cross-correlation, and start-to-start interpretation methods generally produced comparable results; interpretation errors could occur with automated cross-correlation and require visual review.<br /><br />BE and RC measurements agreed well, with RC values averaging slightly higher. The findings support the use of BE testing for Gmax assessment when measurement frequency and signal quality are carefully controlled.

Keywords

bender-element

small-strain shear modulus

Gmax

clean sand

repeatability

resonant-column

measurement uncertainty

soil stiffness

offshore wind turbine foundations

signal quality