Shear Modulus and Damping in Soils: Measurement and Parameter Effects (Terzaghi Leture)

TLDR

Shear modulus decreases and damping increases sharply with strain amplitude, with the rate mainly governed by mean stress, saturation, void ratio, and loading cycles, while other factors such as octahedral stress and frequency have a smaller influence, and cohesive soils behave differently from clean sands. The study employed a pseudo‑static simple shear apparatus and two resonant column setups to accurately measure shear modulus and damping at very small strain levels relevant to earthquake and foundation vibrations.

Abstract

Based on numerous tests on a spectrum of disturbed and undisturbed soils, the shear modulus decreases and the damping ratio increases very rapidly with increasing strain amplitude. The rate of increase or decrease depends on many parameters: (1) Effective mean principal stress; (2) degree of saturation; (3) void ratio; and (4) number of cycles of loading. Ambient states of octahedral shear stress, overconsolidation ratio, effective strength envelope, frequency of loading, and time effects have a less important influence on these properties. Cohesive soils are affected differently than clean sands. The apparatus used to measure shear modulus and damping must be capable of making accurate measurements at very small shearing strains, the range being defined by practical problems in earthquake and foundation vibrations. A pseudo static simple shear apparatus and two different resonant column apparatus were used.