Two response scales
- Dynamic tests span very small strains and larger cyclic loading; stiffness and damping depend on strain level.
- Maximum shear modulus Gmax describes very-small-strain response and relates to shear-wave velocity and soil density.
Main methods
- Bender elements generate and receive shear waves through a specimen; travel time is used to estimate wave velocity and Gmax.
- A resonant column excites the specimen and measures resonant frequency to estimate stiffness and damping at small strains.
- Cyclic triaxial and cyclic simple shear apply repeated loading histories to study degradation and deformation and, under suitable conditions, liquefaction susceptibility.
Preparation and procedure
- Trim or remould a suitable specimen and mount transducers without damaging it or distorting the signal.
- Saturate and consolidate to represent the expected stress state when required; then apply the defined stress and strain sequence.
- Control alignment, frequency, amplitude, confining pressure, and sensor response; check signal quality before interpreting results.
Geotechnical interpretation
- Compare modulus and damping at equivalent strain levels; do not treat Gmax as constant stiffness for every load.
- Use data for dynamic and seismic analysis within the tested range, together with soil characterization and the geotechnical model.
Repeated-load response
- Cyclic deviator stress: σd = σ1 − σ3; state whether the signal value is amplitude, peak, or peak-to-peak.
- Resilient modulus: Mr = σd/εr, where εr is recoverable axial strain after unloading. It is commonly reported in MPa and depends on stress state, moisture, density, and conditioning.
- Permanent strain: εp = ΔHp/H0; track εp and Mr against cycle count to distinguish irreversible accumulation from recoverable response.
- Under cyclic loading, frequency, waveform, confinement, and rest periods affect response. Compare materials only under equivalent sequences and report compaction conditions.
Educational reference guide. Check the current standard, calculation assumptions, and project specification before applying a procedure or acceptance criterion.