Dynamic properties of naturally frozen ice

Graduation date: 1984In recognition of the need to determine dynamic properties of naturally frozen materials, a research project was conducted to measure the dynamic properties (dynamic moduli and damping ratio) of naturally frozen wedge ice samples obtained from the Fairbanks, Alaska area. Cyclic triaxial and resonant column test systems were employed to subject the samples to a range of conditions associated with dynamic loading of frozen ground deposits. Test parameters considered in the program included temperature, confining pressure, strain amplitude, and frequency of loading. Material properties considered were density and sample core orientation. A preliminary laboratory investigation was performed on laboratory prepared ice specimens to develop methods for attaching the naturally frozen ice specimens to the cyclic triaxial and resonant column test systems. Time effects were also evaluated on laboratory prepared ice specimens to determine the time required for dynamic properties to stabilize following temperature and confining pressure changes. The orientation of ice crystals was determined and mapped to establish if the core samples were anisotropic. A preferred crystal orientation was noted for all the naturally frozen ice samples. All naturally frozen ice specimens were tested in the resonant column test apparatus first, due to the potential for specimen damage during cyclic triaxial testing at high strain amplitudes. During resonant column testing the specimens were subjected to both longitudinal and torsional excitation under various confining pressures (0, 30 and 70 psi (0, 207 and 482 KN/m²)), under controlled temperatures of -1, -4 and -10°C (30, 25 and 14°F). During cyclic triaxial testing, dynamic properties were measured at various confining pressures (0, 30 and 70 psi (0, 207 and 482 KN/m²)) strain amplitudes (.0009, .005 and .01%) and loading frequencies (.05, .5 and 5 cps) under controlled temperature conditions of -1, -4 and -10°C (30, 25 and 14°F. Measurements of the dynamic properties of the naturally frozen ice samples under varying conditions of temperature, confining pressure, strain amplitude, and loading frequency indicated that the parameters considered in the program have an influence on the dynamic properties. Specifically, it was found that: (1) the dynamic moduli increased with increased frequency of loading and strain amplitude, but showed a decrease with ascending temperature, and (2) in general, the damping ratio decreased with increased frequency of loading and strain amplitude but showed an increased with ascending temperature. Sample core orientation had little affect on the dynamic properties for the wedge ice samples employed in the test program. It was also found that the dynamic properties were influenced to a greater extent by changes in temperature and confining pressure when the test specimen were subjected to strong motion loading conditions (low frequency, high strain amplitude) rather than weak motion conditions (high frequency, low strain amplitude)