Dynamic properties of naturally frozen Fairbanks silt Public Deposited

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  • As part of a long term study to evaluate the dynamic properties of naturally frozen soils, resonant frequency and cyclic triaxial tests were conducted on Fairbanks silt samples from the U.S. Army Cold Regions Research and Engineering Laboratory Permafrost Tunnel, Fox, Alaska. The test methods subject right cylindrical test specimens to various loading conditions simulating ground motions induced by vibrating machines, blasting, and earthquakes. Dynamic elastic properties (expressed in terms of dynamic Young's modulus, dynamic shear modulus, and longitudinal and compressional wave velocities) and energy absorbing properties (expressed as damping ratio) were determined. Resonant frequency tests were conducted prior to cyclic triaxial tests due to the potentially destructive nature of the cyclic triaxial equipment. During the resonant frequency testing sequence, the frozen specimens were subjected to increasing and decreasing confining pressures (0, 70, and 0 psi (0, 482, and 0 kN/m2)) at ascending test temperatures (-10, -4, and -1°C (14, 25, and 30°F)). At each confining pressure, the specimen was tested in the longitudinal and torsional mode. During cyclic triaxial testing, each specimen was subjected to ascending temperatures (-10, -4, and -1°C (14, 25, and 30°F)) and increasing strain amplitudes (.0009, .005, and .01%) at each temperature. For each strain amplitude, each specimen was subjected to increasing and decreasing confining pressures (0, 70, and 0 psi (0, 482, and 0 kN/m2)) and for each confining pressure the frequency was incrementally increased (.05, 0.5, and 5.0 Hz). The above testing sequence allowed an evaluation of the influence of the various dynamic loading parameters (strain amplitude, frequency, temperature, and confining pressure) on dynamic properties. In addition, the influence of material density, water content, and anisotropy were evaluated. Confining pressure, density, and water content were generally found to have little effect on either the dynamic moduli or damping ratio. Tests results indicate the dynamic moduli decreases with increasing strain amplitude and increases with increasing frequency and ascending temperature. The damping ratio generally increases with increasing axial strain amplitude and decreases with increasing frequency and descending temperature. For the various combinations of test parameters, the dynamic Young's modulus ranges from 0.1 x 10^6 to 2.4 x 10^6 psi (0.7 to 16.5 GN/m2), the dynamic shear modulus from 0.1 x 10^6 from 0.8 x 10^6 psi (0.7 to 5.5 GN/m2) and the damping ratio from .01 to .26. The relative effect of three differing specimen orientations on the dynamic properties could not be established. The test results obtained in the present study are in good agreement with the results obtained by previous investigators who evaluated dynamic properties of both reconstituted artificially frozen and naturally frozen silts. The test results from the present and previous studies indicate that dynamic properties of frozen silt are influenced primarily by material type and ice content. The ice structure (i.e., lense thickness, orientation, and spacing) does not appear to influence dynamic properties of silt. Based on this conclusion, it would appear that it is not necessary to test specimens obtained from naturally frozen in situ samples if artificially frozen test specimens are prepared in the laboratory at the same density and ice content as the in situ material.
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