• Experimental And Numerical Study Of Sonic Wave Propagation In Freezing Sand And Silt

      Li, Hui (2009)
      A numerical model for delineating the temperature-velocity relationship of freezing porous media and soil is developed in Matlab based on Leclaire's Biot-type three-phase theory. Leclaire's theory gives lower sonic velocities than the experimental results because it does not take into consideration the effect of the solid-ice frame when water is freezing. To take the solid-ice effective frame into account, the average bulk and shear moduli estimation are modified with a proposed procedure. The modification gives higher P-wave and S-wave velocities that fit experimental data well. A comprehensive suite of physical and acoustic laboratory experiments are conducted on artificial sands, sand-clay mixtures and Fairbanks silts to investigate the temperature-velocity relationship during the freezing process and the effects of grain size and fine clay content. A Multi-channel ultrasonic scanning system (MUSS) is designed, installed and programmed for the experimental computerized ultrasonic tomography (CUST) study. The inward and outward freezing process and freezing front development in Fairbanks silt samples are observed using computerized ultrasonic tomography (CUST) in the laboratory. The experiments generate sonic wave velocity and temperature distribution during the freezing process. The freezing front is clearly identified in the CUST as a function of time and temperature. Comprehensive numerical finite element method (FEM) simulations, which account for the conduction in porous media, the latent heat effect and the nonlinear thermal properties of soil, are performed on the inward and outward freezing process of Fairbanks silt based on the experimental conditions. In conjunction with the temperature-velocity model developed in the study, sonic wave velocity tomograms are generated. The results are comparable with those obtained by CUST. The study indicates that CUST is an effective method for studying freezing processes and has potential for indirect measurement of unfrozen water content variations in the soil without interfering with the freezing process.
    • The Physical Dynamics Of Patterned Ground In The Northern Foothills Of The Brooks Range, Alaska

      Overduin, Pier Paul; Kane, Douglas L. (2005)
      Periglacial landforms, called patterned ground, change the vegetation, microtopography and organic content of the surface soil horizons. Because they are uniquely products of the periglacial environment, changes in that environment affect their distribution and activity. As surface features, they mitigate heat and mass transfer processes between the land and atmosphere. For environmental change detection, the state of the soil and active layer must be monitored across temporal and spatial scales that include these features. It is suggested here that changes in the state of the active layer due to the abrupt spatial changes in surface soil character lead to changes in the distribution of soil components, soil bulk thermal properties and the thermal and hydrological fluxes result. The determination of soil volumetric moisture content using the relative dielectric permittivity of the soil is extended to include live and dead low-density feathermoss. High temporal resolution monitoring of the thermal conductivity of mineral and organic soil horizons over multiple annual cycles is introduced, along with a new method for analyzing the results of transient heat pulse sensor measurements. These results are applied to studies of frost boils and soil stripes in the northern foothills of the Brooks Range in Alaska. Active layer ice dynamics determine the thermal properties of the frozen soil in the frost boil pedon. Annual heaving and subsiding of the ground surface reflects these changes in ice content and can be used to estimate active layer ice content as a function of depth. These estimates correlate with bulk soil thermal diffusivity, inferred as a function of depth from temperature data. Differences in soil thermal diffusivity determine thaw depth differences between frost boil and tundra, and between wet and dry soil stripes. For the latter, deeper subsurface flow through the high organic content wet stripes is delayed until mid-summer; when it does occur, it has a large component normal to the hillslope as a consequence of differential heave. Dynamics in these periglacial landforms can be identified from surface features, highlighting the potential for scaling up their net effect using remote sensing techniques.