Chuangxin Lyu

Permafrost in marine sediments exhibits a lower freezing point and significant unfrozen water content. This paper investigates the role of the soil freezing characteristic curve (SFCC) in permafrost degradation. Three SFCCs, representing thawing–freezing characteristics of soils with varying clay content and salinity, were established based on experiments and existing data. These SFCCs were then applied in numerical analyses to simulate permafrost thawing under various warming scenarios, using measured ground temperatures and permafrost profiles for a site at Longyearbyen in Svalbard (Norway). It is shown that the ground temperature in non-saline permafrost soil increases more rapidly than saline permafrost, due to a greater downward net heat flux to the permafrost in the former case. Conversely, the thawing rate is more pronounced for saline permafrost soil, attributed to its lower freezing point and latent heat consumption. A more nonlinear ice-melting process is observed for permafrost soil with a lower salinity. The temperature rise follows three stages: a constant-rising, a damp-rising, and an accelerated-rising rates. The duration of the damp-rising rate becomes shorter for saline permafrost under a great warming condition. The study underscores the high significance of the soil-freezing characteristic curve for accurate estimations of permafrost degradation.

The Skempton pore pressure coefficient, B, is defined as the variation in pore pressure with the unit change in confining pressure under undrained conditions. The B parameter is an essential parameter to consider the coupled effects of solid–fluid compressibility and skeleton compressibility in a porous system. It is a key factor in exploring a possible definition of effective stress in frozen soil. However, limited experimental and theoretical research is available in the literature to give insight to the problem. Therefore, a series of B tests on frozen clay was conducted in this study. Results from these tests, along with tests on Ottawa sand, as available in the literature, are analysed considering the effect of the ice crystallisation mode on the skeleton stiffness. The measured B values were lower than expected compared with B values using models that consider single grain bulk stiffness. However, when the difference in bulk stiffness of ice and that of soil grains is considered, even an increase in pore volume, for an increase in fluid pressure, at constant Terzaghi effective stress is possible. The ‘pore stiffness’, which is different from the solid phase stiffness, can take a negative value and can be used to explain the low measured B values.

A systematical testing program on frozen Onsøy clay under isotropic loading and undrained shearing at different temperatures (− 3 ~ − 10 °C), strain rates (0.2~5%/h) and initial Terzaghi effective stress (20~400 kPa) was conducted with the focus on pore pressure development. It is meant to increase the understanding and facilitate the development of an ‘effective stress’-based model for multi-physical analysis for frozen soils. This study adopted the pore pressure measurement method suggested by Arenson and Springman (Can Geotech J 42 (2):412–430, 2005. https://doi.org/10.1139/t04-111) and developed a new testing procedure for frozen soils, including a ‘slow’ freezing method for sample preparation and post-freezing consolidation for securing hydraulic pressure equilibrium. The B-value of frozen soils is less than 1 and significantly dependent on temperature and loading history. The dilative tendency or pore pressure development in an undrained shearing condition is found to be dependent on both unfrozen water content and mean stress, which is consistent with unfrozen soils. Besides, the experimental results reported in the literature regarding uniaxial tests show that the shear strength does not share the same temperature- and salinity-dependency for different frozen soil types. The rate dependency of frozen soils is characterized between rate dependency of pure ice and that of the unfrozen soil and is therefore highly determined by the content of ice and the viscous behavior of ice (through temperature dependency). This paper also explains the pore pressure response in freezing and thawing is dependent on volumetric evolution of soil skeleton.

This paper reviews eight geoacoustic models applied to frozen soils: crystal growth models (grain cementing, grain coating, matrix supporting, and pore filling), the weighted equation (WE) model, Zimmerman and King's model (KT), the Biot‐Gassmann theory modified by Lee (BGTL), and a two‐end member model. We verify the capacity of these models to estimate unfrozen water content (UWC) based on “reference” UWC results and joint P and S wave velocities for different soil types. The satisfactory UWC estimates of saline unconsolidated sand and overconsolidated clay based on V_p data prove that the KT, BGTL, and two‐end member models are capable of modeling “smooth” transitions in the ice crystal growth mode, while they may provide less accurate UWC values when abrupt change of crystallization mode occurs. None of the tested soil types show a single crystallization mode throughout the freezing process, as assumed by individual crystal growth models. V_s‐based UWC estimates are less accurate due to significant but difficult‐to‐estimate influence of effective stress and soil initial cementation. All models, except pore filling and matrix supporting, can match V_s versus V_p measurement results for sands and silts but gradually provide inconsistent estimates with increasing clay content. We conclude that model validation by independent UWC measurements is necessary and that consistency between UWC values estimated from V_s and V_p is insufficient to ensure proper model validation.

The climate change has aroused great concern on the stability and durability of the infrastructure installed on permafrost, especially for frozen saline clay with a large amount of unfrozen water content at subzero temperature. The joint electrical resistivity and acoustic velocity measurements are conducted for frozen saline sand and onsøy clay with 50% clay content and 20~40 g/L salinity in order to determine the unfrozen water content. A systematic program of tests involves the saline sand with different salinity, natural onsøy clay with the variable of temperature and freezing-thawing cycles and reconstituted onsøy clay with distinctive density and salinity. The data analysis of measurement results in combination with previous joint measurements for frozen soil resolves the effect of temperature, salinity, soil type and freezing-thawing cycles on the acoustic and electrical properties. An increase of temperature, fine content and salinity results in a decrease of both acoustic velocity and electrical resistivity. Electrical resistivity is sensitive to salinity, while acoustic velocity changes substantially near thawing temperature. We also find that both natural and reconstituted clay with similar water content and salinity show quite different acoustic velocity and electrical resistivity, which indicates that ice crystal structures are distinctive between natural and reconstituted samples. Besides, P-wave velocity is much more sensitive to the fabric change or induced cracks than electrical resistance during freezing-thawing cycles.  In the end, acoustic models like the weighted equation (Lee et al., 1996), Zimmerman and King’s model (King et al., 1988) and BGTL (Lee, 2002) are applied to the UWS estimates based on P-wave velocity and electrical models like Archine’s law are adopted based on electrical resistance. Both estimated UWS from different methods is not always consistent. The difference can be up to 20%.Keywords: Frozen Saline Clay, Acoustic Velocity, Electrical Resistance, Unfrozen Water SaturationReferences:King, M. S., Zimmerman, R. W., & Corwin, R. F. (1988, May). Seismic and Electrical-Properties of Unconsolidated Permafrost. Geophysical Prospecting, 36(4), 349-364. https://doi.org/10.1111/j.1365-2478.1988.tb02168.xLee, J. S. (2002). Biot–Gassmann theory for velocities of gas hydrate-bearing sediments.Lee, M. W., Hutchinson, D. R., Collett, T. S., & Dillon, W. P. (1996). Seismic velocities for hydrate-bearing sediments using weighted equation. Journal of Geophysical Research: Solid Earth, 101(B9), 20347-20358. https://doi.org/10.1029/96jb01886