mechanical properties of frozen ground
Link: http://w3id.org/permafrost/v/ipaglossary/t0205
Within: IPA Permafrost Glossary
Definition: The properties of frozen ground governing its deformability and strength.
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propriétés mécaniques du gélisol
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mechanical properties of frozen ground en |
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1. Properties under quasi-static loading: The behaviour of frozen soils under quasi-static loading is usually different from that of unfrozen soils because of the presence of ice and unfrozen water films. In particular, frozen soils are more susceptible to creep and relaxation effects, and their behaviour is strongly affected by temperature change. In addition to creep, volumetric consolidation may also develop in frozen soils with large unfrozen water contents. As with unfrozen soils, the strength of frozen soils depends on interparticle friction, particle interlocking and cohesion. In frozen soil, however, bonding of particles by ice is the dominant strength factor. This is complicated by the unfrozen water films surrounding the soil particles, which reduce interparticle bonding. The strength of ice in frozen soil is dependent on many factors, such as temperature, pressure, strain rate, grain size, crystal orientation and density. At very high ice contents, frozen soil behaviour under load is similar to that of ice. At low ice contents, however, when interparticle forces begin to contribute, the unfrozen water films play an important role, especially in fine-grained frozen soils. (a) Deformability and strength: The deformability and strength of a frozen soil can be studied by specially designed tests either in a cold room or in situ. Quasi-static elastic parameters usually determined in such tests are: Young's modulus and Poisson's ratio for short- term response, and creep parameters (used in a creep equation) for long-term response. In addition, the variation of strength with time or with strain rate is also determined from the tests. The strength is usually found to vary from a high short- term value to a much smaller long-term value, which is considered to govern the behaviour of frozen soil under sustained loading. Under high confining stresses and at relatively high freezing temperatures, most frozen soils creep and eventually fail in a plastic manner. On the other hand, under compression at low confining stresses, or at tensile stresses combined with low temperatures, many frozen soils fail in a brittle manner by tensile crack propagation. (b) Compressibility of frozen soils: Although frozen soils are usually considered to be practically incompressible, and volume change deformations can therefore be neglected, investigations show, however, that the compressibility of frozen soils can be significant and should not be neglected in some cases, especially when large areas are loaded. Compressibility and its time dependence in frozen soils are due to several causes, such as instantaneous compression of the gaseous phase, creep of the pore ice due to shear stresses at the grain contacts, and hydrodynamic consolidation due to the expulsion of unfrozen water under stress, the amount of which varies with the applied pressure. 2. Dynamic properties of frozen ground: Information on the dynamic properties of frozen soils is important with regard to the behaviour of structures subjected to seismic or vibratory loads, and the evaluation of results of seismic field surveys in permafrost areas. Dynamic properties are expressed either in terms of two dynamic elastic parameters: the dynamic modulus of elasticity and the dynamic Poisson's ratio, or in terms of the propagation velocities of compressional waves and shear waves in the material. The two sets of dynamic parameters are uniquely related by the theory of elasticity. The dynamic elastic parameters deduced from wave propagation velocities are different from those obtained from any type of static loading tests because the latter contain the additional effects of elastic relaxation and creep. Parameters that influence the compressional and shear wave velocities in soils and rocks include grain size, lithology, total water content, porosity and pore structure, the nature, temperature and degree of freezing of the interstitial water, degree of cementation, and confining pressure. Due to the ice content, seismic velocities are generally higher in frozen soils or rocks than in the same unfrozen materials. The change in velocity can occur gradually as temperatures decrease below 0°C, if freezing point depression conditions exist in the soil.
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https://doi.org/10.4224/20386561
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