Moisture content, ice content, density and porosity are the main physical indices that characterize the engineering-geological aspects of frozen and thawed soils.
Moisture content of frozen soil is the water content, based on drying at a temperature of 100-105 °C to obtain the constant mass of solid material. There are different indices - total moisture content, integral (natural) moisture content and volumetric moisture content. Total moisture content Wtot of the frozen soil is the ratio of water mass in solid and liquid state contained in the frozen soil to the mass of its skeleton, and in salinized materials, to the mass of the material's skeleton and of the salts present (as a percentage or fraction of a unit). The integral moisture content WDat, is the ratio of all phases of water mass to the mass of the frozen ground: the volumetric moisture content Wvol is the ratio of water volume in solid and liquid phases to the volume of the frozen soil.
The total moisture content of frozen soils, unlike those unfrozen, can much exceed the value of total moisture capacity. The value varies within a wide range in frozen soils - from maximum molecular moisture capacity to values 3-4 times exceeding the upper limit of plasticity. In general total moisture content increases with finer grain size. The total moisture content with schlieren cryogenic textures is always higher than that of the soil with massive cryotexture.
Ice content (I) is an index that characterizes the ground ice content of the frozen soil (a percentage or fractions of a unit). For estimating ice content of soils contributed by texture and texturogenous ice use is made of the total ice content index. Quantitatively, it is the ratio of the whole ice mass to that of dry matter - the gravimetric ice content; the ratio of gravimetric ice content to total moisture is relative ice content; the ratio of the total volume of ice to that of the frozen soil is the volumetric ice content.
Depending on the value of the ice content, soils are subdivided into ice-rich, ice-bearing and those with low ice content. Ice-rich soils are those in which ice volume occupies more than a half of the frozen soil volume. After thawing such soils pass into a fluid or fluid-plastic state which accounts for their high subsidence. In the thawed state ice-rich soils are characterized by a low bearing capacity, low water resistance and high compressibility. Soils with a low ice content (lower than 25%) acquire a visco-plastic and semisolid consistency after thawing, and are characterized by a high water resistance and low compressibility. Ice-bearing soils with ice content 2550% have intermediate properties compared with the above categories.
Density p (volume weight, gem"3) of the frozen soil is the ratio of the frozen soil mass with texturogenous ice to the volume of the frozen soil with its structure undisturbed; density of the frozen soil skeleton is the ratio of soil skeleton mass to the volume of the undisturbed frozen soil with its cryogenic structures.
The greatest density is typical of the materials that contain iron oxides or pyrites, while low values are typical of the rocks in which montmorillonite and halloysite prevail. Higher content of organic matter leads to lower density. In approximate calculations the following average densities of the solid mineral component are usually assumed: for sandy materials, 2.65 g cm "3; for silty-clay materials, 2.70-2.73 g cm ~3; for clays, 2.75 g cm "3.
The density of the frozen soil skeleton varies generally from 2 to 0.62gem-3. Since there are always pores filled with ice, frozen water or gas in the frozen material, the unit weight of its volume is always less than the density of the solid mineral component. The density of rock in the frozen state varies within the range of 1.0 g cm "3 for ice-rich materials with ataxite cryogenic structure to 2.73gem"3 and over for highly cemented argillites and sandstones with massive cryogenic structure.
One of the main physical indices of the structure of unfrozen soils is the porosity of the skeleton, the index which usually characterizes compressibility, i.e. total void volume per unit of soil volume irrespective of pore size and degree of saturation. In frozen soil where pores are filled with not only unfrozen water and gas inclusions, but also with ice-cement and ice that forms various cryogenic textures, one should distinguish between porosity (or void space) determined as a ratio of all the pore volume (voids) not filled with unfrozen water or ice to the volume of the frozen soil, and porosity of the frozen soil skeleton, which is the ratio of the whole void space volume not occupied by mineral skeleton to the frozen soil volume. Often, porosity is characterized as the ratio of volume of voids to the volume of the solid component of the soil and is called reduced porosity or coefficient of porosity. As regards unfrozen soil the porosity coefficient usually does not exceed 1.5-2, while in frozen and especially in highly ice saturated soils it often reaches 3-3.5, and with higher ice content increases several times. Usually, the porosity of unfrozen fine-grained soils is 20-40%, while the porosity of the mineral skeleton of the frozen soils can be as high as 60 and even 90%.
Washout capacity and soaking capacity belong to water resistance characteristics of the frozen soils that are indispensable in the evaluation of the thermal erosion hazards and of potential gully formation. These characteristics are to be used in estimating the rates of reservoir bank erosion and in calculations of stability of canal slopes and earth structures that interact with water flows.
Washout capacity of the frozen soils is a property that characterizes the capability to yield aggregates and elementary particles of the soils to the simultaneous thermal and mechanical effects of flowing water.
Washout of the frozen soils is dependent on a number of interrelated factors among which the main ones are the nature of structural bonds in the soil, its ice content and type of cryogenic structure (Table 8.1). In a general case, washout capacity of thawing soils increases when grains become smaller and density and cohesion lower, as well as with disturbance of natural structure. Washout capacity of syncryogenic perennially frozen soils is much higher than that of epicryogenic ones.
Soaking capacity of the frozen soils is such that soil loses cohesion and transforms into a loose mass while interacting with water. Soaking of the frozen ground is the result of ice melting and weakening of bonds existing between particles on swelling. Soaking is characterized by the rate and type of the process. It is accompanied by reduction of strength and determines the stability of the frozen soil with respect to degradation into mineral aggregates in still water (or water resistance). Unlike unfrozen soils, soaking of the frozen soils is dependent on not only lithological characteristics and type of natural cement, but also on ice content and its spatial distribution, i.e. cryogenic structure.
The frozen fine-grained ice-rich soils belong to colloidal systems that possess thixotropic properties on thawing. As shown by studies conducted in West Siberia, tundra areas of Bol'shezemel'skaya Tundra, Southern Yakutia and other regions, the fine-grained soils of the seasonally thawing layer are almost everywhere thixotropic in the absence of drainage on thawing. Under dynamic load their natural structure breaks down, they become liquified, losing their strength completely. After the load is removed a gradual recovery of the primary strength is observed. The main cause that
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