Orlando B. Andersland

Frozen soil subsurface barriers: formation and ice erosion

Abstract Before and during soil remediation, frozen soil barriers are used to provide containment of liquid contaminants so as to prevent their migration to adjacent areas. Questions concerning formation of frozen gravelly sand barriers in the vadose zone and barrier resistance to ice erosion by liquid contaminants provided the impetus for this study. Bench-scale barriers with full and partial ice saturation were studied. For comparison, one barrier was formed in gravelly sand using water injection via a bentonite-water slurry. The bentonite increased the slurry viscosity, thereby providing a water retention period suitable for freezing. Liquid contaminants included a sodium nitrate solution (freezing point of −5°C) and an antifreeze solution (freezing point of −33°C). Results showed that ice erosion in the bench-scale barriers occurred when the contaminant freezing point depression was lower than the temperature of the frozen soil. When liquid contaminants entered the voids of partially ice saturated frozen soil, the rate of ice erosion was significantly increased. Full ice saturation and barrier temperatures below the freezing point depression of the contaminant are needed to minimize ice erosion.

Crack formation in soil landfill covers due to thermal contraction

Soil landfill covers in the northern United States experience ground freezing to depths of 2 m or more. During periods of decreasing winter temperatures, thermal contraction will increase tensile stresses creating the potential for crack formation. If elastic soil behaviour is assumed, a drop of only 2 or 3°C will generate significant tensile stresses. Climatological data examined for three locations, along with computed ground temperatures, show larger drops in temperature. Frozen cover soils are comparatively weak in tension. Cracks, once initiated, can propagate unstably through the frozen soil, and may extend deeper than the tensile stresses to which they owe their growth. Simple elastic soil behaviour used with thermal strains does not provide adequate information for predicting thermal contraction and crack formation. Information is needed on the thermal contraction behaviour of frozen soils, on the extent to which soil creep will reduce the tensile stresses, and on criteria suitable for preventing crack formation.

Movement of liquid contaminants in partially saturated frozen granular soils

Abstract Understanding the migration of nonaqueous phase liquids in frozen subsurface soils is becoming increasingly important in permafrost regions and in temperate zones where frozen subsurface barriers have been proposed to confine contaminants. Tests were performed on 32 specimens of gravelly sands from the Hanford, Washington reservation to determine the relationship between degree of ice saturation and intrinsic permeability. Decane, a representative nonaqueous phase liquid, was employed as the permeant, and was infiltrated through the frozen specimens at −10°C. In addition to pure water, a NaCl brine and a mixture of water and decane were utilized as the pore liquids. For all specimens the intrinsic permeability correlated linearly with the ice saturation, varying from approximately 2.7× 10−7 cm2 at 0% saturation to negligible values at nearly 100% saturation. The different pore liquids did not affect the correlation significantly. Bentonite was added to some of the specimens prior to freezing, reducing the intrinsic permeability to negligible levels.

Heat Flow in Soils

Ground temperatures have a significant effect on soil engineering behavior and must be considered in the design of frozen ground support systems and other constructed facilities in cold regions. In these problems, temperatures within, under, and around the structure depend on the ground surface temperatures together with the geothermal gradient for the area. In addition to dependence on variable surface factors, ground temperatures may also depend on construction activity: for example, the controlled freezing of an earth support system. Where freezing occurs in the ground, the soil latent heat, soil thermal conductivity, and heat capacity will play a part when time dependence is involved. These topics are addressed in this chapter.

Physical and Thermal Properties

Frozen soil is a four-component system consisting of soil particles, ice, water, and air. The particles (mineral and/or organic matter) come in various sizes and shapes with a thin film of unfrozen water coating most mineral grains. The voids are filled with ice, unfrozen water, and air. Ice may be distributed uniformly throughout the soil mass or it may have accumulated in the form of irregular or stratified ice inclusions. Larger ice masses may form as a result of processes associated with ice wedges and pingos. Frozen soil classification involves identification of the soil phase, adding characteristics associated with the frozen soil, and describing ice found in frozen ground.

Mechanical Properties of Frozen Soils

From the point of view of the science of materials, frozen soil is a natural particulate composite, composed of four different constituents: solid grains (mineral or organic), ice, unfrozen water, and gases. The most important characteristic by which it differs from other similar materials, such as unfrozen soils and the majority of artificial composites, is that under natural conditions its matrix, composed mostly of ice and water, changes continuously with varying temperature and applied stress.

Earthwork in Cold Regions

Embankment construction with frozen and unfrozen soil materials is conducted throughout the year on many engineering projects where ambient air temperatures remain below 0°C for a significant portion of the year. Construction for oil field support facilities on the Alaska North Slope required that operations proceed over a 12-month period and not wait for the short 3-month thaw season (Tart, 1983). Operation of open-pit mines requires effective and economic excavation and handling of frozen and thawed ground during the entire year. Difficult problems can be experienced depending on the material type, temperature and water content, and the time of year. Material properties of fill used on the project must be reviewed. Landform analyses, borings, and geophysical techniques help assess potential material sources. Borrow source characteristics determine the techniques required for excavation (e.g., blasting, ripping, and scraping). Material type and volumes involved determine haul equipment most suitable for the project. In addition, placement and compaction techniques are dependent on embankment use and material type.

Thaw Behavior of Frozen Ground

Frozen ground contains ice in several forms, ranging from coatings on soil particles and individual ice inclusions to ice with soil inclusions. On thawing, the ice will disappear and for existing overburden pressures the soil skeleton must now adapt itself to a new equilibrium void ratio. The resulting thaw settlement phenomenon is important to the design of frozen ground support systems, design of building foundations and embankments on permafrost where thaw is permitted, design of buried pipelines, and road and highway design on seasonally and perennially frozen ground. Thaw settlement due to melting of ice-rich permafrost is illustrated in Fig. 4-1. The settlement appears to follow a polygon shape, suggesting melting of ice wedges. In this chapter we introduce concepts relative to thaw settlement, thaw consolidation, and thaw behavior in layered soil systems.

Foundations In Frozen Soils

Foundation design in areas of seasonal frost depends on the choice of an appropriate foundation depth and protection of the foundation from the effects of frost, particularly where there is frost-susceptible soil. Under certain conditions, harmful frost action effects may arise. For this to occur, frost must penetrate down to frost-susceptible soil, and sufficient water must be available to sustain the formation and growth of ice lenses. The lenses usually form parallel to the frost front, producing forces or soil movements directed at a right angle to the frost front. These forces can be very large and can lead to heaving or displacement of all parts of the foundation as the soil freezes. The magnitude of heave forces is generally difficult to determine. As it is impractical to restrain heave fully, one should attempt to eliminate it by proper design. In practice, this means that any frost-susceptible soil that will affect the foundation must be either prevented from freezing by proper insulation, or the water supply to the freezing front should be reduced by drainage. Otherwise, such a soil must be excavated and replaced with a frost-stable material.

Construction Ground Freezing

Controlled ground freezing for construction and mining applications has been in use for over a century. Frozen ground may be used to provide ground support, groundwater control, or structural underpinning during construction. Constructed prior to excavation, the frozen earth wall, for practical purposes, eliminates the need for sheeting of the earth, site dewatering, soil stabilization, or concern for movement of adjacent ground. It is a versatile technique that involves use of refrigeration to convert in situ soil pore water into ice. The ice becomes a bonding agent, fusing together adjacent particles of soil or blocks of rock to increase their combined strength and make them impervious to water seepage. Excavation and other work can then proceed safely inside, or next to, the barrier of strong, watertight frozen earth. It should be noted that it is essential that groundwater be present, supplied either by high water table or artificially.