frost action presentation

Information about frost action presentation

Published on September 20, 2007

Author: Sharck

Source: authorstream.com

Content

CE 565 SOIL BEHAVIOUR:  CE 565 SOIL BEHAVIOUR FROST ACTION IN SOIL DAVUT KÖKSAL Weathering:  Weathering Weathering is the process of breaking down rocks, soils and their minerals through direct contact with the atmosphere. Weathering occurs in situ, or 'without movement', and thus should not to be confused with erosion, which involves the movement and disintegration of rocks and minerals by processes such as water, wind, ice, hail and gravity Slide3:  The materials left over after the rock breaks down combined with organic material creates soil. The mineral content of the soil is determined by the parent material, thus a soil derived from a single rock type can often be deficient in one or more minerals for good fertility, while a soil weathered from a mix of rock types (as in glacial, eolian or alluvial sediments) often makes more fertile soil. Two main classifications of weathering processes exist:  Two main classifications of weathering processes exist Mechanical or physical weathering Chemical weathering Slide5:  Mechanical or physical weathering involves the breakdown of rocks and soils through direct contact with atmospheric conditions such as heat, water, ice and pressure. Slide6:  Chemical weathering, involves the direct effect of atmospheric chemicals, or biologically produced chemicals (also known as biological weathering), in the breakdown of rocks, soils and minerals. TYPES OF MECHANICAL WEATHERING:  TYPES OF MECHANICAL WEATHERING Thermal expansion Freeze thaw weathering Frost wedging Pressure release Hydraulic action Salt-crystal growth (haloclasty) Biotic weathering Freeze thaw weathering:  Freeze thaw weathering This process can also be called frost shattering. This type of weathering is common in mountain areas where the temperature is around freezing point. Frost induced weathering, although often attributed to the expansion of freezing water captured in cracks, is generally independent of the water-to-ice expansion. Slide9:  It has long been known that moist soils expand or frost heave upon freezing as a result of water migrating along from unfrozen areas via thin films to collect at growing ice lenses. This same phenomena occurs within pore spaces of rocks. They grow larger as they attract liquid water from the surrounding pores. The ice crystal growth weakens the rocks which, in time, break up. Intermolecular forces acting between the mineral surfaces, ice, and water sustain these unfrozen films which transport moisture and generate pressure between mineral surfaces as the lens aggregates. Slide10:  Experiments show that chalk, sandstone and limestone do not fracture at the nominal freezing temperature of water of slightly below 0°C, even when cycled or held at low temperature for extended periods, as one would expect if weathering resulted from the expansion of water as froze. For the more porous types of rocks, the temperature range critical for rapid, ice-lens-induced fracture is -3 to -6°C, significantly below freezing temperatures. Slide11:  Freeze induced weathering action occurs mainly in environments where there is a lot of moisture, and temperatures frequently fluctuate above and below freezing point that is, mainly alpine and periglacial areas. An example of rocks susceptible to frost action is chalk, which has many pore spaces for the growth of ice crystals. This process can be seen in Dartmoor where it results in the formation of tors. A rock in southern Iceland fragmented by freeze-thaw action :  A rock in southern Iceland fragmented by freeze-thaw action Controlling Factors:  Controlling Factors For frost action to occur three basic conditions must be satisfied: the soil must be frost-susceptible; water must be available in sufficient quantities; and cooling conditions must cause soil and water to freeze. If one of these conditions can be eliminated, frost heaving will not occur. Slide14:  Frost-susceptibility is related to size distribution of soil particles. In general, coarse-grained soils such as sands and gravels do not heave, whereas clays, silts and very fine sands will support the growth of ice lenses even when present in small proportions in coarse soils. If frost-susceptible soils located where they will affect foundations can be removed and replaced by coarser material, frost heaving will not occur. Water must be available in the unfrozen soil for movement to the freezing plane where the growth of ice lenses occurs. A high groundwater table with respect to the location of the ice lenses will therefore favour frost action. Where proper drainage is prescribed water can be prevented from reaching the freezing zone in frost-susceptible soils. Depth of freezing is largely determined by the rate of heat loss from the soil surface. Besides the thermal properties of the soil, this heat loss depends upon such climatic variables as solar radiation, snow cover, wind, and air temperature, which is the most significant. If loss of heat can be prevented or reduced, frost-susceptible soils may not experience freezing temperatures Frost wedging:  Frost wedging Formerly believed to be the dominant mode, ice wedging may still be a factor for weathering of nonporous rock, although recent research has demonstrated it less important than previously thought. Frost action, sometimes known as ice crystal growth, ice wedging, frost wedging or freeze-thaw occurs when water in cracks and joints of rocks freeze and expand. Water can exert pressures up to 21 megapascals (MPa) (2100 kgf/cm²) at −22 °C. This pressure is often higher than the resistance of most rocks and causes the rock to shatter. Slide16:  When water that has entered the joints freezes, the ice formed strains the walls of the joints and causes the joints to deepen and widen. This is because the volume of water expands by 10% when it freezes. Slide17:  When the ice thaws, water can flow further into the rock. When the temperature drops below freezing point and the water freezes again, the ice enlarges the joints further. Slide18:  Repeated freeze-thaw action weakens the rocks which, over time, break up along the joints into angular pieces. The angular rock fragments gather at the foot of the slope to form a talus slope (or scree slope). The splitting of rocks along the joints into blocks is called block disintegration. The blocks of rocks that are detached are of various shapes depending on rock structure The formation of scree:  The formation of scree The formation of scree is often a result of frost heaving, one of the physical weathering processes that slowly wear mountains down. During the day, water can flow into cracks and crevices in the rock. If the temperature drops sufficiently, for example with the onset of evening, the water freezes. Since water expands when it freezes, it forms a powerful wedge which can eventually break out pieces of rock. A repeated cycle of freeze-thaw can lead to significant erosion and most of the loose rock or scree slopes so common in the mountains have been formed in this way. Climber ascending scree slope on the west flank of Cross Fell, England:  Climber ascending scree slope on the west flank of Cross Fell, England Frost Action during Construction in Winter:  Frost Action during Construction in Winter Frost heaving and damage frequently occur on construction sites in early winter because temporary heating is not begun sufficiently early. Special attention must be given to prevent frost action from affecting foundations during this period. Interior footings, which are often placed only a few inches below basement floors, are particularly vulnerable to frost action at such times. The walls and floors of a partially completed structure act like cooling fins to accelerate the extraction of heat from the ground. This effect is most pronounced immediately beneath the footings. If the soil is frost-susceptible, severe heaving of the structural frame may result. Under the same conditions concrete floors on grade may be heaved, resulting in the crushing of lightweight partitions or distortion of the building frame and subsequent structural damage if the partitions are sufficiently strong to transfer stresses. Straw is sometimes used effectively as temporary insulation over the floors but during sub-freezing weather it cannot prevent heat losses from beneath the footings supporting walls and columns. For buildings designed to be heated, therefore, it is important that foundations at shallow depths and floors on grade be adequately protected against frost during construction in cold weather by either temporary heating or properly placed and suitable insulation. Buildings in which crawl spaces are provided between the foundations and the first floor level are also vulnerable to frost action. Temporary heating is often installed only above the first floor, with no provision made for the crawl space, and in freezing weather frost heaving can result. Frost Action on pavement materials and the underlying subgrade :  Frost Action on pavement materials and the underlying subgrade Frost action, which can be quite detrimental to pavements because of its effect on the underlying subgrade, can be divided into 'frost heave' and 'thaw weakening'.  'Frost heave' is an upward movement of the subgrade resulting from the expansion of accumulated soil moisture as it freezes, while 'thaw weakening' is a weakened subgrade condition resulting from soil saturation as ice within the soil melts. Slide23:  Frost Heave Frost heaving of soil is caused by crystallization of ice within the larger soil voids and usually a subsequent extension of this ice to form continuous ice lenses, layers, veins, or other ice masses. As depicted in Figure 3, An ice lens grows and thickens in the direction of heat transfer until the water supply is depleted or until freezing conditions at the freezing interface no longer support further crystallization. As the ice lens grows, the overlying soil and pavement will 'heave' up potentially resulting in a rough, cracked pavement (see Figure 4). Frost heave occurs primarily in soils containing fine particles (often termed 'frost susceptible' soils), while clean sands and gravels (small amounts of fine particles) are non-frost susceptible (NFS). Thus, the degree of frost susceptibility is mainly a function of the percentage of fine particles within the soil. Many agencies classify materials as being frost susceptible if 10 percent or more passes a No. 200 sieve or 3 percent or more passes a No. 635 sieve.  Slide24:  Slide25: 

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