Soil improvement and ground modification methods chapter 13 thermal treatments

Soil improvement and ground modification methods chapter 13 thermal treatments Soil improvement and ground modification methods chapter 13 thermal treatments Soil improvement and ground modification methods chapter 13 thermal treatments Soil improvement and ground modification methods chapter 13 thermal treatments Soil improvement and ground modification methods chapter 13 thermal treatments Soil improvement and ground modification methods chapter 13 thermal treatments Soil improvement and ground modification methods chapter 13 thermal treatments Soil improvement and ground modification methods chapter 13 thermal treatments Soil improvement and ground modification methods chapter 13 thermal treatments

CHAPTER 13 Thermal Treatments This chapter provides an overview of soil stabilization methods with thermal treatments Thermal treatment refers to the modification and/or stabilization of soils by application of (1) heat (typically by way of combustion of fossil fuels) for improving properties of clayey soils and (2) artificial ground freezing for the temporary treatment and stabilization of soils and fractured rock These two approaches are obviously very different in many respects and will, therefore, be addressed separately herein 13.1 TYPES OF THERMAL TREATMENTS Heat treatment has been utilized for soil stabilization for many years It includes burning petroleum products directly in soil borings and surface heating from the close proximity burners of traveling heaters In general, heating is an effective method of soil treatment for fine-grained (clayey) soils only The high temperatures cause permanent physical reactions in the clay minerals, as well as a drying effect by evaporation of water The increased costs and environmental concerns of using petroleum products have rendered many of these types of processes extinct, although, recently, heating has made a comeback for limited applications in the remediation of contaminated soils Heating the soil at a moderate temperature assists the vapor extraction of volatile organic compounds Soil vapor extraction performance can be enhanced or improved by injecting heated air or steam into the contaminated soil through the injection wells Heating the soil to extremely high temperature is the in situ vitrification by which electrical current is used to heat and melt the soil in place (Terashi and Juran, 2000) The technique is effective for soils contaminated with organic, inorganic, and radioactive compounds Heating, or more properly “firing” of clays to make bricks could also be considered a soil heat treatment Ground freezing is a technique that provides a temporary increase of strength and shut off of water seepage It is used around open cuts and excavations, small and large diameter shaft excavations, underpinning of existing structures, and tunneling Often, it may be the best choice for sinking deep excavations and shafts that extend below the water table It has also found a Soil Improvement and Ground Modification Methods © 2015 Elsevier Inc All rights reserved 319 320 Soil improvement and ground modification methods significant environmental purpose in handling and/or containing contaminated ground or wastes, arresting landslides, and stabilizing underground collapses in emergency situations Another method, called active freezing, is used in northern latitudes to maintain frozen ground in permafrost zones beneath heated structures where passive systems using insulation alone are not considered adequate to maintain a frozen state Thawing of permafrost beneath heated buildings may result in unwanted settlements and/or loss in bearing strength Mageau and Nixon (2004) describe this type of system, which utilizes natural and forced ventilation through air ducts and ventilated granular pads to remove heat from beneath structural foundations Active freezing has also been used to aid in maintaining frozen conditions for artificially frozen ground for stabilizing soil or for water cutoff 13.2 HEAT CAPACITY OF SOILS In order to better understand the mechanics of thermal treatments, one needs to understand the basics of heat energy For example, ground can be artificially frozen when heat energy is removed Here we should also review and/or define some terminology and units Heat energy is transferred as a result of a temperature difference, where energy is transmitted from a body with higher temperature to one with lower temperature Heat energy is commonly defined in units of joules ( J) or calories (cal) A calorie is defined as the amount of heat required to change the temperature of one gram of liquid by  C The Joule is the official SI unit of heat energy: 1cal ¼ 4:184J The transfer of energy due to temperature difference alone is called heat flow The SI unit of power, the watt, is used for reference to heat flow A watt is defined as J/s In reality, the total heat energy of a fluid is dependent on both temperature and pressure This total energy is called specific enthalpy, and refers to the total energy of a unit mass The unit of measure most commonly used is kJ/kg Specific heat is the amount of heat required to change the temperature of kg of a substance by 1 The units of measure would then be kJ/kg K (K ¼ kelvins) Heat capacity can then be defined as the heat required to change the temperature of a whole system by 1 Thermal treatments 321 The heat capacity characteristics of soils, water, ice, and other earth materials must be carefully evaluated for each specific application in order to properly design and monitor successful applications 13.3 HEAT TREATMENT OF SOILS Heat treatment has been utilized as a method of ground modification by improving engineering properties of fine-grained soils Heat can affect clay chemistry and has the ability to alter clay mineralogy through diagenesis, allowing for improved engineering properties of these materials Granular soils are generally unaffected by the application of heat at temperatures less than 1000  C, with the exception of drying, which has little effect on engineering properties of these soil types Heat treatment of clayey soils results in permanent, irreversible changes as a consequence of both the drying effect and changes in the actual mineral structure of these soils A number of significant improvements can be made by utilizing heat treatments, although examples described in the literature indicate that the energy and associated fuel consumption is relatively high Some efforts were made to apply heat treatment to stabilize clay slopes in the former Soviet Union (Turner and Schuster, 1996) Because of the cost of and conscious awareness toward reducing consumption of nonrenewable energy sources and concerns of pollutants, the current and future use of heat treatment for soil modification is likely to be restricted to (biological) control and treatment of contaminated soils One area in which heat treatment may still be viable is in the production of Ferroclay building blocks These may range from earth/mud bricks, still utilized in third-world construction, to fully fired bricks (Hausmann, 1990) 13.3.1 Improvements and Applications of Ground Heating Generally, improvement of engineering properties of clayey soils occurs with an application of at least 400  C Improvements, including decreased compressibility, reduced plasticity, reduced swelling potential, lower optimum moisture content, and increased strength, have been detailed in the literature (Abu-Zreig et al., 2001) Case studies have shown strength increases of up to 10-20 times Heat treatment has been applied to soil through a variety of techniques, including combustion of fuel in boreholes, surface treatments by traveling “burners” in close proximity to the ground surface, and through “baking or firing” of clay blocks (forming a range of 322 Soil improvement and ground modification methods construction elements from crude mud blocks to conventional bricks, as described above) In situ improvement at depth has been successful only where there is a source of relatively low-cost fuels As a result, this approach has all but disappeared, given the rise in fuel costs and other environmental considerations Surface treatment by means of traveling heaters can successfully treat to a limited depth of existing, in situ surface soils or layers of engineered fill One note of caution is to beware of possible ground movement resulting from expansion of water followed by consolidation upon cooling 13.4 GROUND FREEZING The principle of ground freezing is that when the moisture (pore water) in the soil freezes, the soil particles are bound together, creating a rigid structure with considerable strength and stiffness Ground is artificially frozen when heat energy is removed from it This is accomplished by introducing a lower temperature medium that causes a flow of heat energy from higher to lower temperature, thereby reducing the heat (cooling the soil) Understanding the relatively simple mechanics involved points to the fact that, unlike heat treatments, artificial freezing may be applicable to a wide range of soil types, grain sizes, and ground conditions Fundamentally, the only requirement is that the ground has sufficient soil moisture (pore water) Ground freezing and associated improvements and/or stabilization is possible only if continuous artificial cooling is maintained It is, therefore, of critical importance to understand that ground freezing is always only a temporary stabilization technique As a result, consideration should be given to back-up systems as a part of initial planning and design However, once ground is frozen, some time will be needed for it to thaw, so relatively short power interruptions are not necessarily critical The first reported use of ground freezing was in South Wales in 1862 in conjunction with a mine shaft excavation (Schaefer et al., 1997) The strength of frozen soil may be on the order of 1-10 MPa, although it depends on a variety of factors, such as soil type, water content, rate of freezing, and maintained temperature of the frozen soil An important attribute is that frozen soil becomes a nearly impermeable material The technique is currently used for the temporal increase of strength and temporal shut off of water seepage around open cuts, shaft excavations, and tunneling There have been a number of specialty symposia on ground freezing that provide an overview of applications, including the International Symposium on Thermal treatments 323 Ground Freezing that has been held periodically since 1978 In addition, the increasing number of specialty contractors providing ground freezing services has provided even more available literature and case studies 13.4.1 Improvements and Applications of Ground Freezing The fundamentals of ground freezing have been known and used since the 1880s for the mining industry The principle improvements of freezing the ground are typically either strengthening or stabilizing the ground, controlling seepage, or a combination of both Frozen ground can have increased shear strengths of up to 20 times that of unfrozen soil (or nearly twice that of concrete) by combining the inherent soil shear strength with that of ice Seepage is controlled by the formation of a frozen barrier of the pore water acting as an effective cutoff if sufficient pore water is available One caution and/or concern is the disruption of soil structure and associated volume change due to expansion of the pore fluid upon freezing Another issue has been with deformations and loss of soil strength upon thawing of the frozen soil mass Because successful ground freezing fundamentally relies only on there being enough moisture in the ground, it is applicable to virtually all earth materials, making this method more versatile for temporary water cutoff than many others Figure 13.1 demonstrates the range of applicability of freezing compared to other common cutoff methods Ground freezing has been successfully used for temporary construction elements (e.g., excavations (see Figure 13.2), cofferdams, underpinning of existing structures, stabilization for tunneling, etc.), incipient or active slope failure stabilization, containment (or exclusion) of contaminated groundwater, hazardous wastes and toxic “spills,” undisturbed sampling of cohesionless soils, and so forth At the same time, frozen ground provides a hydraulic barrier for temporary seepage control of construction dewatering applications As such, freezing eliminates the need for costly construction of both structural shoring systems and dewatering (hydraulic barrier) systems In addition, freezing can provide a hard, durable working surface even in soft and/or wet soils Figure 13.3 shows a freezing project for excavation of a deep shaft Where accessibility, space limitation, and “sensitive” infrastructure exist, ground freezing has been demonstrated as a workable solution Examples of this are excavations adjacent or in close proximity to historic structures 324 Soil improvement and ground modification methods Clays Sits Sands Gravels Cobbles Man-made Boulders obstructions Rock Ground freezing Jet grouting Permeation grouting Sheet piling Secant plies Slurry walls Deep mixing Figure 13.1 Freezing applicability compared to other improvement methods for ground support Courtesy of Moretrench 13.4.2 Ground Freezing Techniques Freezing is typically induced by insertion of equally spaced pipes circulating supercooled brine (often