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

Thermal Treatments

This chapter provides an overview of soil stabilization methods with thermaltreatments Thermal treatment refers to the modification and/or stabiliza-tion of soils by application of (1) heat (typically by way of combustion offossil fuels) for improving properties of clayey soils and (2) artificial groundfreezing for the temporary treatment and stabilization of soils and fracturedrock These two approaches are obviously very different in many respectsand will, therefore, be addressed separately herein

13.1 TYPES OF THERMAL TREATMENTS

Heat treatment has been utilized for soil stabilization for many years Itincludes burning petroleum products directly in soil borings and surfaceheating from the close proximity burners of traveling heaters In general,heating is an effective method of soil treatment for fine-grained (clayey) soilsonly The high temperatures cause permanent physical reactions in the clayminerals, as well as a drying effect by evaporation of water The increasedcosts and environmental concerns of using petroleum products have ren-dered many of these types of processes extinct, although, recently, heatinghas made a comeback for limited applications in the remediation of contam-inated soils Heating the soil at a moderate temperature assists the vaporextraction of volatile organic compounds Soil vapor extraction perfor-mance can be enhanced or improved by injecting heated air or steam intothe contaminated soil through the injection wells Heating the soil toextremely high temperature is the in situ vitrification by which electrical cur-rent is used to heat and melt the soil in place (Terashi and Juran, 2000) Thetechnique is effective for soils contaminated with organic, inorganic, andradioactive compounds Heating, or more properly “firing” of clays to makebricks could also be considered a soil heat treatment

Ground freezing is a technique that provides a temporary increase ofstrength and shut off of water seepage It is used around open cuts and exca-vations, small and large diameter shaft excavations, underpinning of existingstructures, and tunneling Often, it may be the best choice for sinking deepexcavations and shafts that extend below the water table It has also found a

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Soil Improvement and Ground Modification © 2015 Elsevier Inc.

significant environmental purpose in handling and/or containing nated ground or wastes, arresting landslides, and stabilizing undergroundcollapses in emergency situations.

contami-Another method, called active freezing, is used in northern latitudes tomaintain frozen ground in permafrost zones beneath heated structures wherepassive systems using insulation alone are not considered adequate to main-tain a frozen state Thawing of permafrost beneath heated buildings mayresult in unwanted settlements and/or loss in bearing strength Mageauand Nixon (2004)describe this type of system, which utilizes natural andforced ventilation through air ducts and ventilated granular pads to removeheat from beneath structural foundations Active freezing has also been used

to aid in maintaining frozen conditions for artificially frozen ground forstabilizing soil or for water cutoff

13.2 HEAT CAPACITY OF SOILS

In order to better understand the mechanics of thermal treatments, oneneeds to understand the basics of heat energy For example, ground can

be artificially frozen when heat energy is removed Here we should alsoreview and/or define some terminology and units

Heat energy is transferred as a result of a temperature difference, whereenergy is transmitted from a body with higher temperature to one withlower temperature Heat energy is commonly defined in units of joules( J) or calories (cal) A calorie is defined as the amount of heat required tochange the temperature of one gram of liquid by 1
C The Joule is theofficial SI unit of heat energy:

1cal¼ 4:184JThe transfer of energy due to temperature difference alone is called heatflow The SI unit of power, the watt, is used for reference to heat flow A watt

is defined as 1 J/s

In reality, the total heat energy of a fluid is dependent on both temperatureand pressure This total energy is called specific enthalpy, and refers to the totalenergy 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

1 kg of a substance by 1
The units of measure would then be kJ/kg K(K¼kelvins) Heat capacity can then be defined as the heat required to changethe temperature of a whole system by 1

The heat capacity characteristics of soils, water, ice, and other earthmaterials 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 byimproving engineering properties of fine-grained soils Heat can affect claychemistry and has the ability to alter clay mineralogy through diagenesis,allowing for improved engineering properties of these materials Granularsoils are generally unaffected by the application of heat at temperatures lessthan 1000
C, with the exception of drying, which has little effect on engi-neering 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 mineralstructure of these soils A number of significant improvements can be made

by utilizing heat treatments, although examples described in the literatureindicate that the energy and associated fuel consumption is relatively high.Some efforts were made to apply heat treatment to stabilize clay slopes inthe former Soviet Union (Turner and Schuster, 1996) Because ofthe cost of and conscious awareness toward reducing consumption

of nonrenewable energy sources and concerns of pollutants, the currentand future use of heat treatment for soil modification is likely to berestricted 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/mudbricks, still utilized in third-world construction, to fully fired bricks(Hausmann, 1990)

13.3.1 Improvements and Applications of Ground HeatingGenerally, improvement of engineering properties of clayey soils occurswith an application of at least 400
C Improvements, including decreasedcompressibility, reduced plasticity, reduced swelling potential, lower opti-mum moisture content, and increased strength, have been detailed in theliterature (Abu-Zreig et al., 2001) Case studies have shown strengthincreases of up to 10-20 times Heat treatment has been applied to soilthrough a variety of techniques, including combustion of fuel in boreholes,surface treatments by traveling “burners” in close proximity to the groundsurface, and through “baking or firing” of clay blocks (forming a range of

construction elements from crude mud blocks to conventional bricks, asdescribed above).

In situ improvement at depth has been successful only where there is asource of relatively low-cost fuels As a result, this approach has all but dis-appeared, given the rise in fuel costs and other environmental consider-ations Surface treatment by means of traveling heaters can successfullytreat to a limited depth of existing, in situ surface soils or layers of engineeredfill One note of caution is to beware of possible ground movement resultingfrom expansion of water followed by consolidation upon cooling

13.4 GROUND FREEZING

The principle of ground freezing is that when the moisture (pore water) inthe soil freezes, the soil particles are bound together, creating a rigid struc-ture with considerable strength and stiffness Ground is artificially frozenwhen 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) standing the relatively simple mechanics involved points to the fact that,unlike heat treatments, artificial freezing may be applicable to a wide range

Under-of soil types, grain sizes, and ground conditions Fundamentally, the onlyrequirement is that the ground has sufficient soil moisture (pore water).Ground freezing and associated improvements and/or stabilization is pos-sible only if continuous artificial cooling is maintained It is, therefore, ofcritical importance to understand that ground freezing is always only a tem-porary stabilization technique As a result, consideration should be given toback-up systems as a part of initial planning and design However, onceground is frozen, some time will be needed for it to thaw, so relativelyshort power interruptions are not necessarily critical

The first reported use of ground freezing was in South Wales in 1862 inconjunction with a mine shaft excavation (Schaefer et al., 1997) Thestrength 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, andmaintained temperature of the frozen soil An important attribute is that fro-zen soil becomes a nearly impermeable material The technique is currentlyused for the temporal increase of strength and temporal shut off of waterseepage around open cuts, shaft excavations, and tunneling There havebeen a number of specialty symposia on ground freezing that provide anoverview of applications, including the International Symposium on

Ground Freezing that has been held periodically since 1978 In addition, theincreasing number of specialty contractors providing ground freezing ser-vices has provided even more available literature and case studies.

13.4.1 Improvements and Applications of Ground FreezingThe fundamentals of ground freezing have been known and used since the1880s for the mining industry The principle improvements of freezing theground are typically either strengthening or stabilizing the ground, control-ling seepage, or a combination of both Frozen ground can have increasedshear strengths of up to 20 times that of unfrozen soil (or nearly twice that ofconcrete) by combining the inherent soil shear strength with that of ice.Seepage is controlled by the formation of a frozen barrier of the pore wateracting as an effective cutoff if sufficient pore water is available One cautionand/or concern is the disruption of soil structure and associated volumechange due to expansion of the pore fluid upon freezing Another issuehas been with deformations and loss of soil strength upon thawing of thefrozen soil mass

Because successful ground freezing fundamentally relies only on therebeing enough moisture in the ground, it is applicable to virtually all earthmaterials, making this method more versatile for temporary water cutoffthan many others Figure 13.1demonstrates the range of applicability offreezing compared to other common cutoff methods

Ground freezing has been successfully used for temporary constructionelements (e.g., excavations (seeFigure 13.2), cofferdams, underpinning ofexisting structures, stabilization for tunneling, etc.), incipient or active slopefailure stabilization, containment (or exclusion) of contaminated groundwa-ter, hazardous wastes and toxic “spills,” undisturbed sampling of cohesion-less soils, and so forth At the same time, frozen ground provides a hydraulicbarrier for temporary seepage control of construction dewatering applica-tions As such, freezing eliminates the need for costly construction ofboth structural shoring systems and dewatering (hydraulic barrier) systems

In addition, freezing can provide a hard, durable working surface even in softand/or wet soils.Figure 13.3shows a freezing project for excavation of adeep shaft

Where accessibility, space limitation, and “sensitive” infrastructureexist, ground freezing has been demonstrated as a workable solution.Examples of this are excavations adjacent or in close proximity to historicstructures

13.4.2 Ground Freezing Techniques

Freezing is typically induced by insertion of equally spaced pipes circulatingsupercooled brine (often<25 to 30
C) or, more expensive but muchquicker, by injection of liquid nitrogen (LN2), which boils at196
C Inthe case of using brine, the solution is circulated down a central tube andback up through the annulus to extract heat from the surrounding soil(Figure 13.4) A strong saline (usually calcium chloride) solution has a muchlower freezing point that that of typical pore water and will therefore remainfluid even at temperatures as low as35
C The pipes are usually placed in arow or “line” to provide a continuous wall or temporary “structural” ele-ment to support higher loads and/or provide a hydraulic barrier for ground-water cutoff (Figure 13.5) Freezewall is a term sometimes used to describe acontinuous wall of frozen soil columns As previously indicated, this type of

“frozen wall” construction can be utilized to provide both wall support and ahydraulic barrier In some cases, freezing of a larger mass of soil may bedesired to temporarily stabilize unsafe or unstable conditions This turnsthe ground into the consistency of soft rock, which can then be excavated,drilled, or tunneled through by conventional or more modern techniques.Using liquid nitrogen for ground freezing is more costly due to theexpense of the nitrogen (which is expended and, therefore, must be regularlyreplenished to maintain freezing), but due to the extremely low

Deep mixing

Slurry walls Secant plies Sheet piling Permeation grouting Jet grouting

Figure 13.2 Frozen ground for excavation shoring Courtesy of SoilFreeze.

Figure 13.3 Freezing around the periphery of a deep-shaft construction Courtesy of Moretrench.

temperatures generated (196
C or320
F), freezing will be very rapid.

In addition, the necessary cooling equipment is substantially less involvedand, therefore, less costly than a brine cooling unit, and may not require

a locally available power supply Liquid nitrogen is also nonflammableand nontoxic, and it can be easily transported in tanks These attributes makefreezing with liquid nitrogen advantageous for emergency stabilization atremote sites The liquid gas is pumped directly into copper freeze pipesinstalled in (or in emergencies, driven into) the ground, which immediatelyfreezes adjacent surrounding ground as the liquid nitrogen vaporizes(Figures 13.6and13.7) The vaporized cold nitrogen (i.e., exhaust gas) fur-ther extracts heat as it flows back out of the ground (www.lindeus.com)

Figure 13.4 Schematic example of freezing by circulating super cooled brine Courtesy

of Moretrench.

3⬙ Freeze pipe

Extent of freeze after 1-2 weeks

Extent of freeze after 2-3 weeks

Figure 13.5 Example of how the frozen zone surrounding freeze pipes eventually joins

to form a continuous strong, impermeable “wall.” Courtesy of SoilFreeze.

This process may be practical for small, short-term projects and/or for gency stabilization.

emer-To reliably use and/or design freezing technology requires an standing of the thermal, mechanical, and hydraulic properties of the frozensoil, the equipment used to freeze the ground, and the assessment of the

under-Figure 13.6 Schematic example of freezing by injecting liquid nitrogen Courtesy of Moretrench.

Figure 13.7 Application of freezing by injecting liquid nitrogen Courtesy of SoilFreeze.

construction process Inherent in this approach is a fundamental ing of the basic physics of thermal conductivity, heat capacity, and energyrequirements to move between solid, liquid, and gaseous phases of the mate-rials being treated In reality, energy losses in practical applications result in asomewhat higher consumption than would be predicted by the basic physicsalone A number of variables that will affect the effectiveness of a freezingapplication include soil type (mineralogy and thermal properties), watercontent, velocity and inherent temperature of the ground water, and rate

understand-of freezing

The design of a frozen earth barrier (or support) is governed by the heatcapacity characteristics of soils, water, ice, and other earth materials, asdescribed inSection 13.2 Formation of the frozen earth mass surroundingeach pipe will depend on the thermal and hydraulic properties of each stra-tum being treated Obtainable strengths will depend on soil type, moisturecontent, and temperature For instance, when soft, relatively weak clay iscooled to below freezing, some portion of its pore water begins to freeze,and the clay begins to stiffen and strengthen When the temperature isreduced further, more of the pore water freezes, and the soil strength can

be dramatically increased In contrast, a sandy soil with relatively higher tial strengths may be adequately frozen with substantial strength gains withless temperature differentials required For example, a temperature of5
Cmay be adequate for freezing granular soils, while temperatures as low as

ini-25
C may be required for some fine-grained soils The rate of freezing

is also dependent on thermal and hydraulic properties of the soils materialsinvolved Fractured rock and coarse-grained soils will typically freeze signif-icantly faster than finer-grained silts and clays with lower hydraulic conduc-tivities Freeze pipe spacing, freezing radius around pipes, and freezing timescan be estimated by relatively straightforward computations, such as thoseprovided byHarlan and Nixon (1978) But today most ground freezing pro-jects are designed and analyzed by computer programs such as TEMP/W(www.geo-slope.com) because they can also provide detailed 2-D analyses(Figure 13.8)

Due to the number of specialty geotechnical contractors who havegained sufficient experience in thermal ground treatment (particularlyartificial ground freezing), economical solutions are becoming much morecommon for a wide variety of applications The increased number ofcontractors engaging in ground freezing has also generated some healthycompetition for these services