In Situ Ground Freezing to Obtain Undisturbed Samples - Hofmann (2000)

8/20/2019 In Situ Ground Freezing to Obtain Undisturbed Samples - Hofmann (2000)

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JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING / NOVEMBER 2000 / 979

IN SITU GROUND FREEZING TO OBTAIN UNDISTURBED SAMPLESOF LOOSE SAND

By B. A. Hofmann,1

D. C. Sego,2

Member, ASCE, and P. K. Robertson,3

Member, ASCE

ABSTRACT: In situ ground freezing with liquid nitrogen can be an excellent technique for obtaining highquality undisturbed samples from a wide range of loose sand deposits, with their in situ conditions preserved.This paper describes the techniques used during the CANLEX project to carry out in situ ground freezing and

undisturbed sampling of the loose sand deposits encountered at six test sites. Specific details, with respect tothe effects of difficult subsoil or ground-water conditions encountered at the sites, are provided. Modificationsmade to the ground freezing and sampling system to accommodate difficult ground conditions are described.The actual advance of the freezing front and the consumption of liquid nitrogen are compared with theoreticalpredictions. The dry coring technique utilized to recover undisturbed frozen sand samples is described. The voidratios measured for the frozen core samples are compared with void ratio measurements obtained using geo-physical logging in boreholes located within a 5-m radius of the freeze pipe. The costs associated with in situground freezing utilizing liquid nitrogen and coring of the frozen sand at six test sites are presented.

INTRODUCTION

The Canadian Liquefaction Experiment (CANLEX project)was undertaken to determine the most effective methods of characterizing sand to evaluate the potential for dynamic andstatic liquefaction. One of the primary objectives of the project

was to evaluate the in situ undrained response of various sanddeposits and, ultimately, to provide a framework for linkinglaboratory and field data that could be used for design againstsoil liquefaction (Wride and Robertson 1997). Obtaining un-disturbed samples was seen as an integral part of developingthis framework, to directly assess the undrained response of sand to certain loading conditions. Information obtained fromundisturbed specimens regarding the in situ void ratio andstress-strain response could then be extrapolated beyond theregion where undisturbed samples had been recovered throughcorrelation with in situ testing results.

If a sample of granular soil is to be considered as trulyundisturbed, the sampling and handling processes should notalter the in situ void ratio, fabric, structure, stress history, or

degree of saturation. Changes in the void ratio of granular soilscan affect their strength and deformational properties. Sladenet al. (1985) also noted that utilizing the collapse surface con-cept as a design tool to evaluate flow liquefaction has beenseverely hampered by the lack of techniques available to con-fidently establish the in situ void ratio. Moreover, disturbanceof the in situ fabric anisotropy tends to influence the volumechange behavior of a soil during deformation. Alteration of thein situ degree of saturation can also affect the development of pore pressure and/or volume change. Therefore, disturbanceassociated with conventional sampling often results in mis-leading laboratory test results.

In approximately the last 15 years, the technique of carryingout in situ ground freezing has been demonstrated to be asuperior method for obtaining undisturbed samples of sand by

Japanese and North American researchers (Singh et al. 1982;

1Consultant, #5493 Wildwood Cr., Delta (Tsawwassen), BC, CanadaV4M 3S8.

2Prof., Dept. of Civ. and Envir. Engrg., Univ. of Alberta, Edmonton,AB, Canada T6G 2G7 (corresponding author). E-mail: [email protected]

3Prof., Dept. of Civ. and Envir. Engrg., Univ. of Alberta, Edmonton,AB, Canada T6G 2G7.

Note. Discussion open until April 1, 2001. To extend the closing dateone month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review andpossible publication on September 21, 1998. This paper is part of the

Journal of Geotechnical and Geoenvironmental Engineering, Vol. 126,No. 11, November, 2000. ASCE, ISSN 1090-0241/00/0011-0979–0989/$8.00 $.50 per page. Paper No. 19305.

Yoshimi et al. 1984, 1989, 1994; Hatanaka et al. 1995; Toki-matsu and Hosaka 1986). Provided drainage is not impededand no change in void ratio occurs during freezing, the in situstructure can be preserved, and, therefore, frozen undisturbedspecimens are valuable for assessing the liquefaction potential

of a sand deposit. Hence, for the CANLEX project, undis-turbed samples of loose sand were obtained by in situ groundfreezing with liquid nitrogen at six test sites.

This paper describes the methodology utilized to carry outin situ ground freezing and coring of frozen sand deposits atthe test sites and provides specific details with respect to theeffects of unique subsoil or ground-water conditions encoun-tered. The ground freezing systems utilized at the test sites aredescribed, the data measured during the freezing process arepresented, and the actual advance of the freezing front is com-pared with theoretical predictions. The procedures utilized forcoring of the in situ frozen sand are described. Measurementsof the in situ void ratios determined from the frozen cores arecompared with the void ratios determined by the geophysicallogging conducted within a 5-m radius of the freeze pipe. Fi-

nally, the costs associated with in situ ground freezing utilizingliquid nitrogen and coring of the frozen sand at the CANLEXtest sites are summarized.

SITE LOCATION SELECTION

For large projects where the level of risk associated withsoil liquefaction is high, site characterization is often carriedout in stages. During the preliminary stage, in situ testing isusually used to identify regions where the soil is loosest andwhere the potential for liquefaction is high. If there is uncer-tainty with respect to the prediction of liquefaction suscepti-bility and the costs associated with excessive conservatism arehigh, a more detailed study may be appropriate. This detailedstudy may include undisturbed sampling and associated high

quality laboratory testing. If sampling is cost effective andconsistent with the size and level of risk of the project, poten-tial zones for undisturbed sampling should be identified andmeasures taken to prevent any premature disturbance of thearea. When ground freezing is undertaken to obtain undis-turbed samples, it is imperative that freezing be carried outunder conditions of free drainage, to allow for pore-water ex-pulsion in advance of the freezing front. Therefore, when iden-tifying the zone where undisturbed samples are required, it isimportant to select an area without silt lenses or other lesspermeable strata, if possible. Target freezing, as will be dis-cussed later in this paper, can then be undertaken to freezediscrete zones within the deposit. The target zones for detailedcharacterization at the CANLEX test sites were selected based

J. Geotech. Geoenviron. Eng. 2000.126:979-989.


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on preliminary cone penetration testing (CPT) utilized to iden-tify the loosest zones of interest. An overview of the CANLEXproject is provided by Robertson et al. (2000).

CANLEX Test Sites

The objective of the CANLEX Phase I test site was to char-acterize a relatively loose, hydraulically placed sand depositsituated at significant depth in the southeast corner of the Mil-dred Lake Settling Basin dyke at Syncrude Canada Ltd. (SCL),north of Fort McMurray, Alberta, Canada. The target zone se-

lected for in situ ground freezing and sampling was locatedbetween depths of 27 and 37 m below the crest of the dyke.Liquid nitrogen was used to radially freeze a 2-m-diameter 10-m-long column of relatively loose sand in this region, fol-lowed by undisturbed sampling of the frozen sand utilizing adry coring technique. As described by Wride et al. (2000),various in situ testing techniques were also carried out arounda 5-m radius from the freeze pipe to fully characterize the sandin the target zone.

Natural sand deposits tend to be less homogeneous thanhydraulically placed sand, and, as a result, characterization of natural deposits can be more challenging. Phase II of theCANLEX project therefore carried out a detailed study of characterization techniques in a loose, native sand deposit.Two sites, KIDD 2 and Massey Tunnel, were selected in the

lower mainland of British Columbia, comprising relativelyloose distributary channel sands of the modern Fraser RiverDelta (Monahan et al. 1995). In situ ground freezing and un-disturbed sampling were carried out at each of the two testsites, with various in situ characterization techniques under-taken within a 5-m radius of the freeze pipe (Hofmann et al.1995; Robertson et al. 2000). Target zones with a length of 5m and containing loose sands were selected for characteriza-tion and sampling. At the KIDD 2 site, this included a rela-tively loose zone between 12 and 17 m and, at the MasseyTunnel site, a zone between 8 and 13 m.

Phase III of the CANLEX project involved a full-scale ex-periment designed to statically trigger a liquefaction flow slideby rapidly loading a loose, saturated sand deposit. The rapid

loading was carried out by pumping tailings into a reservoirbounded by a composite clay/sand containment dyke, con-structed on a 10-m-thick deposit of loose sand. The foundationsand was placed hydraulically into standing water in an aban-doned borrow pit (J-pit), at SCL. This resulted in approxi-mately 7 m of loose beach below water (BBW) sand, overlainby 3 m of slightly more dense beach above water sand. Furtherdetails regarding the scope of work for Phase III are presentedby Byrne et al. (1995) and Hofmann et al. (1996a,b).

Detailed characterization of the foundation sands was re-quired to aid in predicting whether or not the loose foundationsands would undergo a flow liquefaction failure under rapidloading conditions. Therefore, prior to rapid loading, the BBWsand was fully characterized utilizing in situ testing and sam-pling techniques that were similar to those carried out during

Phases I and II of the CANLEX project. Detailed characteri-zation was undertaken within an 8-m-radius circle located nearthe south end of the clay/sand containment dyke. To obtainundisturbed samples and to estimate the void ratio of the looseBBW sand, in situ ground freezing and frozen core samplingwas undertaken in the center of the detailed characterizationzone. Liquid nitrogen was used to radially freeze a column of loose sand between depths of 3 and 7 m below the groundsurface. The ground freezing and sampling activities carriedout at the Phase III test site are described in detail by Hofmann(1997) and Hofmann et al. (1996a).

During Phase IV of the CANLEX project, ground freezingand undisturbed sampling were also carried out at two siteslocated at the Highland Valley Copper Mine, south of Kam-

loops, British Columbia. The two sites, LL Dam and HighmontDam, comprised hydraulically placed sand deposits associatedwith the hardrock mining industry. Target zones with a lengthof 4 m were selected at each of the sites, based on CPT screen-ing. This included a relatively loose zone between 6 and 10m at the LL Dam and a zone between 8 and 12 m at theHighmont Dam. Details of the ground freezing and samplingundertaken at the Phase IV test sites are provided by Biggarand Sego (1996).

GROUND FREEZING FEASIBILITY STUDIES

Prior to undertaking in situ ground freezing to obtain un-disturbed samples of loose sand, it is necessary to confirm thatground freezing is appropriate, with respect to the subsoil andground-water conditions. To evaluate the feasibility of obtain-ing undisturbed samples by in situ ground freezing using liq-uid nitrogen, several factors should be considered. These in-clude the frost susceptibility of the soil related to the grain-sizedistribution, fines mineralogy, and unfrozen water content; thesite conditions, characterized by the overburden stress, stratig-raphy, and drainage conditions; and the ground-water temper-ature, salinity, and flow rate. Provided that drainage aroundthe zone in which in situ ground freezing is being undertakenis unimpeded by physical constraints, and that the soil is fro-zen at a rate that is slow enough compared to the hydraulicconductivity of the soil to allow for expulsion of the excess9% pore water, there is little or no disturbance of the in situvoid ratio or fabric. The most effective method of undertakenin situ ground freezing, allowing for free drainage away fromthe freezing front, involves radially freezing from a centralfreeze pipe, as will be described later in the paper. The groundfreezing feasibility studies conducted prior to undertaking insitu freezing at the CANLEX Phase I, II, and III test sites aredescribed in detail by Hofmann (1997) and by Biggar andSego (1996) for the Phase IV test sites. Potential difficultiesassociated with the site specific characteristics of the sand de-posits and/or ground-water conditions identified during thefeasibility studies are discussed below.

Subsoil Conditions

In general, grain-size analyses and mineralogical evaluationindicated that the subsoil conditions at the CANLEX test siteswere favorable for in situ ground freezing and undisturbedsampling (Hofmann 1997). Preliminary assessment of the frostsusceptibility of the sand deposits was carried out utilizing thecriteria using a chart developed by Davila et al. (1992), whichrelates the specific surface area of the fines to the frost heavesusceptibility of the soil. The chart provides an approximateboundary between sandy soils that can and cannot be frozenwithout disturbing the in situ void ratio and fabric (Sego et al.1994). Laboratory frost heave tests were also conducted onbulk samples obtained from each CANLEX test site. The con-ditions imposed on the soil during the frost heave tests were

similar to those that would exist in situ during ground freezing,including a similar temperature gradient, overburden stress,density, and drainage conditions.

It should be noted that the criteria presented by Davila etal. (1992) indirectly takes into account the phenomenon of loosely bonded water freezing in place at temperatures belowthe freezing point depression. Davila’s criterion does thisthrough consideration of the clay mineral specific surface area,which is linked to unfrozen water content (Anderson and Tice1972). In addition, direct evaluation of the frost heave suscep-tibility of a given soil in the laboratory also takes into accountthe effect of the unfrozen water film, provided that the tem-perature gradient used in the laboratory is the same as thatwhich will be used in situ (Hofmann 1997).



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FIG. 1. Effect of Ground-Water Flow on Shape of Frozen ZoneBased on Hashemi and Sliepcevich (1973)

The Phase III test site had the highest fines content; it wasanticipated that the fines content could be as high as 22%.Utilizing the grain-size information obtained from 17 SPT splitspoon samples and the results from a mineralogical evaluationcarried out on the fines fraction, the criteria proposed byDavila et al. (1992) indicated that the risk of frost heave wasnegligible. Frost heave tests were also carried out on recon-stituted samples of Phase III sand with a fines content of 22%and negligible heave was measured.

To avoid disturbance during ground freezing in sands, therate of freezing must be sufficiently slow, compared to the

hydraulic conductivity of the deposit, to allow for pore-waterexpulsion in advance of the freezing front. To estimate thehydraulic conductivity, permeability tests were carried out onreconstituted samples of Phase III sand containing 22% fines.The hydraulic conductivities measured from these tests were7.6 104 cm/s, under an overburden stress of 30 kPa, and6.2 104 cm/s, under an overburden stress of 100 kPa. Usingthe permeability test results, the maximum rate at which watercould be expelled from the freezing front was calculated to beapproximately 2.7 cm/h (Hofmann 1997). The freezing rate,at a radial distance from the freeze pipe where sampling wouldbe undertaken, was estimated to be approximately 0.4 cm/h.Therefore, even if the in situ fines content in some areas of the target zone were 22%, the relatively high permeability of

the sand would not inhibit pore-water expulsion during in situfreezing.

Ground-Water Conditions

The CANLEX Phase I, II, and III test sites had potentiallyadverse ground-water conditions that could affect the shape of the frozen zone. At the Phase I test site, ground-water seepageflows of approximately 0.25 m3 /day m2 were expected. At thePhase II test sites, due to the close proximity to the FraserRiver, the ground-water table at both sites was expected to beaffected by tidal fluctuations. It was also expected that thegroundwater at the Phase II test sites could be saline due tointrusion of seawater into the Frazer River. At the Phase IIItest site the tailings sand was placed hydraulically into the J-

pit to form the foundation sand, with an initial placement tem-perature of about 60C. In situ freezing and sampling wasscheduled to take place approximately 2 months after place-ment of the foundation sands, thereby allowing only minimaltime for cooling of the groundwater.

Each of these adverse ground-water conditions were eval-uated with respect to the heat extraction capability of liquidnitrogen and the ability to freeze sufficient volumes of sand toallow for sampling. Evaluation of the effect of the ground-water flows on the shape of the frozen zone at the Phase I andII test sites will be discussed in this section. The effect of highinitial ground-water temperatures will be discussed later.

To estimate the effects of seepage at the Phase I test site,data presented by Hashemi and Sliepcevich (1973) regardingthe effect of ground-water flow on the shape of the frozen zone

was reviewed. Using a finite-difference technique, Hashemiand Sliepcevich estimated the deformed shape of a frozen col-umn around a freeze pipe for different ground-water flow ve-locities, assuming freezing would be undertaken with brine ata temperature of 35C. The flow velocities considered byHashemi and Sliepcevich were in the order of the ground-water flow expected at the Phase I test site.

A comparison of the shape of the frozen zone predicted forno ground-water flow and for a ground-water flow of 0.4 m3 / day m2 is shown in Fig. 1, assuming a coolant temperature of 35C. Fig. 1 confirms that when ground-water flow exists,the frozen zone tends to be elongated in the downstream di-rection and foreshortened in the upstream direction. However,for a ground-water flow of 0.4 m3 /day m2, which is slightly

greater than that expected at the Phase I test site, the totalfrozen volume is not reduced significantly nor is the shapedistorted sufficiently to restrict sampling. Furthermore, asshown by Hofmann (1997), the critical ground-water flow ve-locity, under which groundwater cannot be frozen in place, is5 times greater when liquid nitrogen (196C) is used as thecoolant than when brine (35C) is used.

To account for some elongation of the frozen column in thedownstream direction at the Phase I test site, the boreholes forsampling were advanced slightly downstream of the freezepipe. Resistance temperature measurement devices (RTDs)were installed at the top of the frozen zone in cased boreholesfor sampling, to confirm that the sand was frozen prior to

recovering samples. In addition, RTDs were installed throughthe target freezing zone in two small diameter boreholes, lo-cated at radial distances of 0.6 and 1.0 m from the freeze pipe.

At the Phase II test sites, to evaluate the effect of tidalfluctuations on the shape of the frozen zone, it was necessaryto estimate the associated ground-water flow velocities. TheKIDD 2 site was located approximately 200 m south of thenorth arm of the Fraser River, and the Massey Tunnel site waslocated approximately 200 m south of the main channel of theFraser River. The average ground-water level measured at thesites were approximately 1.5 m below the ground surface. Ac-cording to locally available information, tidal fluctuations atthe mouth of the Fraser River, during the time of the proposedfieldwork, would average approximately 0.6 m/h. This wouldresult in a change in the river level of 4.8 m/8-h tidal period.

Because the sand deposits were relatively clean, the Hazenformula (Hazen 1911) was used to calculate an upper estimateof the permeability. Using a permeability of 1 102 cm/s,the ground-water flow velocity resulting from the rise and fallof the river level during each tidal period was estimated to bein the order of 0.2–0.4 m/day. However, because the directionof ground-water flow between the river and the test site wouldbe reversed every 8 h and ground freezing would be carriedout over a 200-h period, it was not expected that the frozencolumn would be elongated in any one direction.

The effect of the saline groundwater at the Phase II test siteswas not expected to be significant. Seawater freezes at a tem-perature of approximately 1.8C (Andersland et al. 1991).However, the freezing point depression caused by saline pore



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FIG. 2. Schematic Diagram of Typical Freeze Pipe andSampleBorehole Configuration

water is negligible with respect to the heat extraction capabil-ity of liquid nitrogen as it changes phase at 196C. More-over, the sand encountered at the Phase II test sites had a lowproportion of fines and, therefore, would have an insignificantunfrozen water content (Hofmann 1997).

GEOMETRY OF FREEZING AND SAMPLING SYSTEM

To allow for unimpeded drainage in advance of the freezingfront, the most effective way of undertaking in situ groundfreezing involves radially freezing a column of soil around

the freeze pipe that extends through a target zone of interest.When liquid nitrogen, with a phase change temperature of 196C, is injected into the freeze pipe it absorbs heat fromthe surrounding ground and undergoes a phase change togas. To allow sufficient room for sampling, experience at theCANLEX test sites indicated that it is necessary to freeze ap-proximately a 1-m radius of sand to allow sufficient room forsampling without the risk of unfrozen sand flowing into theboreholes during coring. Sampling can be carried out in sev-eral large diameter (260 mm) boreholes advanced with thecenterline at about 0.6 m from the freeze pipe, thus avoidingthe 100-mm zone around a 50-mm-diameter freeze pipe thatcould be disturbed due to installation of the freezing system(Yoshimi et al. 1978). This also allows for inaccuracies re-garding borehole verticality. Fig. 2 shows the general layout

of the liquid nitrogen ground freezing and sampling systemused for the CANLEX project.

To utilize liquid nitrogen efficiently, a freeze pipe was de-signed at the University of Alberta (Sego et al. 1994; Hofmann1997) with a reservoir for the liquid nitrogen correspondingto a target zone of interest, where undisturbed samples arerequired. Hence, freezing does not take place outside the zoneof interest. Using a 51-mm-diameter steel freeze pipe, a 13-mm-diameter copper pipe can be used to feed liquid nitrogeninto the reservoir and a second copper pipe can be installed tovent gaseous nitrogen back into the atmosphere, with the twopipes being separated at the top of the reservoir by a rubberseal. The length and location of the freezing target zone canbe altered in the field, depending upon the actual ground con-ditions encountered, by simply moving the location of therubber seals within the freeze pipe. Fig. 2 shows a cross sec-tion through a freeze pipe, designed for target freezing. Thissystem was utilized at the CANLEX test sites; therefore, itwas possible to identify discrete zones of interest at each sitefor detailed characterization and undisturbed sampling.

Because the zone where samples are required is often atsome depth below the ground surface, the overlying unfrozensoil can usually be removed with a drilling rig prior to sam-pling. At the CANLEX test sites, wet rotary coring was un-dertaken to install a large diameter casing to just above thetarget sampling zone. Once the casing was installed, provisionswere made to allow for sampling through a dry borehole. Drill-ing fluid inside the casings was replaced with water, and thebottom of each of the boreholes was sealed with bentonite.

After ground freezing was complete, the water in the sampleboreholes was blown out so that sampling could be carried outin a dry borehole, with the bottom frozen approximately0.5 m into the targeted sampling zone. Complete descriptionsregarding the freezing and sampling systems used for theCANLEX project are provided by Hofmann (197). Specificdetails regarding difficulties that were overcome with respectto installation of the ground freezing and sampling systemsutilized at four of the CANLEX test sites are presented below.

Ground Freezing and Sampling Systems Used atTest Sites

Each of the target sampling zones at the CANLEX test siteswas located at different depths. In response to the various den-

sities and overburden stresses at the sites, the freeze pipes wereinstalled utilizing different techniques, as discussed below.

Phase I Test Site

The target freeze zone at the Phase I test site was locatedat a significant depth between 27 and 37 m below the crest of the dyke. Therefore, the material overlying the target zone waspredrilled to a depth of 26.5 m, and casings were installed tofacilitate installation and operation of the ground freezing sys-

tem, as well as sampling in the target zone. To allow for in-stallation of the freeze pipe and to provide sufficient clearancefor the core barrel in the sampling boreholes, large diameterboreholes (approximately 260 mm) were advanced to 26.5 mbelow the ground surface and PVC casing was installed.

The borehole layout included a central freeze pipe with fourboreholes for sampling of the frozen core located within a 1-m radius of the freeze pipe. One borehole was advanced andcased to the top of the target zone for later installation of thefreeze pipe. Four boreholes were then advanced and cased atradial distances of approximately 0.5–0.6 m, measured fromthe centerline of the freeze pipe, for later coring of the frozensand. To reduce the risk of nonvertical boreholes, the drillingrig was leveled, and the drill was centered on the drill table



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prior to commencing drilling. To confirm that the bottom of each of the cases sampling boreholes fell within a 1 m radiusof the freeze pipe, the deflection of each borehole casing wasmapped with a Digitilt Inclinometer. The grooved inclinometercasing was installed in the large diameter PVC casing fromthe surface down to the top of the target zone. Plastic cen-tralizers were used to keep the inclinometer casing in the cen-ter of each borehole. A Digitilt Inclinometer was then lowereddown each borehole, and the actual alignment and location of each of the borehole casings were mapped.

For later installation of the freeze pipe inside the large di-

ameter cased borehole at the center of the test site, a steelcasing, with a diameter of approximately 150 mm (6 in.), wasdrilled through the bentonite seal, at the base of the PVC cas-ing, to a depth of 26.5 m. The steel pipe was installed toprevent problems associated with the bentonite seal squeezinginto the small diameter borehole advanced through the targetzone for the freeze pipe. The borehole for the freeze pipe wasadvanced through the target zone to a depth of 37 m, utilizinga 67 mm (2 5/8 in.) tricone drilling bit. The 51-mm-diametersteel freeze pipe was then lowered into the borehole, where itwas positioned between 27 and 37 m below the ground sur-face. To provide a reservoir for the liquid nitrogen in the targetzone, a rubber seal was installed in the freeze pipe at a depthof 26 m. Thus, the bottom 0.5 m of each sample casing would

be frozen into the top of the sampling zone. To allow forinjection of liquid nitrogen into the reservoir and ventilationof gaseous nitrogen after phase change, a 13-mm-diametercopper inflow and outflow pipes were extended from therubber seal to the ground surface. A schematic illustration of the freeze pipe is shown in Fig. 2. A more detailed descriptionof the freeze pipe installation at the Phase I test site is providedby Hofmann (1997).

RTDs were utilized for monitoring temperatures within andaround the freezing system. After completion of the freezepipe installations, RTDs were installed near the top and bottomof the liquid nitrogen reservoir in the freeze pipe, at depths of approximately 29.5 and 34.8 m. This allowed for monitoringof liquid nitrogen levels inside the reservoir during freezing.RTDs were also installed at the top of the target zone in each

of the sample boreholes to confirm that the sand at the baseof the casing was frozen prior to commencing coring. Forinstallation of RTDs throughout the target zone, two small di-ameter boreholes were advanced at radial distances of approx-imately 0.5 and 1.0 m from the freeze pipe, to monitor theprogress of the freezing front.

Phase II Test Sites

Considering the relatively shallow target zones at the PhaseII test sites, two options were available for sampling of the insitu frozen sand. One option included freezing from the groundsurface to the base of the target zone and then coring throughthe frozen overburden into the target zone. The second optioninvolved freezing in the target zone only and installing cased

boreholes through the unfrozen soil from the ground surfaceto the top of the target zone, in a manner similar to that usedat the Phase I site. At both the KIDD 2 and Massey Tunneltest sites, cost analyses carried out during the feasibility studyindicated that it would be more economical to advance casedboreholes to the top of the target zone for sampling of the insitu frozen core.

At the center of each of the Phase II test sites, a 230–240-mm-diameter borehole was advanced and cased to the top of the target zone. A 51-mm-diameter steel freeze pipe was thenlowered down the cased borehole and jetted into the targetzone, in a manner similar to that used by Sego et al. (1994)at the Duncan Dam. This was done by flushing water througha 25-mm-diameter internal jetting pipe, with a perforated tip,

set just inside the end of the 51-mm freeze pipe. Care wastaken not to hydraulically fracture the ground by exceedingthe minimum principal stress. As the freeze pipe was jettedinto place, the sand cuttings were washed up the annulus be-tween the jetting tool and the inside wall of the freeze pipe.Once the freeze pipe was in place and the jetting tool wasremoved, a 0.35-m-long bentonite seal was placed at the bot-tom of the freeze pipe, and the water inside the pipe was blownout. Reservoirs for the liquid nitrogen in the freeze pipe wereestablished in the target zones utilizing rubber seals and copperpiping as described previously. To improve control of the liq-

uid nitrogen levels within the freeze pipe, a back pressurevalve was installed on the copper exhaust pipe. Sample bore-holes and boreholes for monitoring ground temperatures dur-ing freezing were installed utilizing the same technique as thatdescribed for the Phase I test site.

Phase III Test Site

Prior to installing the ground freezing system, SPTs andCPTs conducted an 8-m radius from the center of the circle,where the freeze pipe would be installed, confirmed that thesand was very loose. The normalized SPT and CPT data in-dicated that the average ( N 1)60 was about 3 and the average qc1was about 2.4 MPa. Given the loose subsoil conditions andthe shallow target zone, the freeze pipe, equipped with a conetip at the bottom end, was slowly pushed directly into place.A 150-mm-diameter PVC casing was first installed through thedenser upper 2.7 m of sand. A 51-mm-diameter steel freezepipe was then slowly lowered down through the center of thecasing and pushed to a depth of 6.9 m, utilizing a drilling rig.The resistance to pushing the freeze pipe into place was min-imal.

Phase IV Test Sites

At the Phase IV test sites the freeze pipes were installed bypredrilling 65-mm boreholes with a wet rotary drill rig to justbelow the target zones at each site. At the previous test sites,the copper inflow pipes extended to the bottom of the freeze

pipe reservoir. However, this resulted in a tendency for theliquid nitrogen to accumulate in only the lower part of thereservoir during the initial stages of freezing. In an attempt toovercome the problems associated with liquid nitrogen poolingat the bottom of the freeze pipe, the copper inflow pipes weremodified.

At the Phase IV test sites, two 13-mm inflow pipes wereinstalled. The first pipe extended to the midpoint of the targetzone and the second pipe extended to the bottom of the targetzone. Both inflow pipes were perforated along their length toallow liquid nitrogen to spray onto the inside wall of the freezepipe. The initial intent was to regularly alternate between in-flow pipes. However, as freezing progressed, it became evidentthat the most effective way to proceed was to utilize the upperinflow pipe only. This resulted in liquid nitrogen being sprayed

against the inside wall of the upper portion of the freeze pipeand then running down the wall to the bottom (Biggar andSego 1996). This method reduced the preferential freezing atthe bottom of the target zone that occurred at the previous testsites.

PREDICTION OF HEAT EXTRACTIONREQUIREMENTS

The heat extraction requirements govern the volume of liq-uid nitrogen required to freeze a column of soil at a specificsite. Heat conduction theory may be used to determine the heatextraction requirements and length of time necessary to freezea given radius of soil. In the development of the equation



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FIG. 3 (a). Growth of Frozen Zone at CANLEX Phase I Test Site

FIG. 3 (b). Liquid Nitrogen Consumption at CANLEX Phase ITest Site

describing heat extraction, Sanger and Sayles (1979) assumedthat the isotherms move slowly enough to resemble steady-state conditions to avoid problems associated with the discon-tinuity that exists at the boundary between the frozen and un-frozen soil column around the freeze pipe. This discontinuityexists due to the change in thermal conductivity and specificheat capacity that occurs with a change in phase. Alternatively,a transient solution may be obtained using numerical analysis.However, large-scale field experiments and the ground freez-ing undertaken at the CANLEX test sites indicated that, al-though the assumption of steady-state conditions is not strictly

correct, steady-state heat conduction theory predicts well theaverage frozen radius and the liquid nitrogen requirements.Moreover, the equations can be set up on a spreadsheet thatcan be used to modify predictions on-site, in the event that theactual field conditions differ from those originally assumedduring the feasibility studies. As will be discussed later in thepaper, difficulties associated with liquid nitrogen flow ratesmay require revisions of the volume predictions.

Based on thermal heat flow equations developed by Sangerand Sayles (1979), predictions were made of the time and vol-ume of liquid nitrogen required to freeze a 1-m-radius columnof sand through the target zones at each of the CANLEX testsites. Example predictions for the Phase I test site will be givenhere. Detailed information regarding predictions made for the

Phase II and III test sites are provided by Hofmann (1997)and Hofmann et al. (1995), and for the Phase IV test sites byBiggar and Sego (1996).

At the Phase I test site, the growth of the frozen columnwas predicted by solving the steady-state heat flow equationsrepeatedly for 0.25-m radial increments up to 1.0 m. The com-putations were based on available information regarding thesite conditions and estimated thermal properties for the sandencountered at the site. The following input parameters wereused for the theoretical predictions:

3k = 1.45 J/s m K; k = 2.8 J/s m K; = 1.80 Mg/mu f d

3 3C = 3.17 MJ/m K; C = 2.34 MJ/m K; = 22%u f

where k u and k f = unfrozen and frozen thermal conductivities,

respectively; C u and C f = unfrozen and frozen volumetric heatcapacities, respectively; d = dry density; and = moisturecontent. The values of k u and k f were estimated based on thecharts published by Farouki (1981, 1986), giving the thermalconductivity for granular soils as a function of moisture con-tent, density, and quartz content. According to Farouki (1981),the thermal conductivity of granular soil is strongly dependentupon quartz content.

The energy and time requirements for undertaking groundfreezing at the Phase I site were first evaluated neglectingground-water flow. The ground-water temperature measure-ments at the Phase I test site indicated an average ground-water temperature of approximately 15.5C. Calculations in-dicated that the estimated time required to freeze the columnof sand in the target zone would be about 13 days (300 h).

Fig. 3(a) shows the predicted growth of the frozen radius withtime, assuming no ground-water flow, with a heavy dashedline.

The effect of ground-water flow was estimated, based on areview of the transient heat conduction modeling carried outby Hashemi and Sliepcevich (1973) for seepage velocities inthe order of those expected at the Phase I site. As discussedin the previous section, elongation of the frozen zone in thedownstream directions was not expected to be significant,given a ground-water flow velocity of approximately 0.25 m3 / day m2. Therefore, the ordinate in Fig. 3(a) was estimated torepresent an approximate average of the elongated downstreamfrozen radius and foreshortened upstream frozen radius. Tak-ing into account slight elongation of the frozen zone in the

downstream direction, three of the four sample boreholes wereinstalled downstream of the freeze pipe.

The volume of liquid nitrogen required to freeze a 1-m-

radius column of sand was determined by dividing the totalenergy extracted from the ground by the volumetric latent heatof liquid nitrogen (0.1098 MJ/m3). Fig. 3(b) shows the totalpredicted volumes of liquid nitrogen required for initialground-water temperature of 15.5C, with a heavy dashed line.The total predicted volume of liquid nitrogen was about107,000 m3 (equivalent gaseous volume). It was recognizedthat the volume of liquid nitrogen required to freeze a 1-mradius may be greater if ground-water flow velocities wouldbe significantly higher than anticipated. Discussion regardingcomparisons between the measured and predicted rates of freezing are given in a later section.

MONITORING GROUND TEMPERATURESDURING FREEZING

The rate of advance of the freezing front, the associatedground temperature changes, and the rate of liquid nitrogenconsumption should be monitored during ground freezing.This allows for early detection of any problems. At the PhaseI test site, ground freezing was carried out over a 12-day pe-riod. During that time the RTDs installed at the top of thetarget freeze zone in each sample borehole and those installedin small diameter boreholes, advanced for installation of RTDstrings through the target zone, were monitored every 6 h.

Based on the temperature measurements from all of theRTDs, a cross section of the frozen zone was established. Thecross section indicated that, after 258 h of freezing, the frozencolumn extended beyond all of the sample borehole locations



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except borehole FS1, located 0.76 m upstream of the freezepipe. To avoid problems with ground-water inflow during cor-ing of the in situ frozen sand, liquid nitrogen was fed directlyinto the bottom of sample borehole FS1. After 24 h, the ma-terial between the column of sand frozen radially and the bot-tom of borehole FS1 had been frozen and coring of boreholeFS1 commenced. This technique allowed for sampling of thefrozen sand column located at a radial distance of 0.76 m fromthe freeze pipe. However, only the sand core extracted at thislocation from a depth of >29 m, where the sand column hadbeen frozen radially from the freeze pipe, was considered to

be undisturbed.

MONITORING LIQUID NITROGEN CONSUMPTION

The consumption of liquid nitrogen was monitored every 6h during ground freezing. The rate of usage of liquid nitrogenwas reviewed to check for any indication that problems maybe developing. Approximately six tanks, containing 15,000 m3

(equivalent gaseous volume) of liquid nitrogen, were requiredto complete ground freezing and coring at the site. Fig. 3(b)compares the actual volume of liquid nitrogen used duringfreezing (shown as a solid black line) with two predictionsbased on either 100% liquid nitrogen (N2) in the freeze pipeor 33% nitrogen gas and 66% liquid nitrogen. Discussion of the comparisons is given in the following section.

COMPARISON OF ACTUAL FREEZING PROCESSWITH THEORETICAL PREDICTIONS

At the Phase I test site, the actual growth of the frozenradius, based on RTD measurements, was compared with theinitial theoretical predictions. Initially, predictions were madeassuming that the freeze pipe reservoir in the target zone wascompletely full of liquid nitrogen at all times during freezing.Fig. 3(a) shows a comparison between the actual data and thetheoretical predictions, based on an approximate average initialground temperature of 15.5C. The figure indicates that theactual frozen radius at a depth of 27 m was less than thepredicted frozen radius. This discrepancy was caused by dif-

ficulties in keeping the phase change interface of the liquidnitrogen at the top of the target zone. Due to insufficient flowof the liquid nitrogen into the reservoir, compared to the rapidheat absorption from the surrounding ground, it was not pos-sible to completely fill the 10-m-long freeze pipe reservoirwith liquid nitrogen. As a result, slower growth of the frozenzone occurred near the top of the target zone, where the freezepipe contained only gaseous nitrogen. In this region coolingtook place due to the volumetric heat capacity of gaseous ni-trogen.

More rapid growth occurred at the bottom of the target zonewhere the freeze pipe was filled with liquid nitrogen. In thelower portion of the target zone, the actual maximum frozenradius, at a depth of 37 m, exceeded the theoretical predictionmade assuming that liquid nitrogen was contained within the

freeze pipe reservoir at all times. The conservatism associatedwith the theoretical equation given by Sanger and Sayles(1979) to predict growth of the frozen radius with time resultsfrom two factors. First, the theory assumes that steady-stateconditions have been attained, whereas, during the initialstages of freezing, the growth of the frozen radius progressesmore quickly than steady-state growth as a result of the highheat extraction capacity of liquid nitrogen. Second, the frozenthermal conductivity used for the theoretical prediction wasdetermined based on Johansen’s method (Johansen 1975; Far-ouki 1986), which neglects the temperature dependence of thisparameter. At temperatures close to that at which liquid nitro-gen undergoes a phase change (196C), the frozen thermalconductivity is probably much higher than that used for the

theoretical prediction, resulting in more rapid growth of thefrozen radius. However, the slight inaccuracy of the theoreticalpredictions did not warrant the expense and difficulties asso-ciated with determination of the frozen thermal conductivityunder the extreme conditions at 196C.

The predicted volume of liquid nitrogen required to freezea 1-m radius was also compared with the actual volume of liquid nitrogen used during ground freezing, as shown in Fig.3(b). The figure indicates that the total volume of liquid nitro-gen used, about 90,000 m3 (equivalent gaseous volume), wasless than the predicted total volume of 107,000 m3 (equivalent

gaseous volume), assuming that the reservoir was full of liquidnitrogen at all times during freezing. This was mainly due tothe fact that freezing was terminated before the frozen radiusextended to 1.0 m. The final frozen radius, just prior to com-mencing coring, was approximately 0.91 m at a depth of 30m. Fig. 3(b) also shows that the actual rate of liquid nitrogenconsumption was considerably less than the predicted rate.This appeared to be due to the fact that, during the initialstages of ground freezing, the flow rate of liquid nitrogen intothe freeze pipe was not high enough to maintain a full liquidnitrogen level in the reservoir at all times during freezing.

To confirm this hypothesis, a revised prediction was made.Based on temperature measurements from RTDs that had beeninstalled near the top and bottom of the freeze pipe reservoir,

it was estimated that on average, during the initial stages of the freezing process, the phase change of liquid nitrogen re-sulted in 2/3 of the contribution to the total heat extractionand conductive heat loss to gaseous nitrogen resulted in 1/3of the contribution to the total heat extraction. It was possibleto increase the flow of liquid nitrogen during the later stagesof in situ ground freezing; hence, the predictions were furthermodified to consider two stages in the freezing process usingthe following equation:

2 2V = P R { L ((a 1)/2 ln(a ))C (v ) C v /(2 ln( R / r ))}LN2 1 r r u 0 LN2 f s 0

L LN2

2 2 P R { L ((a 1)/2 ln(a ))C (v ) C v /(2 ln( R / r ))}2 r r u 0 GN2 f s 0

C GN2v g(1)

where P 1 = proportion of liquid nitrogen contained within thefreeze pipe; LLN2 = latent heat of liquid nitrogen (MJ/m

3); P 2= proportion of gaseous nitrogen contained within the freezepipe; C GN2 = volumetric heat capacity of gaseous nitrogen (MJ/ m3 K); v g = increase in temperature of the gas nitrogen dueto absorption of heat from the surrounding ground (K); (v 0)LN2= initial temperature of the liquid nitrogen (K); and (v 0)GN2 =initial temperature of the gaseous nitrogen (K).

The revised predictions are also shown in comparison withthe actual data in Figs. 3(a and b). The revised predicted frozenradius agrees well with the actual frozen radii measured in

sample boreholes FS3, FS4, and FS5, at the top of the targetzone where the freeze pipe was probably filled with gaseousnitrogen during at least the initial stages of the freezing pro-cess. After approximately 192 h of freezing, it was possible toincrease the flow rate, such that the entire freeze pipe reservoirin the target zone was completely full of liquid nitrogen. Here,the prediction marks the average of the frozen radii measuredat 27 and 37 m. Fig. 3(b) shows good agreement between theactual and revised predictions of liquid nitrogen consumptionduring approximately the first 192 h of freezing. During thefirst stages of freezing, the actual consumption rate follows theprediction made assuming that the freeze pipe was 2/3 full of liquid nitrogen. Corresponding to an increased level of liquidnitrogen in the freeze pipe after 192 h of freezing, the actual



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TABLE 1. Costs Associated with Carrying Out In Situ Ground Freezing at CANLEX Test Sites

Phase

(1)

Site

number

(2)

Core Length (m)

100-mm

diameter

(3)

200-mm

diameter

(4)

Total

(5)

Core volume

(cm3)

(6)

Total cost

(dollars)

(7)

Unit Costs

Site

(dollars/site)

(8)

Length

(dollars/m)

(9)

Volume

(dollars/m3)

(10)

I 1 20 0 20 157,080 94,200 94,200 4,710 0.60II 2 40 0 40 314,159 100,100 50,050 2,503 0.32III 1 3.9 6.9 10.8 247,400 48,000 48,000 4,444 0.19IV 2 31 3 34 337,721 81,822 40,911 2,407 0.24

data agree with the prediction made assuming that the freezepipe was completely full of liquid nitrogen.

However, although it may be possible to determine the rel-ative proportions of gas and liquid nitrogen by installing tem-perature measurement devices that can withstand the temper-atures and turbulence associated with liquid nitrogen in thefreeze pipe, it is preferable to freeze the soil in the form of acylinder by maintaining only liquid nitrogen in the freeze pipe.Freezing a uniform column of soil reduces the risk of distur-bance of the soil, due to subvertical freezing fronts, and fa-cilitates coring of the frozen sand upon completion of in situ

ground freezing. To avoid difficulties associated with coringof the frozen sand and inaccurate prediction of the liquid ni-trogen requirements, modifications to the freezing system wereundertaken at the Phase I test site so that the reservoir couldbe filled completely with liquid nitrogen. Utilizing higher liq-uid nitrogen flow rates and installation of a valve on an ex-haust pipe, to apply a slight back-pressure, aided in keepingthe freeze pipe reservoir consistently full of liquid nitrogenduring freezing.

CORING SYSTEMS UTILIZED FOR SAMPLING INSITU FROZEN SAND

Coring of in situ frozen sand was undertaken utilizing aCold Regions Research Engineering Laboratory core barrel

with a dry coring technique. Both 100- and 200-mm-diametercore barrels can be used. Under conditions where the in situstresses and ground conditions permit, the 200-mm-diameterfrozen core yields three to four times as many triaxial speci-mens, as three to four adjacent specimens can be trimmed fromthe same depth interval. The diameter of the Cold RegionsResearch Engineering Laboratory core barrel used at eachCANLEX test site depended upon the extent of the frozenzone after the budgeted volume of liquid nitrogen had beenused, the depth of the deposit, and the associated in situstresses. A 100-mm-inner-diameter core barrel was used at thePhase I and II test sites and a 200-mm-diameter core barrelwas also utilized at the Phase III and IV test sites. Detailsregarding coring undertaken at each of the test sites is pro-vided by Hofmann (1997), Hofmann et al. (1994, 1995,1996a), and by Biggar and Sego (1996). The general techniqueutilized at the CANLEX test sites is described below.

The core barrel was easily advanced through the frozen sandusing a dry coring technique with a tungsten carbide tippedcutting shoe. Core catchers were installed at the bottom of thecore barrel to prevent loss of the frozen core as the barrel wasbrought to the ground surface. To avoid problems associatedwith melted slough accumulating at the base of the boreholes,they were periodically cleaned out with a 260-mm tricone drillbit. Core runs that were approximately 0.6 m in length wererecovered, and the frozen core was extruded directly into plas-tic bags at the ground surface utilizing a hydraulic core ex-truder. Immediately after extrusion, the frozen core was placedin an insulated box filled with dry ice, to prevent premature

thawing of the outside perimeter of the frozen core. Thelengths and depths of the core samples were recorded, and thesamples were then stored in freezers between layers of insu-

lation and dry ice. At the end of the fieldwork, the frozen coresamples were transported to the laboratory, in a conventionalfreezer filled with dry ice, where they were carefully placedin a cold room for detailed logging and preserved for long-term storage.

No visual evidence of disturbance due to frost heave or dueto freezing and expansion of pore water was noted in any of the frozen core specimens obtained from the CANLEX testsites. A total of approximately 20 m of 100-mm-diameter insitu frozen core was recovered from each of the Phase I andII test sites. At the Phase III test site, 6.4 m of 200-mm-di-

ameter core and 3.9 m of 100-mm-diameter core were recov-ered. At the Phase IV sites, 31 m of 100-mm-diameter coreand 3 m of 200-mm-diameter core were recovered. A summaryof the volume of undisturbed samples obtained by in situground freezing at each site is provided in Table 1.

MEASUREMENT OF IN SITU VOID RATIOS

Frozen core specimens obtained from each of the test siteswere trimmed for either triaxial, or simple shear testing, uti-lizing a lathe. To prevent premature thawing of the specimens,trimming was carried out in a walk-in freezer. At this time,estimates of the in situ void ratios were made based on frozencubes trimmed from one end of the frozen cores. These voidratio measurements served as a guideline to determine which

of the frozen core specimens trimmed for laboratory testinghad the highest void ratios. At the completion of laboratorytesting, the total dry weight of the failed specimens and thevolume change measurements taken during thawing and shear-ing could also be used to back-calculate the actual initial voidratios of the specimens. Further details regarding measurementof the in situ void ratios from frozen core specimens is pro-vided by Hofmann (1997).

The void ratios measured from the frozen core trimmingswere compared with the in situ void ratios determined inde-pendently from geophysical logging undertaken within a 5–8-m radius of the freeze pipe at each test site. The geophysicallogging was carried out using a downhole nuclear density tech-nique with a tool that measures gamma-ray scattering as afunction of bulk density. Details of the geophysical loggingare given by Küpper et al. (1995). In general, at each of thetest sites, the void ratios established from frozen core samplesagreed very well with those measured in situ utilizing geo-physical logging. Fig. 4 shows a comparison between the geo-physical and frozen core void ratio measurements taken at theMassey Phase II test site. Very good agreement is evident, andboth sets of void ratio measurements indicate significantvariations in density vertically and horizontally (Wride et al.2000b). Details regarding similar comparisons at each of theother CANLEX test sites is provided by Robertson et al.(2000). The target zone at the Massey site was located between8 and 13 m, as shown in Fig. 4. The bold, nearly vertical lineshown in Fig. 4 represents the approximate location of thereference ultimate state line (Robertson et al. 2000). The scale

of void ratios shown in the figure represents the approximatevalues of emax and emin; hence, Fig. 4 can be used to estimatethe in situ relative density.



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zen core specimens were obtained from each of the test sitesfor laboratory testing.

A summary of the total costs incurred at each of theCANLEX test sites indicated that in situ ground freezing andundisturbed sampling of a 5-m-thick target zone can be un-dertaken at most sites for approximately $50,000; this corre-sponds to a unit cost per volume of frozen sample of about$0.32/cm3. The cost of ground freezing to obtain undisturbedsamples of granular soils is high. However, for large projectswhere the level of risk associated with soil liquefaction is high,the cost of undisturbed sampling and laboratory testing may

be appropriate and cost effective. This is particularly true incases where the prediction of liquefaction is uncertain, and thecost associated with excessive conservatism is high. A recentcase history illustrates this point. In a recent study of theDuncan Dam, BC Hydro was faced with possible remediationcosts of about $24,000,000 due to possible liquefaction of thefoundation sands. A detailed study was carried out that in-cluded ground freezing to obtain undisturbed samples and highquality laboratory testing. The findings of the detailed studyresulted in a significant reduction in the cost and amount of remediation (Little et al. 1994).

ACKNOWLEDGMENTS

This work was partly supported by CANLEX, a project funded through

a Collaborative Research and Development Grant from the Natural Sci-ence and Engineering Research Council of Canada, SCL, Suncor Inc.,Highland Valley Copper, BC Hydro, Hydro Quebec, and Kennett Cor-poration. The collaboration also includes the following engineering con-sulting companies: EBA Engineering Consultants Ltd., Klohn-CrippenConsultants Ltd., AGRA Earth and Environmental Ltd., GolderAssociatesLtd., and Thurber Engineering Ltd., as well as faculty, staff, and studentsfrom the Universities of Alberta, British Columbia, Laval, and Carelton.

APPENDIX I. REFERENCES

Andersland, O., et al. (1991). ‘‘Mechanical and thermal design of frozensoil structures.’’ Proc., Int. Symp. on Ground Freezing, ISGF—WorkingGroup 2.

Anderson, D. M., and Tice, A. R. (1972). ‘‘Predicting unfrozen watercontents in frozen soils from surface area measurements.’’ Hwy. Res.

Rec. 393, 12–18.Biggar, K. W., and Sego, D. C. (1996). ‘‘In-situ ground freezing and

sampling.’’ CANLEX Tech. Rep., Phase IV , University of Alberta, Ed-monton, Alta, Canada.

Broms, B. B. (1980). ‘‘Soil sampling in Europe: State-of-the-art.’’ J. Geo-tech. Engrg. Div., ASCE, 106(1), 65–98.

Byrne, P. M., Robertson, P. K., Plewes, H. D., List, B. R., and Tan, S.(1995). ‘‘Liquefaction event planning.’’ 49th Can. Geotech. Conf., Ca-nadian Geotechnical Society, Alliston, Ontario, 341 –352.

Davila, R. S., Sego, D. C., and Robertson, P. K. (1992). ‘‘Undisturbedsampling of sandy soils by freezing.’’ 45th Can. Geotech. Conf., Ca-nadian Geotechnical Society, Alliston, Ontario, 13A-1–13A-10.

Farouki, O. T. (1981). ‘‘Thermal properties of soils relevant to groundfreezing. Design techniques for their estimation.’’ Proc., 3rd Int. Symp.on Ground Freezing, U.S. Army Corps of Engineers, CRREL, 139–146.

Farouki, O. T. (1986). Thermal properties of soils, Trans TechPublications, Vol. 11, Clausthal-Zellerfeld, Germany.

Hashemi, H. T., and Sliepcevich, C. M. (1973). ‘‘Effect of seepage stream

on artificial soil freezing.’’ Proc., Ground Freezing Conf.: Devel. inGeotech. Engrg., Vol. 28, 189–201.

Hatanaka, M., Sugimoto, M., and Yoshio, S. (1985). ‘‘Liquefaction resis-tance of two alluvial volcanic soils sampled by in-situ freezing.’’ Soilsand Found., Tokyo, 25(3), 49–63.

Hatanaka, M., Uchida, A., and Oh-Oka, H. (1995). ‘‘Correlation betweenthe liquefaction strengths of saturated sands obtained by in-situ freezingmethod and rotary-type triple tube method.’’ Soils and Found ., Tokyo,35(2), 67–75.

Hazen, A. (1911). ‘‘Discussion of ‘Dams on sand foundations,’ by A. C.Koenig.’’ Trans. ASCE , 73, 199–203.

Hight, D. W., and Georgiannou, V. N. (1995). ‘‘Effects of sampling onthe undrained behaviour of clayey sands.’’ Géotechnique , London,45(2), 237–247.

Hofmann, B. A. (1997). ‘‘In-situ ground freezing to obtain undisturbedsamples of loose sand for liquefaction assessment.’’ PhD thesis, Dept.

of Civ. and Envir. Engrg., University of Alberta, Edmonton, Alta., Can-ada.

Hofmann, B. A., et al. (1996a). ‘‘CANLEX Phase III—Full scale lique-faction test: site characterization.’’ Proc., 49th Can. Geotech. Conf.,Canadian Geotechnical Society, Alliston, Ontario, 579–586.

Hofmann, B. A., Robertson, P. K., Gräpel, C. K., Cyre, G., Lefebvre, M.,and Natarajan, S. (1996b). ‘‘CANLEX Phase III—Full scale liquefac-tion test: Instrumentation and construction.’’ Proc., 49th Can. Geotech.Conf., Canadian Geotechnical Society, Alliston, Ontario, 587–596.

Hofmann, B. A., Sego, D. C., and Robertson, P. K. (1994). ‘‘Undisturbedsampling of a deep, loose sand deposit using ground freezing.’’ Proc.,47th Can. Geotech. Conf., Canadian Geotechnical Society, Alliston,Ontario, 287–296.

Hofmann, B. A., Sego, D. C., and Robertson, P. K. (1995). ‘‘In-situground freezing for undisturbed samples of loose sand—Phase II.’’Proc., 48th Can. Geotech. Conf., Canadian Geotechnical Society, Al-liston, Ontario, 197–204.

Johansen, C. (1975). ‘‘Thermal conductivity of soils.’’ PhD thesis, Uni-versity of Trondheim, Trondheim, Norway.

Küpper, A., Lawrence, M., Howie, J., and Plewes, H. (1995). ‘‘The useof geophysical logging techniques at the CANLEX project.’’ Proc.,48th Can. Geotech. Conf., Canadian Geotechnical Society, Alliston,Ontario, 81–88.

Little, T. E., Imrie, A. S., and Psutka, J. F. (1994). ‘‘Geologic and seismicsetting pertinent to dam safety review of Duncan Dam.’’ Can. Geotech.

J., Ottawa, 31, 919–926.Monahan, P. A., Luternauer, J. L., and Barrie, J. V. (1995). ‘‘The geology

of the CANLEX Phase II sites in Delta and Richmond, British Colum-bia.’’ Proc., 48th Can. Geotech. Conf., Canadian Geotechnical Society,Alliston, Ontario, 59–68.

Plewes, H. D., McRoberts, E. C., and Chan, W. K. (1988). ‘‘Downholenuclear density logging in sand tailings.’’ Proc., ASCE Spec. Conf. on Hydr. Fill Struct., ASCE, New York, 290–309.

Robertson, P. K., et al. (2000). ‘‘The Canadian Liquefaction Experiment:An overview.’’ Can. Geotech. J., Ottawa, 37, 499–504.

Sanger, F. J., and Sayles, F. H. (1979). ‘‘Thermal and rheological com-putations for artificially frozen ground construction.’’ Engrg. Geology,13, 311–337.

Seed, H. B., Singh, S., Chan, C. K., and Vilela, T. F. (1982). ‘‘Consid-erations in undisturbed sampling of sands.’’ J. Geotech. Engrg. Div.,ASCE, 108(2), 265–283.

Sego, D. C., Robertson, P. K., Sasitharan, S., Kilpatrick, B. L., and Pillai,V. S. (1994). ‘‘Ground freezing and sampling of foundations soils atDuncan Dam.’’ Can. Geotech. J., Ottawa, 31, 939–950.

Singh, S., Seed, H. B., and Chan, C. K. (1982). ‘‘Undisturbed samplingof saturated sands by freezing.’’ J. Geotech. Engrg. Div., ASCE, 108(2),247–264.

Sladen, J. A., D’Hollander, R. D., and Krahn, J. (1985). ‘‘The liquefaction

of sands, a collapse surface approach.’’ Can. Geotech. J., Ottawa, 22(4),564–578.

Tokimatsu, K., and Hosaka, Y. (1986). ‘‘Effects of sample disturbance ondynamic properties of sand.’’ Soils and Found., Tokyo, 26(1), 53–64.

Wride, C. E., and Robertson, P. K. (1997). ‘‘CANLEX: Introductory datareview report.’’ CANLEX Tech. Rep., University of Alberta, Edmonton,Alta., Canada.

Wride, C. E., and Robertson, P. K. (1998). ‘‘CANLEX: Final summarydata review report.’’ CANLEX Tech. Rep., University of Alberta, Ed-monton, Alta., Canada.

Wride, C. E., et al. (2000a). ‘‘Interpretation of in situ test results fromthe CANLEX sites.’’ Can. Geotech. J., Ottawa, 37, 505–529.

Wride, C. E., et al. (2000b). ‘‘Ground sampling program at theCANLEX test sites.’’ Can. Geotech. J., Ottawa, 37, 530–542.

Yoshimi, Y., Hatanaka, M., and Oh-Oka, H. (1978). ‘‘Undisturbed sam-pling of saturated sands by freezing.’’ Soils and Found., Tokyo, 18,59–73.

Yoshimi, Y., Tokimatsu, K., and Hasaka, Y. (1989). ‘‘Evaluation of liq-uefaction resistance of clean sands based on high-quality undisturbedsamples.’’ Soils and Found., Tokyo, 29(1), 93–104.

Yoshimi, Y., Tokimatsu, K., Kaneko, O., and Makihara, Y. (1984). ‘‘Un-drained cyclic shear strength of a dense Niigata sand.’’ Soils and Found., Tokyo, 24(4), 131–145.

Yoshimi, Y., Tokimatsu, K., and Ohara, J. (1994). ‘‘In-situ liquefactionresistance of clean sands over a wide density range.’’ Géotechnique,London, 44(3), 479–494.

APPENDIX II. NOTATION

The following symbols are used in this paper:

C f = frozen volumetric heat capacity (MJ/m3 K);

C GN2 = volumetric heat capacity of gaseous nitrogen (MJ/ m3 K);



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C u = unfrozen volumetric heat capacity (MJ/m3 K);

emax = maximum void ratio;emin = minimum void ratio;

k f = frozen thermal conductivity (J/s m K);k u = unfrozen thermal conductivity (J/s m K);

LLN2 = latent heat of liquid nitrogen (MJ/m3);

P1 = proportion of liquid nitrogen contained within freezepipe;

P2 = proportion of gaseous nitrogen contained within freezepipe;

(v 0)GN2 = initial temperature of gaseous nitrogen (K);(v 0)LN2 = initial temperature of liquid nitrogen (K);

v g = increase in temperature of gas nitrogen due to absorp-tion of heat from surrounding ground (K);

d = dry density (Mg/m3); and

= moisture content (%).

In Situ Ground Freezing to Obtain Undisturbed Samples - Hofmann (2000)

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