Introduction

A large diameter pipeline may be subject to frostheave displacements of a meter or more, if the pipe isburied in initially unfrozen soil, and is operated contin-uously at temperatures significantly below 0¡C.Considerable study has been carried out into predictingfrost heave itself (Nixon, 1991), and future laboratoryand field studies will undoubtedly further refine pre-dictions.

Another major input required for frost heave-pipelineinteraction analysis is the uplift resistance offered toupward motion of the pipe by the frozen soil. The rela-tively sudden transition from unfrozen (stable) soil tounfrozen (heaving) soil may result in large pipe bend-ing strains, if the resistance to uplift in the stable frozenzone is sufficiently high. If a cylindrical pipe is forcedupward through a mass of frozen soil, then initially thesoil around and above the pipe will deform as a contin-uum, according to the elastic and viscous properties ofthe soil. At some point, tensile and shear stressesinduced in the soil may be sufficient to cause cracks topropagate through the frozen soil. After this point, theload on the pipe will likely decrease as the pipe dis-placement increases. Resistance to upward pipe motionis now governed partly by the weight of the rectangularsoil blocks on either side of the pipe, and partly by the

flexural resistance of he two cantilever soil "beams"being lifted by the pipe.

Previous tests carried out in the Calgary lab ofImperial Oil Resources Ltd are described by Nixon andHazen (1993) for a 140 mm diameter model pipe in afrozen clayey silt. A total of five tests investigated theeffects of displacement rate, and two soil temperatures.Peak uplift resistance (as represented by pipe contactstress, the load divided by the plan area of the pipe incontact with the soil) ranged from 600 to 1500 kPa,depending on soil temperature and displacement rate.A pronounced peak and post-peak reduction in loadwas noted in all tests. Cracking in the soil adjacent toand over the pipe was observed at around peak load.Peak uplift resistance was rate dependent, and slowerdisplacement rates resulted in lower uplift resistance.Surface warming, simulating natural summer tempera-ture cycling, was extremely effective in reducing theuplift resistance experienced by the pipe. The tensilestrength of the frozen soil as measured in a separate setof uniaxial tests was time dependent, and equal toabout 400 kPa for a time to failure of about 2 days.Uniaxial tensile strains at failure were about 1.0%, andappeared to be largely independent of the time to failure.

J.F. (Derick) Nixon 821

PIPE UPLIFT RESISTANCE TESTING IN FROZEN SOIL

J.F. (Derick) Nixon

Nixon Geotech LtdBox 9, Site 9, RR6

Calgary, Alberta T2M 4L5Canada

e-Mail: derickn@cadvision.com

Abstract

The design of buried chilled pipelines in frost heaving terrain requires a knowledge of the uplift resistance ofpipes buried in frozen ground. Higher uplift resistances will increase the severity of pipe curvatures and strainsat frozen-unfrozen interfaces.

Twelve tests have been carried out in three different test programs, that provide the general shape of the load-displacement curve, and the effects of backfill type, cover depth, displacement rate and soil temperature. Thispaper includes a summary of 5 previous tests, and interprets the results of all 12 tests carried out to date.

Interesting crack patterns in the frozen soil around the pipe were observed, and their role in limiting peakand residual uplift resistance is discussed. Correlations have been developed that will allow pipeline designersto establish the load-displacement characteristics of a buried pipe displacing upwards through frozen soil, asrequired for structural analysis of chilled pipes in discontinuous permafrost.

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Table 1. Summary of uplift test results

One other recent test for uplift resistance for a 270 mmdiameter pipe at the Canada-France frost heave test sitewas reported by Foriero and Ladanyi (1994). This testused a hydraulic air pressure method of applying loadto the ends of the pipe, and therefore did not achieve asuniform a rate of pipe displacement as this program. Inaddition, some of the soil over the pipe may have beendesiccated, and therefore may not have as high astrength as saturated soils. The cover depth was 30 cm,and the soil/pipe temperature was around -2.0¡C. Anaverage displacement rate for the pipe prior to peakuplift was 1.0 mm/day. The peak load when norma-lized with respect to the plan area of the pipe was 480 kPa, which is very similar to the range of valuesreported here.

Test equipment and scope

Details of the test equipment are covered in moredetail in ERCL (1991) and in Nixon and Hazen (1993).The bulk Calgary clayey silt sample was blended withabout 22% water by dry weight. The protruding ends ofthe pipe were connected to the reaction frame and load-ing assembly. The unique aspect of the loading systeminvolved two mechanical actuators (screw jacks) ratedfor a total load of 20 tonnes each. These were turned bya 1/4 HP DC motor with variable power supply, and agear reduction system. The gear reduction system wasdesigned to achieve a near constant displacement rateof around 1 mm/day or less, which is similar to themaximum rate of displacement experienced by a pipeunder field conditions. The gear reducer system was a120,000:1 triple reduction worm gear unit. A hydraulicloading system was discarded in favor of the mechani-cal system, due to potential problems with unequalloading, variable displacement rates, and tilting of thepipe. Load cells and displacement transducers wereused to monitor end loads and displacements automati-cally during the test.

Nine thermistors in the soil, cold room and pipe wereused to monitor temperatures. An array of surface dialgauges was used to measure soil surface displacementin later tests, at three different planes at right angles tothe pipe. Later tests also used an array of colored pinsembedded in one exposed end of the soil to monitordisplacement vectors in the soil during the test. Thedata acquisition system used was an interactive pro-gram for the PC that allows regular sampling of varioussensors. A real time display on the PC screen wasobtained during testing.

Test 1 was the control test using uniform silt at -5.5¡C.Test 2 studied the effects of re-worked backfill over thepipe. Test 4 used a warmer soil temperature, and Test 5involved a much slower displacement rate (Nixon andHazen, 1993). Test 6 applied a warming cycle to the soil

surface, while maintaining a controlled pipe tempera-ture of around -5¡C as before. Test 7 investigated theeffects of shallower pipe burial on the uplift resistancecharacteristics of the pipe-soil system. Test 8 used alarger (317 mm diameter) pipe. Test 9 repeated the sametest at a warmer soil temperature of -2.5¡C, and Test 10investigated the effects of thawing and re-freezing fromthe soil surface, prior to testing. A final tests seriesfunded by NEB was started late in 1993, and exploredthe effects of colder temperatures (Test 11), smaller pipesize (Test 12), and a more plastic clay soil (Test 13).

A summary of the background data for each of themore recent seven tests is given on Table 1, togetherwith the initial 5 tests published by Nixon and Hazen(1993).

Summary of new test results

A summary of load displacement curves for most ofthe tests is given on Figure 1. The results have been nor-malized with respect to the peak uplift load, and thedisplacement at peak load, to allow better comparisonof load-displacement curves. The similarity of the initialsegment of the load-displacement curves is evident,considering the widely different test conditions. Thishas prompted some simplifications in later discussionof generalizing the load-displacement response foranalysis. The mechanical actuator system maintainedvery uniform displacement rates over long time pe-riods, and is far superior to any degree of control thatcould be obtained by any reasonable hydraulicallyapplied displacement system.

Figure 2 shows a typical contour plot of vertical soildisplacement generated from the array of pins embed-ded in the vertical end plane of the soil in Test 12. Mostof the soil displacements are confined to a zone of aboutone pipe diameter on either side of the pipe. Very littlesoil deformation occurs more than a fraction of a pipediameter below the pipe. Figure 3 shows the pattern ofsurface soil displacements observed at one end planeduring test 13 on the clay soil. The soil displacementspeak suddenly at the vertical crack that usually formsdirectly over the pipe center-line. Figure 4 show thetypical soil crack pattern mapped at the end of Test 13.Two cracks radiate laterally from the pipe mid-height,and a further vertical crack develops at the soil surface,and propagates downwards at the pipe center-line.

The effect of greatly reduced applied displacementrate is shown on Figure 5. The initial part of the load-displacement curve is identical with the faster displace-ment rate, and appears to be independent with the rateof load application. However, the peak load is signifi-cantly reduced from the control (faster rate) test. Figure 6 shows the effect of shallower burial on the

J.F. (Derick) Nixon 823

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Figure 1.Normalized load-displacement curves

Figure 2. Contours of vertical displacement for end of Test 12.

shape of the load-displacement curve. The peak is verymuch reduced, and the residual, or post-peak load iseven further reduced. If feasible, this would suggest avery effective method of uplift resistance reductionunder full scale conditions. Figure 7 shows the impor-tant effects of soil/pipe temperature on peak load.Again, the initial (pre-peak) segment of the loading

curve appears nearly independent of temperature.However, the peak resistance is strongly dependent ontemperature, due to greater soil strengths and lowercreep rates at colder temperatures.

When the peak load is normalized with respect toplan contact area, the effects of pipe diameter are not

J.F. (Derick) Nixon 825

Figure 4. Soil crack pattern mapped at end of Test 13.

Figure 3. Soil surface displacement pattern at end plane - Test 13.

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Figure 6. Effect of burial depth on load vs. displacement for test 7.

Figure 5. Effect of slow displacement rate on load displacement curve.

J.F. (Derick) Nixon 827

Figure 8. Effect of soil type on load displacement curve.

Figure 7. Effect of temperature on load displacement curve.

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clear from the testing. As shown on Table 1, no cleartrend with pipe diameter alone is apparent. Finally, theeffects of soil type are shown on Figure 8. A more plas-tic soil type was obtained from a site in Edmonton, andthe properties of both the Edmonton and Calgary soilsare provided on Table 2.

The post-peak loading curves are very similar, but thepeak resistances are quite different. The more plasticclay soil exhibited a much more ductile type of beha-vior than the more brittle silt soil. A near-constant pipeload was maintained for a longer period of the test onthe clay soil, and this is characteristic of soils that exhi-bit secondary creep (constant strain rate at a constantstress level). It appears that all things being equal, higher clay contents will result in lower peak uplift resistance.

A major unknown is whether the frozen soil followingloading, cracking and load reduction can re-heal andsubject the pipe to a similar high load year after year ona seasonal basis. Test 10 was therefore a repeat of Test 8,with the soil structural changes as described aboveincorporated in the soil mass around and over the pipe.The peak end load experienced in the final test wasabout 60 kN, or about three-quarters of the 80 kN peakload observed in the control test 8 on the large diameterpipe. The shape of the load displacement curves for thetwo tests are similar, however. This indicates that thereis a significant reduction in peak load after thaw and re-freezing, and this can be expected in the second andsubsequent years on the backfill around a pipelineheaving under field conditions. Whether continuedreductions in peak load will continue is not known, butit is likely that further reductions after the second

Table 2. Properties of soils tested

Figure 9. Effect of pipe displacement rate on peak pipe contact pressure.

freeze-thaw cycle would not be as significant as thereduction following the first freeze-thaw cycle.

It is further noted that even though the cracked soilstructure is still present in the frozen soil during thesecond loading period, there is still a significant peak topost-peak drop-off in load with increasing pipe displacements.

Analysis of results

One of the primary goals of this program is to provideinput parameters for structural analysis of a buriedchilled pipeline, crossing interfaces between unfrozenand frozen soil zones. Some of the important issuesinclude the scaling from smaller model pipes to larger,full scale pipes in the field, the effects of different soiltemperatures, the effects of different burial depths, andseasonal thermal effects of surface warming, thawingand re-healing effects in the soil. Some functions forpipe uplift resistance are proposed that can be used toaccommodate the above-mentioned effects.

In order to compare results from different tests wherethe pipe diameter or the length of pipe embedded inthe soil might be different, the peak load on the pipe

was normalized by dividing the load by the plan areaof the pipe embedded in the soil, i.e. the product of thepipe diameter and the embedded length. In this way, apipe contact pressure was obtained in kPa, that couldbe related to different controlling variables, as summa-rized on Table 1. The peak pipe contact pressure so cal-culated was further normalized with respect to burialdepth ratio and soil temperature parameter (1-T).

The pipe contact pressure function expresses thatpeak pipe stress is related to Dc/d and (1-T). Therefore,the pipe contact pressure function has been defined as:

Wp / { Dc/d¥ (1-T) } [1]

where Wp is the peak pipe contact pressure as definedpreviously; and Dc is the depth of cover over the pipe.

The strength of frozen fine grained soils is dependenton the rate of strain. As observed in the tensile testingdescribed in Nixon and Hazen (1993), the peak strengthor resistance is a strong, non-linear function of the timeto failure, or the rate of strain application. The theoreti-cal work of Foriero and Ladanyi (1994) also suggeststhat peak uplift stress should be related to normalizeddisplacement rate via a power law relationship.

J.F. (Derick) Nixon 829

Figure 10.Correlation for displacement at peak load.

Figure 9 shows a plot of a pipe contact pressure func-tion against a normalized displacement rate, defined asthe applied pipe displacement rate, Sdot, divided by thepipe diameter, d. A best fit regression line is shown,providing the peak contact pressure function. The Caentest on the 270 mm pipe reported by Foriero andLadanyi (1994) is also shown for comparison on Figure 9. The agreement between the normalizedresults is quite reasonable, considering the different soiltypes and loading methods.

The tests from the present program carried out withthe larger pipe (i.e., tests 8 and 9) plot somewhat lowerthan the equivalent test data for the smaller pipe. Thissuggests one of two or more possibilities, i.e. (a) the testconfiguration used in the larger pipe tests more closelyapproximated a plane stress condition, rather than theplane strain condition approached in the first 7 tests, or(b) pipe uplift resistance is not directly or linearly relat-ed to pipe diameter, but is rather related to some non-linear function of pipe diameter. The displacement atwhich the peak contact pressure is achieved is plottedwith peak load on Figure 10. An approximately linearrelationship is obtained, suggesting that the initial slopeof the load-displacement curve is not heavily depen-dent on the variables discussed above.

Finally, the post-peak or residual load varies betweenone-quarter to one-half of the peak load at large pipedisplacements. A simple and conservative estimate ofthe post-peak load based on the results of Table 1would be to assume the post-peak load falls to one-halfof the peak load at a displacement of 3 times Ymax.Another approach would be to develop a general func-tion for pre-peak and post-peak load with displacement.

Referring to Nixon and Hazen (1993), and Zhu andCarbee (1987), the tensile strength is a function of thetime to failure, and the estimated tensile strength for afailure time of 7-10 days (typical of many of the upliftresistance tests) would be around 300-350 kPa. Thepeak uplift resistance, when expressed as a stress, istypically 2 to 4 times this value, and this is similar toresults that might be anticipated from charted solutionsfor stresses around underground openings available inthe literature. However, no clear trend is apparent, dueto the different test conditions such as burial depth, soiltemperature, etc, and the effects of the rigid pipeembedded in the frozen soil mass.

It would be necessary to carry out a series of finite ele-ment stress analyses to determine the relationshipsbetween these parameters, and reconcile the observeduplift resistance-pipe displacement response with themore fundamental strength and stress-strain properties

of the frozen silt soil. Additional model uplift testingshould include testing on polycrystalline ice, in order toprovide a bound on the behavior of high ice contentfrozen soils.

Summary and conclusions

A special loading device and soil container to measurepipe uplift resistance in frozen soil were used to com-plete a series of 12 tests that have investigated theeffects of different displacement rates, pipe diameters,soil temperatures, and surface thermal history.

A pronounced peak load and post-peak reduction inload is present in all tests, although warmer tempera-tures and slower displacement rates seem to result inless pronounced post-peak reductions in load.

Cracking in the soil commences from the soil surfaceover the pipe, propagating downwards towards thepipe as the test progresses. Meanwhile, at some dis-placement at or just after peak load, cracks propagatedoutwards and sometimes upwards from the pipespringline. Only minor cracking in the soil adjacent tothe pipe was observed in the slowest test at a displace-ment rate of 0.2 mm/day where the soil behavior wasmore ductile, rather than brittle.

Relaxation of load occurs when the applied displace-ment rate is set equal to zero. This implies that a signifi-cant component of the load is creep-related, or depen-dent on the rate of displacement.

Surface warming is extremely effective in reducingthe uplift resistance experienced by the buried pipe.

A series of tensile tests allowed correlation of theobserved uplift resistance tests with the soil tensilestrength.

There appears to be a direct relationship betweenpeak uplift resistance and depth of soil cover.

Larger pipe diameters result in larger overall upliftresistance per unit length of pipe. However, whenexpressed as a load per unit plan area, the larger piperesults in lower unit pipe uplift stresses.

A surface freezing and thawing cycle prior to the test,results in a significant reduction in peak uplift resis-tance. Re-healing of the cracked soil in the regionaround and over the pipe is not completely effective inre-establishing the original peak uplift resistance.

Post-peak loads fall to between 40 and 70% of thepeak load when the pipe displacement reaches threetimes the displacement at peak load.

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An empirical correlation between peak pipe load andburial depth ratio, soil temperature and pipe displace-ment rate can be used by designers to establish the loaddisplacement curve for design.

Peak uplift resistance is strongly related to the time-dependent tensile strength of the soil, however finiteelement stress analysis is needed to reconcile theobserved uplift resistance with the more fundamentalstrength and deformation properties of the frozen soil.

Reasonable agreement with the Caen, France uplifttest suggests that uplift resistance may be similar for arelatively wide range of silt soil types. However, more

plastic clay soils provide a less pronounced and lowerpeak uplift resistance.

Acknowledgments

The support of Stephen Lord, Ibrahim Konuk andGordon Daw of the National Energy Board in Calgaryis acknowledged. Mr. Ron Coutts helped to run Tests 6and 7. Mr. Gary Wostradowski and Mr. Larry Ritco ofHBT-Agra in Calgary ran the later tests 8-13.

J.F. (Derick) Nixon 831

References

Esso Resources Canada Limited (ERCL) (1991). Report onpipe uplift resistance testing in frozen fine-grained soil.Internal Report by J.F. Nixon, November 1991.

Foriero, A. and Ladanyi, B. (1994). Pipe uplift resistance infrozen soil and comparison with measurements.Proceedings ASCE Journal of Cold Regions Engineering, 8, 93-111.

Nixon, J. (1991). Discrete ice lens theory for frost heave insoils. Canadian Geotechnical Journal, 28, 843-859.

Nixon, J. (1994). Role of heave pressure dependency and soilcreep in stress analysis for pipeline frost heave. InProceedings 7th International Cold Regions SpecialtyConference, Edmonton, March 7-9. pp. 397-412.

Nixon, J. and Hazen, B. (1993). Uplift resistance of pipes infrozen soil. In Proceedings, Sixth International PermafrostConference, Beijing, China. pp. 494-499.

Zhu, Y. and Carbee, D. (1987). Tensile strength of frozen silt.U.S. Army CRREL Report 87-15, Hanover, N.H.