Frost-Susceptibility Ratings and Pavement Structure Performance

Transcription

1 Transportation Researh Reord FrostSuseptibility Ratings and Pavement Struture Performane DAVID C. ESCH, ROBERT L. MHATTIE, AND BILLY CONNOR A threeyear study of the relations beteen flexible pavement performane, design methods, materials properties, and environmental fators reently ompleted by the Alaska Department of Transportation and Publi Fa ilities is desribed. The pavement setions hosen for the investigation ere 12 older setions from the various limati regions of the state. Performane as harateried by measurements of fatigue raking, rut depths, and peak springtime defletion levels. Pavement strutures ere measured and sampled to a depth of 4.5 ft. Sample testing inluded grada tions, Atterberg limits, moisture ontents, and frost suseptibility related fators. Additional information from previous frost heave testing programs as also used to supplement the results of the performane study. Conlu sions are presented on the relation beteen frostsuseptibility indiators and performane. Results of the study indiate that lo ontents of partile sies smaller than.75 mm (no. 2 sieve) and.2 mm in unsta bilied pavement strutural layers may be the most important of the many fators that affet strutural performane. Classifiations and analysis of pavementlayer soils and systems by the U.S. Army Corps of Engineers frostsuseptibility system and the reduedsubgradestrength design method shoed signifiant relations ith pavement performane in Alaska, hereas use of the stabilometer Rvalue method in testing and design analysis as of no value in indiating relative performane levels. A major study as reently ompleted that ompared the field performane of Alaska's flexible highay pavements ith the physial properties of the underlying aggregate and soil layers. The primary intent of this study as to evaluate the reliability of existing design proedures and to determine the maximum ontents of partiles smaller than.75 mm and.2 mm that ould be alloed at eah depth ithout resulting in exessive tha eakening and fatigue failure of the pavement system. Soil layers ere investigated from the bottom of the asphalt pavement to a depth of 4.5 ft. Highay pavement strutures in Alaska are urrently designed by a ombination of the U.S. Army Corps of Engineers reduedsubgradestrength design method and the Hveem Rvalue method <lrll Subgrade soils are lassified by the Casagrande riteria for frostsuseptibility value {FSV), based primarily on their.2mm ontents {see Figure 1). Minimal pavement struture thiknesses, termed the "frost overlay", are seleted from design urves based on the traffi index {TI) and the FSV lassifiations of the subgrade soils {see Figure 2). The thiknesses of the pavement and baseourse layers are then seleted on the basis of Rvalue test data. Current speifiations restrit the. 75mm ontents of base and subbase layers to 6 perent or less, to redue the frost suseptibility of these layers, although speifiations in past years have permitted.75mm ontents as high as 112 perent. Results obtained from this study and from prior laboratory frostheave testing of base and subbase materials have led to a series of onlusions regarding the relations beteen soil properties and frostheave test results and pavement performane in various Alaska environments. SITE SELECTIONS AND OBSERVATIONS A total of 12 paved highay setions ith unbound granular base layers {eah setion. 51 mile in length), loated throughout the various limati regions of Alaska, ere seleted for this study. The roaday setions seleted ere those that exhibited either good longterm performane or lear evidene of premature fatiguetype distress. Eah setion as intensively measured for fatigue (alligator) raking and heelpath rutting beause these fators are onsidered to be the primary strutural performane indiators {l). Alligator raking as measured separately in eah heel path and is expressed here as a perentage of the full lane length. Rutting as measured at intervals by using a 5. 5ft straightedge. To determine the degree of seasonal tha eakening, Benkelman beam rebound defletion surveys ere made eekly for one month after the start of spring thaing on eah setion. The Asphalt Institute test proedure as used (3). Additional summer and fall defletion measuremets ere made to further haraterie the setions. Maximum defletion levels ere used in this study to indiate the strutural strength of eah setion (4). To determine the properties of the pavem;nt strutures, to test pits ere exavated to a depth of 4. 5 ft in eah study setion, and samples and thikness measurements ere taken from eah layer. Pavement ores ere taken from both heelpath and nonheelpath areas to determine thiknesses and to orret the surfae rut measurements for the effets of pavement ear and displaement. Environmental fators ere determined for eah site, inluding limatologial data suh as mean annual temperature, preipitation, and mean freeing index (2_), as ell as age and umulative traffi. These fators ere inluded in the analysis to determine hether there ere signifiant differenes in performane beteen similar pavement strutures in armer oastal environments and the older, dry interior regions of Alaska. Mean air temperatures ranged from 22 to 4 F, freeing indies from 3 to 6 degreedays {Fahrenheit), and annual preipitation from 1 to 8 in. The number of equivalent 18 lb axle loadings at the time of site inspetions varied from 2 to 86. Asphalt pavement thiknesses ranged from 1 to 4 in and averaged 1.9 in. LABORATORY TESTING All pavementstruture soil samples ere tested for partile sie gradation, moisture ontent, and Atterberg limits and ere lassified for FSV by using Figure 1. Additional tests ere performed on seleted samples to determine their stabilometer Rvalues and heave rates. Heave Rate Heaverate testing as performed on 41 baseourse and subbase samples seleted from study setions that ranged from exellent to poor in performane. Heave test proedures used in this study ere developed by the U.S. Army Cold Regions Researh and Engineering Laboratory {CRREL) {&_,l) and modified by the Alaska Department of Transportation and Publi Failities over the past 1 years, for evaluating base and subbase aggregates at differing gradations and fines ontents. The proedure involves the removal of partiles larger than. 75 in and onditioning to optimum moisture folloed by ompation

2 28 Transportation Researh Reord 89 Figure 1. FSV soil lassifiation hart based on Corps of Engineers system. Figure 3. Frostheave test abinet. SILT & CLAY NFS F4 (F3 n Pl> 12) ( >SO t) VERY FINE NFS SILTY SANDS SANDS NFS ( % 4 >+ 4) F3 F4 F2 F3 GRAVELS ( 7. +"4>"4) NFS F1 F P.2nn Figure 2. Alaska's pavement design hart for frostsuseptible soils based on soi I FSV lass and Tl FROST OVERLAY (feet) of the samples ith a vibratory hammer in 6indiameter by 5. 5inhigh segmented ring molds. Samples are then saturated by overnight soaking before freeing. Heave measurements are reorded hile applying a fixed +15 F air temperature above the samples for 72 h and maintaining a +4 F temperature and an open ater supply beneath the samples (see Figure 3). Samples are lassified on the basis of the heave rate ourring beteen 48 and 72 h after the start of freeing, during hih time the freeing rate approximates.5 in/day. Stabilometer RValue For evaluation of the Hveem Rvalue design method, 25 road setions ere seleted to over the full range of distress severity, and samples from eah layer ere tested to determine the stabilometer Rvalues in aordane ith ASTM PERFORMANCE ANALYSIS Initial analyses to relate performane levels to soil properties at various depths ere made by using a omputeried multiple regression proedure developed by Kansas Universi ty as part of the Statistial Pakage for the Soial Sienes (SPSS) program. The major dependent variables used to desribe the performane of eah setion ere perentage of alligator raking, maximum defletion level, and average rut depth. Although all possible soil property, layer thikness, and environmental par am eters ere inluded in stepise multiple regression analyses, no reasonable orrelations ere obtained beteen performane and any ombination of variables, in spite of the large number of setions (12) and the large number of independent variables onsidered for eah setion (175). As a result, this approah as abandoned in favor of grouping pavement study setions on the basis of similar levels of performane. Average performane levels of these groups ere then ompared ith the average material properties at seleted depths beneath the pavement of 3, 9, 18, 3, 42, and 54 in. Comparisons among the three primary performane fatorsnamely, fatigue or alligator raking, rut depths, and peak spring levels of Benkelman beam defletionsindiated very good relations among these variables, as shon by Figures 4 and 5. For this reason, generalied omparisons beteen materials properties and performane ould be based on either maximum spring defletion levels or perentag of alligator raking. All referenes in this paper to defletion levels refer to the maximum or "peak" springtime Benkelman beam rebound defletion, alulated as the average of 11 tests on eah setion plus tie the standard deviation of those tests (). Performane Versus Contents Smaller than.75 and.2 mm Partiles smaller than.75 mm [partiles passing a no. 2 mesh sieve (P2 or fines)] and frations smaller than.2 mm are generally used as the primary indiators of frost suseptibility. Finesontent speifiations are most often used to ontrol frost suseptibility in pavement layers due to the ease of testing. To evaluate the effets of P2 ontents on defletion levels and alligator raking, the study setions ere grouped into six defletionlevel ranges and four alligatorraking ranges. Average P2oo ontents for eah group ere then determined at eah of the six depths mentioned above. Plots of these relations at the 3in depth are shon in Figures 6 and 7. The bestfit lines in these plots ere then used to determine the average fines ontents at eah depth that orresponded to seleted defletion and alligator raking levels, summaried in Figures 8 and 9. As a seond indiator of the effets of ing fines ontents on redued roaday during the thaeakening period, five inreasstrength roaday

3 Transportation Researh Reord Figure 4. Maximum rebound defletion versus alligator raking: average pavement age =14.7 years and average total EALs = 155. Figure 8. Average P 2 ontents at various depths related to inreasing defletion levels. 5 (!) 4 u 3 u 2... (!) 1 ::i MAX. DEFLECTION (in) "' Figure 5. Rut depth versus alligator raking DEPTH (ft) J:... n ::>.1 Figure 9. Average P2 ontents at various depths related to inreasing levels of alligator raking ALLIGATOR CRACKING (%) Figure 6. Average P 2 ontents in base ourse for 12 roaday setions by levels of alligator raking. lf'. <!5 8 6 u u (!) ::i P2 % Figure 7. Average P2 ontents in base ourse for 12 roaday setions by pavement defletion levels..8.6 i= u.4 tt x.2 ::!! P2C%) "' DEPTH (ft) setions ere seleted that had progressively inreasing fines ontents in the baseourse layer and leaner soils underlying the base to at least 4. 5 ft. In these ases, the base ourse obviously represents the most frostsuseptible layer in the pavement system. All setions of this series shoed seasonal defletion histories that had very rapid tha eakening and rapid strength reovery folloed by lo summertime defletion levels. Figure 1 shos the peak spring defletion levels for these setions versus the fines ontents of the base ourse and again demonstrates the detrimental effets of inreased fines ontents on pavement strutural performane. Correlations beteen performane fators and.2mm ontents ere similar to, but not quite as signifiant as, those for the P2 ontents in spite of the fat that the.2mm partile sie is a stronger preditor of frostheave rates under laboratory onditions (!!). The ratio of perentage.75mm to perentage.425mm partile sies, termed the dust ratio, also had a signifiant relation ith performane. No other partile sie or shape fators ere observed to signifiantly affet performane. The average soil properties assoiated ith those

4 3 Transportation Researh Reord 89 pavement setions that had no alligator raking ere very similar to those assoiated ith maximum defletion levels of.2 in and ith best overall performane. For the soil samples obtained in this study, the. 2mm partile ontents averaged approximately 6 perent of the o. 75mm ontents hereas soils ith layey fines ere rarely enountered. Therefore, no onlusions ould be made on the effets of plastiity index (PI) on performane. From these observations, the detrimental effets on performane of inreasing f i nes and. 2mm ontents is obvious. Hoever, the signifiane of soil properties at the greater depths listed above ould not be determined from this study, sine this as not a ontrolled experiment in hih singlelayer properties ould be varied hile all other fators ere held onstant. Inspetion of the soils data indiated that, due to geologial fators ontrolling soil distribution, high fines ontents at depths of 24 ft related to a higher probability that base and subbase layers ould also be high in fines. Performane Versus Soil FSV Classifiations After the soils at the six depths listed above ere grouped by similar FSV values, the average maximum defletion values for eah FSV group and depth ere alulated. The results are shon in Figure 11. The folloing observations ere made from inspetion of thes e relations: 1. Nonfrostsuseptible (NFS) (F ) soils shoed slight performane advantages over lasses that have greater frost suseptibility. 2. No signifiant performane differenes are apparent beteen F1 and F2 lasses. This may be due primarily to the organiation of the frost lass hart (Figure 1), here, for instane, nearly idential gravels ith a ontent of 4 perent finer than the. 2mm sie may be assigned to an F1 lass (gravel) or to an F2 lass (sandy gravel). J. FSV lasses F3 and F4 at depths of up to 42 in relate to signifiantly higher defletion levels than the loer FSV laoo. rrformane differenes beteen the F3 and F4 lasses appear insignifiant. 4. Overall, the FSV lassifiation system appears only eakly related to pavement defletion levels. Similar plots of FSV groups by perentage of alligator raking sho even more errati relations. In the plot, shon in Figure 12, NFS soil layers sho major benefits in redued alligator raking to a depth of approximately 3 ft, hereas differenes in alligator raking among lasses F1 to F4 are errati and appear nonsignifiant. In ontrast to the intent of the lassifiation system that higher values represent the orst frost suseptibility, Figure 12 appears to indiate that the F2 soils group may represent the orst FSV lassifiation. The effets of the various FSV lasses on rut depth ere similar to those observed for alligator raking exept that NFS soils did not exhibit suh a deisive performane advantage. In general, it as not demonstrated that the existene of a partiular FSV lass at a given depth an serve as a basis for onfident performane predition beause all layers ontribute to performane. NFS material is, hoever, most strongly indiated for use in the base and subbase layers. In vie of the favorable performane expetations attahed to lo defletion levels, this study indiates the use of NFS materials to be of benefit even at depths of 3 in or greater. Figure 1. Defletions versus P2 in base ourse for seleted roaday setions: data from five roads ith loer fines in subbase and subgrade..8 "2 ;:;.6 i== u...j u..4 Cl P2 IN BASE Figure 11. Average defletions related to different soil frost lasses at various depths..6 '.E.4 lrl ii.2 DEPTH 3 DEPTH = 9 NFS Fl F2 F3 NFS Fl F2 F3 SOIL FROST CLASS (FSV) Figure 12. Perentage of alligator raking related to different soil frost lasses at various depths. 4 DEPTH= 3" DEPTH= 9' (!) ;?; 3 ii:: < 2 I < 1 (!) :::; Fi F2 F3...J < NFS SOIL FROST CLASS (FSVl Evaluations of FSV Design Method A seond approah to evaluating the frostsuseptibility lassifiation system involved the use of Alaska's urrent pavementstruture design proedure (Figure 2) to study the adequay or defiienies of the existing studysetion pavement strutures. The TI for eah setion as first determined from the number of equivalent 18kip axle loadings (EALs) by the folloing formul Additional frost overlay (FSV) thikness requirements ere then derived by exam1n1ng the design overlay thikness required by Figure 2 for eah soil layer to a maximum depth of 54 in. The atual (!)

5 Transportation Researh Reord thikness of the overlying layers as subtrated from the design requirement for the layer being onsidered, and the remainder, if any, as termed the frostoverlay defiit for that layer. The largest net defiit for all layers in a given pavement struture as then defined as the additional FSV overlay requirement. Of all 12 road setions studied, only 8 ere found to require no additional overlay aording to the reduedsubgradestrength design method. By far the most ommon overlay defiit as in the range of 115 in and as required over an F1 or F2 soil type. The pratie observed prior to 1977 of alloing fines ontents as high as 112 perent in base and subbase materials aounts for many of the indiated overlay defiienies. Folloing the determinations of the additional FSV overlay requirements, the relations beteen this fator and the pavement distress fators ere investigated by grouping and averaging data, as shon in Figure 13. Signifiant orrelations are apparent beteen poor performane and FSV overlay defiienies. In summary, analysis of the FSV design method indiated generally signifiant relations beteen performane and overlay requirements. The urrently used FSV overlay design hart may be overly onservative by 1518 in of overlay for average onditions, sine performane did not signifiantly deteriorate until pavement strutures beame defiient by more than this amount. Hoever, some onservatism is neessary to allo for orse onditions. Heave Rates Versus FSV Classifiations and Fines Contents Relations beteen FSV and laboratory heave rates for 41 tests on samples in the NFS, F1, and F2 lasses are shon as "quartile plots" in Figure 14. Diret inreases in the median heave rates ere observed ith inreased FSV lassifiation number, but the range of heave rates also inreases ith lass, hih makes the FSV lass system a relatively poor preditor of laboratory heave rate. The relation of soil fines ontents to laboratory heave rate as also investigated. Preditive equations derived by using leastsquares linear regression analysis are as follos: Heave ratemm/day = (.36)(% 2) +.38 (2) Heave ratemm/day = (1.2)(%.2 mm) 1.1 (3) The oeffiient of determination (R 2 ) values ere.86 for both equations. These equations provide signifiant orrelations beteen fines and heave rate and indiate the generalied rate of hange of heave rate as fines inrease. Hoever, no ombination of soil gradation fators ould be found that predited heave rates ith satisfatory auray. Previous heave test programs performed by the Alaska Department of Transportation and Publi Failities on relatively lean gravels used for pavementsystem subbase and baseourse layers have demonstrated relatively uniform inreases in heave rate ith inreases in the.75mm and.2mm perentages. Figure 15 shos the hanges in heave rate that ourred hen a Fairbanks gravel baseourse aggregate that had idential gradations in the +O. 425mm sieve portions as heave tested ith five different.75mm and.2mm ontents. Test programs on aggregates from several different onstrution projets have onfirmed this omparison beteen heave rates and varying fines ontents. All heave test runs that used varying fines ontents ith the same gravel fration have shon diret relations beteen inreases in. 2mm and.75mm sieve ontents and heave rate and the ourrene of some signifiant heave at fines ontents greater than 12 perent. Laboratory data have onsistently shon that the perentage of the.2mm partile sie is a better indiator of heave rate for aggregates from different soures than is the.75mm partile sie. Performane Versus Heave Rate Heave test samples ere seleted to represent base and subbase materials from 27 pavement study setions that ranged from good to poor in performane and overed the range depth beteen and 3 in. About half of the sampled soils at eah depth had heave rates of less than 3 mm/day. As Figure 16 shos, only a rough relation of loer heave rates to loer distress levels ould be found. For all three distress types, heave rates at 9 and 18 in appeared to be the most onsistent indiators of performane. Lo alligator raking, lo defletion levels (.25 in), and minimal rut depths (O.l in) ere assoiated ith heave rates of less than 23 mm/day, hereas orstase performane levels ere erratially predited by heave rates greater than 34 mm/day. Speifiations Based on FrostHeave Testing Earlier unpublished Alaska Department of Transportation and Publi Failities studies of ertain base and subbase layers that had resulted in very rapid fatigue failures of pavements have shon that heave rates in exess of 3.5 mm/day, resulting from.2mm and.75mm sieve ontents of greater Figure 13. Overlay defiienies by redued subgradestrength design method versus maximum defletions and rut depths, based on group averages from 12 roadays..6.3 ;:;.4 j:: ().2 : ;:; OO B GRAVEL OVERLAY DEFICIT (in). 1.._.._!,....,..., GRAVEL OVERLAY DEFICIT (in)

6 32 Transportation Researh Reord 89 than 8 and 13 perent, respetively, ere totally unaeptable under et oastal limati onditions. Heave rates of 2 mm/day or less under the testing proedure previously desribed appeared to result in aeptable pavement layer performane. On the basis of these early studies, maximum.75mm sieve Figure 14. Quartile plots of laboratory heave rates for different FSV dasses. >. "C "' 8 e&. I < W4 > < J: maximum upper 114 median ontents of base and subbase layers are urrently speified at 6 perent in an attempt to prevent frost ation from ausing exessive tha\'/ eakening of the pavementsystem layers. With normal soures, this ensures that o. 2mm partile ontents ill not exeed approximately 4 perent. Higher laboratory heave rates result from inreased moisture gain on freeing and ould be expeted to result in inreased and prolonged strength losses on thaing. Laboratory heaverate riteria have not been inorporated into any form of pavement strutural design method or projet speifiation at this time. Hoever, heaverate differenes beteen different aggregates ould appear to relate more diretly to strutural performane during the springtime thaeakening period than the simple FSV lassifiation system. More intensive field studies are neessary to provide a basis for pavement strutural design based on frostheave testing. A maximum heaverate riterion of 2 mm/day under the test proedure desribed here is urrently being used by the Alaska Department of Transportation and Publi Failities for evaluation of stabiliing agents intended to redue the frost suseptibility of soils in pavement layers. 2 loer 1/4 Performane Versus RValue Test Data 1 2 FSV 'CLASS minimum Figure 15. Changes in laboratory heave rate from progressive inreases in P2 ontent in a Fairbanks gravel HEAVE RATE (mm dayj Attempts ere made to relate observed pavement performane to soilsample Hveem stabilometer Rvalues, based on data olleted from 25 studysetion loations that overed a ide range of distress severity. Figure 17 shos the average Rvalues at seleted depths assoiated ith high and lo defletion and alligator raking levels, based on a division of groups at O. 4 in of defletion and 5 perent alligator raking. A derease in observed Rvalues ith depth is obvious for eah performane fator. Interestingly, although pavements in the loer defletion group had higher Rvalues at depths greater than 9 in, the plot of alligator raking versus Rvalues indiates that poorer performane as related to the highest average Rvalues at eah depth loation noted. Neither ould any systemati relation be found beteen rut depths and Rvalues at any depth. An analyti approah as then taken that more losely paralleled the atual design use of the Rvalue test. Eah of the 25 setions as examined to see hether the asbuilt pavement strutures satisfied the overall design overlay requirement in aordane ith California's overlay design hart (9). Overlay defiits in terms of additional asphalt pavement thikness ranged from O to 3.5 in and are plotted as group average data points in Figure 18. Equivalent additional gravelbase thikness requirements ould be approximately tie these values, based on a gravel equivalent fator of 2. Figure 16. Heave rates of samples from various depths versus defletion and alligator raking < 2 ::::::., d8flellon DEPTII (in) DEPTII (in)

7 Transportation Researh Reord Figure 17. Rvalues of samples from various depths versus defletion and alligator raking. 8 t, :::> J < 7 > I er f deflet ion (.4" defletion >.4...,..."... alligatoring < 5% 8...._ alligatoring ) 5% ;;! 7 > I er "' '_" DEPTH (in) DEPTH (in) 64 Figure 18. Overlay defiienies versus defletion and rut depth based on Rvalue design method..8 (!) 8.6 j:: (.) i2 (.) < 8 &l.4 er 4 J u. I.2 C3 2 j. < OVERLAY DEFICIT (in) OVERLAY DEFICIT (in) for asphalt onrete pavements. No signifiant orrelations ere found beteen overlay defiits and any of the three prinipal performane variablesnamely, rut depth, alligator raking, and defletion. The absene of any relations beteen Rvalue test results and pavement performane in Alaska is onsidered to be the effet of overriding frostsuseptibility onsiderations that ontrol the field performane of pavements. For Alaska soils, progressively inreasing the fines ontent of a base or subbase aggregate ill not result in any major deline in Rvalues until the fines ontent exeeds 1214 perent, as shon in Figure 19. Hoever, at fines ontents greater than 61 perent, base and subbase aggregate may beome extremely suseptible to frost heave and tha eakening hile retaining high Rvalues. SUMMARY Analyses ere performed to determine the ritial fators in good pavement performane based on 12 roaday setions on hih the pavement strutural layers ere measured, sampled, tested, and lassified for frost suseptibility. This study as onfined to flexible pavements beteen 1 and 4 in in thikness ith unbound granular bases. The soils enountered ere primarily gravels and silts; uniform sands and lays ere onspiuously absent. It must be realied that this as not a ontrolled experimenti i.e., no one variable ould be altered hile all others ere held onstant. For this reason, the effets of a single fator ould not be absolutely determined under speifi onditions. The primary fators omitted from this study ere the relative densities and elasti properties of the pavement strutural layers and quantifiations of the groundater available to support frost ation. In spite of multiple regression analyses of 175 material, dimensional, and environmental fators Figure 19. Rvalues versus P2 ontents for all soils tested in (l.n 5 4 g AVALUE I... ;.k pertinent to the performane of the 12 study setions, no empirial equation ould be derived to predit the relative performane levels of different pavement strutures ith reasonable auray. CONCLUSIONS 1. From analyses of the material and environmental fators ommonly related to good performane of flexible pavement strutures in Alaska, the most important fators in good. performane are lo perentages of.75mm and.2mm partiles in the base and subbase layers. 2. Best performane is related to maximum base and subbase. 75mm ontents of 6 perent and to maximum. 2mm ontents of 3 perent. Poor performane predominates hen base and subbase fines reah 11 perent, and hen.2mm ontents reah 7 perent in these layers. 3. Soil frostsuseptibility ratings of the pavement strutural layers by the Corps of Engineers FSV lassifiation system are only moderately related to pavement performane. Differenes in performane beteen the F1 and F2 lasses ere found to be nearly insignifiant, and differenes beteen the F3 and F4 lasses ere also small. Perentages of soil fines should therefore be onsidered for diret use as a design parameter for frost areas instead of the FSV lassifiation system. 4. The Hveem stabilometer Rvalue test is not a useful indiator of relative pavement strutural performane for frost areas beause it does not indiate differenes beteen soils ith lo and high frost suseptibility. Modifiations to the moistureonditioning phase of this test, suh as the addition of a frostheave stage, should be onsidered to make the Rvalue test more appliable to the strutural design of pavements in frost areas. 5. Results of laboratory heave tests indiated that the best pavement performane is related to 8

8 34 Transportation Researh Reord 89 heave rates in base and subbase layers of less then 3 mm/day. 6. No signifiant differenes in performane relations ere noted beteen the et oastal areas and the dryer interior areas of Alaska. ACKNOWLEDGMENT We ish to express appreiation to the many people hose efforts ontributed to this paper and in partiular to materials engineers Dan Herman, Jerry Roah, Paul Misterek, and Ray Miller for assistane in sampling, soil analysis, and defletion testing operations. This researh ork as aomplished in ooperation ith the Federal Highay Administration, U.S. Department of Transportation. The ontents of this paper reflet our vies and not neessarily those of the state of Alaska or the Federal Highay Administration. REFERENCES 1. Guide for Flexible Pavement Design and Evaluation. Alaska Department of Transportation and Publi Failities, Fairbanks, E.J. Yoder and J.W. Witak. Priniples of Pavement Design, 2nd ed. Wiley, Ne York, Asphalt Overlays and Pavement Rehabilitation, 1st ed. Asphalt Institute, College Park, MD, Manual Series 17, 1969, pp A Guide to the Strutural Design of Flexible and Rigid Pavements in Canada. Canadian Good Roads Assn., Ottaa, Ontario, C.W. Hartman and P.R. Johnson. Environmental Atlas of Alaska, 2nd ed. Institute of Water Resoures, Univ of Alaska, Fairbanks, Kaplar. Experiments to Simplify Frost Suseptibility Testing of Soils. U.S. Army Cold Regions Researh and Engineering Laboratory, Hanover, NH, Teh. Rept. 223, Kaplar. Freeing Test for Evaluating Relative Frost Suseptibility of Various Soils. U.S. Army Cold Regions Researh and Engineering Laboratory, Hanover, NH, Teh. Rept. 25, A. Casagrande. Disussion of Frost Heaving. HRB, Pro., Vol. 2, Part 1, 1932, pp League of California Cities. Strutural Setion Design Guide for California Cities and Counties. DPW Reprodutions, Saramento, CA, Rept , Publiation of this paper sponsored by Committee on Frost Ation. Simulating Frost Ation by Using an Instrumented Soil Column J. INGERSOLL AND R. BERG The use of an instrumented soil olumn in tests to develop a mathematial model of the frostheave proess is desribed. Tensiometers, heatflo meters, thermoouples, and eletrial resistivity gages ere installed throughout a soil olumn filled ith Fairbanks silt, Chena Hot Springs silt, or West Lebanon gravel. The olumn as 1 m long and about 14 m in diameter. An open system as used and absorption as monitored during the freeing proess. Three freeing tests ere onduted on Fairbanks silt, to ith a surharge of 3.45 kpa and one ith 34.5 kpa. The ater level as held at the 45m depth. Eleven tests ere onduted using Chana Hot Springs silt. The ater level as set at 15, 5, or 1m depths, and the surharge as 3.45 or 34.5 kpa. Tests ere onduted by using a onstant rate of frost penetration, a onstant heatflo rate, or three sequentially loer temperature step hanges at the soil surfae. To tests ere onduted ith West Lebanon gravel. Both inluded three step hanges in the surfae temperature and ere surharged ith 3.45 kpa. The ater levels ere at the 15 and 1m depths. The soil olumn has provided ritial data for verifiation of a onedimensional mathematial model for estimating frost heave. As more soils are tested, this equipment ill assist in improving and developing algorithms for the mathematial model and in identifying the most ritial parameters that affet frost heave in a given soile.g., surharge, free ater level, and hydrauli ondutivity. A proedure is also presented for determining the saturated and unsaturated hydrauli ondutivity and moistureretention harateristis of a soil. In July 1975, the Federal Highay Administration (FHWA), the Federal Aviation Administration (FAA), and the U.S. Army Corps of Engineers initiated a ooperative projet to develop a mathematial model of the frostheaving proess. The onedimensional finiteelement omputer program resulting from the study as doumented (!_,, and Guymon and others elsehere in this Reord). The results reported here ere obtained from an instrumented soil olumn that as originally onstruted to provide data for developing an algorithm desribing the effets of overburden (or surharge) on soil moisture stress. The olumn as designed so that the ater table ould also be varied. We have also used data from the soil olumn to aid in verifying the mathematial model of frost heave ( and Guymon and others elsehere in this Reord) and have developed a more refined design that ill be used ith a dual gamma system. With the dual gamma system, e ill have the apability of monitoring hanges in moisture ontent and density nondestrutively. The dual gamma system, in onjuntion ith the other instrumentation in the soil olumn, ill permit developing the moistureharateristi urve and the relation beteen moisture ontent (or matrix sution) and hydrauli ondutivity for the soil ithin the olumn. The soil olumn ill also be used for other portions of the ooperative study e.g., to assist in developing the thaeakening algorithm and in applying the mathematial model to layered pavement systems. TEST APPARATUS The soil as molded ithin a 1mlong irular ylinder that had a diameter of about 14 m. The inside of the upper 15 m of the ylinder tapered slightly and as lined ith Teflon tape to minimie sideall resistane to heaving. The top portion of the ylinder as detahable from the loer portion (see Figure 1). Thermoouples ere inserted through the ylinder alls and into the soil at intervals of 1 m in the upper portion and intervals of 2.51 m in the