Using Time Domain Reflectometry (TDR) - US Forest … · The following report outlines the...

Using Time DomainReflectometry (TDR)andRadio Frequency (RF)Devices to MonitorSeasonal MoistureVariation in ForestRoad Subgrade andBase Materials

United StatesDepartment ofTransportaiion

Federal HighwayAdministration

Prepared byUnited StatesDepartment ofAgriculture

Forest Service

Technology &DevelopmentProgram

July 2001

FOREST SERVICE

DEP A R T MENT OF AGRICUL T U R

E

Using Time DomainReflectometry (TDR)and Radio Frequency(RF) Devices toMonitor SeasonalMoisture Variation inForest Road Subgradeand Base Materials

Gordon L. HanekUmpqua National Forest2900 NW Stewert ParkwayP.O. Box 1008Roseburg, OR 97470541-957-3390 [email protected]

Mark A. TruebeWillamette National Forest211 East Seventh AvenueP.O. Box 10607Eugene, OR 97440541-465-6515 [email protected]

Maureen A. KestlerU.S. Army Cold Regions Research and EngineeringLaboratory72 Lyme RoadHanover, NH 03755-1290603-646-4215 [email protected]

July 2001

Information contained in this document has been developed for the guidance ofemployees of the Forest Service, USDA, its contractors, and cooperating Federaland state agencies. The Department of Agriculture assumes no responsibility forthe interpretation or use of this information by other than its own employees. Theuse of trade, firm, or corporation names is for the information and convenience ofthe reader. Such use does not constitute an official evalution, conclusion,recommendation, endorsement, or approval of any product or service to theexclusion of others that may be suitable.

The U.S. Department of Agriculture (USDA) prohibits discrimination in all itsprograms and activities on the basis of race, color, national origin, sex, religion,age, disability, political beliefs, sexual orientation, or marital or family status. (Notall prohibited bases apply to all programs.) Persons with disabilities who requirealternative means for communication of program information (Braille, large print,audiotape, etc.) should contact USDA’s TARGET Center at (202) 720-2600 (voiceand TDD).

To file a complaint of discrimination, write USDA, Director, Office of Civil Rights,Room 326-W, Whitten Building, 1400 Independence Avenue, SW, Washington,D.C. 20250-9410 or call (202) 720-5964 (voice and TDD). USDA is an equalopportunity provider and employer.

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AcknowledgmentsThe authors wish to acknowledge and thank thefollowing individuals for their support andassistance during the course of this study: JohnSteward (whose support started the whole thing),Tom Moore (for his patience), Joe Barcomb andTina Welch (for providing field test sites and supportpersonnel on the Kootenai and Ochoco NationalForests), John Sloan (for his support), DaveKatagiri, Barry Womack, Gene Stalnaker, GaryEvans, Susan Ortiz, Bill Kimball, Ernie Orr, RayJohnson, Martha Hendrickson, Dale Bull [CRREL],Sue Niezgoda [CRREL] and Sae-Im Nam [CRREL](for their willingness to work long hours undersometimes less than ideal conditions to get theinstrumentation installed, data collected, andanalysis completed). The authors also thank thetechnical reviewers, Gary Hicks, Dick Berg, JimPadgett, Doug McClelland, and Pete Bolander fortheir helpful suggestions and comments.

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Executive Summary/AbstractThe U.S. Department of Agriculture Forest Serviceand the U.S. Army Cold Regions Research andEngineering Laboratory have been evaluating aquantitative technique for the application andremoval of load restrictions by observingrelationships among pavement stiffness, pavementdamage, soil moisture, and seasonal freezing andthawing. Laboratory tests of the Time DomainReflectometry and Radio Frequency sensorsshowed both to be reasonably accurate andrepeatable when compared to known moisturevalues in several soil types. Laboratory tests ofthe probes under repeated adverse freeze-thawcycling showed the probes to be durable. Analysisof field data collected at seven locations on fournational forests showed that permanently installedsensors strategically located on a forest roadnetwork can provide an affordable method forquantitatively determining the beginning and endof critical periods of pavement weakeningassociated with spring thaw. This information wouldbe useful in administering periods of spring thawload restriction.

The following report outlines the laboratory andfield test programs conducted to monitor soilmoisture and pavement stiffness and describesinstrumentation used for the study. It discussesobservations, analyses, and results from thelaboratory and field tests as part of an overall effortto develop a reliable, objective, cost-effectivemethod to determine when to place and removeload restrictions associated with periods of criticalpavement weakening (i.e., spring thaw). Thetechnique is applicable to any secondary roadsubjected to seasonal freezing.

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Table of Contents

Background ................................................................................................................................................... 1

Purpose .......................................................................................................................................................... 2

Project Overview .......................................................................................................................................... 2Instrumentation ................................................................................................................................. 3

TDRRF

Thermistor Strings ............................................................................................................................ 5Standpipes ......................................................................................................................................... 5FWD ................................................................................................................................................ 5Laboratory Tests ............................................................................................................................... 6Field Sites—General ........................................................................................................................ 7Laboratory Testing Program ........................................................................................................... 8Soil Types Tested .............................................................................................................................. 8Test Procedures ................................................................................................................................ 9Test Results ..................................................................................................................................... 10Conclusions on Lab Testing .......................................................................................................... 10

Willamette Field SitesLocation and General Layout ........................................................................................................ 15Site Surfacing Structure and Material Properties ....................................................................... 15TDR, RF, and Thermistor Instrumentation ................................................................................... 19Data Collection and FWD Testing Program ................................................................................. 19Analysis and Results ...................................................................................................................... 19

TemperatureTDR and RF ..................................................................................................................................... 19Groundwater .................................................................................................................................... 25Area Parameter ............................................................................................................................... 25Layer Moduli .................................................................................................................................... 25Subgrade Strain .............................................................................................................................. 30Conclusions from Willamette Test Site ........................................................................................ 30

Ochoco Field SitesLocation and general Layout ......................................................................................................... 30Site Surfacing Structure and Material Properties ....................................................................... 31TDR, RF, and Thermistor Instrumentation ................................................................................... 31Data Collection and FWD Testing Program ................................................................................. 31Analysis and Results ...................................................................................................................... 31

TemperatureTDR and RF ..................................................................................................................................... 36Groundwater .................................................................................................................................... 36Area Parameter ............................................................................................................................... 36Layer Moduli .................................................................................................................................... 36Subgrade Strain .............................................................................................................................. 37Damage Factors .............................................................................................................................. 37Conclusions from Ochoco Sites .................................................................................................... 47

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Kootenai Field SitesLocation and Climate...................................................................................................................... 48Instrumentation Layout and Site Description .............................................................................. 49Data Collection and FWD Testing Program ................................................................................. 52Asphalt Coring ................................................................................................................................. 52Analysis and Results ...................................................................................................................... 53

TemperatureTDR and RF ..................................................................................................................................... 53Groundwater .................................................................................................................................... 58Area Parameter ............................................................................................................................... 58Layer Moduli .................................................................................................................................... 59Critical Strains and Damage Factors ........................................................................................... 61Conclusions from Kootenai National Forest Field Sites ............................................................ 61

White Mountain National Forest Test SiteLocation and Climate...................................................................................................................... 67Site Description and Instrumentation Layout .............................................................................. 67Analysis and Results ...................................................................................................................... 68

Subsurface Temperature RegimeSubsurface Moisture Regime ........................................................................................................ 68Area Parameter ............................................................................................................................... 69Pavement Layer Moduli ................................................................................................................. 69Critical Strains and Associated Damage ..................................................................................... 69Damage Factors as Related to Environmental and Structural Parameters ............................ 75Conclusions from White Mountain National Forest Site ............................................................ 75

Overall Summary and Conclusions ....................................................................................................... 75Principle Overall Conclusions of the Study ................................................................................. 76

Future Plans and Recommendations .................................................................................................... 77

Literature Cited .......................................................................................................................................... 79

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BackgroundThe U.S. Department of Agriculture (USDA) ForestService (FS) has management responsibility formore than 380,000 miles of road. These generally“low-volume” roads have various surface typesranging from thin, hot-mix asphalt cement andbituminous surface treatments (BSTs) to crushedgravel and native soils. Historically, the major heavyvehicle use of these roads has been for log haulduring timber sales. A large portion of the totalmileage is located in areas subjected to seasonalfreezing and thawing or seasonal wet/dry climates.The potentially detrimental effect of heavy vehicleson thaw-weakened or saturated pavementstructures is well known (figure 1). In an effort toreduce road surfacing maintenance costs andextend pavement life, it has been a commonpractice within the FS to restrict heavy vehicle useduring periods of severe seasonal weakening.Decisions on the timing of when to place and liftload restrictions are generally made by local roadmanagers and are based on visual distressindicators, past experience, and local economicimpacts. Short of observing actual damageoccurring to pavement structures, objective datademonstrating structural weakening is usually notavailable.

Current spring-thaw load restriction practices,including the level of restriction, correct timing,and length of restricted haul, vary appreciably

among road maintenance agencies (Kestler et al.2000). During the past 15 years, several forestsin the northwestern United States have installedsubsurface temperature monitoring probes underpaved roads to help determine periods of thaw-weakened conditions (figure 2). The probes consistof thermistors, spliced into a multistrand cable thatis encapsulated into casting resin within a clear,small-diameter plastic tube several feet long. Thetube is placed into a drilled hole through thepavement and backfill is compacted around it.Temperatures at depths below the road surfaceare monitored at the end of the instrumented cable.The readout end of the cable terminates in a small,weatherproof, electrical type box at the side of theroad, often nailed to a tree. Readings have beentaken both manually, with a digital thermometerunit, and with automated data loggers (figure 3).Current practice when using the thermistors is toplace load restrictions when thawing below theasphalt is indicated by subsurface temperatures above32 °F. Based on past limited studies of paved roadswith nonplastic, coarse granular subgrades, therestrictions were generally maintained until allground ice has melted (McBane and Hanek1986a,b). While the use of thermistor probes havebeen effective in determining the beginning ofstrength loss and when to initiate load restrictions,the timing of strength recovery and restrictionremoval is less well-defined.

Figure 1—Examples of pavement damage due to thaw weakening.

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Figure 2—Taking subsurface temperatures at a typicalthermistor probe installation. Sites like this exist on manynational forests in the northwestern United States.

Figure 3—The Handar 555 data-logger is used for auto-logging of thermistor probe readings. Data-logger isshown installed inside a weather-tight utility box.

PurposeThis report documents a project intended toinvestigate and better define the interrelationshipthat occurs in freeze/thaw and wet/dry climatesamong subsurface temperature, moisture, andpavement load capacity. In cooperation with theU.S. Army Cold Regions Research and EngineeringLaboratory (CRREL), the FS investigated thefeasibility of using Time Domain Reflectometry(TDR) and Radio Frequency (RF) soil moisturesensors in conjunction with thermistor probes tobetter define periods of seasonal weakening atseven field test sites located on four national forestsin the northwestern and northeastern United States(figure 4). In particular, this study focused ondetermining if these technologies would be helpfulin assessing when the hauling of heavy loads canbe resumed on low-volume roads followingseasonal load restrictions.

Figure 4—Field monitoring sites.

Project OverviewIn 1993, a proposal was made to the FS RoadsTechnology Development Program (RTDP) toinvestigate the feasibility of using TDR technologyto monitor real-time changes in moisture contentof road base and subgrade materials. The proposalwas subsequently approved and funded by theFS RTDP and U.S. Department of Transportation,Federal Highway Administration (FHWA), FederalLands Highway Coordinated TechnologyImplementation Program (CTIP). A cooperativeagreement was reached with CRREL for assistancebecause they had substantial experience with boththe TDR and RF technologies. Interest in theserelatively new technologies (new in terms of usefor measuring soil moisture) was high, since no

KootenaiNational Forest

OchocoNational Forest

White MountainNational Forest

WillametteNational Forest

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easy-to-use, rugged, reliable method fornondestructive, in situ, real-time measurement ofsubsurface moisture contents at multiple depthsexisted.

A secondary objective of this study was to providefield based information on the timing, duration, andmagnitude of seasonal strength and moisturechanges in road base and subgrade materials foruse in pavement design. Mechanistic-empiricalpavement designs generally require an estimationof seasonal strength and/or moisture variations(Uhlmeyer et al. 1996). Field measured data of thissort is extremely limited, and the results from thisstudy added to the limited database.

Commercially available TDR and RF soil moistureprobes were investigated, evaluated in thelaboratory, and installed at field sites. Laboratoryprograms consisted of calibration testing and freeze-thaw cycling of the probes. The purpose of thelaboratory testing was to evaluate the operationalcharacteristics of the devices with respect to theplanned field applications. Field installationsoccurred at sites on four national forests: theWillamette National Forest in western Oregon, theOchoco National Forest in central Oregon, theKootenai National Forest in northwestern Montana,and the White Mountain National Forest in NewHampshire. Sites were monitored 1 or more years.Additional site instrumentation included thermistorprobes for monitoring subsurface, pavement, andair temperatures; and open well standpipes formonitoring groundwater levels. Falling WeightDeflectometer (FWD) testing was performedperiodically at all sites.

Instrumentation

TDR

TDR probes were used to monitor seasonalvariations of in situ moisture contents of road baseand subgrade soils at multiple depths. TDR probes(figure 5) determine a soil’s volumetric water contentby measuring its apparent dielectric constant (Ka).A material’s dielectric constant is a function of theratio between the speed of light in a vacuum, (c),and the velocity (v) of an electromagnetic pulsethrough the material of interest. It can be writtenas Ka = (c/v)2

Since the length of the tines of the waveguideprobes (length the pulse travels), (L), is known,the equation can be rewritten as Ka = (ct/L)2

where t is the transit time of the electromagneticpulse along the probe tines. The TDR device,similar to radar, transmits an electromagnetic pulsethrough a cable to the waveguide probe. The transittime of the pulse along the probe length ismeasured, and the dielectric constant of thematerial surrounding the probe is calculated usingthe above equation. The dielectric constants ofair, dry soil, ice, and water are approximately 1,3–5, 3–4, and 80, respectively. Because thedielectric constant of water is so much greaterthan that of dry or frozen soil, the contribution ofliquid water to the overall soil-water-air mixturedominates the dielectric constant, andconsequently, the percentage of water, by volume,can be determined. Additional detail on theory isprovided in a number of references (Baker 1990;Klemunes 1998; Look and Reeves 1992; Topp etal. 1980; Topp et al. 1994).

Figure 5—Soilmoisture Equipment Corp. TDR buriableprobe and testing unit.

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TDRs have previously been used for geotechnicaland pavement monitoring applications (Cramer1995; Kane 1986). Both two- and three-tine probeshave been permanently buried in the soil orstructure to be monitored. Probes are availablecommercially from a number of companies or canbe fabricated in any machine shop. Probes canbe read by using a Tektronix (or similar) cable-testing unit or by a commercially available specialTDR readout unit. The cable testing units requiremore time and effort than TDR readout units forwaveform analysis to determine Ka. The TDRreadout unit used on this project was a TRASEModel 6050X1 built by Soilmoisture EquipmentCorp. of Santa Barbara, CA (figure 6). The $8,800cost was more expensive than a standard Tektronixcable-testing unit, but the TRASE device providesdirect volumetric water content values. Individualprobes cost less than $100 each. Literature forthe project TDR equipment is available fromSoilmoisture Equipment Corp. (SoilmoistureEquipment Corp. 1993).

Figure 6—TRASE TDR unit taking moisture contentreading from probe installed below pavement. Theweather-tight utility box shown is used to protect TDRprobe cable ends between readings.

RFRF probes were also used to monitor in situseasonal changes in subgrade moisture contentat multiple depths. RF probes (figure 7), developedby Dartmouth College and CRREL, are nowcommercially produced by Vitel, Inc. of Chantilly,VA (Atkins et al. 1998; Campbell 1988; Kestler etal., in preparation). The Vitel RF probes used inthis study allow measurements of both the realand imaginary dielectric constants. The frequencyof operation of the probes is 50 MHz, whichminimizes the effects of soil salinity and reduces

the need for soil specific calibrations. All requiredelectronics for Ka determination are containedwithin the buriable probes. Four output voltagesfrom the probes can be read with standardmultimeters and automated data acquisitionsystems. Software provided with the equipmentcan be used for determination of a soils salinity,volumetric moisture content, and temperature. Thecost of a probe, connector, and 50 feet of cablewas approximately $300 and a portable readoutunit was approximately $750 (figure 8). Typicalmanufacturer’s literature on the RF probes andthe readout unit are provided by Vitel, Inc. (Vitel1994a,b).

Figure 7—Vitel, Inc. RF probe and reader.

Figure 8—Vitel, Inc. RF reader is shown inside a weather-tight utility box. It is used to protect cable ends betweenreadings.

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Thermistor Strings

Thermistor probes were installed at the test sitesto measure air, pavement, and subsurfacetemperatures for determining the timing and depthof freezing and thawing. Thermistor probes for theproject were constructed by Intercity EngineeringInc. of Springfield, OR to FS specifications usingYSI 44004 thermistors from Yellow SpringsInstrument Co., Inc. The probes were of variouslengths, depending on site climate, and consistedof 9 to14 individual thermistors, spliced into amultistrand cable that was encapsulated usingcasting resin into a small-diameter clear plastictube. A thermistor lead was also provided to monitorpavement temperature, and another thermistor,on a separate cable, monitored air temperatures.A typical probe costs approximately $300. Both amanually operated switchbox (built by IntercityEngineering, Inc., at a cost of approximately $92),in combination with a commercial digitalthermometer (Tegam, Inc. Model 865, at a costof approximately $190), as well as an automateddata logger system (Handar, Inc., Model 555A, ata cost of approximately $1,850) were used to collectsite temperatures. As previously mentioned, similarprobes are currently used by several nationalforests in the northwestern United States to assistin determining critical periods of weakening duringspring thaw (Baichtal 1990; Barcomb 1989; Collins1991; DeJean et al. 1991; McBane and Hanek1986a,b; Utterback 1995).

Standpipes

Open well standpipes were installed to monitorseasonal fluctuations of groundwater at the sites.Standpipes for the project consisted of geotextilewrapped, slotted, 1-inch-diameter PVC water pipeplaced into multiple geotechnical exploration drillholes at each site. The drill holes were continueduntil bedrock was encountered or the hole wasapproximately 20 feet in depth, whichever occurredfirst. Groundwater levels were monitored manuallyusing an electronic water meter from SlopeIndicator Co. of Seattle, WA.

FWDFWD testing (figure 9) was performed at each siteto monitor seasonal changes in pavement stiffness,allowing calculations to be made of seasonalchanges in load carrying capacity and pavementdamage potential. The FWD device applies a fallingweight load to the pavement surface, and theresulting deflection basin is measured by velocitytransducers or linear variable displacementtransducers (LVDTs) at preselected distances fromthe applied load. Internal integration (or directreadout in the case of the LVDTs) yieldsdisplacement, which can be used to manuallydetermine pavement layer moduli viabackcalculation (FHWA 1994).

Figure 9a—FWD used at Willamette, Ochoco, andKootenai National Forest sites.

Figure 9b—FWD used at White Mountain National Forestsite.

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The FWD testing for the three western UnitedStates locations, each location with two test sites,was contracted with Pavement Services, Inc. ofPortland, OR. The device used was a KUAB 2mModel 150 FWD. A 5.91-inch-diameter segmentedplate transferred the load pulse to the pavement.Deflections were recorded by seven LVDTs thatmeasured the displacement of the sensor bodyrelative to an internally suspended damped massthat acts as an inertial reference. The LVDTs hada measurement range of 0 to 200 mils within theaccuracy tolerances of American Society of Testingand Materials (ASTM) D 4694. The FWD load anddeflection measurement system was calibrated atthe Strategic Highway Research Program’s (SHRP)Western Region Calibration Center prior to theproject (calibration date October 18, 1994) andagain prior to the start of testing on the KootenaiNational Forest sites (calibration date October 19,1995). FWD testing of the White Mountain NationalForest site was performed by a Dynatest device,provided by CRREL, that had previously beencalibrated at the Pennsylvania Department ofTransportation (DOT) SHRP calibration center inHarrisburg (calibration date December 3, 1997).

The FWD testing was conducted by first applyinga “seating” drop of approximately 12,000 pounds.After the seating drop, three drops were appliedat each of the target loads of 6,000, 9,000, and12,000 pounds. Peak load and deflection valueswere recorded for all drops except the seating drop.In addition, deflection and load time histories wererecorded for the three drops targeted at 9,000pounds. Deflection sensor spacing varieddepending on the project test site and localstructural and environmental conditions.

EVERCALC, a backcalculation computer programdeveloped for the Washington State Departmentof Transportation (WSDOT), was used for thebackcalculation of pavement moduli for this study(FHWA 1994; WSDOT 1995a,b). EVERCALC is amechanistic pavement analysis program based onthe program CHEVRON. It uses an iterativeapproach to match measured and theoreticalsurface deflections calculated from assumedmoduli, and the root mean square (RMS) value isminimized. It can handle up to five layers with orwithout a rigid base. Seed moduli can be estimated

based on relationships among layer moduli, load,and a variety of deflection basin parameters. Itshould be noted that one of backcalculationsrecognized weaknesses is its questionableaccuracy during thawing, particularly with frozenand thawed layers during spring freeze-thawcycling. Backcalculation does not adequatelyconsider thin layers well nor does it handle soft(thawed) layers between two stiffer (frozen) layers.However, it is believed the backcalculated modulivalues determined here were reasonable.

Laboratory Tests

Laboratory testing was a dual-phase program. Thefirst phase consisted of laboratory tests performedto evaluate the durability, accuracy, andrepeatability of TDR and RF moisture contentsdetermined using the manufacturer suppliedcalibration curves. Measurements were comparedto known gravimetric values determined in large-scale soil molds using four different soil types(figure 10).

Figure 10—Large diameter soil mold used in calibrationtesting.

The second phase required the development of asimple inexpensive laboratory freeze-thaw moisturesensor testing device (figure 11) used to evaluaterepeatability and accuracy of the probes whensubjected to freeze-thaw cycling (Kestler et al.1997).

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Figure 11—Soil mold developed by CRREL for repeatedfreeze/thaw testing of TDR and RF probes.

Field Sites – General

Instrumentation was installed at two test sites eachon the Willamette National Forest, the OchocoNational Forest, and the Kootenai National Forest.A single test site was instrumented on the WhiteMountain National Forest as part of a largerunassociated road drainage study conducted byCRREL. Details in the instrumentation andmonitoring activities varied slightly among the sitesand will be discussed in the respective site-specificsections that follow. In general, each site consistedof a road segment approximately 80-feet long. Oneor two thermistor probes, depending on the site,were installed at each location with the probesgenerally near either end of the site. Near themiddle of the site, a vertically spaced series ofTDR probes were installed in a horizontalorientation, either into the side of a borehole orwithin the borehole backfill. Similarly, in a secondborehole, a series of vertically spaced RF probeswere installed. The depth of the instrumentedvertical profile varied from site to site and wasintended to slightly exceed the anticipated frostdepth. Five FWD impact points, 20-feet apart, werelocated along the length of each site and markedto allow repeated testing on the same spots. Ingeneral, the FWD impact points were locatedbetween the wheel paths. Measurements of all

instrumentation were taken at the time of eachFWD test, as well as periodically between tests.

Pavement structures at the sites are representativeof typical paved and aggregate surfaced FScollector and arterial roads. All of the test sites,except the two on the Willamette National Forest,had asphalt surfacing. The asphalt thickness variedfrom 3.0 to 5.5 inches, depending on the site.Asphalt quality varied from new hot-mix asphalton the White Mountain National Forest, to amoderately aged hot-mix asphalt on the OchocoNational Forest, to a composite asphalt surfaceconsisting of several types of asphalt treatment(bituminous surface treatments, cold mix,chipseals) placed over a period of many years onthe Kootenai National Forest. Both sites on theWillamette National Forest had crushed aggregatesurfacing. The Willamette National Forestaggregate surfaced sites were chosen toinvestigate the potential for utilizing moisturemonitoring devices for road management ofunpaved roads in nonfrost climates.

The range of subgrade soils at the test sites istypical of those found in mountainous terrain andrepresentative of FS roads. Subgrade soils, underthe Unified Soil Classification System, varied widelyincluding silty sands (SM), silty-sandy gravels(GM), clayey sands (SC), silts (ML), and elasticsilts (MH).

Climates among the sites are representative ofthe wide range of conditions commonly found onmountainous FS roads throughout the UnitedStates. Climates ranged from a wet but nonfrostcondition at the Willamette sites, to a mild andrelatively dry climate with repeated freeze/thawcycling on the Ochoco National Forest, to relativelysevere, wet, deep frost at the Kootenai and WhiteMountain sites.

The Willamette, Ochoco, and White Mountain siteswere instrumented in the fall of 1994. Sitemonitoring and associated FWD testing occurredfor approximately 1 year following instrumentation,with additional data collected in subsequent yearsat the White Mountain site. The Kootenai siteswere instrumented in the fall of 1995 withmonitoring and FWD testing occurring during 1995and 1996.

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Laboratory Testing Program

The laboratory testing program was a multi-phaseprogram. The first phase, conducted by theWillamette National Forest Materials Laboratory,was initiated for the purpose of gaining familiaritywith the operational aspects of the TDR and RFequipment in the range of soil types and moistureand density conditions expected at the field testsites. The second phase of the testing program,conducted by CRREL, evaluated the TDR and RFprobes when subjected to freeze-thaw cycling(Kestler et al. 1997). Both studies evaluated thedurability, accuracy, and repeatability of probesand provided information for installation proceduresand limitations of the equipment.

The TRASE TDR equipment utilizes the “Topp”correlation equation programmed into the TDR unitto relate the dielectric constant to the volumetricmoisture content (Soilmoisture Equipment Corp.1993). The Topp equation is an experimentallyderived third degree equation based on laboratorytests of silts and clays. The equation is as follows:

θv

= -0.053 + 0.0292 Ka -5.5 × 10-4 Ka 2 + 4.3 ×

10-6 Ka 3, where

θv

= the volumetric moisture content of a soilexpressed as a percent, and

Ka = the dielectric constant of the soil.

The soil types, bulk densities, and moisturecontents used to develop the Topp equation weretypical of agricultural soils. The Topp equation hasbeen shown to adequately address these soil typesas a “universal” correlation equation (Topp et al.1994).

The Vitel RF equipment utilizes experimentallyderived polynomial equations for three standardsoil types (sand, silt, and clay) in correlating thevolumetric moisture content to the dielectricconstant. The equations for each soil type can beformulated using the following generic equationand the coefficients shown below.

θv = a + b Ka + c Ka 2 + d Ka

3, where

Soil Type a b c d

Sand -8.63 3.216 -0.0954 0.001579

Silt -13.04 3.819 -0.09129 0.007342

Clay -20.93 6.553 -0.2464 0.0032414

Some of the road building materials, in which thegauges for this study were installed, were outsidethe range of soil types used to derive theseequations. This testing procedure was initiated todetermine the appropriate relationship betweenthe dielectric constant and moisture for commonroad building materials.

Soil Types Tested

Four soils were tested in the first phase of thelaboratory testing program. The Rexius Loam wasselected because it was similar to an agriculturalsoil used in the derivation of the Topp equation.The Rexius Loam was 1/2-inch-minus with 48.4percent passing the #200 sieve, nonplastic, anda classification of SM.

The aggregate soil was selected because probeswere planned for instal lat ion in roadbaseaggregate, as well as forest soils that are moregranular than agricultural soils. The aggregateselected was 1/2-inch-minus with 9.6 percentpassing the #200 sieve. Aggregate base materialis typical ly 1- inch-minus or coarser, but thespacing of the RF gauge’s probe tines were only0.47 inches. The finer aggregate was used toallow for better material contact around the probetines. The other two soils were subgrade soilsfrom the Willamette and Ochoco field test sites.These materials were an elastic silt and siltysand, respectively. The Ochoco soil was 1/2-inch-minus with 47.8 percent passing the #200 sieve,had a liquid limit of 39 percent, a plasticity indexof 4 percent, and a classification of SM. TheWil lamet te soi l was 3/8- inch-minus with 72percent passing the #200 sieve, a plasticity indexof 6 percent, and a classification of MH.

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Subgrade soil from the White Mountain NationalForest test site was used for the laboratory freeze/thaw testing by CRREL. The soil was a nonplasticGM under the Unified Soil Classification System.It contained 15 percent gravel, 53 percent sand,and 32 percent fines passing the #200 sieve.

Test Procedures

During testing at the Willamette Laboratory, theexperimental procedure consisted of placing bothTDR and RF wave-guides in a large mold withsoil compacted to a specific moisture content anddensity. The mold was constructed from a 24-inch-long section of 18-inch-diameter PVC pipe (id of17.44 inch) mounted onto a plywood base. Themold was scribed horizontally on the inside every3 inches. Soil was batched to the specified moisturecontent and kept in sealed buckets. A specifiedweight of the soil-water mixture was placed in themold and compacted in 3-inch lifts to meet thespecified density. Compaction was accomplishedwith a 5.5-inch-diameter vibratory plate. Moisturecontents were obtained before, during, and aftercompaction with testing on approximately everyother lift to assure quality control.

Tests were performed on each of the four soils ata density of approximately 85 percent of themaximum density as defined by AASHTO T-99 andat various moisture contents above and belowoptimum moisture. This density level was selectedto facilitate insertion of the probes into the soil.

The RF probes consist of four 0.16-inch-diameterby 2.3-inch-long tines. Three of the tines werespaced at 120 degrees along a 0.50-inch-radiuscircle with the fourth tine at the center of the circle(figure 7). The RF probes were pushed verticallyinto each layer approximately 3.5 inches from theoutside of the mold at 90 degree increments.

Various configurations of probes or waveguideconnectors are available with the TRASE TDRequipment. This study used both a connector-typeand buriable-type waveguide. The connector probehad two 0.25-inch-diameter by 5.9-inch-longdetachable tines spaced 2.1 inches apart. Theburiable probe had three 0.13-inch-diameter by7.9-inch-long tines spaced 1 inch apart. The

connector probe was made to be installed verticallyfrom the soil surface, while the buriable probe ismade to be buried horizontally in a soil, but canbe pushed vertically. Generally, the buriable probeswere installed horizontally. Typically, the TDRconnector probes were pushed vertically into everyother lift (6 inches) approximately 3.5 inches fromthe outside of the mold, 180 degrees apart. TDRand RF probes were placed in the same lift, butwere placed away from each other as not to affectthe readings. Initially, the buriable probes wereplaced at the layer interfaces. This resulted in someproblems with erratic readings. It was believedthe problems were due to density variations causedby uneven compaction around the probe. Theprocedure was then modified by scarifying thebottom layer prior to placing the probe and upperlayer. This appeared to improve the situation, butmore consistent results were obtained when aprobe was placed at middepth of an uncompactedlayer. The last of these methods was employedfor this study.

The CRREL testing program consisted ofdetermining the freeze-thaw behavior of a 17-inch-diameter soil sample subjected to three freeze-thawcycles instrumented with temperature, RF, and TDRsensors. The test apparatus and general testingprocedure were modeled after CRREL’s frostsusceptibility laboratory test. The purpose of theprogram was to determine the reliability anddurability of the sensors when subjected torepeated freeze-thaw behavior.

The White Mountain test site subgrade soil wascompacted by CRREL into a 17-inch-diameter cellin 9 lifts, 90 to 95 percent of the optimum densityas defined by AASHTO T-99. The test apparatuscell consisted of 12 clear plastic ringsapproximately 1.1-inches high, placed on top ofa porous plate and an aluminum base plate. Thebase plate allowed water entry into the samplefrom a reservoir. Cold plates at the top and bottomof the sample enabled control of a thermal gradientacross the soil sample. The test apparatus canbe seen in figure 11. The sample was instrumentedwith three TDR sensors, three RF sensors,thermistors, thermocouples, and screws mountedinto the clear plastic rings for monitoring frostheave.

10

Test Results

During the Willamette Laboratory calibration checktesting, with the TDR buriable probe installedhorizontally, readings of volumetric moisture weretaken after the construction of every lift. Theconnector probe was installed vertically, read, thenremoved and reinstalled after every lift. The RFprobes were also inserted vertically, read for Ka,then removed and reinstalled after every lift. Thevolumetric moisture results for a given probe typeand orientation were averaged for a test setup(soil type, density, and moisture content). Thevolumetric moisture content was converted to thegravimetric moisture content of the test point bythe following relationship:

θg = θv × γw / γd, where

θg = the gravimetric moisture content,

θv = the volumetric moisture content,

γw = the unit weight of water, and

γd = the dry density of soil.

The RF Ka data was converted to the volumetric

moisture content (and then gravimetric moisturecontent using the above relationship) using thecorrelation equations for sand, silt, and clay.Figures 12 through 15 show the lab measuredgravimetric moisture contents versus thosecalculated from the gauge readings with the 1:1line indicating no deviation between measured andcalculated values.

As shown in the figures, the RF results were allclose to the 1:1 line. For the Willamette site soil(Lowell Silty Clay), the RF values for the “clay”curve are essentially on the 1:1 line, as was the“silt” curve values for the Rexius Loam. For theOchoco soil, the “silt” and “clay” curves are veryclose to the 1:1 line. The aggregate results werefarther from the 1:1 line, but still good. In all cases,the graphs plotted approximately parallel to the1:1 line, indicating good correlation betweenchanges in gravimetric and volumetric moistures.

The TDR results, with the exception of theaggregate soil and the Ochoco silt, generally variedmore from the 1:1 line than the best RF fits, butwere still reasonably close. The TDR results were

better for the Rexius Loam and Ochoco soils asthey are more similar to the silty soils modeledby the Topp curve. As with the RF probe, the TDRdata generally plotted approximately parallel tothe 1:1 line.

During the CRREL freeze-thaw testing, the frostpenetration rate was fairly constant and wasselected to be similar to the rates expected at theMontana and New Hampshire sites. The frost heaveafter the first cycle was minimal, probably due tothe position of the water reservoir being belowthe base of the actual sample. The level was raisedfor cycles 2 and 3 and the sample heaved by 2.5inches or nearly 15 percent.

The initial moisture contents for each probe typemeasured by the three TDR probes or three RFprobes were close to one another, but the RFvalues were different than the TDR values. Thisis because the RF probes were installed in frozensoil and inserted into the mass while the TDRprobes were compacted into the mass. After thefreeze-thaw cycles, the TDR and RF valuesapproached one another as expected.

The relationships between frost penetration, frostheave, and water added to the system wereanticipated. The total frost heave correspondedwell to frost penetration and to the water addedto the system. Sharp increases and decreases involumetric water contents also correspond wellwith the freezing front. The reader is referred tothe previously referenced paper (Kestler et al.1997) for additional details on this portion of thestudy.

Conclusions on Lab Testing

1. During the Willamette Laboratory testing, bothgauges did reasonably well correlating Ka tomoisture. The RF did slightly better, probablybecause of the separate curves for various soiltypes.

2. The internal RF calibration was a poor choicefor the aggregate test soil, probably because ofthe difficulties obtaining soil compactionbetween the tines.

11

a) Lab Testing: Lowell Silty Clay

b) Lab Testing: Lowell Silty Clay RF Calibration

Figure 12—TDR and RF results compared with laboratory determined values—Lowell Silty Clay.

)

)

GR

AV

IME

TR

IC M

OIS

TU

RE

(%

)(f

rom

gau

ge

read

ing

)G

RA

VIM

ET

RIC

MO

IST

UR

E (

%)

(fro

m g

aug

e re

adin

g)

GR

AV

IME

TR

IC M

OIS

TU

RE

(%

)(f

rom

gu

age

valu

e)

GRAVIMETRIC MOISTURE (%)(from gauge reading)

GRAVIMETRIC MOISTURE (%) - (from lab value)

)GRAVIMETRIC MOISTURE (%)(from gauge reading)


GR

AV

IME

TR

IC M

OIS

TU

RE

(%

)(f

rom

gau

ge

read

ing

)G

RA

VIM

ET

RIC

MO

IST

UR

E (

%)

(fro

m g

uag

e va

lue)

12

a) Lab Testing: Aggregate

b) Lab Testing: Aggregate—RF Calibration

Figure 13—TDR and RF results compared with laboratory determined values—Aggregate.

GR

AV

IME

TR

IC M

OIS

TU

RE

(%

)(f

rom

gau

ge

valu

e)

)

)

TDR-BUR.HOR.

)

GR

AV

IME

TR

IC M

OIS

TU

RE

(%

)(f

rom

gau

ge

read

ing

)G

RA

VIM

ET

RIC

MO

IST

UR

E (

%)

(fro

m g

uag

e va

lue)





GR

AV

IME

TR

IC M

OIS

TU

RE

(%

)(f

rom

gau

ge

read

ing

)G

RA

VIM

ET

RIC

MO

IST

UR

E (

%)

(fro

m g

uag

e va

lue)

13

a) Lab Testing: Rexius Loam

b) Lab Testing: Rexius Loam—RF Calibration

Figure 14—TDR and RF results compared with laboratory determined values—Rexius Loam.

GR

AV

IME

TR

IC M

OIS

TU

RE

(%

)(f

rom

gau

ge

valu

e)G

RA

VIM

ET

RIC

MO

IST

UR

E (

%)

(fro

m g

aug

e va

lue)

)

(from lab value)

(from lab value)

TDR-CONN.-VERT.

TDR-BUR.-HOR.

RF - SILT

14

a) Lab Testing: Ochoco Silt

b) Lab Testing: Ochoco Silt—RF Calibration

Figure 15—TDR and RF results compared with laboratory determined values—Ochoco Silt.

GR

AV

IME

TR

IC M

OIS

TU

RE

(%

)(f

rom

gau

ge

valu

e)G

RA

VIM

ET

RIC

MO

IST

UR

E (

%)

(fro

m g

aug

e va

lue)

)

)

15

3. Both the TDR and RF results tended toapproximately parallel best-fit 1:1 correlationlines. This indicates that small soil-specificcorrection values could be developed, if needed,to further improve the correlation accuracy.

4. During freeze-thaw testing, rapid decreases andincreases in the apparent water content wereobserved as the freezing front advanced andretreated. These observations showed that TDRand RF moisture sensors are good indicatorsof frozen versus thawed conditions.

5. Laboratory testing demonstrated that TDR andRF probes are reliable, repeatable, and durablewhen used to measure unfrozen water contentsin soils subjected to freeze-thaw cycling.

Willamette Field SitesLocation and General Layout

The Willamette National Forest test sites werelocated on the Lowell Ranger District, ForestService Road No. 1821190 at milepost 0.6. Road1821190. Typical of forest roads west of theCascade, it is a single-lane road with turnouts,side-hill construction, aggregate surface, and roadgrades between 6 and 12 percent (figure 16). Theclimate west of the Cascades is maritime, withmild temperatures and a distinct rainy and dryseason. Average annual precipitation measuredat the closest weather station (Lookout Point Dam,approximately 13 miles west of the site) is 40inches, delivered primarily as rainfall betweenNovember and May.

Figure 16—Willamette National Forest test site duringinstrumentation installation. The narrow, aggregatesurfaced road is typical of many western Oregon FSroads.

The test site location was within the road segmentused for a road sedimentation study (Foltz andTruebe 1994) and an aggregate thickness designstudy (Truebe and Evans 1994) that weremonitored between 1992 and 1994. The two siteswere approximately 50 feet apart and set upsimilarly. Each site had five FWD test locationpoints spaced 20 feet apart. Instrumentation pointsfor TDR, RF, and thermistor sensors were locatedbetween the FWD tests location points as shownin figures 17 and 18.

The TDR and RF probes were installed horizontallyin the sidewall of the augered holes when possible.The Willamette sites were the first sitesinstrumented in this study and the installationequipment consisted of simple pry bars. Some ofthe probes had to be compacted in the backfillbecause of these limitations. Specialized pry barheads were constructed for the Ochoco andKootenai sites, and more probes wereconsequently installed into the sides of the augeredholes (figure 19).

Site Surfacing Structure and MaterialProperties

The Willamette sites were aggregate surfaced roadsegments over an elastic silt subgrade (ML to MH).The surfacing material was a good quality 1-inch-minus, dense graded, crushed aggregate rangingin thickness from 10 to 11 inches at site 1, and 11to 12 inches at site 2. A thin layer (2 to 5 inches)of an older, 4-inch-minus aggregate wasencountered at some locations at site 1.

A subsurface investigation was performed at eachsite for the purpose of determining the depth to amaterial change or “hard layer” and for monitoringgroundwater fluctuations. Two drive probe points(depths up to 27 feet) and one augured hole (depthsup to 17 feet) were installed at each site.Standpipes were placed in all of the holes for waterlevel monitoring.

16

Figure 17—Instrument Layout: Willamette National Forest—Site 1.

DP-3,-4 AND AH-3Z: GROUNDWATER A-E: FWD

VERTICAL SCALE: 1"=20"

HORIZONTAL SCALE: 1"=20’

SCALE: 1"=20’

AH-3Z

E

E T1

D

RF

C

D C

B

DP-4TDR

B

DP-3

A

A

10"

15"

21"

27"

33"

45"

7"

13"

17"

21"

27"

33"

45"

54"

6"

10"

14"

18"

22"

26"

30"

36"

42"

AGGREGATE

1" MINUS

DENSE GRADED

4" MINUS AGGREGATE

SUBGRADE:

T1: THERMISTOR PROBE

NOTES:

1 - PROFILE VIEW SHOWS PLACEMENT DEPTHS OF INSTRUMENTATION

2 -

3 - BOREHOLE DIAMETER NOT DRAWN TO SCALE

PROBE INTO SIDE OF BOREHOLE

PROBE COMPACTED INTO BOREHOLE BACKFILL

PROBE PLACED VERTICALLY IN BOREHOLE BACKFILL

TDRT1RF

MH - ELASTIC SILT

4 -

B) PROFILE VIEW

A) PLAN VIEW40'

14'

17

Figure 18—Instrument Layout: Willamette National Forest—Site 2.

DP-1, -2 AND AH-8Z: GROUNDWATER A-E: FWD



SCALE: 1"=20’

E

E T1

D

RF

C

D C

B

TDR

B

DP-2

A

A

6"

10"

14"

18"

22"

26"

30"

36"

42"

AGGREGATE

1" MINUS

DENSE GRADED

SUBGRADE:

DP-1

AH-8Z

11"

16"

21"

27"

33"

45"

6"

12"

16"

20"

27"

33"

45"

57"

NOTES:


2 -

3 - BOREHOLE DIAMETER IS NOT DRAWN TO SCALE



RF T1 TDR

T1: THERMISTOR PROBE

MH - ELASTIC SILT

4 -

A) PLAN VIEW

B) PROFILE VIEW

14'

18

Figure 19a—Modified (slotted ends) pry bars are used for inserting probes into the side of a borehole.

Figure 19b—Using a pry bar with an attached wooden block, the TDR probe is inserted into the side of a borehole.

19

TDR, RF, and Thermistor Instrumentation

The TDR probes were installed in 10-inch-diameteraugured holes. Soil and aggregate was saved fromspecific depths of the excavation and reused asbackfill at corresponding depths. In some instances,extra material was necessary to backfill the hole.An effort was made to push the probes into thesides of the boreholes, but when this was notpossible, probes were installed in the backfill. AllTDR probes were installed horizontally and rotatedapproximately 45 degrees from the probe installedimmediately below. Depths and installation methodof the TDR probes are shown in figures 17 and18.

The RF probes were installed in 6-inch-diameteraugured holes. Soil and aggregate was conservedand utilized similar to the TDR holes. Similar tothe TDR sites, an effort was made to install theRF probes into the sides of the augured holes.When this was not possible, the probes wereinstalled vertically into the backfill. Depths andinstallation method of the RF probes were alsoshown in figures 17 and 18.

The thermistor strings were installed with 9thermistors located between depths of 6 and 42inches and a 10th thermistor located to monitorair temperatures.

Data Collection and FWD TestingProgram

Data was collected for temperature (thermistors)and weather conditions, moisture content (TDRand RF), and groundwater from August 19, 1994through August 29, 1995. Readings were obtainedat least every month during the dry season andat least twice per month during the wet time ofthe year. All readings were obtained manually.Additional information was recorded and storedin the TDR and RF units for review and dataverification purposes.

FWD data was obtained throughout the year forthe purpose of determining material layer strengthsand seasonal strength variations. Data wasobtained every other month from October 1994through August 1995. Temperature and moisturedata was always collected on these test dates.

Analysis and Results

Temperature

Subsurface and air temperatures were monitoredat the Willamette sites from July 1994 through May1995. Temperatures were read manually at leastmonthly, generally late in the morning or in theafternoon as shown in figure 20. All measuredtemperatures were above 32 °F. The lowesttemperature measured in the ground was 34.5 °Fat a 6 inch depth. At a depth of 10 inches (top ofsubgrade), the lowest temperature measured was35.7 °F. The average minimum monthlytemperatures (1961 to 1990) at the nearest weatherstation (Lookout Point Dam, approximately 13 mileswest of the site) were 37.4, 30.6, 29.5, and 34.7 °Ffor November through February. Based on themeasured temperatures and historic averages, itwas likely the air temperatures on some nightshad dropped below freezing, but it was unlikelyfor freezing of more than the top 1 or 2 inches ofthe aggregate to have occurred.

TDR and RF

The volumetric moisture contents as shown infigures 21 through 24 do not show a rapid increaseor decrease in moisture such as would be expectedduring freezing and thawing. The TDR figures showsmall but gradual decreases in moisture fromsummer to the beginning of the wet season, whichbegan in November. The moisture contentsremained fairly stable throughout the winter andthen decreased back to the summer values startingin June. This trend, however, was not as clearwhen viewing the RF results. The Willamette siteswere the first sites where the RF probes wereinstalled in the field. Problems with erratic readingswere experienced. This was determined to be theresult of corrosion of the aluminum connectors.These connectors were replaced with copperconnectors on March 6 and 7, 1995.

20

a) Temperatures Willamette—Site 1.

b) Temperatures Willamette—Site 2.

Figure 20—Willamette seasonal temperature variations. Legend shows depth below road surface of individual thermistorsensors within probe.

TE

MP

ER

AT

UR

E (

F)

TE

MP

ER

AT

UR

E (

F)

DATE

DATE

21

a) TDR Volumetric Moisture: Willamette—Site 1.

b) Selected TDR Volumetric Moisture: Willamette—Site 1.

Figure 21—Seasonal variation in TDR measured moisture. Legend shows probe depth below road surface.

VO

LU

ME

TR

IC M

OIS

TU

RE

(%

)V

OL

UM

ET

RIC

MO

IST

UR

E (

%)

DATE

DATE

7/21/94 9/15/94 11/10/94 1/5/95 3/2/95 4/27/95 6/22/95 8/17/95 10/12/95

SUBGRADE

SUBGRADE MID-BASE

7"

13"

17"

21"

27"

33"

15

20

25

30

35

40

45

50

55

60

65

7"13"17"21"27"33"45"54"

22

a) TDR Volumetric Moisture: Willamette—Site 2.

b) Selected TDR Volumetric Moisture: Willamette—Site 2.

Figure 22—Seasonal variation in TDR measured moisture at indicated depths below road surface.

VO

LU

ME

TR

IC M

OIS

TU

RE

(%

)V

OL

UM

ET

RIC

MO

IST

UR

E (

%)

DATE

DATE

7"

13"

17"

21"

27"

33"

45"

SUBGRADE

TOPSUBGRADE

15

20

25

30

35

40

45

50

55

60

65

6"

12"

16"

20"

27"

33"

45"

57"

7/21/94 9/15/94 11/10/94 1/5/95 3/2/95 4/27/95 6/22/95 8/17/95 10/12/95

23

a) RF Volumetric Moisture: Willamette—Site 1.

b) Selected RF Volumetric Moisture: Willamette—Site 1.

Figure 23—Seasonal variation in RF measured moistures at indicated depths below road surface.

VO

LU

ME

TR

IC M

OIS

TU

RE

(%

)V

OL

UM

ET

RIC

MO

IST

UR

E (

%)

DATE

DATE

ALL INSUBGRADE

15.0

20.0

25.0

30.0

35.0

40.0

45.0

50.0

55.0

60.0

65.0

10"

15"

21"

27"

33"

45"

7/21/94 9/15/94 11/10/94 1/5/95 3/2/95 4/27/95 6/22/95 8/17/95 10/12/95

24

a) RF Volumetric Moisture: Willamette—Site 2.

b) Selected RF Volumetric Moisture: Willamette—Site 2.

Figure 24—Seasonal variation in RF measured moistures at indicated depths below road surface.

VO

LU

ME

TR

IC M

OIS

TU

RE

(%

)V

OL

UM

ET

RIC

MO

IST

UR

E (

%)

DATE

DATE

ALL IN SUBGRADE

15.0

20.0

25.0

30.0

35.0

40.0

45.0

50.0

55.0

60.0

65.0

11"

16"

21"

27"

33"

45"

7/21/94 9/15/94 11/10/94 1/5/95 3/2/95 4/27/95 6/22/95 8/17/95 10/12/95

25

In viewing the average TDR results from

bothsites,the average subgrade volumetricmoisture content during the dry season was 31percent, while the volumetric moisture contentaveraged 41 percent during the wet season. Thiscorresponded to gravimetric moisture contents of23 and 30 percent, respectively. The percentsaturation for the dry and wet season was 61 and81 percent using a dry density of 82 pounds percubic foot (average value measured at these sitesin 1993) and assuming the specific gravity of thesolids was 2.60. The average wet season subgrademoisture content, measured as part of thepreviously mentioned sediment study at these sitesduring 1993, was 37 percent (gravimetric), whichis reasonably close to the gravimetric valuecorrelated from the TDR readings.

Groundwater

Groundwater levels were monitored in the threeopen standpipes installed at each site. As seenin figure 25, the groundwater fluctuations at eachsite were similar, occurring at essentially the sametimes. In general, the groundwater appeared tofluctuate at each site during the wet season froma depth of 7 to 17 feet.

Area Parameter

The area parameter is an index of the overallstiffness of a pavement section. The larger thearea parameter, the stiffer the pavement section(FHWA 1994). It is defined as the normalized areaof the deflection basin divided by the maximumdeflection. The area parameter was calculated asfollows:

Ap = 6[d0 + 2d

1 + 2d

2 + d

3]/d

0, where

Ap is the area parameter,

d0 is the deflection measured at the centerof the applied load,

d1 is the deflection measured at a distanceof 1 foot, and

d2 is the deflection measured at a distanceof 2 feet, and

d3 is the deflection measured at a distanceof 3 feet.

The maximum possible value of 36 (i.e.,d0=d1=d2=d3) would indicate an extremely stiffpavement system. Low values would suggest thepavement is not much different from the underlyingsubgrade.

As shown in figure 26, the area parameters forthe Willamette sites are fairly constant from October1994 to May 1995 and then increase byapproximately 10 percent during the dry season.

Layer Moduli

The subgrade moduli were backcalculated for eachFWD testing point from the deflection data usingEVERCALC (version 5.11), the WSDOT elastic-layered analysis procedure. The pavementstructure was modeled as a four layer system, withlayers 1 and 2 each being half the total aggregatethickness (including the 4-inch-minus aggregatewhere present). Layer 3 was modeled as thesubgrade soil to the top of the stiff layer. The topof the stiff layer ranged between 75 and 80 inches,defined by an increase in driving resistance of thedrive probe. The RMS error ranged from 0.13 to4.22 percent and was typically in the range of 1to 2 percent. The RMS error is slightly higher thandesired (<1 percent), but considered acceptablesince the tests were performed on aggregatesurfaced roads as opposed to an asphalt pavement.

As shown in the graphs of subgrade modulusversus date in figure 27, the moduli ranges from8 to 30 ksi at site 1 and 8 to 19 ksi at site 2. At agiven test location, there was a trend of the modulidecreasing from the dry season to the wet seasonand increasing again during the summer. Themoduli values were fairly constant through the wetwinter months, again reflecting the higher moisturecontents from rainfall as opposed to any effectsof freeze-thaw.

26

a) Groundwater Depth: Willamette—Site 1.

b) Groundwater Depth: Willamette—Site 2.

Figure 25—Seasonal groundwater variation.

DE

PT

H B

EL

OW

GR

OU

ND

SU

RFA

CE

(ft

)

DATE

DE

PT

H B

EL

OW

RO

AD

SU

RFA

CE

(ft

)

DATE

27

Area Parameter (D0-D36): Willamette–Sites 1 and 2

Figure 26—Seasonal variation in area parameter.

AR

EA

PA

RA

ME

TE

R (

in2 /

in)

DATE

28

a)Subgrade Modulus: Willamette—Site 1.

b)Subgrade Modulus: Willamette—Site 2.

Figure 27—EVERCALC determined seasonal change in subgrade modulus. Legend refers to the five individual FWD

MO

DU

LU

S (

ksi)

DATE

MO

DU

LU

S (

ksi)

DATE

29

a) Vertical Subgrade Strain: Willamette—Site 1.

b) Vertical Subgrade Strain: Willamette—Site 2.

Figure 28—EVERCALC determined seasonal change in subgrade compressive strain. Legend refers to individual FWDimpact points.

ST

RA

IN (

in/in

x 1

0-6)

DATE

ST

RA

IN (

in/in

x 1

0-6)

DATE

30

Subgrade Strain

Subgrade vertical compressive strain is animportant parameter in predicting the amount ofpavement rutting. The subgrade strain wasdetermined during the EVERCALC backcalculationprocess. As seen in figure 28, the strain at a giventesting location was fairly constant through theyear. There was a general trend of increasing strainin the wet season and decreasing strain in thedry season, but only a slight trend. The variationin strain (and thus, rutting potential) among thefive test spots within a test site exceeded theseasonal variation at any given test spot. Thissuggests that for these sites, the natural variabilityof site conditions is more important than seasonalvariations in determining pavement structuralneeds.

Conclusions from Willamette Test Site

1. No short-term dramatic changes in siteproperties were observed, only relativelygradual, annual variations in moisture content,area parameter, and modulus.

2. TDR (and to a lesser degree RF) probesperformed well in tracking seasonal changes inmoisture contents.

3. Road management load restrictions based onseasonal variation in subgrade moisture (shortof “dry-season-only haul”) are probably notfeasible for this sort of environment (i.e., wet/dry). No short-term period of susceptibility tohigh intensity damage was observed.

Ochoco Field SitesLocation and General Layout

The two Ochoco National Forest test sites werelocated on the Ochoco Ranger District, ForestService Road No. 22 at mileposts 0.1 and 0.4(figures 29 and 30). Forest Road 22 is a double-lane paved road with road grades less than 2percent. Site 1 was located at an elevation of 4,020feet in a generally shaded area while site 2 wasat an elevation of 4,140 feet in a more open area.The average annual precipitation in the vicinityof the sites is 14 to 30 inches, depending onelevation. Design freezing indices for the area

(average of three coldest winters in the last 30years) range from 500 to 750 degree-days F,depending on elevation. The area’s mean freezingindex for 30 years of records range from 100 to250 degree-days F. Freezing index is a measureof the severity of a winter climate. It is calculatedby summing the daily difference between the dailyaverage temperature (sum of the daily high andlow divided by two) and 32 °F for days when thedaily average temperature is less than 32 °F. Forexample, if the daily low was 10 °F and the dailyhigh was 40 °F, then the daily average temperaturewould be 25 °F and the freezing index for thatday would be 7 degree-days F. The daily freezingindices are summed over a winter to determine alocations freezing index for a particular winter.Freezing indices measured at sites 1 and 2 were501 and 355 degree-days F, respectively, for thewinter of 1994/1995.

Figure 29—Ochoco National Forest—Site 1 during theJanuary 25, 1995 FWD testing.

Figure 30—Ochoco National Forest—Site 2 during FWDtesting. Paint marks indicate instrumentation locations andFWD test points.

31

Each test site had five FWD test location points,spaced 20 feet apart located between the wheelpaths. Instrumentation points for TDR, RF, andtwo thermistor strings were located between theFWD test location points as shown on figures 31and 32. The TDR and RF probes were all installedhorizontally, either into the sidewall of the auguredholes or compacted within the backfill.

Site Surfacing Structure and MaterialProperties

The pavement structure at site 1 consisted of 3inches of asphalt concrete over 15 inches of a1-inch-minus, dense graded, crushed aggregateoverlying a clayey-sand to silty-sand subgrade soil.The pavement structure at site 2 consisted of 4.25inches of asphalt concrete over 12 to 18 inchesof a 1-inch-minus, dense graded, crushedaggregate overlying a clayey sand subgrade soil.

A subsurface investigation was performed at eachsite for the purpose of determining the depth to amaterial change or “hard layer” used in thebackcalculation analysis and for monitoringgroundwater fluctuations. Three drive probes wereinserted at each site to depths ranging from 8 to12 feet at site 1 and 9 to 12 feet at site 2.Standpipes were placed in the holes for water levelmonitoring.

TDR, RF, and Thermistor Instrumentation

The TDR probes, RF probes, and thermistor stringswere installed in 10-, 6-, and 4-inch-diameteraugured holes, respectively. Soil and aggregatewas saved from specific depths of the excavationand reused as backfill at corresponding depths.In some instances, extra material was necessaryto backfill the hole. The subgrade TDR and RFprobes were all pushed into the sides of theaugured holes while probes within the base coursewere compacted into backfilled aggregate. Allinstalled probes were rotated approximately 45degrees from the probe installed immediatelybelow.

TDR probes at the 34-inch depth at site 1 and the38-inch depth at site 2 failed to operate following

installation, probably caused by damage duringthe installation process. The RF probe at the 21-inch depth at site 2 also initially failed to operate,but began producing readings in March 1995.

The thermistor strings were installed with 13thermistors located between the top of the asphaltconcrete, at a depth of 64 inches, and the 14ththermistor located to monitor air temperatures.


Data was collected from temperature (thermistor)and weather conditions, moisture content (TDRand RF), and groundwater from September 30,1994 to August 30, 1995. Readings were obtainedat least once per month during the summer andincreased to approximately weekly during thewinter. Readings were collected manually whensite visits were made. One thermistor string at eachsite was monitored with an automated dataacquisition unit throughout the duration of theproject.

FWD data was obtained throughout the year forthe purpose of determining material layer strengthsand seasonal strength variations. Data wasobtained approximately monthly during the summerand biweekly during the winter. At five times inthe winter, FWD data was collected on 2 or 3consecutive days.


Temperature

Temperature was monitored two to four times perhour at each site from October 30, 1994 throughSeptember 30, 1995 by means of the Handar dataacquisition units. The data was used to producethe frost profiles displayed in figure 33. Typically,frost penetration occurred at rates of approximately2 to 3 inches per day. Typical rates of both bottom-up and top-down thawing were 1 to 2 inches perday. These roughly equal bottom-up and top-downthaw rates contrast with the colder climate testsites in Montana and New Hampshire, wherebottom-up thaw rates were only 10 to 20 percentof the top-down rate.

32

Figure 31—Instrument Layout: Ochoco National Forest—Site 1.

A B C D E

T1 RF TDR T2

SCALE: 1"=20’

DP-1 DP-2

DP-3

SUBGRADE: SC/SM

AGGREGATE BASE-

A CB D E

19"

23"

33"

27"

39"

45"

51"

63"

15"

11"

7"

3"1.5"

63"

51"

45"

39"

7"

27"

33"

23"

11"

15"

19"

1.5"3" ASPHALT

18"

24"

30"

36"

42"

52"

11"

18"

21"

27"

34"

39"

48"

58"

1" MINUS GRADATION



A-E: FWD DP-1,-2,-3: GROUNDWATER

T1,T2: THERMISTOR PROBE

NOTES:

1 - PROFILE VIEW SHOWS PLACEMENT DEPTH

OF INSTRUMENTATION

2 -

3 - BOREHOLE DIAMETER NOT DRAWN TO SCALE



BEDROCK

T1 TDRRF T2

CLAYEY SAND TO

SILTY SAND

4 -

A) PLAN VIEW

B) PROFILE VIEW

20'

33

Figure 32—Instrument Layout: Ochoco National Forest—Site 2.

A B C D E

T1 RF TDR T2

SCALE: 1"=20’

AGGREGATE BASE-

A CB D E

20.25"

24.25"

34.25"

28.25"

40.25"

46.25"

52.25"

64.25"

16.25"

12.25"

8.25"

4.25"

1.5" 1.5"ASPHALT

21"

25"

29"

34"

38"

52"

11"

19"

23"

28"

33

38"

50"

59"

1" MINUS GRADATION



A-E: FWD DP-4,-5,-6: GROUNDWATER

T1,T2: THERMISTOR PROBE

DP-4DP-6DP-5

4.25"

34.25

16.25"

28.25

24.25

20.25"

12.25"

8.25"

64.25"

52.25"

40.25

46.25

SUBGRADE: SC/SM -

NOTES:


2 -

3 - BOREHOLE DIAMETER IS NOT DRAWN TO SCALE



T2TDRRFT1

BEDROCK

CLAYEY SAND TOSILTY SAND

SC - CLAYEY SILT

4 -

A) PLAN VIEW

B) PROFILE VIEW

22'

34

a) Frost Profile: Ochoco—Site 1

b) Frost Profile: Ochoco—Site 2

Figure 33—Seasonal frost depth profiles. Note that most of the thawing occurred from bottom up at these sites. There arealso shallow frost penetrations and multiple freeze/thaw events.

DE

PT

H B

EL

OW

AS

PH

ALT

(in

)

DATE

0

5

10

15

20

10/28/94 11/11/94 11/25/94 12/9/94 12/23/94 1/6/95 1/20/95 2/3/95 2/17/95 3/3/95

FROZEN

THAWED

TOP OF SUBGRADE

DE

PT

H B

EL

OW

AS

PH

ALT

(in

)

DATE

0

5

10

15

20

10/28/94 11/11/94 11/25/94 12/9/94 12/23/94 1/6/95 1/20/95 2/3/95 2/17/95 3/3/95

FROZEN

TOP OF SUBGRADE

35

VO

LU

ME

TR

IC M

OIS

TU

RE

(%

)V

OL

UM

ET

RIC

MO

IST

UR

E (

%)

DATE

DATE

45

40

35

30

25

20

15

10

5

0

11"

18"

21"

27"

39"

48"

58"

9/17/94 11/12/94 1/7/95 3/4/95 4/29/95 6/24/95 8/19/95 10/14/95

a) TDR Volumetric Moistures: Ochoco—Site 1

b) Selected TDR Volumetric Moistures: Ochoco—Site 1

Figure 34—TDR measured moisture contents. Note the moisture spikes in base and upper subgrade during earlyFebruary thaw.

36

Also reflecting the less severe winter conditionsin eastern Oregon were three complete freeze-thaw cycles (freezing 4 inches or greater belowthe asphalt concrete) observed at each site. Themaximum freezing depths at sites 1 and 2 were14 and 18 inches, respectively. Partial thawingfollowed by refreezing occurred on three separateoccasions during December and January at site1. During these time intervals, as well as at theend of the major freezing period, water was trappedabove the ice. At site 2, water was trapped abovethe ice for only short periods of time during theend of the major freezing period. At both sites,for the most part, thawing occurred from the bottom up.

TDR and RF

The volumetric moisture content fluctuations foreach site are shown in TDR and RF figures 34through 37. In general, most probe locations showonly moderate moisture fluctuations in that theywere below the freezing depth. In addition, themoisture variation decreases with increasing depth.The TDR probes at 11 and 18 inches at site 1,the RF probe at 18 inches at site 1, and the TDRprobe at 11 inches at site 2 are within the freeze-thaw zone. These probes clearly show the decreasein volumetric moisture when frozen (actuallyreflecting the low dielectric constant of ice) and asubsequent increase in moisture after thaw.

Groundwater

Groundwater levels were monitored in the threeopen standpipes installed at each site (figure 38).At site 1, the groundwater did not fluctuatesignificantly throughout the year, but was only 2feet from the ground surface on the north end ofthe site. At site 2, the groundwater levels fluctuatedby approximately 4 feet in each well. The increasein water levels at site 2 corresponded closely tothe rapid melting of the frost profile at site 2 afterJanuary 6, 1995. This may be a consequence tosite 2 being a much sunnier site than site 1.

Area Parameter

The area parameter, a convenient index of overallpavement stiffness, was calculated for the Ochocosites in a similar manner to the Willamette sites.

As seen in figure 39, the area parameter increasedmarkedly from the fall value of 18 square inchesper inch to a peak value of 29 square inches perinch corresponding to the date of the maximumfrost penetration. After the thaw, the value droppedto 14 to16 square inches per inch then graduallyincreased back to the fall value of 18 square inchesper inch.

It is interesting to note that the two sites havesimilar area parameter values with the exceptionof the FWD tests performed during the third weekof January. From the frost profiles, it can be seenthat site 1 was still frozen approximately 8 inchesbelow the asphalt, while site 2 had little or no frost.This difference is clearly reflected in the calculatedarea parameters.

Layer Moduli

The layer moduli values were backcalculated foreach FWD testing point from the deflection datausing EVERCALC. The pavement structure wasmodeled as a four-layer system when no ice waspresent and a five-layer system with ice present.With no ice present, layer 1 was the asphaltconcrete, layer 2, the aggregate base course, layer3, the subgrade to the “hard layer,” and layer 4,the hard layer. When ice was present, layer 2 wasthe thickness of frozen aggregate (and top ofsubgrade when frozen), layer 3, the unfrozenaggregate, layer 4, the subgrade to the “hard layer,”and layer 5, the hard layer.

As shown in figures 40 and 41, above 32 °F, therelationship between backcalculated asphaltconcrete modulus and temperature is fairly flat atapproximately 500 and 700 ksi for sites 1 and 2respectively. Below 32 °F, at site 2 the modulusincreased above 1,000 ksi for some test locationsand over 900 ksi for some locations at site 1. Thebackcalculated subgrade modulus values for bothsites show moduli values in the fall of 13 ksi, adecreased modulus value of 10 ksi through mostof the winter, with a “spike” of 27 ksi at site 1 inJanuary. The spike value for site 1 occurred whenfreezing penetrated 2 inches into the subgrade.Backcalculated base modulus values for both siteswere 24 to 33 ksi during the fall, decreasing inthe spring to 12 to 17 ksi. Results during the winteron several test dates at both sites indicated basemodulus values exceeding 100 ksi when frozen.

37

Subgrade Strain

Both vertical compressive strain at the top of the subgrade and tensile strain at the bottom of theasphalt concrete were calculated from the deflection data for each testing date using EVERCALC andEVERSTRESS. EVERSTRESS is a companion WSDOT PC computer program to EVERCALC and canbe used to calculate stresses, strains, and deflections throughout a pavement structure (WSDOT 1995b).Subgrade vertical compressive strain is an important parameter in predicting pavement rutting. Asphalttensile strain is likewise an important parameter in predicting fatigue cracking (WSDOT 1995).

As shown in figure 42 for site 1, the subgrade strain in the fall was -500 × 10-6 inches per inch (thenegative indicating compression). The strain decreased to zero strain on January 7, 1995 when freezingpenetrated through the aggregate base and 2 inches into the subgrade. After the thaw, the strainincreased to approximately -650 × 10-6 inches per inch and gradually decreased back near the fallvalue of -500 × 10-6 inches per inch. A similar trend, shown in figure 43, occurred at site 2.

Damage Factors

Relative damage factors were computed for both subgrade rutting and asphalt cracking. The relativedamage factors are defined as the amount of heavy traffic the pavement could support at a referencetime (in this case, the initial fall test point) divided by the amount of heavy traffic the pavement couldsupport at any other given time. Table 1 presents the equations for rutting and asphalt fatigue crackingused in this study and as used by WSDOT in their EVERPAVE software (EVERPAVE is WSDOT’sasphalt pavement overlay PC computer design program). Rutting failure is described in the EVERPAVEmanual (WSDOT 1995b) as a 0.75-inch-deep rut. No definition of asphalt fatigue failure is provided inthe EVERPAVE manual, but similar equations to the one presented define failure as cracking occurringover 10 percent of the wheel path area (FHWA 1994).

Table 1—Damage Factor Equations.

Rutting:

Damage Factor = Nrf /Nri, where

Nri is the loads to failure (rutting) at any time, i

Nrf is the loads to failure (rutting) at the reference time (fall), and Nr = a x evb, where

Nr = loads to failure,

a = 1.077 ¥ 1018,

ev = vertical subgrade compressive strain (in/in ¥ 10-6), and

b = -4.4843

Fatigue (AC cracking):

Damage Factor = Nff /Nfi, where

Nfi is the loads to failure (fatigue) at any time, i

Nff is the loads to failure (fatigue) at the reference time (fall), and

Log Nf = 14.82 – 3.291 log (et) – 0.854 log (Eac), where

Nf = loads to failure,

et = tensile strain at the bottom of the asphalt (in/in x 10-6)

Eac = stiffness of asphalt (ksi)

38

a) TDR Volumetric Moistures: Ochoco—Site 2

b) Selected TDR Volumetric Moistures: Ochoco—Site 2

Figure 35—TDR measured moisture contents. Note the sharp drop of the mid-base sensor indicating freezing conditionsin early January.

VO

LU

ME

TR

IC M

OIS

TU

RE

(%

)V

OL

UM

ET

RIC

MO

IST

UR

E (

%)

DATE

DATE

60

50

40

30

20

10

0

11"

19"

23"

28"

33"

50"

59"

9/17/94 11/12/94 1/7/95 3/4/95 4/29/95 6/24/95 8/19/95 10/14/95

39

a) RF Volumetric Moisture: Ochoco—Site 1

b) Selected RF Volumetric Moistures: Ochoco—Site 1

Figure 36—RF measured moistures. Note the sharp drop of the sensor at an 18 inch depth indicating the approachingfreezing front in early January.

VO

LU

ME

TR

IC M

OIS

TU

RE

(%

)V

OL

UM

ET

RIC

MO

IST

UR

E (

%)

DATE

DATE

45.0

40.0

35.0

30.0

25.0

20.0

ALL IN SUBGRADE

18"

24"

30"

36"

42"

52"

9/17/94 11/12/94 1/7/95 3/4/95 4/29/95 6/24/95 8/19/95 10/14/95

40

a) RF Volumetric Moisture: Ochoco—Site 2

b) Selected RF Volumetric Moistures: Ochoco—Site 2

Figure 37—RF measured moistures. All sensors are located below frost penetrated depth.

VO

LU

ME

TR

IC M

OIS

TU

RE

(%

)V

OL

UM

ET

RIC

MO

IST

UR

E (

%)

DATE

DATE

30.0

35.0

40.0

45.0

ALL IN SUBGRADE

21"

25"

29"

34"

38"

52"

9/17/94 11/12/94 1/7/95 3/4/95 4/29/95 6/24/95 8/19/95 10/14/95

41

a) Groundwater Depth: Ochoco—Site 1

b) Groundwater Depth: Ochoco—Site 2

Figure 38—Seasonal groundwater depths.

DE

PT

H B

EL

OW

PA

VE

ME

NT

SU

RFA

CE

(ft

)D

EP

TH

BE

LO

W P

AV

EM

EN

T S

UR

FAC

E (

ft)

DATE

DATE

42

Area Parameter (D0-D36): Ochoco—Sites 1 and 2

Figure 39—Seasonal change in area parameter. Note the sharp increase in December and early January as freezingoccurs.

AR

EA

PA

RA

ME

TE

R (

in2 /

in)

DATE

43

a)Backcalculated AC Moduli: Ochoco—Site 1

b)Base and Subgrade Modulus: Ochoco—Site 1

Figure 40—EVERCALC backcalculated moduli.

MO

DU

LU

S (

ksi)

TEMPERATURE (F)

MO

DU

LU

S (

ksi)

DATE

BASE MODULUS

SUBGRADE MODULUS

BASE MODULUS GREATER THAN 100 ksi (FROZEN)

NOTE: FREEZING 2 INCHESINTO SUBGRADE

44

a) AC M(r) vs. Temperature: Ochoco—Site 2

b) Moduli and Backcalculated RMS Values: Ochoco—Site 2

Figure 41—EVERCALC backcalculated moduli.

MO

DU

LU

S (

ksi)

AV

ER

AG

E R

MS

(%

)

MO

DU

LU

S (

ksi)

DATE

y = 3029.5x-0.4431

SUBGRADE

3 TEST DATESW/FROZEN

BASE

BASE

RMS

45

a) Critical Strains at 55F and TDR Moisture at Mid-Base: Ochoco—Site 1

b) Damage Factors: Ochoco—Site 1

Figure 42—Critical pavement strains and damage factors. Note the near zero damage potential during frozenconditions of December and January.

DA

MA

GE

FA

CTO

R(R

EL

AT

IVE

TO

10/

25/9

4 C

ON

DIT

ION

S)

VO

LU

ME

TR

IC M

OIS

TU

RE

S (

%)

ST

RA

IN (

in/in

x 1

0-6)

DATE

DATE

46

a) Critical Strains [at 55F AC M(r) Field Curve] vs. TDR Moisture: Ochoco—Site 2

b) Damage Factors vs. TDR Moisture: Ochoco—Site 2

Figure 43—Critical pavement strains and damage factors compared with selected TDR moistures.

TD

R M

OIS

TU

RE

(%

)

ST

RA

IN (

in/in

x 1

0-6)

DATE

TD

R M

OIS

TU

RE

(%

)

DA

MA

GE

FA

CTO

R(R

EL

AT

IVE

TO

10/

25/9

4 C

ON

DIT

ION

S)

DATE

47

As was shown in figure 42 for site 1, the damagefactor for rutting is 1.0 for the reference initial falltest point. (Note the first three data points in figures42b and 43b are the same for rutting and ACcracking.) The damage factor approached zerofor approximately 5 weeks when the freezing profilepenetrated the base course and into the subgrade,increased to a maximum value of 4.7 in April, thendecreased back to near 1 in August. A similarrelationship is presented in figure 43 for site 2that had a maximum damage factor for rutting of11.8.

For site 1, the damage factor for asphalt fatiguecracking is 1.0 for the reference initial fall test point.The damage factor decreased to 0.0 duringmaximum freezing, increased to a maximum valueof 3.9 in March, then decreases to 1.0 in August.Again, site 2 followed a similar trend and had amaximum value for fatigue cracking of 3.4.Maximum damage factor values for subgraderutting were greater than those for asphalt crackingat both sites. The damage factor for rutting wasmuch greater than the damage factor for ACcracking at site 2, but that was not the case atsite 1. This is probably because the asphaltthickness is greater at site 2. The figures also showdamage factor variations are reflective of changesin the TDR measured moisture at selected probeswithin the base and upper subgrade. In particular,the site 2 TDR probe at the top of the subgrademeasured peak values during February, at a timewhen the damage factors were also at theirmaximums.

Conclusions from Ochoco Sites

1. Freeze-thaw conditions existed at these sitesfrom November through March. The conditionswere cyclic and included periods of both partialand total midwinter thaw. Maximum frostpenetrations were relatively shallow rangingfrom 14 to 18 inches below the asphalt.

2. Due to the relatively shallow frost depths andapparently high residual ground heat from theprevious summer, rates of thawing from thebottom up were high relative to other sites withinthis study. Consequently, most thawing on theOchoco sites occurred from the bottom up,

allowing for vertical drainage as thawingprogressed. Only during isolated cases, lastingfor less than a week at a time, did top-downthawing occur, impeding vertical drainage.

3. Due to the highly cyclic nature of freezing andthawing at the Ochoco sites, FWD testing onlycaptured the concluding portion of the strengthloss occurring after thawing of the most severefreeze-thaw cycle (18 inches maximum freezingdepth). The maximum relative damage factorcalculated was 4 to 5 times the fall conditionsfor cracking and approximately 11 times fallconditions for subgrade rutting. The recovery toa damage factor of 2 required approximately 3to 4 months. This is probably due to the fine-grained nature of the soil and the associateddrainage properties.

4. Maximum damage factors for subgrade ruttingwere greater than those for asphalt fatiguecracking at both sites implying that, for theserelatively “thick” pavement structures, increasedrates of rutting may be the most likelyconsequence of heavy vehicle use during thelate winter/early spring.

5. It appears that the relatively modest increasesin damage factors at these sites are only partlyexplained by seasonal freezing-thawingconditions. Only very brief, subdued “spikes” inpost-thaw moisture contents were observed,and then only from probes within the aggregatebase or upper subgrade. Elevated moisturecontents (and groundwater levels) persisted formonths beyond thawing. It seems likely theseincreased moisture contents and the associatedincrease in damage factors, are more likelyrelated to snow melt and spring runoff than directeffects of thaw-weakening.

6. Critical periods of thaw weakening monitoredat the Ochoco sites appear to have been verybrief due to the relatively mild winter conditions.Use of thermistors and TDR (or RF) equipmentallows monitoring of the timing, duration, andindirectly, the severity of these sporadicconditions and could be used to limit vehicle useaccordingly. The longer periods of sustainedelevated moisture contents observed areprobably better handled as a pavement designissue. The observations obtained at these sites

48

would be helpful in determining seasonaladjustment factors for use in current mechanistictype design programs.

7. TDR (and to a lessor degree RF) probes did agood job of tracking seasonal moisture changes.

Kootenai Field SitesLocation and Climate

The Kootenai National Forest is located in thenorthwest corner of Montana, bordering bothCanada and Idaho. The two field test sites on theforest were located on the Canoe Gulch RangerDistrict, approximately 15 miles north of Libby, MTat mileposts 12.4 and 17.0 along Forest ServiceRoad No. 6800 (Pipe Creek Road, figures 44 and45). Pipe Creek Road (in the vicinity of the testsites) is typical of many “low-volume,” asphaltsurfaced FS roads; road widths vary from 18 to20 feet, cut/fill and shallow turnpike constructionwere used, grades vary from 2 to 5 percent, witha “composite” asphalt surfacing consisting of aseries of paving and maintenance activities overa period of more than 25 years. The test sitelocations were intentionally selected to providerelatively uniform aspect and shadingcharacteristics throughout the site lengths.

Figure 44—Kootenai National Forest—Site 1 duringinstrumentation installation.

Figure 45—Kootenai National Forest—Site 2.

Climate in this part of Montana, approximately 100miles west of the Continental Divide, is affectedby both modified maritime and continentalinfluences. Maritime influences generally dominatein the winter and result in rain or snow when warmPacific air masses are cooled as its passes throughthe mountain ranges. Continental influencesgenerally dominate in the summer with low-pressure areas from the hot southerly interiorcausing convection type showers and occasionalcloud bursts. Winters are neither as wet nor aswarm as the Pacific coastal area, but are generallyless severe than areas to the east of the ContinentalDivide. The mean daily July temperature in Libby(nearest long-term weather station) is 67 °F withan extreme maximum of 109 °F. January is thecoldest month with a mean daily temperature of23 °F and an extreme low of -46 °F. Extremelylow temperatures are not common however, andtemperatures of 0 °F are reached on only 12 daysin an average year (figure 46).

49

Figure 46—Kootenai National Forest Site 2 on March 14,1996 near beginning of subsurface thawing. Note thewater is “bleeding” up through the pavement.

Elevations of the test sites were 2,800 and 3,100feet at sites 1 and 2, respectively. Libby’s elevationis approximately 2,100 feet with an average annualprecipitation of 18 inches. Precipitation varieswidely in the forest, ranging from 14 inches to over100 inches at higher elevations, approximately 70percent of the total falling as snow. Due to theirhigher elevations, both sites would be expectedto have colder temperatures and more precipitationthan Libby. Design freezing indices (average ofthree coldest winters in the last 30 years) rangefrom 1,000 to 1,500 degree-days F, depending onelevation. The areas mean freezing index for 30years of records range from 500 to 1,000 degree-days F. Freezing indices for sites 1 and 2 were1,056 and 1,114 degree-days F, respectively, forthe winter of 1995/1996.

Instrumentation Layout and SiteDescription

Plan and profile views of the test site layout areshown in figures 47 and 48. Each site was 80 feetin length. The five FWD test points (A–E), spaced20 feet apart, were painted on the asphalt surfaceat each site to facilitate repeat testing at the samelocations. The FWD testing schedule was selectedto focus on the critical thaw-weakening period,with additional tests conducted during 1995 through1996 to provide late summer/early fall referencevalues and other typical seasonal values.

Two thermistor probes, each containing 12thermistors, were installed in boreholes at oppositeends of each site to monitor subsurfacetemperatures to a depth of approximately 65inches. Additional thermistors monitored theasphalt pavement and air temperatures. At eachsite, eight TDR and six RF probes were installedin 10-inch and 6-inch-diameter auger holes,respectively, to monitor subsurface moisturecontents to depths of 50 to 60 inches. Due to thecoarse, rocky nature of the subgrade soils, all TDRprobes were compacted into the borehole backfill.Backfill material was conserved from the augeringprocess and supplemented, where necessary, withsimilar on-site material. Ten of the 12 RF probesat the two sites were inserted horizontally into thesides of the boreholes, using modified pry barsand wooden blocks to provide leverage. Theremaining two RF probes (at site 1) were placedvertically. All TDR and RF probes functionednormally following installation with the exceptionof RF probe No. 5 at site 1, which was apparentlydamaged during the installation process.

Two open-well standpipes consisting of geotextilewrapped, slotted, 1-inch-diameter PVC pipe wereinstalled in boreholes at opposite ends of eachsite to monitor groundwater elevation changes.The boreholes were drilled to approximately 20feet in depth at both sites through silty, sandygravels containing cobbles and small boulders(GM). No bedrock was encountered. Groundwaterwas encountered during installation of thestandpipes at site 1; however, none was observedat site 2. Native materials consisted of glacial tilland glacial outwash deposits, some of which hadbeen reworked through alluvial processes toproduce the large terraces traversed by presentday Pipe Creek Road.

Surfacing materials at the sites are variable bothin quality and thickness. Site 1 was a build-up ofasphalt surfaces (chipseals, cold mix, patches)varying from 2.5 to 3.5 inches in total thickness,overlying, in most cases, a weaker (“crumbly”)asphalt coated layer from 1.25 to 2.00 inches inthickness. This weaker asphalt layer, in turn,overlays a 0.5-inch-thick BST (bituminous surfacetreatment). Sporadically, a 1.25- to 2.00-inch-thicklayer of 1/2-inch-minus, fine, sandy aggregate wasfound sandwiched above the oldest BST.

50

Figure 47—Instrument Layout: Kootenia National Forest—Site 1.

A B C D ET1 RFTDR T2

SCALE: 1"=20’

DH-2 DH-1

A CB D E

1.5"



A-E: FWD DH -1,-2: GROUNDWATER

4.5"

8.5"

12.5"

16.5"

20.5"

24.5"

28.5"

34.5"

40.5"

46.5"

52.5"

64.5"

1.5"

5.5"9.5"

13.5"

17.5"

21.5"

25.5"

29.5"

35.5"

41.5"

47.5"

53.5"

65.5"

11.5"

25"

18.5"

33"

42"

53.5"

13"

19"

24"

29"

34"

40"

51"

61"

T1 TDR RF T2

NOTES:

1 - PROFILE VIEW SHOWS PLACEMENT DEPTHS OF INSTRUMENTATION.

2 -

3 - BOREHOLE DIAMETER NOT DRAWN TO SCALE.

PROBE PLACED VERTICALLY IN BOREHOLE BACKFILL



AGGREGATE BASE

SUBGRADE: GM - SILTY GRAVEL

T1, T2: THERMISTOR PROBE

WITH SAND,

4 -

COBBLES, AND BOULDERS

A) PLAN VIEW

B) PROFILE VIEW

BST (1/2")POOR QUALITY AC (2")

GOOD QUALITY AC (3")

51

Figure 48—Instrument Layout: Kootenia National Forest—Site 2.

ABCDET1RF TDR

T2

SCALE: 1"=20’

DH-2

AC B

19.5"

23.5"

33.5"

27.5"

39.5"

45.5"

51.5"

63.5"

15.5"

11.5"

7.5"

3.5"

1.5"

63"

51"

45"

39"

7"

27"

33"

23"

11"

15"

19"

1.5"

3"

20"

26"

32"

37"

52"

9"

14"

20"

26"

31"

38

62"



A-E: FWD

DH-1,-2: GROUNDWATER

E

T2

D RF TDR T1

9.5"

50"

DH-1

SUBGRADE:

GM - SILTY GRAVEL

WITH SAND,

NOTES:

1 - PROFILE VIEW SHOWS PLACEMENT DEPTHS OF INSTRUMENTATION.

2 -PROBE INTO SIDE OF BOREHOLE.

PROBE COMPACTED INTO BOREHOLE BACKFILL.

3 - BOREHOLE DIAMETER NOT DRAWN TO SCALE.

T1, T2: THERMISTOR PROBE

4 -

COBBLES AND BOULDERS

A) PLAN VIEW

B) PROFILE VIEW

BST (1/2")"BLACK" AGGREGATE (2")GOOD QUALITY AC (2.5")

52

Underlying the asphalt layers was a “pit-run” basecourse, 10 to 12 inches in thickness, which wasfield classified as a GP-GM gravel with smallcobbles. The subgrade at site 1 was a silty-gravel(GM) with cobbles and boulders, approximately65 percent gravel, 20 percent sand, and 15 percentnonplastic fines. For moduli backcalculationpurposes, all degraded asphalt layers, below theuppermost 2.5 to 3.5 inch thickness, werecombined with the underlying base coursethickness.

Site 2 had a surface layer of intact asphalt from2.25 to 3.00 inches in thickness overlying a 2.0-to 2.5-inch-thick layer of asphalt-coated, fineaggregate that was no longer a cohesive material.Sporadically, underlying this fine aggregate, wasa 1/2-inch thick BST, similar to that found at site1. No discernible difference could be seen betweenthe material underlying the lowest BST and deepersubgrade materials. For this reason, it is believedthat no distinct base course exists at site 2. Aswith site 1, during moduli backcalculations, alldegraded asphalt layers under the uppermostasphalt surface were combined into the underlyinggranular material layer (i.e., subgrade in the caseof site 2).


Data collection consisted of temperatures(thermistors), moisture contents (TDR and RF),groundwater, and site conditions (i.e., sunny,cloudy, raining, dry, snowing, etc.) from September23, 1995 to September 27, 1996. Readings wereobtained at least monthly during the summer, fall,and early winter, increasing to daily during the 6-week thaw period that occurred between mid-Marchand early May. Readings were collected manuallyduring all site visits. One thermistor probe at eachsite was connected to an automated data collectiondevice. Temperature readings from the thermistorsin this probe were taken either two or four timeseach hour, depending on the depth of the individualthermistor; the more frequent readings were takenon the upper thermistors.

FWD data was collected throughout the year todetermine material layer strength variations. A totalof 33 FWD testing events occurred during the studyperiod. Two of these were performed duringOctober 1995 to establish reference conditions.The remainder occurred between March 18 andMay 8, 1996 to monitor the progressive changein material strengths throughout and immediatelyfollowing spring-thaw. Testing generally occurred,alternately, in the midmorning and midafternoonon consecutive days to observe the effect ofdifferences in asphalt temperature on thebackcalculated strengths.

Asphalt Coring

On May 9 and 10, 1996, following completion ofthe FWD testing, a total of 10 asphalt cores fromeach of the sites were obtained for laboratorymodulus testing (figure 49). Two cores were takenat each FWD test spot: one directly from the paintedFWD impact point and another approximately 1to 1.5 feet from the test spot, along the line of thedeflection measuring sensors. The cores weretested at temperatures of 35, 55, and 80 °F in anenvironmental chamber at the WillametteLaboratory.

Figure 49a—Drilling AC pavement cores for laboratorymodulus testing. Kootenai National Forest—Site 2.

53

Figure 49b—Typical AC pavement core from KootenaiNational Forest sites. Note the thin, older BST surfacefound below thicker AC.


Temperature

Frost profiles depicting the timing, extent, andduration of freezing and thawing at the test siteswere constructed using the temperature datacollected. As seen in the plots shown in figure 50,a single major freezing and thawing eventdominates winters in this part of the country(contrasting with the multiple, relatively minorevents observed at the Ochoco National Forestsites). Two minor periods of freezing and thawingdid occur: one in early November and another inthe later part of March; however, the overall patternis clearly one of massive progressive freezing,followed in the spring by nearly continuous thawing,predominately from the top down. Again, thispredominately top-down thawing and associatedimpeded vertical drainage contrasts sharply withthe results from the Ochoco sites. Typical ratesof freezing in early December at the Kootenai siteswere approximately 2.0 to 2.5 inches per day. Ratesof thawing during the later part of March and earlyApril were approximately 1.5 to 2.5 inches per dayfrom the top down and approximately 0.2 to 0.5inches per day from the bottom up. In addition,approximately 80 percent of the thawing at thesesites occurred from the top down, with total thawingoccurring over a period of about 5 weeks.

TDR and RFTDR and RF volumetric moisture contentfluctuations for each site are shown in figures 51through 53. The monitored changes are moredramatic than those observed at the Willametteor Ochoco sites. Both Kootenai sites and both typesof instrumentation show the same general patternof relatively stable values until freezing occurs inearly December. After freezing, the values sharplydrop to reflect the lowered dielectric constant ofthe frozen soil. Interestingly, the deepest sensors(both TDR and RF) at site 1, which did not freeze,show increased moisture contents as freezingprogresses, possibly due to groundwater beingdrawn toward the freezing front as a result ofincreases in capillary tension of the soil moisture.With the initiation of thawing in early March, bothTDR and RF sensors recorded a large, sharpincrease in moisture contents to levels well abovethose observed immediately prior to freezing. Thissuggests that moisture may have been added tothe structures during the freezing process; it mayalso, in part, be a result of surface infiltration ofmoisture from adjacent melting snow-banks. Sinceboth vertical and horizontal drainage is impededby frozen ground, distinctive peaks formed duringthe thawing process. It can be seen that the peaksof individual sensors occur sequentially as thawingprogressed in depth. As thawing progressed, anequally rapid sequential decrease in moisturecontents was observed until a few days followingcomplete thawing, at which point the slope of themoisture content recovery curve significantlyflattens. This suggests that the moisture trappedabove the ground ice, at least for coarse-grainedsoils, such as those found at these sites, drainsrapidly following thaw. A secondary, much less rapid“drying” phase follows the initial “drainage” phase,with moisture contents eventually returning to levelsapproximating those observed the previous fall.

Interestingly, as previously mentioned, groundwaterwas not encountered in the 20 foot deep monitoringholes of site 2; yet, even in these very coarse-grained soils, a moisture peak upon thawing, similarto that observed at site 1, is apparent. There aretwo possible sources for this moisture peak: it isthe result of moisture desiccated from underlyingunsaturated soils, or it could be a result of surfaceinfiltration from adjacent melting snow banks onthe road shoulders.

54

a) Frost Profile: Kootenai—Site 1

b) Frost Profile: Kootenai—Site 2

Figure 50—Seasonal frost profiles. Note the majority of thawing occurred from top down at these sites.

DE

PT

H B

EL

OW

AS

PH

ALT

(in

)

DATE

0

5

10

15

20

25

30

35

40

45

50

10/28/95 11/25/95 12/23/95 1/20/96 2/17/96 3/16/96 4/13/96

UNFROZEN

THAWED

THAWED

FROZEN

DE

PT

H B

EL

OW

AS

PH

ALT

(in

)

DATE

0

5

10

15

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25

30

35

40

45

50

55

60

65

7010/28/95 11/25/95 12/23/95 1/20/96 2/17/96 3/16/96 4/13/96

UNFROZEN

THAWED

THAWED

FROZEN

55

a) TDR Volumetric Moistures: Kootenai—Site 1

b) Selected TDR Volumetric Moistures: Kootenai—Site 1

Figure 51—TDR measured moisture contents. Note the pronounced peaks during thaw. Sensors at 51 and 61 inches werebelow depth of frost penetration.

VO

LU

ME

TR

IC M

OIS

TU

RE

(%

)

DATE

VO

LU

ME

TR

IC M

OIS

TU

RE

(%

)

DATE

35

30

25

20

15

10

5

0

35

30

25

20

15

10

5

09/25/95 10/28/95 12/23/95 2/17/96 4/13/96 6/8/96 8/3/96 9/28/96

9/25/95 10/28/95 12/23/95 2/17/96 4/13/96 6/8/96 8/3/96 9/28/96

13"

19"

24"

29"

34"

40"

51"

61"

13"19"29"

MID-BASE

TOP SUBGRADE

10" INTO SUBGRADE

TOP SUBGRADE

MID-BASE

10" INTO SUBGRADE

56

a) TDR Volumetric Moisture: Kootenai—Site 2

b) Selected TDR Volumetric Moistures: Kootenai—Site 2

Figure 52—TDR measured moisture contents. Note the pronounced peaks during thaw. All sensors were within frostpenetration depth.

VO

LU

ME

TR

IC M

OIS

TU

RE

(%

)

DATE

VO

LU

ME

TR

IC M

OIS

TU

RE

(%

)

DATE

40

35

30

25

20

15

10

5

0

BASE/SUBGRADE

20" INTO SUBGRADE

8" INTO SUBGRADE

9/2/25 10/28/95 12/23/95 2/17/96 4/13/96 6/8/96 8/3/96 9/28/96

9 "

1 4 "

2 0 "

2 6 "

3 1 "

3 8 "

5 0 "

6 2 "

9"14"36"

57

a) RF Volumetric Moistures: Kootenai—Site 1

b) RF Volumetric Moistures: Kootenai—Site 2

Figure 53—RF measured moisture contents moisture contents. Similar pronounced peaks were observed by TDRs duringthaw. Sensor at 53.5 inch depth (site 1) was below depth of frost penetration.

VO

LU

ME

TR

IC M

OIS

TU

RE

(%

)

DATE

35

30

25

20

15

10

5

09/2/95 10/28/95 12/23/95 2/17/96 4/13/96 6/8/96 8/3/96 9/28/96

VO

LU

ME

TR

IC M

OIS

TU

RE

(%

)

DATE

35

40

30

25

20

15

10

5

0

9/2/95 10/28/95 12/23/95 2/17/96 4/13/96 6/8/96 8/3/96 9/28/96

11.5""

18.5""

25"

33"

53.5""

9.5"

20"

26"

32"

37"

52"

58

Groundwater

Figure 54 shows groundwater measurements obtained from site 1. As mentioned above, groundwaterwas not observed within the 20-foot depth of the observation wells at site 2. Both observation wells atsite 1 indicated similar groundwater depths and rates of change. In general, groundwater depths at site1 were 12 to 13 feet below the pavement surface during the late summer and early fall. The highestmeasured groundwater levels, approximately 6 feet below the pavement, occurred in mid-Februarywhen ground-freezing was near its maximum and prior to any thawing. Levels varied from 7 to 9 feetbelow the pavement surface during the period of thaw, gradually decreasing to the previous fall levels.

Groundwater Depth: Kootenai—Site 1

Figure 54—Seasonal groundwater depths.

Area Parameter

Area parameters were calculated for the Kootenai sites in a manner similar to the Willamette andOchoco sites. The resulting values are shown in figure 55. Values indicated that, unlike the Ochocosites, there is a significant difference in overall stiffness between sites 1 and 2, with site 2 being weaker.This is reasonable given the thinner asphalt surface, lack of base course, and weaker subgrade. Inaddition, it is apparent that both Kootenai sites are less stiff throughout the year than sites on theOchoco National Forest. Again, this is reasonable given the superior asphalt quality, greater asphaltthickness, and graded, crushed base that exist at the Ochoco sites. The Kootenai sites had fall referencevalue area parameters of approximately 15.5 and 12.5 square inches per inch for sites 1 and 2, respectively.Values were somewhat erratic during the thawing process, cycling, roughly, with changes in asphalttemperature (due to sequential testing alternately performed in midmorning and midafternoon). A sharppeak was observed on March 25, 1996, following 3 days of continuously subfreezing temperatures.Thermistor data indicates several inches of refreezing below the asphalt occurred during this period oftime. Beginning approximately April 3, 1996, with 12 to 18 inches of thaw below the asphalt, a period ofless erratic strength recovery occurred. By April 18, 1996, within a week following complete thawing,relatively stable values were achieved, only slightly below values of the previous fall.

DE

PT

H B

EL

OW

PA

VE

ME

NT

SU

RFA

CE

(ft

)

DATE

DH-1DH-2

59

Area Parameter (D0-D36): Kootenai—Sites 1 and 2

Figure 55—Seasonal change in area parameters. Note the sharp spike on March 15, 1996 coincides with a period ofpartial refreezing of base materials.

Layer Moduli

Layer moduli values were backcalculated for each FWD testing point from the load and deflection datausing EVERCALC. During backcalculations, sites 1 and 2 were modeled somewhat differently. Whenno ground ice was present, site 1 was modeled as a four layer problem with layer 1 being asphalt,layer 2 being base course, layer 3 being subgrade above the watertable, and layer 4 being subgradebelow the watertable. Since the depth to the watertable was always known, EVERCALC was allowedto backcalculate the layer 4 moduli. When ground ice was present, site 1 was modeled as a five-layerproblem, layer 1 being asphalt and layer 2 being one half the thawed thickness (until the 14-inch-thickbase was completely thawed, at which point layer 2 became the thawed base). Layer 3 was the lowerone half of the thawed thickness (until the 14-inch-base was completely thawed, at which point layer 3became the thawed subgrade). Layer 4 was the frozen subgrade, and layer 5 was the unfrozen subgradebelow the ground ice.

When no ground ice was present, site 2 was modeled as a four layer problem with layer 1 beingasphalt, layer 2, the upper 18 inches of subgrade, layer 3, a 39-inch-thick subgrade unit, and layer 4,a “hard” layer at approximately 60 inches in depth. EVERCALC was allowed to backcalculate themodulus of layer four. The 60-inch-hard-layer depth was based on preliminary runs that showed thisdepth produced the lowest RMS errors. Site 2 (with ground ice) was modeled as a four or five layerproblem depending on the thaw depth. With less than 14 inches of thaw, layer 1 was the asphalt, layer2, the thawed subgrade, layer 3, the frozen subgrade, and layer 4, the unfrozen subgrade below theground ice. With greater than 14 inches of thaw, layer 1 was the asphalt, layer 2, the thawed subgradethickness minus 6 inches, layer 3, was the lowest 6 inches of thawed subgrade, layer 4, the frozensubgrade, and layer 5, the unfrozen subgrade below the ground ice.

AR

EA

PA

RA

ME

TE

R (

in2 /

in)

DATE

10/25/95 (OFFSET FOR CLARITY)

SITE 1SITE 2

60

In general, backcalculation RMS errors were less for unfrozen conditions than for frozen conditions, asmight be expected. For unfrozen conditions, average RMS values for all FWD tests were 1.0 and 1.2percent for sites 1 and 2, respectively. For tests where ground ice was present, the overall averagevalues were 1.3 and 1.8 percent, respectively.

Backcalculated moduli for the asphalt layers were compared with moduli obtained from laboratorytesting on site cores. Figure 56 shows the results of laboratory testing on cores from sites 1 and 2along with the temperature/modulus curve for WSDOT B Mix for comparison. The general shape of allthree curves is very similar, with the cores from site 1 indicating a somewhat stiffer mix than WSDOT“B”, while site 2 cores indicate a softer mix. Figure 57 compares the laboratory core results withbackcalculation results for the asphalt layers. Considerable scatter is evident in the backcalculatedresults, as might be expected when attempting backcalulations with such thin asphalt layers. Eachbackcalculated value represents the average result for the five FWD test points at each site for aparticular testing date. Even with the scatter, it is apparent the backcalculated best-fit curves for thesites are quite similar in slope and considerably flatter than the respective laboratory curves. This maybe attributed to the greater lateral confinement that exists in the field compared to the unconfinedlaboratory tests.

Figure 56—Laboratory modulus tests on site AC cores. WSDOT B mix is shown for comparison.

Seasonal variations in the backcalculated moduli for base and subgrade layers are shown in figures 58and 59. Also displayed on these figures is the thawed depth on each test date. Reference values fromthe previous fall are also shown. Both base and subgrade moduli show a trend toward stabilized values,though still reduced from the previous fall values, coinciding with the time all ground ice thaws. Minimumsubgrade moduli were calculated to be approximately 4 ksi at both sites 1 and 2. This compares toearly fall values of approximately 14 ksi and 21 ksi for sites 1 and 2, respectively. Minimum subgrademoduli occurred with thaw depths of 18 inches. The base course fall value was approximately 26 ksiand was reduced to a minimum of approximately 18 ksi during spring thaw. Several sharp spikes canbe seen in the base moduli during the thawing period. From the thermistor data, it is apparent that

MO

DU

LU

S (

psi

)

TEMPERATURE (F)

SITE 1 CORES

SITE 2 CORES

61

several inches of refreezing of the base occurredjust prior to the March 25, 1996 tests and this isreflected in the backcalculated values. Althoughthe subsurface temperature data is inconclusiveconcerning the other base moduli spikes duringthe later half of March, the monitored airtemperatures do suggest that some minor amountsof refreezing may have occurred.

Critical Strains and Damage Factors

Both vertical compressive strain at the top of thesubgrade and horizontal tensile strain at the bottomof the asphalt were calculated using EVERSTRESSwith the EVERCALC backcalculated moduli values.As explained earlier, these parameters allowestimates to be made of a pavement’s ability towithstand rutting of the subgrade and fatiguecracking of the asphalt surfacing. For the straincalculations, a fixed asphalt modulus equal to thevalue determined for 55 °F from the best-fit curvefor backcalculated values (figure 57) was used;this was done to model a typical sunny spring dayduring thaw and was intended to reduce the scatterin the results that would be introduced by usingvarying asphalt moduli values.

The resulting strains were used to calculatedamage factors in a manner similar to thatdiscussed for the Ochoco sites. Figures 60 and61 show the results of the calculations for Kootenaisites 1 and 2, respectively. Also displayed on thesefigures are the TDR moisture contents measuredat the top of the subgrades and the thaw depthbased on thermistor temperature measurements.Moisture contents at the top of the subgrades areshown because moisture changes at this positionreflect conditions within the overlying base, as wellas the upper subgrade. Changes in moisturecontent of these uppermost layers is likely todominate changes in overall pavement strength.

Since site 2 did not have a true base course, the“subgrade” depth was selected 6 inches under theasphalt for strain and damage factor calculations.Both sites reacted similarly with both strains(particularly for subgrade rutting) and damagefactors starting out relatively low, increasingsignificantly as the thawing process progressesand moisture contents increase, and finally

decreasing rapidly as ground thawing is completedand moisture contents decrease. These trends arethe same as those observed earlier in the areaparameter. Maximum damage factors for ruttingof 32 and 45 were calculated for sites 1 and 2,respectively. Maximum damage factors for asphaltfatigue cracking were approximately 2 for bothsites. The results suggest, at least for thesepavement structures, that during spring thaw, therutting potential increases more than the potentialfor fatigue cracking. This may be a consequenceof the relatively high strength subgrade soils foundat these sites and their ability to provide supportfor the asphalt surfacing even during thaw-weakened conditions.

Conclusions From Kootenai NationalForest Field sites

1. A generally single, relatively deep, frost occurredat these sites. The sites were continuouslyfrozen from early December through earlyMarch. A single, brief period of frost occurred inearly November, but thawed from the bottomup within a week. Another isolated period ofrefreezing of several inches below the asphaltoccurred during the primary thaw in late March,but rethawed within a few days. Maximum frostdepths below the asphalt were approximately43 and 63 inches for sites 1 and 2, respectively.Initial freezing in early December occurred at arate of approximately 2.0 to 2.5 inches per day.

2. Thawing predominantly occurred from the topdownward, in contrast to the Ochoco sites wheremost thawing was from the bottom upward.Typical rates of thawing from the top downwardwere 1.0 to 2.5 inches per day while bottomupward rates were typically 0.2 to 0.5 inchesper day. Over a period of 5 weeks, between earlyMarch and mid-April, 80 percent of total thawingoccurred from the top downward.

3. Both TDR and RF devices worked well inmonitoring seasonal changes in base andsubgrade moisture contents. Both clearly showpost-thaw increases in moisture contents aboveprefreeze conditions. The increased moisturecontents may be a result of ice lens growthduring freezing, surface water infiltration from

62

a) Lab vs. Backcalculated AC Moduli: Kootenai—Site 1

b) Lab vs. Backcalculated AC Moduli: Kootenai—Site 2

Figure 57—EVERCALC backcalculated AC moduli compared to laboratory core test values. Note that backcalculatedvalues indicate less temperature sensitivity.

MO

DU

LU

S (

ksi)

MO

DU

LU

S (

ksi)

LAB (SITE 1 CORES): y = 0.2768x2 - 65.583x + 4052.7

LAB (SITE 1 CORES: y = 7318.1e-0.0429x

BACKCALCULATED: y = 1562.2e-0.0099x

BACKCALCULATED: y = 1288.5e-0.0028x

63

a) Base Modulus vs. Thawed Depth Below Asphalt: Kootenai—Site 1

b) Subgrade Modulus vs. thawed Depth Below Asphalt: Kootenai—Site 1

Figure 58—EVERCALC backcalculated base and subgrade moduli. Note that values stabilize as thawing concludes.

MO

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ksi)

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ch)

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(in

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64

Upper Subgrade Modulus vs. Thawed Depth Below Asphalt: Kootenai—Site 2

Figure 59—EVERCALC backcalculated subgrade modulus. Note the value stabilizes as thawing concludes.

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ksi)

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10/25/95(OFFSET FOR CLARITY)

UPPERSUBGRADETHAWEDDEPTH

65

a) Critical Strains [at 55F AC, Field Curve M(r)] vs. Thaw Depth and TDR Moisture: Kootenai—Site 1

b) Damage Factors vs. Thaw Depth and TDR Moisture: Kootenai—Site 1

Figure 60—Comparison of changes in critical pavement strains, damage factors, selected TDR moistures and thaw depth.

ST

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x 1

0-6)

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AW

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in)

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THAW DEPTHBELOW AC

TDR: TOP OFSUBGRADE

COMPRESSIVE STRAIN:TOP OF SUBGRADE

10/25/95(OFFSET FOR CLARITY)

66

a) Critical Strains [at 55F AC, Field Curve M(r)] vs. Thaw Depth and TDR Moisture: Kootenai—Site 2

b) Damage Factors vs. Thaw Depth and TDR Moisture: Kootenai—Site 2

Figure 61—Comparison of changes in critical pavement strains, damage factors, selected TDR moistures and thaw depth.

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67

adjacent melting snow, or both. In any case, thevertically impeded drainage as a result of thefrozen subgrades produced a distinctive,temporary peak in moisture contents thatcorrelated with increases in critical strain levelsand damage factors. The distinctive peak, andmost of the increased moisture content abovefall values, subsided within 1 week followingcomplete thaw. Maximum damage factors of 32and 45 for rutting and a factor of 2 for asphaltfatigue cracking were calculated for the twosites.

4. Results indicate that environmental monitoringwith TDR (or RF) devices in combination withthermistors can provide a viable means ofidentifying periods of lowered pavement strengthduring thaw. This information would be helpfulin determining the optimal timing and durationof load restrictions. For the soils present at thesesites, the critical period of thaw weakeningstrongly coincided with the period of thawing asidentified by the thermistor probes and theperiod prior to “drainage” as identified by theTDR and RF probes.

5. The relative seasonal moisture contentsmonitored at these sites provide usefulinformation for estimating seasonal factors foruse during mechanistic pavement design.

White Mountain National Forest TestSiteLocation and Climate

The White Mountain National Forest project sitewas located on York Pond Road No. 13, an asphaltsurfaced forest road in northern New Hampshire,approximately 10 miles north of Berlin, NH (figure62). Historically, daily air temperatures,precipitation, and snow depths have been recordedat the New Hampshire State Fish Hatchery, anofficial weather monitoring site approximately 2miles west of the project site at an elevation of1,381 feet above mean sea level (MSL). Meanannual air temperature at the weather station is42.8 °F, and the design air-freezing index, oraverage of the coldest 3 years of the last 30 yearsof record, is 1,848 °F days. The mean and designlengths of the freezing season are 127 and 148days, respectively. The lowest minimum daily

temperature in January is -35 °F. Conditions arerepresentative of much of the northern UnitedStates and of many northern national forests.

Figure 62—White Mountain National Forest site onDecember 15, 1994. The shed on the side of the roadhoused the data-logger equipment.

Site Description and InstrumentationLayout

York Pond Road is, for the most part, in extremelypoor condition. It is a narrow, winding, asphaltconcrete surfaced forest road that exhibitsexcessive differential frost heaving each spring.Differential heave as great as 8 inches over ahorizontal distance of 5 feet is not uncommon.The original road foundation consisted of a sandy-silty subbase with a silty-sandy base course rangingfrom 0 to 2 feet in thickness. Past road repairsconsisted of simply adding more material inlocalized areas. This repair method contributedtoward an already highly variable gradation,moisture content, and subgrade strength.

The worst 1-1/2-mile-long section of York PondRoad was reconstructed through a contractawarded by the FS during the summer of 1994.Construction of three 50-foot long drainage testsections for a research project conducted byCRREL was included in this endeavor. In acooperative agreement with the USFS, CRRELagreed to install, monitor, and analyze data froma TDR/RF test site located near the drainage testsections.

68

The two 100-foot USDA FS test sections (YorkPond Road stations 34+50 to 35+50 and 32+00to 33+00) lie immediately to either side of, andserve as control sections for the three drainagetest sections. The two USDA FS test sectionsconsist of a 2-inch-thick hot-mix asphalt concretesurface course, a 12-inch-thick layer of crushedaggregate, a 6-inch-thick layer of bank run gravel,and a 1- to 1.5-foot-thick layer of sandy-gravelbase from the original road. The subgrade is sandy-silt. Initially, most measurements were recordedmanually; however, data-logging systems wereinstalled in the middle of the first winter ofmonitoring. The site instrumentation layout wassimilar to the Ochoco and Kootenai sites.

York Pond Road traffic is composed of loggingtrucks hauling from several timber sales, occasionalconstruction traffic, water trucks associated witha state fish hatchery, a school bus, and automobiletraffic. The volume of truck traffic depends primarilyon the number of active timber sales and thereforevaries from year to year.

Shading, aspect, and road grade are relativelyconsistent across each site. The primary testsection from station 34+50 to 35+50 (fullyinstrumented and monitored) is generally exposedto the sun with shade cover only a short part ofthe day. The secondary site, from station 32+00to 33+00 (in which pavement strength andtemperature are monitored, but moisture is not)is primarily shade covered. Future reference to“the test section” refers only to the primary testsection from station 34+50 to 35+50. The asphaltalong this reconstructed portion of York Pond Roadis in good condition, except for one transversecrack that occurred during the first year ofmonitoring. Groundwater monitoring wells wereinstalled at each side of the test section.

This study reports FWD results collected duringthe winter and spring of 1997 through 1998.Although additional testing was performed duringother years, this data represents the best FWDcoverage obtained while moisture probes wereoperating properly.


Subsurface Temperature Regime

Figure 63 shows frost penetration through the laterportion of the 1997/1998 winter-spring season. The32 °F isotherm was determined by interpolatingsubsurface temperatures recorded by thermistors.Information, such as state of ground andcorresponding depths, is necessary to redefinelayer thickness of materials of similar moduli forbackcalculation during frozen conditions. A periodof partial thawing and refreezing, lastingapproximately 9 days, occurred during early March.

Similar to the sites on the Kootenai National Forest,these sites experienced primarily top-down thawingin the spring with approximately 80 percent ofthawing occurred in this manner. Initially, top-downthaw progressed at 3.5 inches per day throughthe coarse base/subbase materials. This ratedecreased to 1.2 inches per day once the finergrained, wetter subgrade materials wereencountered.

Subsurface Moisture Regime

Because TDR devices measure dielectric constant,which is directly proportional to unfrozen watercontent, freezing and thawing the pavement canalso be shown by TDR readings over time. Figure64 shows the TDR measured moisture contentsat the test site. TDR probes at depths of 9.2 and15.2 inches beneath the pavement surface showrelatively low, constant unfrozen water contentbetween early December and early March. OnMarch 9, these probes recorded a spike in moisturecontent which coincides with a period of partialthaw as measured by the thermistor string (figure63). The period of partial thaw ended on March13 with both the thermistors and TDR probesindicating that fully frozen conditions had returned.Thawing began again on March 25 with the TDRprobes showing a rapid increase in measuredmoisture contents peaking during the first weekin April. Moisture contents then declined rapidlyon April 15, at which point the rate of decreasesubstantially slows. This transition from a rapidto slower decrease in moisture contents occursat roughly the same time as the thawing of thelast of the ground ice (figure 63). The process of

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thawing of the entire pavement structure from thesurface downward is shown by the progressionof TDR curve peaks.

Although RF probes worked well at other USDAFS test sites, RF probes at this particular site didnot function properly, consequently RF moisturecontent graphs are unavailable. It was noted byCRREL that problems with the RF probes havebeen experienced at a variety of non-USDA FSmonitoring sites. They have observed that probesare typically functional when read manually;however, the failure rate is high when used withan automated data acquisition system. Becausethe electronics are located in the sensor head andthe sensors are then buried, evaluation/assessmentof the problem would entail excavation and removalof nonfunctional probes. In contrast to the RFprobes, TDR probes used for these monitoring sitescontained minimal electronics in the sensor head.

The groundwater table at the test site is shown infigure 65. No significant fluctuations were observedduring the spring of 1998. Depth to the water tableis typically used to define a stiff layer forbackcalculation. Water depth shown in the figurecorresponds to depth below the top of the openobservation wells to either side of the road, notthe pavement surface.

Area Parameter

As with the other test site locations, a normalizedarea parameter was calculated as an index ofpavement stiffness.

For the Berlin data, averages of deflections at eachdrop height were used. An increase in areaparameter followed by a general leveling off isshown in figure 66. The values stabilized duringearly April when thaw depths were approaching 3feet and TDR moisture contents were rapidlydeclining.

Pavement Layer Moduli

Pavement layer moduli were backcalculatedthroughout the spring season using the EVERCALCprogram. Backcalculated base course andsubgrade modulus values are shown through the

spring season in figure 67. Note the markeddecrease in modulus of the base course as thawprogresses followed by a gradual recovery. Incontrast, the subgrade modulus does not show asimilar trend in the time during which monitoringtook place. The lack of a sudden drop and recoveryof the subgrade modulus is unusual since mostroads monitored during past research projects haveshown a particularly low modulus for a short timeduring spring thaw. It may be that the subgradeunderwent only minor changes in moisture content,as shown by the 36-inch TDR probe in figure 64,resulting in similarly minor changes in subgrademodulus. It is also possible this could be attributedto backcalculation procedure (spring thaw)weaknesses previously mentioned.

Figure 68 shows a comparison of thebackcalculated AC modulus on the FWD test datesverses a standard WSDOT B mix. The resultingcurves are similar to the backcalculated curveindicating a somewhat stiffer (i.e., less temperaturesensitive) mix than the WSDOT B mix.

Critical Strains and Associated Damage

Strains at the critical locations were determinedusing EVERSTRESS. Figure 69 shows a generallyincreasing horizontal tensile strain at the bottomof the asphalt concrete that somewhat levels out,comparable to strains determined at site 1 on theKootenai. Vertical compressive strain shows thesame trend (with compression and tension simplyopposite in sign). Strain peaks somewhat later inthe top of the subgrade compared with in the base,as would be expected, as thaw progresses fromthe surface downward and excess moisturedissipates. Both tensile and compressive strainsstabilize at approximately the same time as allground ice thaws and the rapid decrease in TDRmoisture transitions to a more moderate rate ofdecline.

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Figure 63—Partial season frost profile. Note that thawing occurred predominately from the top down with a significantpartial thaw during early March.

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a) TDR Volumetric Moistures: White Mountain National Forest Site

b) Selected TDR Volumetric Moistures: White Mountain National Forest Site

Figure 64—TDR measured moisture contents. Note the drop in values as freezing occurs. The sharp spike on March 9,1998 at 9.2 and 15.2 inches coincides with period of partial thaw. Peaks during spring thaw are similar to Kootenai sites.

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Groundwater Depth: White Mountain Site

Figure 65—White Mountain test site seasonal groundwater depths.

Area Parameter: White Mountain National Forest Site

Figure 66—Seasonal change in area parameter. Note the stabilization of values as thawing concludes.

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Modulus vs. Thawed Depth: White Mountain Site

Figure 67—EVERCALC backcalculated base and subgrade moduli compared to thaw depth.

Backcalculated AC Moduli: White Mountain National Forest Site

Figure 68—EVERCALC backcalculated asphalt moduli compared to WSDOT B mix.

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Critical Strains (at 55F AC) vs. Thaw Depth and TDR Moisture: white Mountain National Forest Site

Figure 69—Seasonal change in EVERSTRESS determined critical pavement strains compared to TDR moisture (at 15.2inches) and thaw depth.

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Damage Factors vs. Thaw Depth and TDR Moisture: White Mountain National Forest Site

Figure 70—Seasonal change in damage factors compared to TDR moisture (at 15.2 inches) and thaw depth.

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Damage Factors as Related toEnvironmental and StructuralParameters

Figure 70 ties together environmental and structuralparameters. As with the other study sites, damagefactors (defined as the ratio of the number of loadsto reach failure under normal summertimeconditions to the number of loads to reach failureunder freeze-thaw conditions) were determinedfor cracking (function of horizontal tensile strainat the bottom of the asphalt) and rutting (as afunction of vertical compressive strain at the topof the subgrade). The figure shows damage inrelation to thaw depth and moisture content asmeasured by the TDR at 15.2 inches (subbase).As believed to be the case, but to be evaluated inthis study, the highest potential for damage occursduring early April when trapped moisture withinthe pavement structure is at its peak. This alsocoincides with subgrade modulus being at its lowestas shown in figure 67.

Conclusions from the White MountainNational Forest Site

1. For this site, moisture content appears to be agood indicator of when load restrictions couldbe removed; after the moisture has dissipated,damage potential dramatically decreases.However, the authors’ recommendations wouldbe to wait a given length of time (1 week) afterwater content had dropped before allowing trafficto resume. The recovery of base coursemodulus and the decrease in correspondingdamage is a gradual process, not a markedchange as with the onset of thaw.

2. Another recommendation is to install multipleTDRs at depths deemed to be critical (near thebottom of the subbase and within the top of thesubgrade in this instance).

3. Because of reliability problems experienced withthe RF probes at this and other non-USDA FSsites, CRREL would currently recommend TDRsfor this and other similar moisture monitoringapplications.

4. The observation that moisture content providesa good indicator of when to resume hauling issimilar to preliminary conclusions reached whenthe study was approached slightly differently

without using backcalculation (Kestler et al.1999). Soil moisture was found to correlate wellto deflection basin area (defined slightlydifferently than in this report), which itself canbe correlated to modulus according to the U.S.Army Corps of Engineer’s design procedure forpavements in seasonal frost areas (Bigl andBerg 1996).

Overall Summary and ConclusionsTo minimize road damage during spring thaw, theUSDA FS needs an affordable, reliable, quantitativemethod for determining when to suspend andcommence hauling heavy loads. Usingtemperatures measured by thermistors has provento work well for determining when to suspendhauling. This study focused on how to determinewhen hauling can be resumed.

TDR and RF moisture sensors underwentlaboratory testing with several soils at knownmoisture contents to validate internal moisturecorrelations, as well as cyclic freeze-thaw testingto examine probe durability. Sensors proved tobe durable, accurate, and repeatable understandard and adverse weather conditions. TDRand RF moisture sensors, as well as thermistorsprobes and groundwater monitoring wells, wereinstalled at seven test sites on four national forests.Seasonal changes in subgrade and base moisturecontents, subsurface temperatures, groundwaterlevels, and pavement stiffness were monitored forapproximately 1 year at all test sites. Correlationsbetween these various parameters were observedand reported.

To maximize pavement life and minimizemaintenance costs, traffic can be restricted orprohibited during critical periods when high intensitydamage tends to occur and be allowed to resumeonce damage susceptibility is reduced. By usingsensors, monitoring a damage-related parametercan provide information that can be used to controlthis re-establishment of traffic. Therefore,environmental and structural variations duringwhich the major portion of damage actually occurswere evaluated in this study. Because strain istraffic induced, it is not a viable parameter thatcan be measured. While modulus is independent

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of traffic and is a good indicator of when trafficshould be allowed, it is tedious to calculate andrequires an FWD, which is a costly piece ofequipment and none are currently owned by theFS. However, based on the observationsdiscussed, it appears that monitoring subsurfacesoil moisture using TDR probes can provide anefficient, alternative method for determining whendamage susceptibility is reduced and heavy loadscan be resumed. Only minimal damage occurs onceexcess moisture has dissipated. The reducedmoisture contents needed to minimize damage willvary according to soil type. However, it was foundin this study that the exact moisture content valuesare unimportant for this particular application,rather, it is the distinctive shape of the moisturerecovery curve observed which can be useful.

Principal Overall Conclusions of theStudy

1. Both the TDR and RF technologies providedreasonably accurate and reliable volumetricmoisture contents under laboratory-controlledconditions with a variety of soil types. This wastrue for testing under both room temperature andfreezing-thawing conditions. The RF probestended to be slightly more accurate comparedwith gravimetric determined values. This resultis possibly due to the RF’s three internal soilmoisture correlation curves verses the singlecurve used by the TDR unit. Laboratory testingshowed the TDR and RF probes to be a durable,reliable, and repeatable method for measuringunfrozen water content in soils subject to freeze-thaw cycling. TDR and RF probes can be usedto determine frozen verses thawed conditions.

2. The TDR devices proved to be more rugged andreliable than the RF devices during fieldoperations. Changing RF cable connector endsdue to continual oxidation of the originalaluminum contacts was necessary early in thefield tests. Repeated problems were alsoencountered with the Vitel RF probe reader unitswhich had a tendency to indicate probe failures.Opening the unit and removing a battery, thus“rebooting” the device, would often correct theproblem. Because of these and other difficulties,feedback from field operators responsible for

collecting data indicated a preference for theTDR units. All RF probes located at the WhiteMountain National Forest site failed early in thefield test portion of the study for unknownreasons.

3. As expected, freezing conditions were mostsevere at the Kootenai and White MountainNational Forest sites. Maximum frost depths of4 to 5 feet were observed at these sites.Freezing generally consisted of a single,massive, continuous event with “spring thaw”beginning in mid- to late March. Thawingextended over a period of 3 to 4 weeks, with 80percent occurred from the top downward.

4. TDR monitored volumetric moistures at theKootenai and White Mountain National Forestsites responded similarly during thaw. Initially,a dramatic increase in moisture contents tovalues well above those immediately precedingfreezing were observed. These were followedby a similarly rapid peaking and decrease inmoisture followed by a much less extremedecline through the spring and summer. Thisrelatively sharp transition of the moistureresponse curve from a rapid “drainage” phaseto a slower “drying” phase was found to roughlycoincide with the thawing of all ground ice.Recovery and stabilization of area parameters,moduli, critical strains, and damage factors werealso found to coincide with the end of the“drainage” period as indicated by the TDRs.

5. In contrast to Kootenai and White MountainNational Forest sites, the Ochoco test sitesexhibited several episodes of relatively mildfreezing and thawing. Maximum frost depthsranged from 14 to 18 inches below the asphaltsurface. Also contrasting with the more severesites, approximately 80 percent of the thawoccurred from the bottom upward, thus drainagewas less impeded during most thawing events.

6. TDR data obtained from the Ochoco NationalForest sites indicate moisture peaks associatedwith thawing are neither as extreme nor aslengthy as those observed at the Kootenai andWhite Mountain National Forest sites. However,subdued peaks on selected sensors locatedwithin or just below the frost zone wereobserved. These results suggest the Ochoco

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site would experience a similar moistureresponse curve as that observed on theKootenai and White Mountain sites if a moresevere winter than that of 1994/1995 occurred.

7. Seasonal variations in area parameter, moduli,critical stains, and damage factors were less atthe Ochoco National Forest sites than observedat the Kootenai and White Mountain NationalForest sites. The less severe effects at theOchoco National Forest sites was likely theresult of less frost penetration into the subgrade,as well as the predominately bottom-upwardthawing observed. More severe effects wouldbe expected during a colder winter than the oneobserved during this study.

8. Though significantly less in severity, damagefactors at the Ochoco National Forest sitesexperienced an extended period of recoverycompared to the Kootenai National Forest sites.The finer grained, plastic subgrade soils at theOchoco National Forest sites appear to extendthe period of weakening well beyond the thawingof all ground ice.

9. In contrast to all other study sites, the WillametteNational Forest test sites did not experiencemeasurable freezing. Only relatively gradual,long-term (annual) variations in site propertieswere observed. The variations coincided withthe annual wet/dry cycle experienced at thelocation.

10.The use of field installed TDR (or improved RF)probes and thermistor strings for roadmanagement purposes appears best suited forrelatively severe winter climates which undergodeep, persistent freezing. Based on the resultsof this study, the timing and duration of themarked weakening occurring in these climates,due to thawing, can be monitored through theuse of these technologies. Less severe freeze-thaw climates, such as those found in theOchoco National Forest, may also benefit fromthe use of these technologies, particularly duringmore severe winters. Little or no benefit is seenfrom the use of these technologies in wet/dryclimates, such as those observed on theWillamette National Forest test site. Such wet/dry climates offer no relatively short-term periodof susceptibility to high intensity road damage.

Thus, designing for use that includes the wetterperiods appears, in most cases, to be a moreappropriate strategy.

11.Though not analyzed in detail for this study, datacollected that details the timing, duration, andmagnitude of moisture variations in subgradeand base materials provide useful examples oftypical values to assist in determining seasonalfactors for mechanistic pavement design.

In summary, it is believed that by using thermistorsto quantitatively determine the start andprogression of thaw and provide data on frostdepth, and by using soil moisture sensors toquantitatively determine recovery from a thaw-weakened condition, the optimal balance betweenreducing road damage and maximizing road usecan be achieved. Monitoring these environmentalparameters will provide information useful inscheduling both the beginning and the end of loadrestriction periods.

Future Plans and RecommendationsAn implementation guide based on experiencesfrom this study is funded and scheduled forpublication in 2001. The guide will provideequipment descriptions, installation guidelines,current costs, and data interpretation guidancebased on results from this study.

It is believed that results from this study will aidin the development of guidelines for reopeningroads to hauling after the spring thaw, based uponsoil moisture dissipation. However, additionalmonitoring and expansion of the database tovalidate and develop more comprehensiveguidelines for different soil types and environmentsis highly recommended. Data collected during thisstudy may also be useful for validating variouscomputer models used to predict groundtemperatures and moisture contents.

In addition, future studies correlating TDRmeasured moisture changes with dailyprecipitation, groundwater fluctuations, and percentsubgrade saturation would be of interest.

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Literature CitedAtkins, Ronald T.; Pangburn, Timothy; Bates, Roy E.; Brockett, Bruce E. 1998. Soil moisture determinationsusing capacitance probe methodology. CRREL Special Report 98-2, Hanover, NH, January.

Baichtal, J.F. 1990. Monitoring subgrade frost penetration using constant data loggers with thermistorinstallations. Engineering Field Notes, Washington, DC: vol. 22. USDA Forest Service.

Baker, J.M. 1990. Measurement of soil water content. Remote Sensing Review. 5(1): 263-279.

Barcomb, Joe. 1989. Use of thermistors for spring road management. Transportation Research Board1252.

Bigl, Susan; Berg, Richard. 1996. Material testing and initial pavement design/modeling; Minnesotaroad research project, CRREL Report 96-14, Hanover, NH.

Campbell, Jeffrey Earle. 1988. Dielectric properties of moist soil at RF and microwave frequencies.Hanover, NH: Dartmouth College. September.

Collins, R.K. 1991. Thermistors are helping road managers reduce pavement damage on forest roadsduring spring thaw conditions. Submitted to Civil Engineering Department, Oregon State University(unpublished).

Cramer, R. 1995. Monitoring the moisture content of highway subgrade. Washington DC. TransportationResearch Board, National Research Council.

DeJean, K. et. al. 1991. Ochoco National Forest, region 6, policy for managing log haul on pavedroads under freeze/thaw conditions for Deschutes, Malheur, and Ochoco National Forests. Unpublished.

Foltz, Randy; Truebe, Mark A. 1994. The effect of aggregate quality on sediment production from aforest road, low volume roads conference. Washington DC: March. Transportation Research Board.

Federal Highway Administration. 1994. Pavement deflection analysis participant workbook, NHI Course13127, Publication No. FHWA-HI-94-021. Washington, DC.

Kane, D.L. 1986. Soil moisture monitoring under pavement structures using time domain reflectometry.Federal Highway Administration. Report No. FHWA-AK-RD-87-08.

Kestler, Maureen A.; Bull, Dale; Wright, Brenda; Hanek, Gordon; Truebe, Mark. 1997. Freeze-thawtesting of time domain reflectometry (TDR) and radio frequency (RF) sensors. Proceedings: internationalseasonally frozen soils symposium; June; Fairbanks, AK.

Kestler, Maureen A.; Stark, Jeffrey A.; Gwilliam, Bruce. [In preparation]. Field deployable soil moistureprobe for mobility predictions. CRREL Special Report, Hanover, NH.

Kestler, Maureen A.; Hanek, Gordon; Truebe, Mark; Bolander, Pete. 1999. Removing spring thaw loadrestrictions from low volume roads, development of a reliable cost-effective method, TRB Low VolumeRoads Conference, Baton Rouge, LA.

Kestler, Maureen A.; Knight, Thomas; Krat, Audrey S. 2000. Thaw weakening on low volume roads andload restriction practices, CRREL Special Report 00-006, Hanover, NH.

Klemunes, J. 1998. Determining soil volumetric moisture content using time domain reflectometry.FHWA-RD-97-139.

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Look, B.G.; Reeves I.N. 1992. The application of time domain reflectometry in geotechnical instrumentation.Geotechnical Testing Journal. GTJODJ, 15 (3): 277-283. September.

McBane, J.; Hanek, G.L. 1986a. Determination of the critical thaw-weakened period in asphalt pavementstructures. Transportation Research Record 1089, Washington DC: TRB, National Research Council.

McBane, J.; Hanek, G. 1986b. Determining the critical thaw-weakened periods in asphalt pavementstructures. Engineering Field Notes, Washington, DC: USDA Forest Service. 18, vol.

Soilmoisture Equipment Corp. 1993. TRASE system 1 manual. Santa Barbara, CA.

Topp, G. C.; Davis, J.L.; Annan, A. P. 1980. Electromagnetic determination of soil water content:measurements in coaxial transmission lines. Water Resources Research. 16. (3): 574-582. June.

Topp, G.C.; Zegelin, S.J.; White, I. 1994. Monitoring soil water content using TDR: an overview ofprogress. Proceedings: symposium and workshop on time domain reflectometry in environmental,infrastructure, and mining applications. September; 67-80 pp. Northwestern University, Evanston, IL.

Truebe, Mark; Evans, Gary. 1994. Lowell surfacing thickness design test road final report. EM-7170-15, Federal Highway Administration. Report No. FHWA-FLP-94-008. September.

Uhlmeyer, J.S.; Mahoney, J.P.; Hanek, G.L.; Wang, G.; Copstead, R.L.; Janssen, D.J. 1996. Estimationof seasonal effects for pavement design and performance. Federal Highway Administration. ReportNo. FHWA-FLP-95-006.

Utterback, P. 1995. The effects of winter haul on low volume forest development roads, technology &development program, 9577 1207-SDTDC. December.

Vitel. 1994a. Hydra logger manual, version 1.1. Chantilly, VA. February.

Vitel. 1994b. Hydra soil moisture probe user ’s manual, version 1.1. Chantilly, VA. February.

WSDOT. 1995a. Pavement guide, volume 2: pavement notes for design, evaluation, and rehabilitation.

WSDOT. 1995b. Pavement guide, volume 3: pavement analysis computer software and case studies.

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