Geocomposite capillary barriers to reduce frost heave in soils

Geocomposite capillary barriers to reduce frostheave in soils

Karen S. Henry and Robert D. Holtz

Abstract: We investigated the potential for geosynthetic capillary barriers to reduce frost heave in soils by freezing up-right, cylindrical soil specimens with horizontal disks of geosynthetics placed in them. During freezing, water wasfreely available at 25 mm above the base of 150 mm high specimens. The geosynthetics were located 5 mm above thewater supply. We measured frost heave and final water content profiles of specimens containing geosynthetic capillarybarriers and control specimens. The thermal conditions of the tests were typical of pavements in cold regions.Geotextiles prepared to simulate field conditions (i.e., moistened and containing soil fines) failed to significantly reducefrost heave. However, geocomposites comprising needle-punched polypropylene geotextiles sandwiching a drainage net,prepared in the same way as the moistened geotextiles containing soil fines, reduced frost heave when the soil watersuction head in the overlying soil was 1800 mm or more. The geocomposites did not significantly reduce heave whenthe soil water suction head in the overlying soil was 800 mm or less. This is probably due to water migration betweenthe two layers of soil, through the geotextiles and along the net of the geocomposite.

Key words: capillary barrier, frost heave, geosynthetic, geotextile, geocomposite, soil freezing.

Résumé: On a étudié le potentiel de barrières capillaires géosynthétiques pour réduire le soulèvement dû au gel dansles sols en gelant en position verticale des spécimens cylindriques de sol avec des disques de membranes géosynthéti-ques placés à l’intérieur. Durant le gel, l’eau libre était disponible à 25 mm au-dessus de la base des spécimens d’unehauteur de 150 mm. Les disques de géosynthétiques ont été placés à 5 mmau-dessus de la réserve d’eau. Nous avonsmesuré le soulèvement dû au gel et les profils de teneur en eau des spécimens contenant les barrières capillaires géo-synthétiques et des spécimens de contrôle. Les conditions thermiques des essais étaient typiques de celles des chaus-sées en régions froides. Les géotextiles préparés pour simuler les conditions de terrain (i.e., humidifiés et contenant dessols fins) n’ont pas réussi à réduire de façon significative le soulèvement dû au gel. Cependant, des géocompositescomprenant des géotextiles de polypropylène poinçonnés à l’aiguille en sandwich avec un filet drainant, préparés de fa-çon identique aux géotextiles humidifiés contenant des sols fins, ont réduit le soulèvement dû au gel lorsque la chargede succion de l’eau dans le sol sus-jacent était de 1800 mm ou plus. Les géocomposites n’ont pas réduit de façon si-gnificative le soulèvement lorsque la charge de succion de l’eau dans le sol sus-jacent était de 800 mm ou moins. Ceciétait probablement dû à la migration de l’eau entre les couches de sol, à travers les géotextiles et le long du filet dugéocomposite.

Mots clés: barrière capillaire, soulèvement dû au gel, géosynthétique, géotextile, géocomposite, gel du sol.

[Traduit par la Rédaction] Henry and Holtz 694

Introduction

Frost heave and resulting thaw weakening cause signifi-cant damage to pavements each year. Frost heave causescracking during freezing and subsequent thaw weakening,which in turn leads to more cracking and pothole formation.It is difficult to estimate the cost of frost-related damage, butgovernment transportation engineers in cold regions have

credited a minimum of half of road maintenanceexpenditures to the effects of freezing and thawing.

Three conditions are required to cause frost heave(i) freezing temperatures, (ii ) frost-susceptible soil, and(iii ) water supply to the freezing front. A variety of ap-proaches have been used to prevent or minimize frost dam-age to paved roads by modifying one or more of these threeconditions. For example, freezing temperatures can be de-layed or prevented from penetrating by using insulation inthe pavement structure (e.g., Nordal and Refsdal 1989). Theplacement of non-frost-susceptible materials for most or allof the depth of the pavement section that is subjected tofreezing is common (e.g., Berg and Johnson 1983). In addi-tion, capillary barriers have also been used to cut off the wa-ter supply to the freezing front (e.g., Casagrande 1938;Rengmark 1963).

In this paper we report research on the use of geotextilesand drainage geocomposites as capillary barriers to limit thewater supply to frost-susceptible soil that is exposed to

Can. Geotech J.38: 678–694 (2001) © 2001 NRC Canada

678

DOI: 10.1139/cgj-38-4-678

Received April 4, 2000. Accepted January 2, 2001. Publishedon the NRC Research Press Web site at http://cgj.nrc.ca onAugust 3, 2001.

K.S. Henry.1 U.S. Army Cold Regions Research andEngineering Laboratory, 72 Lyme Road, Hanover, NH 03755-1290, U.S.A.R.D. Holtz. Department of Civil and EnvironmentalEngineering, University of Washington, 260 Wilcox,Box 352 700, Seattle, WA 98195-2700, U.S.A.

1Corresponding author (e-mail:[email protected]).

freezing temperatures. Our idea is to place a large-poredlayer (the capillary barrier) between the water table and thefreezing front (located above the water supply) to stop theupward flow of water during freezing. The subgrade soil canthen be placed on top of the capillary barrier until other ma-terial such as gravel is needed for structural support. In areaswhere gravel or crushed rock is scarce or expensive, thispractice could lead to savings in construction costs com-pared with replacing the full depth of frost penetration withnon-frost-susceptible material.

We investigated the potential for geosynthetics to reducefrost heave in soils by freezing soil specimens at thermalconditions that are typical of pavements in cold regions. Thehydraulic conditions of the freezing tests were severe in thata constant level water supply was freely available at 25 mmabove the base of 150 mm high specimens.

Henry (1998) presented the experimental procedure andresults of this investigation in detail. In this article, we pres-ent the results of the freezing tests and discuss them in lightof knowledge gained from related experimental results pub-lished by Henry and Holtz (1997) and Stormont et al. (1997).

Past research on capillary barriers

A capillary barrier is a layer of large-pored material (soiland (or) geosynthetic) in a finer pored soil that (i) reducesupward2 flow of soil water, and (or) (ii ) reduces or preventswater from infiltrating from the overlying fine-pored unsatu-rated soil into the soil below the capillary barrier. In the lat-ter case, if the capillary barrier is sloped, the infiltratingwater flows in the fine soil downwards along the interfacewith the capillary barrier (e.g., Ross 1990) (Fig. 1).

Capillary barriers function because of the contrast in hy-draulic conductivity that exists near the interface betweenthe fine and coarse soils when they are not saturated (Fig. 2).Referring to Fig. 2, suction head is the suction of water inthe pores of an unsaturated soil, expressed as a head of wa-

ter. The fine soil has a measurable hydraulic conductivity atsuction heads of 1000 mm and greater, whereas the coarselayer (the capillary barrier) does not. When the hydraulicconductivity of the coarse layer is negligible, water flowingdownward in the fine soil will not move into the underlyingcoarse layer. In this case, the water content of the fine layerwill increase at the fine–coarse interface, and if the interfaceis sloped the water will be diverted laterally downdip(Fig. 1). Water flowing upward in the fine-pored, underlyingsoil will not enter the coarse layer.

The water-entry suction head is the head at which themovement of water into the smallest pores that form a con-tinuous network through an initially dry porous material oc-curs. For water infiltrating downward in soil, breakthroughinto the coarse layer occurs when the suction head at the in-terface equals the water-entry suction head of the coarselayer (Hillel and Baker 1988). As the suction head decreasesbelow the water-entry suction head, the hydraulic conductiv-ity of the coarse layer increases rapidly and eventually ex-ceeds that of the fine layer. Under these conditions, thecapillary barrier layer no longer impedes vertical watermovement.

The suction head,ht (m), of soil water is defined by

[1] –uw = γLht

whereuw is the soil water pressure (Pa), andγL (N·m–3) isthe unit weight of the liquid. Note that because the soil wa-ter pressure is negative for water subjected to capillary ac-tion, ht is a positive number. Assuming that water above thewater table is due only to capillary rise,ht is simply the dis-tance above the water table. Lambe (1950) noted that thereis a maximum height at which soil water is still in hydrauliccontact with the water table (i.e., a maximum height towhich eq. [1] can be applied). If soil pores are modeled ascylinders, this maximum height,htmax, will occur in thesmallest pores in the soil (Castellan 1983):

[2] hT

rtmax

s

L

= 2 (cos )αγ

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Henry and Holtz 679

Fig. 1. A capillary barrier to cut off upward-flowing water anddivert infiltrating water. The diversion length,L, is the horizontaldistance that water in unsaturated soil overlying the capillarybarrier will be diverted and is a function of the hydraulic proper-ties of the capillary barrier and overlying soil, the infiltrationrate, and the angle of the fine–coarse interface.

Fig. 2. Typical hydraulic conductivities of fine- and coarse-grained soils used in capillary barriers (after Stormont and An-derson 1999).

2Soil water flows towards the surface when there is a suction gradient due to capillary effects, evaporation, and (or) freezing.

whereTs (N·m–1) is the surface tension of the water,α is thecontact angle of the liquid with the cylinder wall, andr (m)is the radius of the smallest pores. Therefore, the maximumsuction at which water can be held in the pores is defined bysubstituting eq. [2] into eq. [1] to obtain

[3] ( )(cos )− = =u h

Tr

w max L tmaxsγ α2

If a capillary barrier is placed above the water table in a soil,water will not enter it unless the water is at a suction of lessthan (–uw)max of the capillary barrier defined by eq. [3]. Inother words, if the base of the capillary barrier layer isplaced above the water table at a distance greater thanhtmax,the water flowing upward cannot enter the smallest pores ofthat material because the pores are too large to support thewater at that suction. The capillary barrier thus “breaks” thecapillary rise of water.

Based on the above analysis, the maximum capillary riseof water into a porous material should be equal to its water-entry suction head. However, this is an idealized concept:soil pores are not cylindrical and there is considerable wet-ting–drying hysteresis in porous materials. Baker and Hillel(1990) found that the suction required to initiate flow in acoarse-pored soil was significantly lower than the water-entry suction estimated by observing the capillary rise ofwater in it. Therefore, water-entry suction head should notbe equated to observed capillary rise. Nonetheless, theseequations serve as an initial conceptual model.

Taber (1929) found that placing a layer of coarse sandabove the water supply in frost-susceptible soil specimensbeing frozen from the top down eliminated frost heave.Casagrande (1938) and Beskow (1946) described placing alayer of sand or gravel above the water table in road con-struction to reduce frost heave of overlying fine-grained soil.Later, Rengmark (1963) and Taivenen (1963) documentedusing a sand layer above the water table to help prevent frostheave in overlying frost-susceptible soil.

Hoover et al. (1981) and Allen et al. (1983) independentlyperformed experiments indicating that certain geotextiles re-duced frost heave when they were placed horizontally in up-right, cylindrical soil specimens that were frozen from thetop down with water freely available at the base. Henry(1988) noted the importance of the surface properties of thegeotextiles, i.e., that hydrophobic geotextiles were much moreeffective in reducing frost heave than hydrophilic geotextiles.

Research objectives

Our objectives were to (i) more clearly define the soil andthermal conditions under which capillary barriers effectively

reduce frost heave, and (ii ) determine the geosynthetic prop-erties required for optimal performance as capillary barriers.

The research involved (i) quantifying the range of thermalconditions that exist in the field and establishing a freezingexperiment that realistically simulates these conditions; and(ii ) establishing soil, thermal, and hydraulic conditions andgeosynthetic properties that are most favorable to usinggeosynthetic capillary barriers effectively.

Based on previous laboratory work (e.g., Allen et al. 1983;Henry 1988), we thought that some geotextiles would be ef-fective capillary barriers to reduce frost heave. Thus, theoriginal experimental design included plans to systemati-cally vary geotextile pore-size distribution and thickness tofind optimum values of these geotextile properties. The out-come of these experiments was to be used as a basis to de-cide how to select geotextile capillary barriers. We includeddrainage geocomposites in our original experimental planbecause we felt that the very large pore sizes of the drainagenet would make an optimal capillary barrier, thus providinga standard for comparing the performance of the variousgeotextiles.

Early results from the freezing experiments significantlydiffered from those expected (phases 1 and 2, Table 1). Insome tests geotextiles and geocomposites did not reducefrost heave, whereas in others they did. This led to consider-able deviation from the original test plan to explore thecauses of the unexpected behavior, and the research programbecame a “learn as you go” process. We compared the con-ditions under which geosynthetics reduced heave to thoseconditions which existed when they did not reduce heave.Measurements of water capillary rise into geotextiles weremade (Henry and Holtz 1997) and the soil moisture reten-tion behavior of the test geotextiles was studied (Stormont etal. 1997). The results of the initial freezing tests, when takentogether with the capillary rise and soil moisture retentionstudy, led to the hypothesis that moist geotextiles containingsoil fines, a condition that can be reasonably expected to oc-cur in the field, will not behave as capillary barriers to re-duce heave. A final freezing experiment was conducted toconfirm this hypothesis (test 10, the first test in phase 3).Thereafter, tests focused on the use of geocomposites as cap-illary barriers in freezing tests (tests 11–16).

Experimental program

We froze 100 mm diameter by 150 mm high cylindricalsoil specimens from the top down, with water freely avail-able at the base, and measured frost heave and final watercontent profiles to study the influence of the geosyntheticcapillary barriers on frost heave. The tests were conducted at

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680 Can. Geotech J. Vol. 38, 2001

Phase Tests Description

1 4–6 Initial tests with geotextiles and geocomposites; it was difficult to force water into the soil above thegeosynthetics by raising a water supply through the base; if we were successful in soaking the soil above thegeosynthetic, the specimens heaved similar to that of control specimens

2 7, 8 Compared frost heave of control specimens and specimens containing geotextiles soaked prior to freezing with frostheave of specimens not soaked prior to freezing; in each test, two specimens were soaked and two were not soaked

3 10–16 Examined the influence of moist geosynthetics containing soil fines on the frost heave of three different soils;test 10 specimens contained geotextiles, and tests 11–16 specimens contained geocomposite capillary barriers

Table 1. Phases of experimental program.

thermal conditions close to those which occur in pavementsthat seasonally freeze. A 100 mm diameter, 30 N surcharge

was placed on the tops of the specimens to simulate approxi-mately 0.2 m of overburden pressure.

Horizontal disks of geocomposites or geotextiles wereplaced 30 mm above the bottom of specimens, only 5 mmabove the water level during freezing. This is a severe hy-draulic condition, since observations in pavements indicatethat the water table often descends 200–1000 mm ahead ofthe freezing front during a typical freezing season (e.g.,Henry 1990; Saarelainen 1992). We used this condition be-cause if capillary barriers will reduce frost heave when thesuctions in the underlying soil are very low, they will alsoreduce heave when the suctions are higher (i.e., the water ta-ble is lower).

Each experimental phase was based on observations andknowledge gained previously (Table 1)3. Each test had fourspecimens; the test number followed by the specimen num-ber identifies specimens. For example, the fourth specimenin test 10 is labeled 10-4. Table 2 lists experimental vari-ables for each test. The test parameters listed in Table 2 arediscussed in more detail below.

Materials tested

SoilsSoil A was the primary test soil (Table 3; Fig. 3). It has

75% passing the 0.075 mm sieve and 60% finer than0.05 mm. Soil A was collected from the Anchorage Interna-tional Airport by the Alaska Department of Transportationand Public Facilities, whose design guidance for facilitiesthat can function with some frost heave requires removal of1.2 m of this material and replacement with clean, non-frost-susceptible gravel. One other soil from Fort McCoy, Wiscon-sin, was used in test 14.

GeosyntheticsGeosynthetics selected for this program are described in

Tables 4 and 5. All of the geotextiles are needle-punchedpolypropylene. Geotextiles A10 and A16 were obtainedfrom one manufacturer and geotextiles ST750 and ST1000obtained from another manufacturer. The geotextiles from

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Henry and Holtz 681

TestspecimenNo.

Drydensity(Mg·m–3)

Bath temperatureduring freezing (°C)

Geosynthetic Soaked Top Bottom

4-1 A16 1.98 Yes* –1.4 0.74-2 A16 1.95 Yes* –1.4 0.74-3 None 1.98 Yes –1.4 0.74-4 None 1.95 Yes –1.4 0.75-1 A16 1.96 Yes –1.4 1.05-2 A10 1.95 Yes* –1.4 1.05-3 ST750 1.93 Yes –1.4 1.05-4 None 1.95 Yes –1.4 1.06-1 None 1.93 Yes –1.3 1.46-2 ST750-TN 2.04 Yes –1.3 1.46-3 A16-TN 2.08 Yes –1.3 1.46-4 A10-TN 2.09 Yes –1.3 1.47-1 A10-TN 2.07 Yes –1.4 1.77-2 A10-TN 2.01 No –1.4 1.77-3 None 1.95 No –1.4 1.77-4 None 1.98 Yes –1.4 1.78-1 A10-TN 2.10 Yes –4.2 1.38-2 A10-TN 2.05 No –4.2 1.38-3 None 1.99 No –4.2 1.38-4 None 1.97 Yes –4.2 1.3

10-1 None 1.97 No –1.4 0.710-2 ST1000 1.93 No –1.4 0.710-3 A16 1.98 No –1.4 0.710-4 A16 1.97 No –1.4 0.711-1 ST750-TX 1.92 No –3.0 0.711-2 ST750-TX 1.97 No –3.0 0.711-3 A10-TX 1.99 No –3.0 0.711-4 None 1.98 No –3.0 0.712-3 A10-TX 2.01 No –3.1 0.712-4 A10-TX 1.98 No –3.1 0.713-1 A10-TX 1.89 No –1.4 0.613-2 ST750-TX 2.03 No –1.4 0.613-3 None 1.96 No –1.4 0.613-4 ST750-TX 1.99 No –1.4 0.614-1 A10-TX 1.99 No –1.5 0.814-2 A10-TN 2.02 No –1.5 0.814-3 ST750-TN 2.02 No –1.5 0.814-4 None 1.98 No –1.5 0.815-1 A10-TX 1.87 No –1.5 0.715-2 A10-TN 1.87 No –1.5 0.715-3 ST750-TN 1.91 No –1.5 0.715-4 None 1.85 No –1.5 0.716-1 A10-TX 1.88 No –1.5 0.716-2 A10-TX 1.86 No –1.5 0.716-3 A10-TX 1.86 No –1.5 0.716-4 None 1.83 No –1.5 0.7

*The appearance of the top surface of these specimens did not changewith soaking.

Table 2. Test specimen number, geosynthetic, dry density, satura-tion procedure, and thermal conditions.

Fig. 3. Grain-size distribution of soil A, the main test soil, andFort McCoy soil, used in text 14.

3Although 17 freezing tests were conducted, we discuss only tests 4–8 and 10–16 in this paper. The other tests were educational to us inlearning how to conduct tests, but yielded no research results of interest.

different manufacturers were used to obtain different pore-size distributions for the geotextiles (for details, see Henryand Ellis 1996). Geotextiles ST750 and ST1000 had, on av-erage, larger pores than geotextiles A10 and A16, eventhough the apparent opening size (AOS) values are approxi-mately the same. Thickness of the products was varied byselecting geotextiles with values of mass per unit area of340 and 540 g·m–2. We made geocomposites by placing onetype of geotextile on each side of a geonet (Table 5).

Thermal conditions of the freezing tests

Net heat flux and temperature gradients in freezing soilare important variables controlling frost heave (e.g., Konrad1984; Henry 1998), and the effectiveness of capillary barri-ers will depend on the thermal conditions that occur. For ex-ample, if all the heat loss from a specimen occurs at a veryhigh heat flux and results in freezing of pore water accompa-nied by rapid frost penetration, a capillary barrier will not behelpful. On the other hand, if the heat flux is relatively low,there will be a stationary frost front with a growing ice lensbeing fed by water flowing upward from the water table. Inthis case, a capillary barrier will cut off the water supply tothe freezing front.

The same value of net heat flux can occur with a range oftemperature gradients, and the temperature gradients also in-fluence rate of frost heave (Fig. 4). To use realistic freezingconditions for this test program, we used data collected frominstrumented pavements to estimate net heat flux. We alsodocumented temperature gradients. The net heat flux fromthe freezing front,qnet (W·m–2) was estimated from the ratesof frost heave and frost penetration according to the follow-ing relation (e.g., Loch 1979):

[4] q Lv S

net fI

w1.1 t

= +

ϑ ∂

∂

whereLf is the latent heat of fusion of water per unit mass(333.52 × 106 J·m–3); νI is the rate of frost heave (m·s–1); ϑwis the volumetric water content of the soil; and∂S/∂t is therate of penetration of the freezing front (m·s–1), whereS isthe freezing front location andt is time.

We used field data in which frost heave, frost penetration,soil layer density, and water content are well documented toestimate the heat flux using eq. [4]. Data from Henry (1990,1998), Saarelainen (1992), and Vikström (1997) were used.Each source of data included the appropriate records for atleast 1 year. A range of climates from the temperate regionof Hamilton, Montana, to the subarctic was represented. Thetime intervals used to determine the net heat flux rangedfrom 8 to 64 days. The details of the estimation procedureare found in Henry (1998).

Estimated heat flux from freezing pavements ranged from5 to 66 W·m–2, and temperature gradients ranged from 0 to12°C·m–1 (Henry 1998). We selected 30 W·m–2 as anapproximate lower value of net heat flux for testing and55 W·m–2 as an upper value. Temperature gradients were

© 2001 NRC Canada


Maximum dry density(Mg·m–3)

Optimum water content(%) Frost

susceptibilitySoil Classification* Standard Modified Standard Modified

A ML 1.77 1.95 14.6 11.0 F4Fort McCoy SM 1.89 nd 11.4 nd F2

Note: Characteristics according to U.S. Army Corps of Engineers criteria (e.g., Berg and Johnson 1983); F2, F3, and F4 roughly correspond toincreasing degrees of frost susceptibility. Standard and modified denote Standard Proctor compaction effort (ASTM D698-91) and Modified Proctorcompaction effort (ASTM D1557-91), respectively. nd, not determined.

*According to ASTM D2487-98.

Table 3. Characteristics of test soils.

Geotextile ConstructionMass/area(g·m–2)

AOS(mm)

A10 Staple fibers 339 0.150A16 Staple fibers 543 0.150ST750 Continuous filament 340 0.180ST1000 Continuous filament 540 0.150

Note: Properties were obtained from Industrial Fabrics AssociationInternational (1994, 1995). All geotextiles consisted of polypropylenefibers. AOS, apparent opening size and equivalent to the diameter of thelargest particle that passes through the geotextile.

Table 4. Geotextiles tested.

Aperture (mm)

Geonet Thickness (mm) Largest Smallest

TN 6.4 15.3 7.2TX 8.8 10.1 6.0

Table 5. Geonets used in the test program (Industrial Fabrics As-sociation International 1995).

Fig. 4. Rate of frost heave versus net heat flux for soils testedby Loch (1979a) and Konrad (1984) under different temperaturegradients (from Henry 1998). The straight line depicts the rate ofheave when no frost penetration occurs, i.e.,υI = [(qnet)/(Lf)]1.1.

selected so that the temperature gradient in the specimenwas about 10 ± 2°C·m–1 for the specimens tested at the lowrates of heat loss and 27 ± 2°C·m–1 in the specimens testedat high rates of heat loss. We could not obtain lower temper-ature gradients for the specimens because the specimenheight was limited to 150 mm. Note that much larger tem-perature gradient differences are needed to significantly in-fluence the rate of frost heave (e.g., Fig. 4).

Preparing the specimens

For each test, we screened approximately 12 000 g of drysoil on a 4 mmmesh sieve, added tap water to achieve thedesired moisture content (optimum moisture content accord-ing to American Society for Testing and Materials (ASTM)standard D1557-91 or D698-91), and mixed it well with thesoil. The soil and water equilibrated overnight in a closedcontainer, and we prepared the specimens the next day.

The specimens were held in six split acrylic rings thatwere stacked, taped into the stacked position, and lined witha 100 mm diameter, 0.3 mm thick latex membrane andplaced on the base plate. We placed a compaction moldaround the rings, lowered the collar over the mold, andbolted the collar onto the base plate to hold the mold inplace during compaction (Fig. 5). Geosynthetics were placedabout 30 mm above the base of the specimen.4

Soaking the specimensIn an attempt to saturate the specimens prior to freezing in

phases 1 and 2 tests, Mariotte tubes were connected to thebase plates and the water level raised at approximately25 mm·h–1 through the height of the specimens. This is astandard procedure to determine the frost susceptibility ofsoil (e.g., ASTM D5918-96). For control specimens, thispractice resulted in the soil specimens becoming visibly verywet at the top within 1 day of the procedure. For specimenswith geotextiles and geocomposites, however, water did notreadily migrate through the geosynthetic into the overlyingsoil (the geosynthetics were blocking the flow). Thus, intests 4–8, if it was difficult to force water across thegeosynthetic layer, the water level was raised until it was upto 200 mm above the specimen. After the water level wasraised we waited from 6 to 16 days for water to migrate intothe soil overlying the geosynthetic. Yet, some specimens stilldid not become visibly wetter at the top than they were dur-ing compaction. Due to a change in the experimental pro-gram, specimens in tests 10–16 were not soaked beforefreezing.

Treatment of geosynthetics prior to freezing in tests 10–16

In tests 10–16 we moistened and emplaced soil fines inthe geotextiles and geocomposites during specimen compac-tion. A cardboard template with a 95 mm diameter circularhole was placed into a standard laboratory 45 mm meshsieve. A 100 mm diameter geotextile or geocomposite wascentered over the hole and taped around the edges. A 20 mmthick layer of air-dried test soil was placed over thegeosynthetic in the sieve, and a mechanical shaker then

shook the sieve for 5 min, but without the hammer.Moistening was then accomplished by placing the geotextileor geocomposite into the bottom of a 100 mm diameterrigid-wall permeameter, pouring a 20 mm thick layer of air-dried test soil on top of it, then gently pouring deaired tapwater into the cylinder until 50 mm of water was above thesoil. This water flowed through the soil and thegeosynthetic, and upon completion the geosynthetic was re-moved and soil was gently rinsed off the top surface withtap water. The geotextile was then placed on paper towels toremove approximately 10 g of water from each geotextilethrough evaporation and absorption.

The mass per unit area of each of the geotextiles in thegeocomposite was determined by dividing the mass of thegeotextile (measured to the nearest 0.01 g) by the area(based on the average of five equally spaced diameter mea-surements made to the nearest 0.01 mm). Geotextile poros-ity, ng, was determined as follows:

nm

tg

g

f

= −1ρ

wheremg is the mass per unit area of the geotextile (g·m–2),ρf is the fiber density (g·m–3), and t is the thickness of thegeotextile (m) (ASTM D5199-95).

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Henry and Holtz 683

Fig. 5. Mold placed on specimen base plate; surrounds mem-brane-lined acrylic rings during compaction.

4 In test 4, the geotextiles were placed 60 mm above the base of the specimen; however, this had no significant influence on the experimentalresults.

Geotextiles were weighed before being moistened andhaving soil fines emplaced and immediately prior to placingthem in the specimens. The soil content of each geotextilewas assumed to remain constant during the test and was de-termined after the geotextiles were removed from the speci-mens and oven-dried. Initial and final percent watersaturation determinations were based on the effective porosi-ties of the geotextiles, described as follows.

The effective porosity of the geotextile (with soil fines in-corporated),ngeff, was determined byngeff = ng – Ssoil, whereSsoil is the ratio of the volume of pore space filled with soilto the total volume of pores in the geotextile:

S ns m

tsoil g

c g

s

=

ρ

wheresc is the soil content and is defined as the mass of thesoil divided by the mass of the geotextile, andρs is the den-sity of the soil particles (2.72 Mg·m–3). The initial porosity,soil content, and effective porosities of geotextiles used intests 10–16 are provided in Table 6.

Once specimens were prepared, they were immediatelymoved into the insulated cabinet of the cold room (main-tained at 0–4°C) and the Mariotte tube water supplies wereconnected to the base plates. The thermocouples were placedand the specimens were insulated as soon as possible toavoid any chance of radial freezing (Fig. 6). The temperaturebaths were then turned to the desired temperatures for condi-tioning prior to freezing (the top and bottom baths wereabout –0.5°C and 1.9°C, respectively), and the specimenswere conditioned until there was a stable temperature distri-bution along the specimen from the top to the bottom of

© 2001 NRC Canada


SpecimenNo. Geotextile

Initialporosity,ng

Soilcontent

Mass/area,mg (g·m–2)

Thickness,t (mm)

Effectiveporosity,ngeff

10-2 ST1000 0.872 2.05 561.5 4.83 0.79610-3 A16 0.856 1.84 608.0 4.65 0.78010-4 A16 0.820 1.38 694.2 4.24 0.75211-1, top ST750 0.888 1.62 306.2 3.00 0.83411-1, bottom ST750 0.882 2.22 403.4 3.76 0.80511-2, top ST750 0.898 1.38 364.1 3.94 0.85611-2, bottom ST750 0.883 0.93 403.5 3.78 0.85111-3, top A10 0.879 0.86 423.4 3.84 0.84811-3, bottom A10 0.882 1.78 443.4 4.14 0.82012-2, top A10 0.895 1.12 417.2 4.37 0.86012-2, bottom A10 0.896 3.22 402.6 4.27 0.79612-3, top ST750 0.888 1.99 323.0 3.18 0.82212-3, bottom ST750 0.887 1.07 267.0 2.59 0.85112-4, top A10 0.896 1.24 441.3 4.67 0.85812-4, bottom A10 0.879 2.41 480.2 4.34 0.79313-1, top A10 0.874 2.85 464.3 4.06 0.77013-1, bottom A10 0.887 1.20 445.6 4.32 0.84613-2, top ST750 0.883 1.12 343.3 3.23 0.84413-2, bottom ST750 0.880 1.84 407.4 3.73 0.81513-4, top ST750 0.896 2.06 253.7 2.69 0.83213-4, bottom ST750 0.872 1.67 440.2 3.78 0.81014-1, top A10 0.875 0.64 462.8 4.06 0.85114-1, bottom A10 0.878 0.61 441.0 3.96 0.85614-2, top A10 0.883 0.61 450.5 4.22 0.86114-2, bottom A10 0.880 0.43 442.8 4.06 0.86514-3, top ST750 0.890 0.46 362.2 3.61 0.87514-3, bottom ST750 0.881 0.52 367.7 3.40 0.86315-1, top A10 0.877 1.47 444.9 3.99 0.82515-1, bottom A10 0.878 2.19 452.9 4.06 0.79915-2, top A10 0.882 1.91 424.5 3.94 0.81515-2, bottom A10 0.886 2.07 422.3 4.06 0.81615-3, top ST750 0.880 1.31 384.7 3.53 0.83415-3, bottom ST750 0.887 1.22 370.6 3.61 0.84616-1, top A10 0.882 1.17 469.5 4.37 0.84116-1, bottom A10 0.883 2.25 433.0 4.06 0.80516-2, top A10 0.884 1.59 436.3 4.14 0.83016-2, bottom A10 0.888 1.44 436.6 4.29 0.84116-3, top A10 0.878 0.40 491.7 4.42 0.86316-3, bottom A10 0.891 0.36 430.5 4.34 0.880

Table 6. Initial geotextile porosity, soil content of the geotextiles, and effective porosity after soil fines were placed in geotextiles fortests 10–16.

about 0.3°C at the top to 0.6°C at the bottom. This took aminimum of 12 h.

Conducting the tests

Heat was supplied to and extracted from the soil speci-mens through heat-transfer plates placed directly on top ofthe specimens and below the porous base plates. Propyleneglycol – water solutions were circulated through the platesfrom two refrigerated circulating baths, one to control thetop temperature of the specimens and one to control the bot-tom temperature. We manually adjusted the top and bottombath temperatures during the test.

The data-acquisition system consisted of an HP3421Adatalogger and an IBM-compatible PC. The PC was con-nected to the datalogger via a general-purpose interface bus(GPIB) and programmed with LabView, a graphically basedcomputer code, to control the data acquisition.

Test results

Repeatability of resultsFrost heave and penetration of control specimens of soil A

from tests 4, 10, and 13, conducted with an applied top bathtemperature of –1.4°C and bottom bath temperatures of0.7°C for tests 4 and 10 and 0.6°C for test 13, are shown inFig. 7. The specimens in test 4 were soaked before freezing,whereas those in tests 10 and 13 were not. The results fromtest 4 indicate the repeatability within a given test, and theresults from tests 10 and 13 indicate repeatability betweentwo tests conducted with similar conditions. Even though thefrost-penetration curves are slightly different during the firstday of freezing between tests 10 and 13, the subsequentrates of frost penetration are quite similar.

Influence of heat flux on frost heave rateHeat flux versus heave rate for freezing tests conducted on

control specimens of soil A compacted to about 2.0 Mg·m–3

was estimated using eq. [1] (Fig. 8). Because the heave rate

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Henry and Holtz 685

Fig. 6. Schematic of soils freezing test setup: (a) plan view, without insulation; (b) cross section, with insulation surrounding the specimens.

at the high heat flux value (55 W·m–2) is greater than that atlower heat fluxes (i.e., the curve does not pass a maximum),capillary barriers effective at the high heat flux rates shouldbe at least as effective as those at lower heat flux rates. ForFort McCoy soil, the heave rate was relatively independentof heat flux for heat flux values ranging from 5 to 70 W·m–2,also suggesting that a capillary barrier will provide benefit.

Geotextiles as capillary barriersIn test 4, the geotextiles significantly reduced frost heave;

however, only one of three geotextiles was effective in test 5(Fig. 9). For the specimens in which geotextiles reducedfrost heave, the water content (gravimetric) in the soil abovethe geotextile decreased from the top of the specimen downto the level of the geotextile. Below the geotextile, the watercontents were about 16% or greater (the saturation watercontent of soil A at the compacted density) (Figs. 10–12).For the control specimens and the specimens in which thegeotextiles did not reduce frost heave, the water content in-creased from the top of the specimen down.

Upon reviewing the results of tests 4–8, we realized thatwhenever excess water appeared at the top of a specimenduring the soaking procedure, the geotextile later failed as acapillary barrier during freezing. On the other hand, if thetop of the specimen did not become visibly wetter than itwas during compaction, the geotextile reduced frost heave.

This led to the idea that if moistened, the geotextiles areineffective capillary barriers. Two additional experimental pro-cedures were then used to test this idea: (i) moisture-retention

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Fig. 7. Frost heave (a) and penetration (b) of control specimensfrom tests 4, 10, and 13, in which soil A was compacted toabout 2.0Mg·m–3 and frozen with approximately the same tem-peratures applied to the top and bottom.

Fig. 8. Heat flux versus frost heave rate for freezing tests conductedwith soil A. A linear regression analysis was performed to fit asecond-order polynomial to the results; theR2 value is shown.

Fig. 9. Frost heave (a) and penetration (b) of test specimens intests 4 and 5, in which geotextiles (indicated after specimennumber) were placed in some specimens. Note that specimens 4-1 and 4-2 experienced an increased heave rate between day 2.0and day 2.5, somewhat after the time that frost penetrated thegeotextiles.

functions of the geotextiles were determined, and (ii ) thecapillary rise of water into geotextiles was measured.

Stormont et al. (1997) determined the soil moisture reten-tion characteristics of our test geotextiles and found that thegeotextiles have considerable wetting–drying hysteresis.Thus, moistened geotextiles absorb and transmit water moreeasily than dry geotextiles subjected to the same suction.Henry and Holtz (1997) also found that placing soil fines

into geotextiles that were originally hydrophobic made themhydrophilic.

When interpreted together, the soil moisture retentionfunctions and the capillary rise measurements strongly sug-gested that moist geotextiles containing soil fines will notbehave as capillary barriers. Test 10 was conducted to con-firm this hypothesis. In test 10, we placed soil fines intogeotextiles and moistened them prior to inserting them into

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Fig. 10. Specimens 4-1 (on left) and 4-4 (control specimen). Note dry soil above the capillary barrier in specimen 4-1.

Fig. 11. Water contents in test 4 specimens after freezing. Linesconnect points where specimen was frozen at the end of the test.

Fig. 12. Water contents in test 5 specimens after freezing. Linesconnect points where specimen was frozen at the end of the test.

the soil specimens; however, we did not saturate the speci-mens. To moisten the geotextiles, we saturated them firstand allowed them to dry to about 30–40% saturation as de-scribed earlier. None of the geotextiles in test 10 reducedfrost heave (Fig. 13), thus confirming our hypothesis.

Geocomposites as capillary barriersFor phase 1 and phase 2 tests with geocomposite capillary

barriers (tests 6–8) we observed that, as with the geotextiles,the geocomposites did not significantly reduce frost heavewhen water migrated across the geocomposite into the over-lying soil during pretest soaking. When no water migratedinto soil above the geocomposite, the geocomposite com-pletely eliminated heave (e.g., specimen 7-2 shown inFig. 14). However, even when the geocomposites did noteliminate frost heave, the amount of water above thegeocomposite was less than the amount of water at theequivalent heights in the control specimens in all three tests(e.g., Fig. 15).

Tests 11–16 (phase 3 tests) utilized geocomposites thatwere moistened and had soil fines emplaced prior to freez-ing. Table 7 summarizes the soil, compaction, and thermalconditions used in these tests. Tests 11 and 12 were experi-ments with soil A compacted at the same conditions (Mod-ified Proctor) as those for the previous tests withgeotextiles.5 These tests were conducted at the high heat fluxcondition because if the geocomposites reduce frost heave atthis more critical condition, they will be successful at doingso at the lower heat flux values as well. There were two cy-

cles of frost heave, with a thaw cycle in between (Figs. 16,17). Unfortunately, there were considerable problems withcontrolling the water level in specimens 11-1 and 11-2 in thefirst freeze cycle, and the water levels were 40 mm abovethe bottom of the geocomposite. During the first freeze cy-cle, specimen 11-2 heaved as if it were a control specimen,whereas specimen 11-1 did not heave at all, indicating capil-lary-barrier performance even in the presence of positivewater pressure. At the end of the first freeze cycle, the top ofspecimen 11-1 was moist but not saturated, and the tops of

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Fig. 13. Frost heave of specimens in text 10, in which moist geo-textiles containing soil fines were placed in some soil specimens.

Fig. 14. Frost heave of test specimens in test 7, in which geo-composites (indicated after specimen number) were placed insome specimens.

Fig. 15. Water contents in test 6 specimens after freezing. Linesconnect point where the specimen was frozen.

TestNo. Soil Compaction method Freezing condition

11 A Modified Proctor High heat flux, two freezing cycles12 A Modified Proctor High heat flux, two freezing cycles13 A Modified Proctor Low heat flux14 Fort McCoy Standard Proctor Low heat flux15 A Standard Proctor Low heat flux16 A Standard Proctor Low heat flux

Table 7. Experimental parameters for tests 11–16.

5All geocomposites except the one in specimen 16-3 were moistened prior to freezing.

specimens 11-2, 11-3, and 11-4 appeared to be very satu-rated. During the thaw, the water level remained high inspecimens 11-1 and 11-2, and it was not successfully low-ered until just before the start of the second freeze cycle.Figure 16 shows that although specimen 11-1 had an effec-tive capillary barrier in the first freeze cycle, it did not in thesecond freeze cycle. Water-content distributions in speci-mens 11-1 and 11-2 indicate that the capillary barrier re-duced water migration into the upper portion of thespecimen (Fig. 18).

In test 12, with the water levels below the base of thegeocomposite, the moist geocomposites containing soil finesvery effectively reduced frost heave (Figs. 17, 19). Speci-mens 12-2, 12-3, and 12-4 resumed heaving at about days1.5 and 10.5 when frost penetrated below the geocompositelayer. Ice was also present in the geocomposites removedfrom the specimens containing geocomposites (e.g., Fig. 20).

To more fully determine conditions under which themoistened geocomposites reduce frost heave, we varied theexperimental parameters as follows: (i) different soil—FortMcCoy soil, a silty sand with lower frost susceptibility thansoil A, compacted at Standard Proctor conditions to achievea minimum of 90% of the maximum dry density and slightlydry of optimum water content (test 14); (ii ) compaction con-ditions—soil A, compacted at Standard Proctor conditions toachieve at least 90% of maximum dry density and optimum

water content (tests 15 and 16); and (iii ) initial moisturecontent of geocomposite—test 16 used identicalgeocomposites with a range of initial moisture contents, in-cluding a dry geocomposite. Low heat flux conditions wereused in tests 13–16 because this allowed the influence of thegeocomposites to be carefully studied.

The frost heave of control specimens of soil A from testsfrozen at low heat flux conditions (tests 10, 13, 15 and 16)but two different compaction conditions (i.e., Standard andModified Proctor) is shown in Fig. 21. There are no signifi-cant differences in frost heave of the control specimens.However, the (initially moist) geocomposites eliminatedfrost heave in the Modified Proctor specimens but not in theStandard Proctor specimens (Fig. 22). Final water contentsalso suggest that the geocomposites which failed to preventfrost heave did reduce the amount of water that migrated tothe overlying soil compared with the control specimen (e.g.,Fig. 23). The geocomposites removed from these specimenswere not saturated.

Figure 24 shows the frost heave of the Fort McCoy siltysand. The geocomposites successfully reduced heave afterinitially heaving at a somewhat greater rate than that of thecontrol specimen. The total amount of frost heave of thecontrol specimen was quite low compared with that of con-

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Fig. 16. Frost heave of specimens in test 11.

Fig. 17. Frost heave of specimens in test 12.

Fig. 18. Water contents in test 11 specimens after freezing. Linesconnect points where specimen was frozen.

Fig. 19. Water contents in test 12 specimens after freezing. Linesconnect points where specimen was frozen.

trol specimens of soil A frozen at the same temperatures(note the scale in Fig. 24).

Discussion

Capillary barrier effectiveness as a function of heat fluxBecause the moist geocomposites containing soil fines

significantly reduced frost heave in tests conducted at highflux, we believed that the geocomposites would be at least aseffective at lower heat flux. For example, using eq. [4], tests11 and 12 (the first freeze cycle) had estimated steady stateheat flux values of 32.3 and 32.4 W·m–2, respectively. Theestimated steady state heat flux value for test 13 was25.6 W·m–2. The geocomposites eliminated heave com-pletely in test 13, in which the lower heat flux condition wasused (Fig. 22).

Water flow across geocompositesWhen geocomposites successfully reduced heave (tests 13

and 14), the top geotextile dried during the test, indicatingthe effectiveness of the net as the capillary barrier layer (Ta-ble 8). When the geocomposites did not reduce frost heave(e.g., tests 15 and 16), however, the top geotextiles did notlose a significant amount of moisture.

When the geocomposites failed as capillary barriers, theinitial degree of saturation in soil above the geocomposite

was greater than or equal to 81% (Table 9). For all of thespecimens in which the geocomposites successfully reducedfrost heave, the initial degree of saturation was less than orequal to 75%. We used Gardner’s equation to estimate thesoil moisture suction head, hp, as a function of saturation(Gardner 1958; Guymon et al. 1993):

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Fig. 20. Photograph of geonet removed from specimen 12-2 that had frozen through the geocomposite. Ice between the bottom and topgeotextiles was concentrated in the pore walls of the geonet.

Fig. 21. Frost heave of control specimens in tests 10, 13, 15,and 16, all conducted at low heat flux. Tests 10 and 13 were onsoil A at Modified compaction conditions, and tests 15 and 16were at Standard compaction conditions.

θu

w p

=+

n

A ha

1

whereAw and a are coefficients determined by fitting mea-sured data,n is the soil porosity, andθu is the volumetricwater content. To obtainAw anda, the values of suction andwater contents at the highest and lowest suctions used in thesoil moisture retention tests that we conducted were insertedinto the equation and two equations in two unknowns weresolved to estimate values of 0.014 and 0.628, respectively(Fig. 25). The shape of the curve extrapolated is typical ofsilt (e.g., Hillel 1980). At 81% saturation, the soil moisturesuction head in soil A is estimated to be 800 mm, and 75%saturation corresponds to a soil moisture suction head ofabout 1800 mm.

The geocomposite probably allowed water to migrateacross when the overlying soil A was 81% saturated becausethe water migrated through the top geotextile and onto thedrainage net surfaces, resulting in hydraulic contact betweenthe water supply and the soil during freezing. This was per-

mitted by the relatively low suction head of 800 mm at thisdegree of saturation. Although advances are being made in

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Fig. 22. Frost heave of specimens in tests 13 (Modified compac-tion), 15 (Standard compaction), and 16 (Standard compaction)containing moist geocomposites.

Fig. 23. Water contents in test 16 specimens after freezing. Thespecimens contained identical geocomposites that had differentinitial amounts of moisture. Lines connect points where the spec-imen was frozen.

Fig. 24. Frost heave of specimens from test 14 on Fort McCoysilty sand.

SpecimenNo.

Saturation (%) %changeGeotextile Before test After test

12-2 A10 90 8 –9112-3 ST750 65 7 –8912-4 A10 100 8 –9213-1 A10 52 3 –9413-2 ST750 53 4 –9213-4 ST750 28 2 –9314-2 A10 76 4 –9514-3 ST750 69 5 –9315-1 A10 89 58 –3515-2 A10 84 28 –6715-3 ST750 65 13 –7716-1 A10 79 34 –5716-2 A10 51 43 –1616-3 A10 0 1

Table 8. Percent saturation of the top geotextiles of thegeocomposites used in tests 12–16.

Specimen No. SoilCompactionmethod

Saturation(%)

Failed to reduce frost heave7-1 A Modified Proctor 1008-1 A Modified Proctor 9414-1, 14-2, 14-3 A Standard Proctor 8215-1, 15-2, 15-3 A Standard Proctor 81

Significantly reduced frost heave7-2 A Modified Proctor 678-2 A Modified Proctor 7511-3 A Modified Proctor 6712-2, 12-3, 12-4 A Modified Proctor 6813-1, 13-2, 13-4 A Modified Proctor 6514-1, 14-2, 14-3 Fort McCoy Standard Proctor 69

Table 9. Percent saturation of soil above geocomposites beforefreezing.

measuring the unsaturated flow in geotextiles (e.g., Lafleuret al. 2000), water migration across geocomposites at vari-ous suctions has not yet been determined.

The geocomposites removed from specimens in test 12, inwhich frost penetrated below the geocomposite, had ice ex-tending from the bottom geotextile to the top geotextile andconcentrated along the geonet, suggesting that liquid watermay migrate along the net pore surfaces between the bottomand top geotextiles. Related to this are our observations thatthe geotextiles of the geocomposite were not saturated afterthey failed to reduce frost heave and that the water contentof these specimens was somewhat lower than that in controlspecimens. Thus, it appears that only portions of thegeocomposite were allowing water to migrate from the un-derlying soil to the overlying soil.

Determining the soil moisture retention and unsaturatedflow characteristics of the geocomposite would help identifyat what suction the soil water must exist to prevent flowacross the geocomposite, that is, the water-entry suction ofthe geocomposites in various states of moisture. The influ-ence of a capillary barrier on the frost heave of a pavementsection might then be studied using existing frost-heavemodels (e.g., Sheng 1994; Konrad and Shen 1996).

Field conditionsThere are three principal ways in which field conditions

will vary from the laboratory tests conducted for this re-search: (i) freezing is two- and (or) three-dimensional;(ii ) water flow is two- and (or) three-dimensional and willinclude infiltration from above; and (iii ) the water table islikely to be much farther below the geosynthetic capillarybarrier during the freezing season than it was in these tests.

Almost all frost heaving in the field results from two- andthree-dimensional freezing. The freezing of a pavement keptclear of snow in the winter will have insulating snow bankson the shoulders and would best be modeled by two-dimensional freezing. Figure 26 shows the shape of the frostline for five different dates during the freezing period undera road near Rutvik, Sweden. During soil freezing, waterflows and ice lenses form perpendicular to these lines, whichcauses the centerline to heave more than the sides of thepavement. If a capillary barrier was horizontal and relativelyhigh in the pavement section, frost could penetrate below it

early in the season. This might exacerbate the differentialheave between the centerline and the sides of the pavement;however, this may not be a concern if the capillary barrier isrelatively deep in the pavement system and frost does notreach it until late in the season.

Water will infiltrate the pavement and come into contactwith the capillary barrier when the soil is not frozen. There-fore, capillary barriers should be tilted so that infiltratingwater will flow downdip before saturation of the soil abovethe capillary barrier occurs. Based on Stormont (1995,1996), slopes of 5–10% are likely to work. Furthermore,there should not be wrinkles of any significance that wouldcause water to pond on top of the material.

During the thaw season and after heavy rainstorms, satu-rated soil conditions may lead to breakthrough of water intothe capillary barrier. If it becomes saturated, and it is tilted,the capillary barrier will easily drain. Thus, the capillarybarrier should be tied into a drainage system or a ditch. In-deed, a large-pored layer will probably help drain the pave-ment section relatively rapidly during spring thaw or afterlarge rainstorms (e.g., Hayden et al. 1999). It is likely thatwith adequate drying conditions, a capillary barrier will healas Stormont and Anderson (1999) observed for experimentalgravel capillary barriers.

Little is known about the water balance and subsurfaceflow of water in pavements at any time of the year, espe-cially winter. However, the fact that in a few carefully moni-tored sites water tables increase in depth while frost heaveoccurs indicates that the moisture migrates upwards and thatcapillary barriers will cut off a significant portion of watersupply to the freezing front. In well-monitored pavementsections (e.g., Henry 1990; Saarelainen 1992) the water levelis about 200–1000 mm below the freezing front, and de-scends at approximately the same rate as the freezing. Thus,field conditions may be more favorable to the successful useof capillary barriers in preventing frost heave than were theconditions of our tests.

Due to the very large pores in a geonet, it is likely to bean effective capillary barrier at any distance above the watertable. As demonstrated in the tests with soil A at standardcompaction, however, the capillary barrier will not reducefrost heave when the water in soil above the capillary barrier

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Fig. 25. Relationship between soil moisture tension and percentsaturation for soil A, estimated using the equation of Gardner(1958).

Fig. 26. Frost line at different dates during the winter of 1957–1958 under a road near Rutvik, Sweden (after Rengmark 1963).

is at relatively low suctions and the capillary barrier is ini-tially moist.

It is important to remember that effective capillary barri-ers promote rapid frost penetration. Even though the soil be-low them may heave, the soil above them will have muchlower water contents and thus retain strength during thaw.This might be a consideration for depth of placement in apavement section or embankment. It is possible that thaw-weakened soil can be tolerated at some depth, given that theoverlying soil retains strength.

Future researchSeveral future research efforts may be useful. Determina-

tion of soil water retention functions and unsaturated hy-draulic conductivities of geocomposites would help defineconditions under which water will flow into the capillarybarrier. The influence of a capillary barrier on the frostheave of a pavement section could then be modeled usingexisting frost-heave models.

The behavior of capillary barriers when subjected tofreezing and thawing and infiltration should also be studied.This should include the drying of geosynthetic capillary bar-riers in which breakthrough has occurred. Field or laboratorystudies, accompanied by monitoring of water flows, heaverates, etc., would be informative. If possible, field studiesshould document the water balance and subsurface flow ofwater in pavements during freezing and thawing. The resultsof the field trials could be used to determine whether exist-ing models that predict frost heave in pavements can predictfrost heave in pavements that contain capillary barriers.

Lastly, if frost-heave models can be used to predict thefreezing behavior of pavements that contain capillary barri-ers, the models should then be used to conduct a parametricstudy to establish the conditions under which capillary barri-ers will provide optimum benefit.

Conclusions

Results of freezing tests reported here, capillary rise mea-surements (Henry and Holtz 1997), and soil moisture reten-tion determinations (Stormont et al. 1997) indicate thefollowing:

(1) Geotextiles containing soil fines and moistened to asaturation of 30–40%, by being initially saturated then dried,failed as capillary barriers and did not significantly reducefrost heave. This was not product dependent.

Freezing tests conducted with geocomposites placed insoil specimens revealed the following:

(2) Moist geocomposites containing soil fines reducedfrost heave when the water level was not above thegeocomposite, and they did so at heat fluxes representativeof those which occur in pavements. The geocomposites per-formed as capillary barriers in highly frost susceptible silt(ML) and silty sand (SM).

(3) When the geocomposites successfully reduced frostheave, the suction head in soil A overlying the geocompositewas at least about 1800 mm. However, the geocompositespermitted significant heave when the soil moisture suctionhead in soil A was approximately 800 mm or less.

(4) The geocomposites were not saturated when theyfailed as capillary barriers, and water probably migrated

from one geotextile to the other in films adhered to the netmaterial, and the geotextiles of the geocomposite do nothave to be saturated to transmit liquid water from one to theother in amounts great enough to sustain the frost heave ob-served.

(5) Conclusions 2–4 were not dependent on the specificgeocomposite tested.

(6) Research is needed to determine the soil moisture re-tention characteristics of the geocomposite. This would helpidentify the soil-moisture conditions that must exist to pre-vent flow across the geocomposites and to study the influ-ence of the geocomposites on frost heave using numericalmodels.

(7) If a capillary barrier were to be placed in a horizontalposition and relatively high in a pavement section, it may ex-acerbate differential frost heave between the centerline andthe shoulders.

(8) Capillary barriers should be tilted to divert infiltratingwater downdip when the soil above it is not frozen. This willhelp keep the overlying soil from becoming saturated andmay help drain water during spring thaw.

(9) The freezing experiments were conducted at severehydraulic conditions, so field conditions may be more favor-able for the successful use of capillary barriers to preventheave than were the conditions of the freezing tests. Unfor-tunately, very little is known about the water balance andsubsurface flow of water in pavements during freezing. Sucha seasonal or annual mass-balance study in pavements is afuture research need.

Acknowledgments

We gratefully acknowledge the financial support of theAlaska Science and Technology Foundation and the U.S.Army Corps of Engineers for the completion of this re-search. We also acknowledge and appreciate Dr. JohnStormont’s enthusiastic collaboration in our capillary barrierwork. Ms. Rosa Affleck, Research Civil Engineer of theCold Regions Research and Engineering Laboratory(CRREL), assisted numerous hours in the laboratory andtook responsibility for specimen preparation in many of thefreezing tests. Mr. Jeffrey Stark, Research Civil Engineer ofCRREL, also gave great advice on the experimental setup ofthe soil freezing tests. Lastly, we thank the reviewers formany helpful comments that resulted in improvements tothis article.

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Geocomposite capillary barriers to reduce frost heave in soils