Frost can damage pave m e n t sand building structures in twoways: heaving of the froze n

g round, caused by ice lenses form-ing in the soil, and collapse of theg round, caused by the thaw of thesesame ice lenses. In the winter, asf rost penetrates the ground, waterin the pores of the soil freezes, leav-ing the soil near the frost line dry.Because water flows from wet to dryregions by capillary action, waterf rom below the frost line is dra w nup. When it reaches the frost line, itf re eze s, too. The ice lenses growthicker much as the ice on a pondthickens from the bottom side. Asthe frost line penetrates deeper intothe soil, new ice lenses form belowthe first ones. The expansioncaused by the water fre ezing heave sthe ground a distance equal to thetotal thickness of all the ice lenses(Reference 1).

The amount of frost heave can bet re m e n d o u s. In one case, a seve n -

s t o ry building heaved 2 to 3 inches( Re f e rence 2). Ve rtical gro u n dm ovements of 4 to 8 inches arecommon and as much as 24 inchesh a ve been re p o rted (Re f e rence 3).Va riations in the amount of heave,due to different soil and water con-d i t i o n s, can crack stru c t u res easily.Wa l l s, slabs, footings, and pave-ments all can be damaged.

In early spring, the frozen soil be-gins to thaw—from both the top andbottom. For a while, a layer of froze nsoil remains below the ground sur-f a c e. Unable to drain through thisf rozen layer of soil, the water- l a d e ns u rface layer loses practically all ofits bearing capacity. The pave m e n tor other stru c t u re built over it sinksand may break up, especially if it isheavily loaded.

Three conditions required forfrost heave

For frost heave to occur, thre ethings must be pre s e n t :

• Fre ezing tempera t u re s• Wa t e r• Frost-susceptible soil

If any one of these three condi-tions is not present, the soil will notbe subject to frost heave.

Freezing tempera t u res. The depthof frost penetration (and theamount of frost heave) depends onthe intensity and duration of thewinter fre eze. Frost penetra t e sdeeper in areas with colder, longerwinters (Fi g u re 1). Frost heave alsois more seve re when tempera t u re sd rop gra d u a l l y, instead of suddenly.Thicker ice lenses form during ag radual fre eze because capillary wa-ter has more time to rise from thewater table.

Wa t e r. Water can come from thewater table or surface infiltra t i o n ,but a high water table causes theg reatest frost heave. If surface infil-t ration is the only source of water,only the water in the pores of thesoil fre eze s. When a high water tableis present, capillary water dra w n

How to prevent frost heaveReduce frost penetration or keep water or frost-prone soil out of the frost zone

Figure 1. Frost penetrates deeper in areas with colder, longer winters. This map shows total degree-days below freezing for thecoldest year in a 10-year period. It does not show local variations, which may be large, par ticularly in mountainous areas. Thetable shows how deep frost may penetrate a pavement foundation made of well-drained material that’s not frost prone. Youcan calculate degree-days for one day, by subtracting the average temperature for that day from 32 degrees F (Reference 4).

Total Degree- Frost PenetrationDays for Coldest of PavementYear in 10 Ye a r s F o u n d a t i o n

(from map) ( i n c h e s )

5 0 1 0

1 0 0 1 4

2 5 0 2 2

5 0 0 3 1

7 5 0 3 8

1 0 0 0 4 4

1 5 0 0 5 4

2 0 0 0 6 2

2 5 0 0 7 0

3 0 0 0 7 7

3 5 0 0 8 3

f rom the water table also fre eze s.Mo re water fre eze s, so more fro s th e a ve occurs.

Frost-susceptible soil. The fro s th e a ve potential of different soils iss h own in Fi g u re 2. Silty soils are themost prone to frost heave. Theirp o res are small enough to dra wm o i s t u re through capillary actionand large enough to allow this mois-t u re to move through the soil quick-l y. In a way, they soak up moistureand hold it like a sponge.

Ca p i l l a ry rise for clays is high also,but permeability is low so waterm oves through the soil slow l y. Wi n-ter may end before ice lenses growthick enough to cause harm f u lheaving. Compacting clay re d u c e sits permeability and frost heave po-tential even more.

Gra vel is the soil least susceptibleto frost heave. Water can’t ri s et h rough it by capillary action, andbecause it’s highly permeable itw o n’t hold water. Fre e - d ra i n i n gg ra vel is thus an excellent base ma-t e rial for foundations.

How do you determine if the soilis silty and prone to frost heave? Asoil test conducted by a materi a l stesting labora t o ry is the most re l i-able way. No more than 10 perc e n tof the particles of a uniformly gra d-ed soil should be smaller than 0.02millimeter (passing the No. 200s i e ve). Fairly we l l - g raded soilsshould not contain more than 3 per-cent of this size (Re f e rences 1 and 4).These are useful rules of thumb butthey are not exact tests for frost sus-c e p t i b i l i t y. Although ra rely re q u i re d ,a slow labora t o ry fre ezing test canbe perf o rmed. The test measure sthe rate of heave in a cylindrical soilspecimen with one end set in waterand the other end exposed to cool-i n g .

You also may be able to get infor-mation about areas with fro s t - s u s-ceptible soils from local buildingcode authori t i e s, highway depart-m e n t s, geological survey offices,a g ri c u l t u ral extension agents, orconsulting engineers.

Three ways of preventing frostheave

Because of heat loss to the sur-rounding soil, heated buildingsra rely suffer frost heave damage.Frost heave also is avoided by ex-tending footings below the fro s tl i n e. Pa ve m e n t s, dri ve w a y s, side-w a l k s, and floor slabs of unheatedbuildings are not supported by foot-i n g s, howe ve r. To protect theses t ru c t u re s, you must eliminate orm i n i m i ze at least one of the thre econditions that lead to frost heave :• Reduce frost penetra t i o n .• Keep water out of the fre ez i n g

zo n e.• Make sure soil in the fre ezing zo n e

is not susceptible to fro s t .Reduce the depth of frost penetra -

tion. Clean sand and gra vel arecommonly used for bases underfloor slabs and pave m e n t s. They al-so provide some insulating va l u e,but usually not enough to adequate-ly reduce frost penetration. A fewyears ago, howe ve r, the Po rtland Ce-

ment Association (PCA) experi-mented with a mortar containingexpanded polystyrene beads (Re f-e rence 5). Placed under asphalt andc o n c rete pavement sections, them a t e rial reduced pavement stre s s-e s, deflections, and depth of fro s tp e n e t ration. One inch reduced fro s tp e n e t ration as much as 3 inches ofg ranular base did.

Keep water out of the fre e z i n gzone. You can keep water out of thef re ezing zone by lowe ring the watert a b l e, blocking the rise of waterf rom the water table, or re m ov i n gs u rface water before or after it en-ters the soil.

L owe ring the water table slow sthe growth of ice lenses by incre a s-ing the distance that water mustclimb to reach a frost line. In manyc a s e s, doubling the distance to thewater table can cut the amount of

FROST HEAVE POTENTIAL OF DIFFERENT SOIL TYPES

Soil Type Capillary Rise Permeability Frost Heave(feet) Potential

Clay Greater than 8 Low Medium

Silt Greater than 8 High High

Fine Sand 3 to 8 Medium Medium

Coarse Sand 1 to 3 High Low

Gravel 0 High Low

Figure 3. Floor slabs for unheatedbuildings should be built with a coarse-grained base and subbase that are notfrost susceptible (Reference 6).

Figure 2. Permeable soils that exhibit strong capillary forces are the most prone tofrost heave. Silty soil is thus very frost prone, and gravel is not.

f rost heave in half. This usually isnot economical, but it can be doneby adding fill to raise the grade lineof the slab in relation to the watert a b l e. Unless the soil is more than 70p e rcent saturated, frost heave usu-ally does not occur if the water tableis more than 10 feet deep ye a r-round (Re f e rence 4).

In some cases, you can block cap-i l l a ry rise or slow it to a tolerable lev-el. Water cannot rise by capillary ac-tion through a layer of gra vel orc rushed stone installed between thewater table and fre ezing zo n e. Ablanket of nonswelling clay underthe slab is effective, too. Clay hass t rong capillary forces but its lowp e rmeability pre vents much waterf rom passing through it.

Good drainage reduces the extentof frost heaving, but usually itd o e s n’t dry the soil enough to elimi-nate heaving entire l y. In pave m e n tc o n s t ruction, try to pre vent surf a c ewater from entering the base or sub-g ra d e. Seal joints and cra c k s, espe-cially joints at the pavement edge.Install drain tile or build open sideditches to intercept water fro mhigher ground nearby. In constru c-tion of retaining walls, provide we e pholes for drainage at the foot of theexposed wall. El e vate building slabsat least 3 to 4 inches above the nor-mal surface water drainage plane(which is not the same as the water

table). Di rect runoff water awayf rom the building with proper land-s c a p i n g .

Make sure soil in the freezing zo n eis not susceptible to frost. Gra vel orc rushed stone drains surface waterquickly and does not draw waterf rom the water table. This makesthese materials good re p l a c e m e n t sfor frost-susceptible soil in thef re ezing zo n e. Coarse aggre g a t ewith all particles about the sames i ze is best; it should be free of fineswhich might fill voids and prov i d ec a p i l l a ry channels around large par-t i c l e s. Fi g u re 3 shows a good floordesign for unheated buildings.

Replacing all the fro s t - p rone soilin the fre ezing zone often is impra c-tical in nort h e rn re g i o n s, howe ve r.Frost in these areas may penetra t em o re than 4 feet. The cost of re m ov-ing and disposing of that much soiland replacing it with gra vel orc rushed stone usually is too gre a t .Even so, replacing some of the fro s t -susceptible soil reduces frost heave.

Use a granular backfill that’s notf rost prone around footings andfoundation walls, too. In some cas-e s, even if the foundation extendsb e l ow the frost line, fro s t - s u s c e p t i-ble soil that sticks to the foundationwalls may lift the foundation if it un-d e rgoes frost heave.

—by Mark Wallace

References1. Spangler, Merlin G., and Richard L.Handy, Soil Engineering, Third Edition,Intext Educational Publishers, NewYork, 1973.2. Penner, E., and C. B. Crawford,Frost Action and Foundations, NRCC21089, Division of Building Research,National Research Council of Canada,Ottawa, 1983.3. Penner, E., “Ground Freezing andFrost Heaving,” Canadian Building Di-gest, CBD 26, Division of Building Re-search, National Research Council ofCanada, Ottawa, 1962.4. Krebs, Robert D., and Richard D.Walker, Highway Materials, McGraw-Hill Book Company, New York, 1971.5. “Properties of Expanded Poly-styrene Concrete and Applications forPavement Subbases,” RDO55.01P,Portland Cement Association, Skokie,Illinois.6. Ringo, Boyd C., “Frost Heave: WhatIt Is and How to Avoid It,” The BuildingOfficial and Code Administrator, No-vember/December 1984, page 20.

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