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Refrigerating Engineering, 66, 10, pp. 33-35, 1958-10-01
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Frost action under cold storage plants
Pearce, D. C.; Hutcheon, N. B.
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Ser
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Nzt tz
n o . 55
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BI,DG
NATIONAT
RNSIARCH
C0UNCII,
CANADA
Irost
Aetion
Under
Cold
Stora$e
Plants
By
D. C. PEARCE AND N. B. HUTCHEON
DIVISION
OT
BUITDING
RTSTARCH
A N A L Y Z E D
Reprinted, From
Rnrnrcnnarrnc ENcTNEERTNc
I/ol. 66. No. 10. October 1958 Issue
TECHNICAL PAPER NO. 56
OF THE
DIYISION OF BUILDING RESEARCH
Ottawa
0ctober 1958
Price lO Cents
NRC 4939
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NAfIO$AI RESEARCH
COUNCII'
Drvrsro' r" g#tlt'c RE'EAR'II
Ebrata to frFrogt Action Und.er Cold Storage ?lante by D.C. Poarce
and N"B" Hutcheon.
(NRC 4939)
p. 5, para
1 (r +
+
-
ft,
should
read(r
+ 3! "
ft,
p. 5, yara 2 where ro is the equivalent radlus of a rectangular
slab of llnear
d.imenslons a and b "
should reacl
where ro 1s the equlvalent radlue of a rectangular
I
F
Frost
Aetion
Under
Cold
Stora$e
Plants
D. C. PEARCE
There have been frequent reports in recent years of extensive struc-tural damage to cold storage ware-houses, especially those of slab-on-grade construction, caused by frost action in the underlying ground.
In a dramatic example recently studied at Cornwall, Ontario, a re-frigerated locker room 50 by 50 ft, which had been operated at -10 F for about seven years, had heaved at the center by about 1 ft. The distortion of the building was so extensive that serious consid-eration was given to abandoning it. This structure is shown diagram-matically in Fig. 1; Fig. 2 shows an interior view. Other failures of a similar nature have been reported in the technical literature.l, 2,:t,4,5,6 Unfortunately many designers of cold storage plants have not heeded or have not known of the evidence lrovided by these documented ex-rmples. The problem is assuming yreater importance with the
con-inued expansion of the frozen food industry which now requires large storages operated at temperatures down to -10 F. Many such build-ings are now constructed with floor slabs laid directly on the ground or
This paper is a contribution frcm the Div. of Builtling Research, National, Resrch Council of Camda, and is published with the alrproval of the Director of the Division.
D. C. Pearce is Resarch Ofrcer, N. B. Ifut-cheon ie Assistsnt Director, Div. of Building Resmch. National Rsearch Council of Oanada. Ottarva. Canada.
N. B. HUTCHEON
on a moderate depth of fill on the ground.
The damage which o€curs rep-resents an appreciable economic loss, since the portions of the build-ings affeoted may eventually have to be withdrawn from service. Care-ful consideration should be given to this possibility in the design of all low temperature storages. There is need for publicity about the p'rob-lem so that designers and builders will be made more aware of the difficulties that can arise, and will be encouraged to recogaize the con-ditions that may lead to difficulty. Frost Action. Although a number of undesirable effects can be pro-duced by frost action in soils, by far the most dramatic and the most damaging is that commonly known as "frost heaving". This is caused by the freezing of water within the soil. Freezing alone, however, is not sufficient to produce extensive heav-ing even though water expands 9 per cent in volume on being frozen, Extensive heaving is produced only when the conditions are such as to cause water to be drawn through the unfrozen portions of a soil bed to a surface within the soil at which freezing is taking place. When these conditions exist the water migrates to the ice crystals already formed and, upon freezing, contributes to
the formation and growth of layers or lenses of ice. It is the grourth of these lenses that causes the frozen soil to increase excessively in vol-ume and to heave. Extremely high pressures, much hig:her than those normally encountered under
foun-dations, can be developed, and thus
even heavy b,uildings can ,be heaved in this way. When the heaving is of different magnitude under differ-ent parts of a building it can be racked and twisted so that it be-comes unusable.
Frost heaving occurs frequent-ly in hig:hway subgradei, where it is readily noticed, as well as in the ground, generally where it rnay not alwaSs be noted. It occurs only in certain soils, usually those of fine-grained structure, which are said to be frost-suscep'tible. It ds only in such soils that "suction" can be developed to transport water to the freezing interface. There rnust also be a supply of water for frost heaving to occur; heaving can take place only as fast as the water can be drawn to and frvzen at the freezing zone in the soil bed. Under conditions of slow steady freezing, very uniform soil, and an adequate water supply within the ground, the ice layers can grow almost in-definitely, forming pure ice within the soil. In nature, however, condi-tions are seldom so favourable and
\<:O€PrH OF FeOSr PEflErnafQy/, saflD NNr//"ANV,/,,4.\N}\4\y,/i{A\\\yt,/.a\\\t//,N\Va/aAr<\yN\v.N/4sN- - \ _ _ /
c L A r
the freezing front can at times ad-vance into the soil without forming ice lenses when one or another of the conditions is disturbed. Alter-nate layers of ice combined with layers of normally frozen soil can then be formed. The total heave is largely determined by the total thickness of ice which is formed. Freezing Under Storage Build-ings. It is probable that, unless special precautions are taken, freez-ing will occur under any low-tem-perature storage having a floor in direct contact with the ground. A few simple calculations of heat flow will suffice to show this. Consider a floor having cork insulation 6 inches thick, laid on the ground, which is typical of many designs. Assume that the cold storage tem-perature is -10 F, that the ground is just at 32 F below the slab and that the coefficient of conduc-tivity, k, is 0.28 Btu/ft2/d,ee.F/in. For this case, the heat flow from the ground through the floor slab into the storage will be 1.96 Btu/ ft2/hr. Now unless heat is avail-able at the bottom of the slab at this rate, the ground temperature will be lowered and freezing will result.
If the slab is extremely large, the heat available under the ce,nter of the slab will be that which flows upward in the ground itself. The normal average temperature gradi-ent in the ground is only about 1 F in 140 ft. Assumins k : 6.0 for the ground, the upward heat flow can be calculated for this gradient to be 0.0035 Btu/ft2/hr. This is clearly a totally inadequate heat supply to balance the heat flow through the slab and to keep the ground from freezing.
When a cold storage building is first put into operation almost all of the heat which flows upward through the floor is extracted from the ground immediately below. The
temperature gradients which are set up in the soil cause heat to flow into the cooled area from below, and from the ground around the building. As the size of a slab is increased the distance through which the heat has to flow from the edge to the center is also increased, so that proportionately less heat can be obtained from the edges of the slab. In the extreme case of a slab of infinite extent. all the heat must come from below and the ground will continue to freeze to greater and greater depths until the normal ground thermal gradi-ent of about 1 F per 140 ft is re-established. In the ultimate case the heat supplied will be 0.0035 Btu/ ft2 /hr. as previously calculated. This will also be the heat flow through the insulated slab, since no other heat is available.
For the slab, infinite in extent, insulated with 6 in. of cork, with a storage temperature of -10 F, the temperature under the slab will eventually be only 0.075 F above the s t o r a g e t e m p e r a t u r e , a n d t h e ground will be frozen to a depth of
about 5,000 ft. This example,
though clearly unpractical since it refers to the condition after
hun-dreds of years for a slab covering many square miles, serves to em-phasize that the amount of heat which can be obtained on a steady-state basis from the glound im-mediately below is extremely small, and that additional heat can be drawn vertically from below only at the expense of continued cooling of the ground. It illustrates further that in large, low-temperature stor-ages placed directly on the ground, the frost penetration ea'n, with time, be extraordinarily great. Effect of Storage Size on Frost Penetration. Since the area cov-ered by a cold storage affects the depth of frost penetration, it is of great interest to know just what the exact relationship is. The ac-tual situation is so complicated, however, that exact solutions can-not be obtained.
It can first of all be assumed that the variations in temperature in the ground from winter to sum-mer can be ignored, and only the mean temperature considered. This is, in effect, assuming that the higher summer temperatures are offset by lower winter temper-atures. Ground temperatures meas-ured at Ottawa at various depths and for different times of the year are shown in Fig. 3, the mean ground ternperature being 48 F.
Additional simplification is ob-tained by representing the rec-tangular slab by an equivalent cir-cular slab. Finally, if it can be as-sumed that heat flow in the ground can be treated as simple conduction in a uniform medium, a relatively simple solution can be found.
rvorE' oA,HEO LIVES tilOrcAt€ O6roqrpil tilfRooucEo 8r FRo57 acftov
FIO. I SKETCH of a CoId Storage Warehouse
Fle. 2 INTERIOR view of refrigerat-ed locker room at Cornwall, Ontario
Others have already made analyses on this basis,?'8 and a method similar to that used by Ward and Sewell8 has been followed in deriv-ing the followderiv-ing equation. The temperatures ultimately re{ched at various depths below the centre of an insulated slab placed directly on the ground can be estirnated from the following expression:
( T . - T - ) T ( z ) - T " , :
l . , 2 r
K " \
1 1 a _ _ _
|
\ r ' K , / l z z ' 2 z l z ' \ \ ' * ; - ; { ' 1 ; ) where T (z) is the minimum groundtemperature al depth "z" ft;
T- is the mean ground tem-peratuTe;
T, is the cold storage tem-perature I
I is the thickness of the in-sulation, (ft);
ro is the equivalent radius of the slab (ft);
K" is the thermal conductiv-ity of the ground; Kr is the thermal
conductiv-ity of the insulation. The derivation of this and other equations is being presented elsewhere in another paper. The equation used for the determination of the radius of the circular equiva-lent of a square or rectangular slab, as propo'sed by Ward and Sewell is as follows:
r l a b \ r o - - l _ f
2 \ t / a " *b , /
where r. is the equivalent radius of a rectangular slab of lin-ear dimensions a and b. The Cornwall storage case pro-vides an opporfunity to apply the equation. The following constants are appropriate: T^: 48 F (measured value at Ottawa) ; T . : - 1 0 F l : 1 / z ft r " : 2 8 . 2 f t Kg: 0.50 Btu/ft.hr.deg. (value for clay measured at Ot-tawa)
Kr :0.02 Btu/ft.hr.deg. (cork board)
The depth of frost penetration is given by the value of z corue-sponding to T (z) : 32 F. The solution is 9.3 ft. The actual depth of frost at Cornwall was about 10 ft at which level it is probable that the water table was encountered. Since heaving was still progressing at a substantial rate it is apparent
FIg. 3 GROUND TEMPERATURE Profiles Ottawa, Ontario (1955) that the frost had not reached its limiting depth. Even so, the check with the calculations can be con-sidered satisfactory in view of the approximations involved.
Thickness of Insulation to Prevent Freezing. The equation given can also be solved for the thickness of insulation which will yield T : 32 F when z : O. This will be the thickness of insulation required to prevent any freezing below the slab. The value obtained for the Cornwall case is 18 in. The equation also shows that the depth of frost p e n e t r a t i o n v a r i e s d i r e c t l y a s the temperature ratio (T- - T.)/
(T- - 32). Thus the thickness of insulation required to prevent freezing will increase as the cold storage temperature decreases and as the mean ground temperature decreases. The thickness of insula-tion for no freezing is proporinsula-tional to the equivalent radius of the structure. Thus a cold storage of 100 x 100 ft for the Cornwall con-ditions would require about 3 ft of insulation to prevent freezing. With only 6 in. provided, the depth of frost penetration eventually would be about 18 to 20 ft. These thick-nesses of insulation which are re-quired to prevent any freezing un-der a low-temperature storage of any appreciable size laid directly on the ground would normally be con-sidered prohibitive in cost.
Freezing Not Always Serious. The major difficulty to be experienced when the ground freezes is that of heaving. Freezing may therefore be tolerated when conditions are such that ice lenses will not be formed. Since both frost-susceptible soil and available water in the ground are required for heaving, no great difficulty need be expected in any
soil which is always extremely dry, or in rock or course sands and gravels. This still leaves, however, a large proportion of the sites which are likely to be considered in which the soils will contain at least some layers of clay, silt, or fine sand. Well-drained sites in Western Canada where the annual precipita-tion is 15 in. or less may turn out to be so dry as to give little trouble. IrI Eastern Canada, however, with rainfalls of 25 in. or more, the depth of the water table is seldom more than 26 ft. and irr fall and winter it frequently rises to the surface.
The assessment of soil and water conditions to determine when frost heaving will occur is cur-rently under intensive study in con-nection with highways" Accurate prediction is not yet possible. The situation for cold storages will be more critical than for highways since the freezing is slow and steady and can penetrate to much greater depths than in nature. This naturally greatly increases the chances that adequate moisture conditions and a frost-susceptible layer will be encountered.
It will be possible to find many sites for small storages at which freezing will cause no difficulty, since the depth of frost penetration may be no more than 5 to 10 ft. It may ven be feasible in some cases to replace a suspected soil with a granular fill to the depth which freezing is expected to reach. It may be noted that so far as freez-ing is concerned it will make rela-tively little difference whether the granular fill is used as a replace-ment for material below grade, or is used to provide an elevated fill. However, in the first case if the water table is naturally high, or there is excessive surface water at the site, the porous flll below grade may act as a sump, thus increasing the chances of difficulty should it subsequently be frozen when com-pletely saturated with water.
In all cases the penetration of the frost line will be relatively slow, and frost heaving may not occur for several years, until heaving conditions are encountered at depth. The ,Cornwall case is inter-esting in this respect. No heaving was noticed for the first flve years after the storage was put into
.o 50 60 IEMPERATURE ('FI
operation. During this time the frost Iine slowly penetrated the first 7 or 8 ft of sandy soil under the slab. At this depth, however, the soil changed to a Leda clay which, in addition to being frost susceptible, provided a barrier to drainage of water fro,m the lower part of the sand layer. Heaving occurred by the formation of ice lenses at the 8- to 10-ft level at the rate of about 6 in. per year, for the next two years.
Ventilated and Heated Floors. All of the foregoing discussion has as-sumed that the floors are in contact with the ground so that all the heat flowing through the insulated slab has to come from the ground itself. The situation as to freezing of the ground can be greatly changed by elevating floors and providing ven-tilated crawl spaces, or by intro-ducing heat in a variety of ways into the fill beneath the slab. One of the obvious disadvantages of this is the increase in heat gain through the floor, representing an increased load on the cooling equipment when the underside of the slab is at32F or higher instead of at some lower temperature between 32 F and the interior storage temperature.
Constructions providing crawl spaces for ventilation under the floor have several disadvantages over slab floors on grade. The main one is cost of the floor. An estimate made recently by an engineering firm on a large frozen food storage indicated that the use of a simple insulated slab-on-grade construc-tion would save up to 20 per cent on the cost. The elevated floor may also impose some. limitations in selecting the floor levels for han-dling of goods in and out of the
warehouse. Unfortunately also,
with elevated floor construction, the freezing problem may not be en-tirely avoided. With outside air used for ventilation the ground in the crawl space will freue in win-ter to some appreciable depth, to be thawed out again the next summer when warm outside air is again available. Heaving caused by ice Iensing of the unloaded soil will cause no difficultl. but freezing with the possibility of ice lensing must be avoided under footings. Not onll' will there be cooling of the ground by the air' in winter, 6
but the supporting construction, particularly interior columns, will often provide conducting paths from the ground through the in-sulation into the storage which are extremely difficult to avoid. There is thus an added tendency for freez-ing under footfreez-ings.
The same problem of heat paths along columns can be en-countered in slab-on-grade con-struction where roof loads are cap ried on columns through the floor to footings below the slab. This can be avoided, however, by transmitting column loads to, and distributing them, in a main slab located above the floor insulation. The insulation can then be made continuo'Js be-tween the cooled slab and the ground. Some ingenuity may be re-quired rn avoiding heat paths at outside walls.
The introduction of heat into the ground below a slab so as to control the depth of frost penetra-tion appears to be entirely prac-ticable. The simple calculation pre-viously made of the heat transfer through a slab having 6 in. of cork insulation with 32 F on one side and -10 F on the other, yielded a heat flow of 1.96 Btu/f.tz/ hr. This amounts to 17,150 Btu per sq ft of floor per year. At a gen-erous estimate of the cost of heat, based on domestic fuel rates. of about $1.50 per million Btu deliv-ered, the cost will be about 2t/2 cents per sq ft per year. To this must be added the yearly cost of the capital investment in the equip-ment necessary to deliver the heat below the slab plus the yearly cost of maintenance and operation of this equipment. Even so, the cost of heating must be considered quite acceptable.
A yearly cost of as much as 5 cents per sq ft for heating, includ-ing both fuel and equipment oper-ation, corresponds to a capital vestment at 5 per cent simple in-terest of $1.00 per sq ft. The inter-est on the saving in original build-ing cost by gobuild-ing to slab-on-grade instead of an elevated slab with crawl space, might well pay for the cost of heating several times over. There are several ways in u'hich heating may be accom-plished. The most direct and ob-viou,s way is to obtain heat from a domestic fuel to be introduced
un-der the slab through a pipe or duct system using water or air as the heat carrier as in domestic heating systems. It 'may be noted that some saving in heat might be made with such a system by arranging it in such a way that the ground is held at 32F at some selected depth rather than at the lower side of the slab. In effect this may be consid-ered as adding to the slab insulation the insulation equivalent to the depth of soil. As moist soils con-duct heat 20 or more times as rapidly as does cork or other in-sulation, this may not be a con-venient way to augment the slab insulation.
' Another possibility is to use the heat'rejected from the refrig-eration co,mpressors for heating under the slab. This may turn out to be more intriguing than prac-tical, especially when it is consid-ered that during the winter the heat gain to the storage is small and sufficient heat may not be avail-able.
There is not, however, any in-superable difficulty in using sea-sonal heating. It would seem to be quite feasible to use the heat from the ground for the winter months allowing it to freeze to a depth of several feet under the slab and then to thaw it out during the summer months in readiness for the follow-ing winter. With this approach, there need be no difficulty about a source of heat since this can be obtained from the warrn summer air. It is believed that this will in most cases be the preferred ap-proach. It will be necessary to en-sure that no serious frost heaving will occur over the depth of soil penetrated by frost during the winter, and there is the problem of designing and installing an under-floor air system. Studies are now beirrg carried out on the design of such systems and will be reporte'd later.
coNcLusloNs
In the design and construction of low ternperature cold storages on slabs in contact with the ground all possibility of difficulty from freez-ing of the ground can be avoided by providing continuous heating under the slabs. This, it appears, can be done without too much
dif-flculty or expense and may often be much cheaper than the construction of an elevated floor.
Unless heat is to be introduced under a cold storage slab-dn-grade, freezing of the ground is almost centain to occur to conside,rable depths. The depth of eventual frost penetration will be in proportion to the,size of the slab. The approx,imate depttr of penetration can be pre-dicted.
Before contemplating a design that will allow any freezing of the ground, careful study should be rnade of the soil tlpe and ground-water conditions. Where the soil is fine grained and tlere is likely to be a supply of water within the soil, there is a definite possibility of frost heaving. This possibility should be avoided.
The designer should then con-r
sider for each individual case what depth of frost penetration, if any, can safely be permitted under the storage. The possibility of replac-ing frost-susceptible soil with granular fill or of adding a porous fill above ground, or both, should be kept in mind. Consideration should then be given to the means by which frost penetration can be con-trolled. Increasing the thickness of insulation under the slab may be feasible in small storages but will be prohibitive in cost for larger buildings. The cost of various sys-tems for introducing heat under the slab should also be studied. Where some penetration of frost on a seasonal basis can be tolerated, the use of an under-floor air duct system using warm summer air for heating is likely to be a preferred system.
EIBtIOgRAPHY
l. Cooling, L. X'., and .w. II. Ward. Some examples of foundation movements due to causes other than structural loads. pro-ceeddng,s, Seconil Interna.tiomal C onlerence on Soil Mechanios onil, Found,atlon Engi-neering, Vol. 2, p. 162, 1948.
2. Cooling, L. X'., and .w. II. .Ward. Dam-age to cold stores due to frost thawins. Proc, Inst. Refrig., Vol, 41. p. 3?, 194i. 3. Coolins. L. tr'.. and W. If. Ward- Dam-age by ice to store foundations. Modern Eellisgration, yol. 47, p. 298, 1944. 4. Dockstatler, E, -d Effect of freezing and thawing of soil under founalations of cold storag:e warehouse. Proceed.ings, First International Conference on Soil Mechan-ics and, Foundation Engineeri,ng, Yol. 3, p . 1 7 1 . 1 9 i i 6 .
7. Ruckli, R. I'. X. Heat flow towards the floor of cold stores, situa,ted in the ground level, and calculation of the insulation or the heating system to-prevent frost pene-tration in the ground. Proceeilings, Secoild. Intematiollal Conf erence on Sodl Mechanics and, Found,ation Engineering, Vol. 1, p. 18, 1 9 4 8 .
L .ward, -w, Ir., a,nd E. C. Sewell. Pro-tection of the ground from thermal effects of inalustrial plant. Geotechnique, yol. 2, p . 6 4 , 1 9 5 0 .
