SHOULD SILO FOUNDATIONS BE PLACED BELOW FROST?

J. E. Turnbull1, J. A. Munroe1 and J. P. F. Darisse2

'Engineering and Statistical Research Institute, Research Branch, Agriculture Canada, Ottawa K1A 0C6;and ExperimentalFarm, Research Branch, Agriculture Canada, L'Assomption, QuebecJOK 1G0.

Contribution no 1-1591, received 2 March 182

Turnbull, J. E., J. A. Munroe, and J.Agric. Eng. 24: 149-155.

P. F. Darisse. 1982. Should silo foundations be placed below frost. Can.

A6.1 x 15.8-m concrete silowas built onanextended-ring foundation according to Canada PlanService, Plan7411,based on a bearing safety factor of 2.97 as determined by vane shear tests in situ. Thefooting-bearing surface was setat only 0.75 m below grade, on a softclayknown to produce large frost-heaves. Recorded annual cycles of winter frostheavefollowed by summer reload settlements have shown a maximum 75-mm heave at the end of the first winter; thisgavea wall tilt of 175 mm out of plumbto NNE; subsequent frostheaves were not as great. Loadedsummersettlementshave been increasing each year but with decreasing increments; maximum settlement from 1977 to 1981 was 61 mmat the outside footing edge, giving a westward wall tilt of 64 mm out of plumb, which is not unusual. A surprisingoutward rotation of the top of the footing with respect to the wall is so far unexplained.

INTRODUCTION

Designs have been developed (Turnbullet al. 1977) for a reinforced concrete ringfoundation for tower silos, such that mostof the footing can extend beyond the silowalls. These unbalanced footings weredesigned to increase the total bearing areaunder silo foundation and floor without

wasting concrete beneath the silo floor.Circumferential reinforcing steel wasplaced near the bottom of the footing ringto resist the movements predicted fromeccentricity of the footing with respect tothe silo wall. Based on this, designs forextended ring silo foundations were published as Plans 7411 and 7412, CanadaPlan Service (CPS). Figure 1 gives a sequence of construction steps for the CPSextended ring foundation.

These plans give the usual 'safe' recommendation, namely to place the footingbearing surface 'below frost.' In manycolder parts of Canada, however, this implies a footing depth of at least 2 m. Thisdepth adds considerably to silo construction costs and, furthermore, it is likely toplace the bearing surface too deep to takeadvantage of the stronger surface crustwhich is often typical of deep agriculturalsoils found at silo sites.

Silo builders in Canada have frequentlyquestioned this 'below frost' recommendation. In fact, few silo foundations areplaced below 1.0 m depth, and many areplaced only to the depth of the concretefooting ring itself. Furthermore, ring footings of this type are seldom reinforcedagainst possible movements due to eccentricities of silage, wall and soil reactionloads.

In 1977 a new 6.1 x 15.8-m cast-in-

place concrete tower silo (Fig. 2) wasplanned for the Agriculture Canada Experimental Farm at Normandin, Quebec.This location is about 250 km north of

Quebec City, where January design temperature (1% basis) is —33°C. Soil at Normandin is a soft, frost-sensitive clay witha history of excessive frost-heave and settlement problems. An experiment wasproposed to evaluate an extended-ring silofoundation to be constructed exactly as

recommended in CPS Plan 7411. The onlyimportant difference would be that thefooting would bear on soil well above theexpected depth of frost for that location.This experiment was planned (1) to provide a field test of an existing CPS planbased on theory, and (2) to determine ifconcerns over frost-heave are justifiedwhen applied to tall structures capableof loading a soil to its full allowablepressure.

This paper was presented as ASAE paper no. 79-4506 at the 1919 Km. Soc. J\gfvc.ling. -winter meeting and is published with their permission.

Figure 1. Steps in building a reinforcedextended ring foundation for a cast-in-place concretetower silo. Numbered steps are referred to in text.

CANADIAN AGRICULTURAL ENGINEERING, VOL. 24, NO. 2. WINTER 1982 149

3740 mm2'Figure 2. Section through the Normandin silo with a reinforced extended ring foundation and

nearby dairy barn. Encircled members (3) through (11) are the same constructionsteps as shown in Fig. 1 and are explained in text. Threaded gauge-rods (12) are formeasuring footing vertical movements.

MEAN TORQUE, ft-lb, TO DEPTH OF 2/3 FOOTING DIAMETER

0 5 10 15 20 25 30

152 mm

MEAN TORQUE, Nm.TO DEPTH OF 2/3 FOOTING DIAMETER

Figure 3. Calculated design curves for converting mean shear vane torque to allowable soilbearing pressure.

CONSTRUCTION AND

INSTRUMENTATION

Because there was no time for a propersite investigation before preparing contract documents, the footing details weretaken from Plan 7411, assuming that thedesign soil bearing stress would equal orexceed 96 kPa. The contract price agreedupon for the job was based on this assumption, but on the understanding thatthe work would go or stop depending onthe results of soil tests at the site, and that

these tests would take place at the time ofexcavation.

Determination of Soil Bearing StrengthFor quick field tests to estimate allow

able foundation pressures on clay soils,the value of the in situ vane shear test was

recommended by Bozozuk (1974, 1977).Two vane shear-testing tools were constructed as shown in Fig. 3. Each was designed to be coupled to an extendable shaftof coupled pipe lengths, permitting soilpenetration to the recommended depth of2/3 times the footing outside diameter.Torque required to shear out a cylinder ofsoil (as defined by the revolution surfaceof the four radial vanes) was measured bya precalibrated mechanic's torque wrenchfitted to the top shaft section. In fact onlythe larger of the two shear vanes was usedbecause of the low shear strengths foundat this site.

As recommended by Bozozuk (1974),the following equation from Skempton(1951) was used, together with the shearedsoil area calculated for each shear vane,to prepare the design curves shown in Fig3:

cNyd (1)

where

<7a = allowable soil bearing pressure(kPa)mean soil shear strength to depth of2/3 of footing outside diameter(kPa)6.6 (shape factor for a circularfoundation, see Skempton (1951))soil surcharge load (kN/m1)footing depth below soil surface(m)safety factor (3.0 recommended)

Safety factor F is subject to the engineer's judgement, but the value F = 3.0is frequently recommended. The realsafety factor against shear failure may, infact, be considerably less than 3.0 due toeccentric silo filling, wind effects, unequal settlement causing silo tilt, and errorsattributable to the vane shear test itself.

N =

7 =

d =

F =

150CANADIAN AGRICULTURAL ENGINEERING. VOL. 24. NO. 2. WINTER 1982

TABLE I. VANESHEAR TEST RESULTS AT NORMANDIN SILO SITE, AUGUST 1977

Depth rom

(m)

Vane shear torque (N-m)

surface Silo site Site 40 m west Site 30 m south

0.9 61 72 88

1.5 51.5 41 502.1 27 IS 272.7 24 7 23

3.3 13.6 31 24

3.9 9.5 27 23

4.5 13.6 24 18

5.1 17.6

Means 27.2 34.2 36.1

line. Basic grid spacing was 0.6 m horizontally and vertically beneath the footing. Additional thermocouples wereplaced at the same elevation as the top ofthe footing ring. The north-south orientation was chosen to observe the influence

of the free-stall dairy barn located about2.4 m north of the silo wall (Fig. 4).

Thermocouple leads were bundled intoa wiring harness, laid under the footingand through an underground pipe into theadjacent silo feed room. To read temper

atures, a 48-point rotary selector switchwas used together with a portable digitalpotentiometer. Temperatures were recorded weekly after filling the silo.

Constructing the Footing RingConstruction closely followed the steps

given in CPS plan 7411, Reinforced Extended Ring Foundation for 6-inchCast-in-place Concrete Tower Silos.In brief, Fig. 1 shows these steps as follows: (1) the footing circles were markedout, and (2) a small backhoe was used todig the circular trench, followed by a littlehand trimming to smooth the trench wallsand bottom and to remove loose soil. Ra

dial rebars (3) were staked out 600 mm oncenter and about 100 mm above the trench

bottom. Circumferential rebars (4) wererun through a bender, coiled into a continuous flat spiral and wired in place on topof the radial rebars (3).

A previous paper (Turnbull et al. 1979)gave the design procedures used to determine footing dimensions and steel re-

One of these errors can result from non-isotropic soil; that is, soil shear strengthin the horizontal plane may be somewhatless than the same soil sheared in the ver

tical plane. The vane test as used here determines shear in the vertical, but the bottom part of the hemispherical shearenvelope of a 'ball' of clay being shearedout by a tilting silo approaches the horizontal. Bozozuk (1974) found that the vertical vane shear could over-estimate the

horizontal shear strength by up to 25%,which could reduce the real safety factorfrom 3.0 to something approaching 2.4,a reasonable number for design. Nevertheless, the vane shear method gave aquick and relatively inexpensive assessment of soil bearing strength, especiallywhen it was used with design curves prepared in advance (Fig. 3). Since the footing was to be placed near the surface, the7 d term of Eq. 1 was disregarded in preparing the design curves.

At the time of excavation, one series ofvane tests was taken at the center of the

footing ring and, for comparison, twoothers were taken nearby, with results asshown in Table I.

One of the authors conducted the vane

tests and was responsible for the decisionto build or not. Working in the excavatedtrench, his instincts said "don't build."However, a mean torque of 27.2 N-m, asgiven in Table I, indicated an allowablebearing pressure of 95 kPa (Fig. 3), whichwas just under the pressure assumed previously in preparing the contract documents. This was good luck, not good judgment. Nearby vane shear results alsoshown in Table I were slightly greater,which further increased his confidence to

build.

Soil Temperature MeasurementsTo compare foundation movements

with frost penetration into the supportingsoil, a grid of precalibrated thermocoupleswas pushed into unexcavated soil underand about the foundation trench. The ther

mocouples were pasitianfid in, a verticalnorth-south plane along the silo center-

DAIRYBARN

N

W -6L_58.

i7(~i\INSTRUMENT CABINET

FEED ROOM

\

\\

W \

\\ \X % \

>V-4-73J*-31- E

Figure 4. Silo foundation plan. Negative numbers at each levelling pin are settlements (mm)after five annual filling cycles.

CANADIAN AGRICULTURAL ENGINEERING, VOL. 24, NO. 2. WINTER 1982 151

quirements. The silo contractor encountered no special difficulties sincereinforcing procedures here were almostidentical to those used later in handlingreinforcing for the walls of the silo. Onemore turn of steel (5) was added to resisthorizontal pressuresanticipatedwithin theplug of unexcavated soil surrounded bythe footing ring. The footing concrete (6)was placed and levelled ready for the silowalls (7). Sand fill (8), polyethylene floormembrane (9), perimeter drain (10) andgravel drainage floor (11) were placedlater.

Measuring Footing MovementsTo observe vertical movements, 14 lev

elling pins of 16-mm threaded steel rodwere set into the wet concrete footing.These pins were paired in two concentriccircles, one as close as possible to the outside footing edge, and the other close tothe silo wall. Pin pairs were located radially on seven cardinal compass points E,SE, S, SW, W, NW and N (Fig. 4). The

NE pins had to be omitted since these arrived under the feedroom floor.

Benchmarks were set at the barn foun

dation nearby, then the as-built elevationsof the footing pins were read with a surveyor's level and rod, prior to filling thesilo. This was found to be a slow, two-man operation; fourteen, 1-m aluminumscales were clamped to the levellingpins and two more to the benchmarks.This permitted the instrument man aloneto take readings rapidly to the nearest millimetre, at convenient intervals after filling silo.

RESULTSSettlement and Heave

The silo was completed on 23 Aug.1977, and first filled 13 days later. Fall-winter-spring soil temperature profileswere taken at convenient time intervals for

two annual storage cycles (25 mo). Corresponding readings of vertical foundationmovements were also taken, but for alonger period extending to the end of consolidation after the 5th filling in 1981. Fig

ure 5 gives the storage history and footingmovements at East, South, West andNorth; data were also taken at the SE, SWand NW points, but these are omitted forbrevity.

As shown in Table II and Fig. 5, theinitial consolidation occurred quite rapidlyafter first filling, and averaged -25 mm(negative meaning 'settlement') just outside the silo wall. This early consolidationeffect was not quite uniform around thefooting circle, the greatest differentialbeing 8 mm (E to W), representing a westward 'tilt' of 0.069°, or a silo wall 19 mmout of plumb in 15.8 m of height.

With arrival of freezing weather in lateNovember 1977, frost began to penetrateunder the perimeter of the footing ring,particularly at the more windswept sidefacing S and SW. After 23 Dec. 1977,elevation readings were discontinued dueto cold weather and were resumed on 14

Apr. 1978 with the arrival of warmerweather. The shape of the heave curves(Fig. 5) indicates this timing was appro-

77.9 kN/m2

o o

* ig i1977

o o

i i 2r — ♦-■? .2. a-

>oc •* i i * ft

> — *jCO D q.

£ - 81980

>oc

.2, c <S a a O

1981

inner levelling pinsouter levelling pins

1978 1979

TIME (months)

Figure 5. Heave/settlement profiles ofthe Normandin silo footing from 23 Aug. 1977 to 26 Oct. 1981. Average soil bearing pressures underfooting and floor include combined masses of footing, walls and silage, indicated as 90.43 kN/m2, etc.

152CANADIAN AGRICULTURAL ENGINEERING, VOL. 24, NO. 2, WINTER 1982

TABLE II. SETTLEMENT HISTORY,AFTER FILLING AND BEFORE WINTER FROST HEAVE

Fall of Mean settlement Angle Out of plumb Direction

1977 -25 mm 0.069° 19 mm W1978 — 37 mm 0.122° 34 mm W1979 — 41 mm 0.196° 54 mm w1980 -41 mm 0.188° 52 mm w1981 -44 mm 0.231° 64 mm w

priate to anticipate the maximum heaveabout a week later. As will be discussedlaterthere was less frost penetration at theN side than at S side, due apparently tothe adjacent warm dairy barn nearby andthe snowdrift that accumulated each winter between barn, feedroom and silo. Theslight westward tilt due to unequal settlement after first filling disappeared duringthe winter. By 20 Apr., frost heave underthe SSW side produced a NNE tilt estimated at 0.63°, or 175 mm out of plumbtowards the feed room. This has been the

greatest tilt recorded to date.With return of warm weather in 1978,

rapid spring resettlement was followed byslower but continued summer settlement

as the frost-heaved soil material became

reconsolidated(Fig. 5). Settlement historyto 1981 is summarized in Table II.

Footing Ring DistortionFigure 6 is an east-west vertical section

of the silo foundation, showing four annual settlements (maximum values, latefall), and one heave (late winter, firstcycle). Footing movements are exaggerated at 25 times the footing scale to make

ttti ii i' /' i

/

1 i46 46 -^ APR.20, 1978

\ \

relative movements more obvious. Silowall tilt was not directly measured, butwas projected from the vertical footingmovements assuming the silo walls remained intact.

Looking at the west side (Fig. 6) it isinteresting to note that an outward rotationoccurred with settlement on the first load

ing cycle, with subsequent settlementsproducing identical rotations each yearthereafter. In fact, this unexpected distortion occurred at all sections investigated,except at East and North where observedannual heaves and settlements were minimal.

Figure 6 also quite clearly shows that,although settlement is increasing witheach annual cycle, the settlement increments are decreasing in magnitude.

Frost Penetration

Figure 7 shows, for two subsequentwinters, progressivemonthlyfreezing linepenetrations into the soil at the N and Ssides of the silo. Note that during bothwinters the depth of freezing was muchgreater at the exposed S side than at theN side, which is partly protected by the

l li Ii Il

I tJ i

11 14.. .-• -*-,

-U-i iI !

AS BUILT (ZERO DATUM) AUG. 23,1977 H-4-1-

11

1-67\

-.-Hi:

WEST

"' OCT. 22,1977

J NOV. 23,1978-

17"""-58 i0CT- 26'1981

mil":::"'

l !

EAST

Figure 6. Section through silo foundation on an east-west vertical plane. Vertical movementscale is 25 X that of section scale, for illustration purposes; numbers at top of footingare actual movements (mm) above or below as-built datum.

CANADIAN AGRICULTURAL ENGINEERING, VOL. 24, NO. 2, WINTER 1982

adjacent dairy barn. In the colder1977-1978 winter (Fig. 7A), frost hadpenetrated completely under the footingwidth on the south side by mid-January,and by late April soil was frozen to a maximum depth of 2.0 m. Then, during Mayand June, frost receded until only a verysmall frozen 'pocket' (shown in dottedlines) remained at the end of June.

The 1978-1979 winter was milder,with maximum recorded frost penetrationonly 1.4 m deep (Fig. 7B), and the corresponding heave effects on the silo weremuch less pronounced than in the previouswinter (Fig. 5).

DISCUSSION

Footing Depth and Frost PenetrationFrom these results there is no doubt that

ice-lensing due to groundwater and frostcan develop forces sufficient to lift a massive structure such as a farm silo. How

ever, silo builders have said that in colderclimates it is often impractical to put silofootings deep enough to be below frost.At Normandin, for example, a footingplaced as deep as 2.0 m could still heavein a winter as cold as the 1977-1978 winter period.

However, a silo footing placed deepenough to avoid frost heave would bear onsofter, water-saturated soil, as shown bythe shear vane tests recorded in Table I,thus the builder could not take advantageof the better-drained 'crustal zone' nearer

the surface. The increasing 7 d term ofEq. 1 would not compensate for lowershear strengths found at greater depth, andthe footing area would, therefore, have tobe increased to maintain a specified safetyfactor.

Another concern of excavating a footing trench into softer soil below 2.0 mdepth, would be possible unstable and unsafe working conditions for the contractorunless shoring was used. Thus, deeperfoundations could add considerably to silocosts without necessarily improving reliability. A better approach might be to provide insulation above the footings andaround the silo wall, to reduce frost penetration. This could be done either by additional soil backfill around the silo, or byadding polystyrene insulation board laidhorizontally above and beyond the top ofthe footing ring.

Allowable Settlement and TiltingAt the end of the first winter, the Nor

mandin silo wall tilted 175 mm out of

plumb toward the feedroom (NE), due tononuniform frost heave. This tilt exceeds

the maximum of 132 mm (or 1:120) alj

153

aataaar •*»».& &»*%;•&,:&?

NORTH SIDE SOUTH SIDE

Figure 7. Frost-line penetration by month, 1977-1978winter(A) and 1978-1979winter (B).The dots give the thermocouple locations, spaced mainly at 600 x600 mm on avertical north-south plane.

lowed in the Canadian Farm BuildingCode (1977). Second-year tilt was less,probably due to a milderwinter. Less tiltwould be desirable, but there seems to belittle danger that frost heave alone wouldupset the silo; frost effects have not beenaccumulative so far. The only real problem with this nonuniform frost heave isthat excessive tilt might affect the operation of the silo unloader.

Settlement and resettlement on the

other hand have been accumulative, although at a decreasing increment eachyear. The accumulated settlement indicated in Table II (- 44 mm mean to 1981)is not unusual or alarming; the disquietingpart is that tilt due to settlement has beenincreasing slightly each year, and in thesame westerly direction.

Moment Effects in the Footing RingDesign of the footing ring (Turnbull

1979) was based on the assumption that

soil reaction pressure would be uniformand purely vertical under the whole footing ring. On this basis, it was predictedthat at any point around the ring therewould be considerable eccentricity between the load imposed by the silo wall(close to the inside footing edge) and thecentroid of soil reaction pressure. This eccentricity should cause the inner part ofthe footing ring to settle more than theouter circumference. Subsequent soil settlement calculations by Sokhansanj (Pers.commun., Agricultural Engineering Dep.,University of Saskatchewan, Saskatoon,Sask.) using finite element analysis techniques, and assuming elastic behavior ofa uniform soil under the footings, havealso predicted that footing settlementshould be greater at the inside edge (heel)of the footing ring than at other outsideedge (toe).

Readings taken on radial pairs of levelling pins indicated exactly the reverse.

With winter frost penetrating first underthe outer edge of the footing, elastic theory would further indicate that this apparent outward rotation of the footing shouldreverse during the winter. In fact this reversal did not occur, for reasons unexplained at this time.

The observations suggest that the circumferential reinforcing steel (Fig. 2,item (4)) might possibly be redistributedand reduced for improved design efficiency, if soil and silage pressures couldbe better predicted. One possibility is thathigh horizontal radial pressures have developed within the 'plug' of unexcavatedsoil contained within the footing ring, andthat these horizontal pressures have upsetthe moment predictions on which the reinforcing steel design was based.

It is interesting to speculate on the effects of frost heave on a ring footing asdescribed in this paper, but constructedwithout circumferential reinforcing.Breakup of the unreinforced footing ringinto several segments would be almost inevitable. The separated segments could befree to rotate about the bottom outside

edge of the silo wall, and the effectivebearing area under the silo and footingwould be dramatically reduced. Assumingthe effective footing diameter would thenbe reduced to that of the outside edge ofthe silo wall (6.1 + 0.3 = 6.4 m), theindicated safety factor would reduce from2.97 (as built) to only 1.8.

SUMMARY

An experimental tower silo extendedring foundation was built above expectedfrost penetration on soft, frost-sensitiveclay soil. In situ soil shear strengths weremeasured by vane shear down to 2/3 of theproposed footing diameter. Using thevane shear results, mean soil bearingstrength was calculated from an equationby Skempton (1951), indicating a safetyfactor of 2.97 in bearing.

Five annual filling/unloading cycleshave resulted in accumulative foundation

settlements but with decreasing annual increments. Nonuniform frost heave the

first winter produced a temporary tilt of1.3 times the recommended construction

tolerance, but the silo resettled to almostplumb the following summer. Unlikelightly-loaded buildings where the greatest footing load is due to roof snow load(which coincides with frost heave), thesilo is heavily loaded after frost hasthawed and appears to be capable of completely reconsolidating the soil heave effects of the previous winter. Therefore,there appears to be little risk that frost-

154 CANADIAN AGRICULTURAL ENGINEERING, VOL. 24, NO. 2, WINTER 1982

heave alone would cause enough tilt to Further study of these pressures is re- Int. Conf. on Soil Mechanics and Founda-seriously endanger the silo, provided that quired to improve the placement and ef- tion Engineering. 1: 55-59.the footing ring is adequately reinforced ficiency of reinforcing steel and possibly SKEMPTON, A. W. 1951. The bearing ca-against breakup. to reduce steel requirements. Pacity of clays- Proc- Build- Res- Congr.,

Circumferential reinforcing steel in the London, pp. 180-189.footing was designed to control radial REFERENCES STANDING COMMITTEE ON FARMcracking due to anticipated bending mo- BOZOZUK, M. 1974. Bearing capacity of BUILDfNG^o^77xTCafc(i1^ ^ ^ments. However, unexpected outward ro- c.ays for tower sUos. Can. Agric. E„g. ,6: g^^^X^Z^tation of the footing nng implies that ac- ^^ M ,97? Evaluatin£ strength TURNBULL, J. E., H. A. JACKSON, and D.tual soil and silo pressures about the ring B^Zf^ ™uJ™n S "f Proc IX h LOWE- 1979' Reinforced extended ringwere not as simple as originally assumed. tests trom toundatlon failures- Proc> IXth foundations for top-unloading concrete

tower silos. Can. Agric. Eng. 21: 111-116.

CANADIAN AGRICULTURAL ENGINEERING, VOL. 24, NO. 2, WINTER 1982 155