FOUNDATION PERFORMANCE OF A STEEL TOWER SILO

J.D. Scott1, G. Haile2, and M. Bozozuk3

1Hardy Associates (1978) Ltd., P.O. Box 746, Edmonton, Alberta, T5J 214; 2Geocon (1975) Ltd., 1130 Ouest rue Sherbrooke,Montreal, Quebec, HiA 2R5; and }Geotechnical Section, Division of Building Research,

National Research Council of Canada, Ottawa, Ontario, K1A 0R6

Received 26 February 1979.

Scott, J.D., G. Haile, and M. Bozozuk. 1979. Foundation performance of a steel tower silo. Can. Agric. Eng. 21: 85-89.

The foundation performance of a steel tower silo founded on a concrete circular raft in a deposit of soft clay was monitored inorder to determine the applicability of standard engineering foundation methods to this type of structure. The allowable bearingcapacity of the soil determined from field vane shear tests was more than adequate for the satisfactory performance of thefoundation. The measured settlement of the silo was about one third of that predicted. The settlement gauges showed that themaximum soil compression occurred immediately below the foundation although the soil was highly overconsolidated bydesiccation. Becausefrost action occurred, the need to design for frost protection when silo footings extend beyond the silo wallata shallow depth is discussed.

INTRODUCTION

On 30 Sept. 1975 a new concrete towersilo 32.3 m high, 9.14 m in diameter,overturned when it was being filled for thefirst time, because of a bearing capacityfailure in the underlying soil (Bozozuk 1977;1979). Other similar foundation failures oftower silos have occurred (Bozozuk 1972;Eden and Bozozuk 1962) and numerouscases of tilting of large tower silos because ofexcessive soil settlements can be seen.

The concern created among farmers andsilo contractors about poor foundationperformance resulted in the initiation ofresearch programs at the Division ofBuilding Research of the National ResearchCouncil of Canada and at several

universities, to study the foundation designand performance of large tower silos. Thispaper reports on the results of one suchinvestigation.

The foundation performance studied isfor a steel tower silo founded on a rigidcircular concrete raft erected in 1976

adjacent to the failed concrete silo. Theengineering properties of the soil at the sitewere used to specify the dimensions for thefoundation for the new silo. The

performance of the foundation was thenmonitored during loading in order toconfirm the application of engineeringfoundation theory and practice to this typeof structure.

CHOICE OF FOUNDATION

A concise summary concerning theessentials for good foundation design wasgiven by Sowers (1970): (1) it must be placedat an adequate depth to prevent frostdamage, heave, undermining by scour, ordamage from future construction nearby; (2)it must be safe against breaking into theground; and (3) it must not settle enough todisfigure or damage the structure.

The first requirement involves a numberof decisions based to a targe extent on thesound judgement and experience of thedesigner or builder. For the other tworequirements, i.e., the allowable bearingcapacity and settlement of the structure, anunderstanding of the principles and methods

of foundation engineering is essential.In general, the selection of foundations

for large tower silos should follow the sameprocedure as that for other earth-supportedstructures such as buildings, warehouses andbridges. The soil conditions at the silo siteshould be ascertained and its strength andcompressibility characteristics determined.These properties may be determined by fieldor laboratory tests on undisturbed samplesof the soil. The procedure for calculating theallowable bearing capacity for tower silosfounded in clay from these soil properties isgiven by Bozozuk (1974).

The second step in foundation designinvolves specifying the thickness of thefoundation and providing the necessary steelreinforcement. The structural design of thefoundation should be done in accordance

with standard reinforced concrete designprocedures, such as presented by Turnbulland Bellman (1977) for ring footings.Alternatively, for most soil conditions andcommon silo dimensions, information onthe size of footing and on reinforcing can beobtained from design tables prepared byCanada Plan Services (Plans 7411 and7412). These plans give in tabulated form themaximum sizes of tower silos that can be

safely supported on soils with allowablebearing capacities varying from 75 kPa to250 kPa.

SITE DESCRIPTION

The site, located 10 km south-west ofOttawa and 7 km north of Richmond,Ontario, is underlain by an overconsolidateddeposit of marine clay. Known as Leda clay,the soil was deposited in the saline water ofthe Champlain Sea during the recent episodeof continental glaciation (Gadd 1975).Detailed discussions concerning the geologyof the clay are available elsewhere (Gadd1975; Fransham and Gadd 1977).

The soil profile together with watercontents, Atterberg limits, undrained shearstrengths and preconsolidation pressures ispresented in Fig. 1. The top 2.4 m consists ofdesiccated and oxidized brown clayey siltbelow which the soil is predominantly softgray silty clay. The Atterberg limits are fairly

CANADIAN AGRICULTURAL ENGINEERING. VOL. 21, NO. 2, DECEMBER 1979

constant with depth. The plasticity indexand the liquid limit are approximately 20and 40%, respectively. Within the oxidizedlayer, the liquid limit and the natural watercontent are about the same. In the softer claylayer below the desiccated crust, however,the natural water contents average 52% andare higher than the liquid limits.

Figure 1 also shows the undrained shearstrength and the remoulded shear strengthdetermined by field vane tests. Within thetop layer, the average undrained shearstrength was about 60 kPa. Below the crustand to a depth of 15 m, the undrained shearstrength was fairly constant with an averagevalue of 36.5 kPa. Then it graduallyincreased to 60 kPa at 18 m.

The remoulded strength of the soil givesan indication of the sensitivity of the clay todisturbance. The average remoulded shearstrength of 5 kPa compared to the averageundisturbed shear strength of 36.5 kPaindicates that the soil is very sensitive todisturbance.

From the distribution of the most

probable preconsolidation pressure and thein situ vertical effective stress, it wasconcluded that the desiccated layer wasoverconsolidated by about 100 kPa, whilethe underlying clay was overconsolidated byonly about 20 kPa.

From Bozozuk (1974), a soil shearstrength of 36.5 kPa will allow a foundationbearing value of 81 kPa with a safety factorof 3 against bearing capacity failure for acircular foundation.

The possible settlement of a structurewith a foundation pressure of thismagnitude depends on the distribution ofthe maximum vertical stress in the soil mass

in relation to the overconsolidation pressureof the soil. The increase in vertical stress at

the base of the foundation would be 81 kPa

minus the pressure from the excavated soil,and it would decrease with depth. For thesoil conditions at the site, the soil in theupper 3 m would undergo little compression,while significant but tolerable settlementswould occur in the soil beneath this depth.An allowable bearing value of 81 kPa,therefore, was chosen as the limit for theconcrete raft to support the new silo.

85

DEPTH

METRES

SOIL

TYPE

WATER CONTENT ANDATTERBERG LIMITS

(%)

SHEAR STRENGTH AND VERTICAL STRESS

(kPa)

^Ground. Surface

El. 107m

20 40 60_J . I . L_

20 40 60 80 100 120 140—I 1 1 1 1 1 L i I • i . i

-Raft Depth• 0.72 m

- 5

clay, slltybrown I *n.

i K-.

'—AIt 1 •..

10

clay, slltygrey -u* :•.*••

i 1

-15

•* :1.

-20

clay, slltybrittle grey

Wp w,

DESCRIPTION OF STRUCTURE

The steel tower silo was erected in June

1976. It had an inside diameter of 7.6 m,height of 26 m excluding the roof, and acapacity of about 950 tonnes.

In order to ensure that the remoulded soil

from the failed concrete silo did not affect

the performance of the new structure, theedge of the new foundation was locatedabout 6 m from the zone of disturbed clay.Fig. 2 shows the relative position of the raftfoundation to the failed silo and the barn.

The raft foundation, 13.7 m in diameter,was founded at a depth of 0.72 m (Fig. 3).The figure also shows the concrete pedestalcast on top of the raft. The pedestal is thestandard Harvestore foundation normallyused to support the silo when soil conditionsare adequate. The standard Harvestorefoundation anchors the steel silo at its base

and contains the trough for the unloadingconveyor system.

Steel reinforcing for the raft wasprovided by adding 20 mm radial bars 3.1 mlong placed 8 cm above the bottom of theexcavation. The reinforcing bars werepositioned 3.2 m from the center of the raftat a uniform spacing of 0.15 m around thesilo perimeter. To hold them in position,they were fastened at both ends to 20 mmcircumferential steel reinforcement.

The total dead mass of the foundation

and silo was approximately 630 tonnes.

86

Figure 1. Soil profile at raft location.

Figure 2. Raft foundation for tower silo. Portion of failed silo in the background.

INSTRUMENTATION AND

OBSERVATIONS

The performance of the silo duringloading and the reaction of the underlyingsoil to the load were monitored by theinstallation of instruments to measure the

settlement and deformation of the raft, thezone of compression in the soil mass, thepore water pressures beneath the silo, and

the contact pressure distribution on the baseof the raft. The instrumentation consisted of

a deep bench mark driven to the bottom ofthe clay deposit, 24 levelling points on theraft (12 around the circumference of the siloand 12 around the edge of the raft), six deepsettlement gauges, two Gloetzl pneumaticand four Geonor standpipe piezometers,and eight Gloetzl hydraulic earth pressurecells. The locations are given in Fig. 3.

CANADIAN AGRICULTURAL ENGINEERING, VOL. 21, NO. 2, DECEMBER 1979

GROUNDSURFACE

STEEL SILO WALL

CONCRETE PEDESTALON TOP OF RAFT

'P-3

LEGEND

C-l to C-8 LOAD CELLS

P-l to P-6 PIEZOMETERS

* LEVELLING POINTS (Concrete Nails)

P-5

,,p-4

-,.., _ 0 1.0 2.0mSCALE I i i

The load-settlement relationship isplotted in Fig. 4. The load was appliedcontinuously during construction andsubsequent filling of the silo with haylageover a period of about 50 days, which raisedthe applied net pressure to about 55 kPa.The settlement due to this loading was about7 mm. In the following 150 days when asmall additional load was applied andremoved, a further settlement of 4 mm tookplace. During the last 50 days of this periodending in December 1976, the settlementwas less than 1 mm, indicating a very slowrate of settlement. Frequent settlementsurveys were discontinued in February 1977,when it was discovered that the silo had

heaved more than 3 mm due to frost

penetration of the underlying clay throughthe concrete raft that extended beyond thesilo wall.

By October 1978, when the silo hadundergone two cycles of loading andunloading, and was loaded for the third timeto a net applied pressure of 65 kPa, the

P-6

6 DEEP BENCH MARK

Figure 3. Section through silo, showing foundation and instrumentation.

settlement had reached a total of 21 mm.The differential settlement of the raft

monitored with the 24 levelling pointsshowed that the raft was very rigid. Theaverage differential movement was in theorder of 1 mm, even when the entirestructure was lifted by frost action on only apart of its base.

The settlement gauges showed that about90% of the soil compression took place in theupper 14 m of the soil profile and almost onehalf of this settlement occurred in the 4 m ofsoil directly below the raft. This settlementpattern was fairly constant over the entireloading range and has not changed withtime.

The excess pore water pressures werevery low compared to the pressures appliedduring construction. The excess pore waterpressures of approximately 6 kPa indicatedthat the silty clay drained rapidly during theconstruction and loading period of less than50 days and that the observed settlementswere due to both elastic soil movements and

CANADIAN AGRICULTURAL ENGINEERING, VOL 21, NO. 2, DECEMBER 1979

consolidation. The excess pore pressureswere almost completely dissipated 60 daysafter the silo was filled.

The earth pressure cells at the base of theraft indicated that the contact pressuredistribution across the foundation was

nonuniform and consistent with

elastoplastic theory for such foundations.When the average gross contact pressurefrom the dead and live loads was 72 kPa, theactual contact pressure was approximately80 kPa at the edge, reducing to about 50 kPatowards the center of the raft.

SETTLEMENT OF SILO

Although reliable methods are availablefor estimating the allowable foundationpressure to ensure an adequate factor ofsafety against a bearing capacity failure, it isdifficult to predict the settlement that thisload will produce. The total settlement of astructure will be composed of: (a) animmediate settlement due to lateraldeformation of the underlying soil under theinfluence of the applied load; and (b) long-term consolidation of the soil due to

dissipation of excess pore water pressures.The estimated theoretical immediate

settlement was compared to the observedsettlement for pressure levels of up to 55kPa. The estimate was based upon the secantmoduli obtained from triaxial stress-straincurves. At the higher pressures, the loadswere applied slowly allowing consolidationto take place at the same time; consequentlythe immediate settlements were notdetermined.

Stress-strain curves for 11 isotropicallyconsolidated undrained (CIU) triaxial testsat various depths were available. Thecompressible zone below the bottom of theraft was therefore divided into 11 soil layersand the calculated compressions of thelayers were added to give the total settlementof the raft. The compression of each layerwas calculated from: (a) estimated verticaland lateral stress increases due to the

foundation pressure at the center of the layerfrom elastic theory, (b) the secant modulusdetermined from the particular triaxialstress-strain curve corresponding to thevertical stress increase estimated above and(c) the compression of the soil layerdetermined from elastic theory. Elasticformulae relating stress and strain are foundin many soil mechanics textbooks. It iscommon to use a Poisson's ratio of 0.5 for

calculations of immediate settlement insaturated clay soils.

In general, the estimated settlementswere greater than observed. For example,corresponding to a net applied pressure of 55kPa, the calculated and measuredimmediate settlements were 13.5 and 7 mmrespectively. The main reason for thediscrepancy was the low moduli obtainedfrom the uniaxial tests. Low moduli arecaused by disturbance in obtaining andpreparing samples. The total immediatesettlement at the maximum pressure applied

87

NET APPLIED PRESSURE (kPa)

30 40_ _50_ 60 70

1

Figure 4. Settlement of silo raft foundation.

of 65 kPa was calculated to be 16 mm.

Based upon the maximum net appliedpressure of 65 kPa, the predicted long-termprimary consolidation settlement,determined from standard odometer test

results, was about 46 mm. The totalpredicted settlement was therefore 62 mm.In addition, creep-type settlements couldoccur in the clay because the total verticalapplied stress was about equal to orexceeded the preconsolidation pressure ofthe clay between 4 and 11 m of depth.

The eventual total settlements will,however, not be much greater than the 21mm measured in October 1978. Theimmediate and consolidation settlements

appeared to be complete but additionalsettlement will probably occur due to (a)loading and unloading of the silo, and (b)soil creep.

Both the calculated and measured total

settlements discussed previously were basedon the maximum net applied pressure of 65kPa. The raft was, however, dimensioned toexert a net increase in average verticalpressure of 72 kPa. If the silo was filled withheavier silage so that the applied pressurewould be greater than 65 kPa, both thepredicted and measured settlements wouldbe larger.

FROST ACTION

The unexpected frost heave of the siloraft indicated a relatively minor problemwith this shallow foundation. Other silosnormally have their foundations placeddeeper than the depth of frost penetration. A

deeper foundation in this case would havemeant a substantial increase in materialcosts and changes in design to allow thenormal unloading of the silo. In general atall, heavy silo such as this one may require awide footing extended well beyond the silowall which would not be protected by theinsulating and heating effects of the silage.

Frost heave pressures under the toe couldbreak the footing. When the silo raft heaved,the earth pressure cells showed that thecentral part of the foundation was liftedclear of the underlying soil by the upliftpressures acting around the edge. Theconsiderable thickness and rigidity of theraft prevented it from being damaged but anormal footing would probably have beencracked and damaged. Such footingstherefore should be placed below the depthof frost penetration, or reinforced towithstand the uplift pressures. It may bemore economical to protect the foundationby placing insulation below or above thefooting extension or by building an earth fillaround the silo to prevent excessive frostpenetration.

SUMMARY AND CONCLUSIONS

The allowable gross bearing pressure of81 kPa determined by commonly usedfoundation engineering methods resulted ina safe design for bearing capacity. Theperformance of this foundation supportedthe conclusion by Bozozuk (1977) that claysoil strengths measured in situ with a fieldvane can be used to predict the ultimatebearing capacity.

The observed total settlements were onlyabout one third of those predicted fromlaboratory tests on soil samples. Thedifficulty in estimating small settlementsresulted from the sensitivity of soil stress-strain characteristics to disturbance that

normally occurs during sampling andhandling.

The relatively small settlement of 21 mmindicated that the factor of safety of 3against bearing failure, commonly used forbuildings sensitive to settlement, could bereduced for farm silos. The allowable total

settlement of most structures is limited to

appproximately 25 mm to ensure thatdifferential settlements will not disfigure ordamage the building. The rigidity of circulartower silos should allow greater totalsettlements without damage; consequently itmay be appropriate to reduce the factor ofsafety to about 2.5, which would result insmaller and more economic foundations.

This may, however, increase the stresses inthe foundations because of variations in soil

compressibility. It would then become mostimportant to include adequate reinforcing inthe footings.

The large percentage of the totalsettlement which occurred in the 4 m of soil

immediately below the foundation was notpredictable from consideration of the soilprofile or results of the laboratory tests. Thiszone of soil was overconsolidated bydesiccation and appeared to be relativelyincompressible compared to the underlyingsofter clay. This finding emphasizes theimportance of placing footings deep enoughso that the highly stressed region immediatelybeneath foundations does not occur in soil

that has been excessively disturbed byweathering and frost action.

Foundations that may be affected byfrost action should be placed below thedepth of frost penetration. If this is notpossible, the foundation should be protectedwith adequate insulation or reinforced withsteel to withstand the bending moments. Iffrost action may cause differentialmovement, hence tilting of the silo, however,then frost protection rather than foundationstrength should be the criterion for design.

ACKNOWLEDGMENTS

The steel tower silo was manufactured by A.O.Smith, Harvestore Products Inc., Illinois, anderected by Ontario Harvestore Systems Ltd. Thisresearch study was made possible by financialassistance from the Ontario Ministry ofAgriculture and Food, Ontario HarvestoreSystems Limited, and the Division of BuildingResearch of the National Research Council.

BOZOZUK, M. 1972. Foundation failure of theVankleek Hill tower silo. Proc. Spec. Conf.Earth and Earth-Supported Structures,ASCE, SMFD, 1(2): 885-902.

BOZOZUK, M. 1974. Bearing capacity of claysfor tower silos. Can. Agric. Eng. 16(1): 13-17.

BOZOZUK, M. 1977. Evaluating strength testsfrom foundation failures. Proc. 9th Int. Conf.

Soil Mech. Found. Eng.. Tokyo, 1: 55-59.

88 CANADIAN AGRICULTURAL ENGINEERING, VOL. 21, NO. 2. DECEMBER 1979

BOZOZUK, M. 1979. Problems with concrete landslides in Ottawa Valley, Ontario. Can. Introductory soil mechanics and foundations.tower silos. Can. Agric. Eng. 21: 69-77. Geotech. J. 14(4): 531-537. The Macmillan Company, London.

EDEN W.J. and M. BOZOZUK. 1962. GADD, N.R. 1975. Geology of Leda clay. Proc. TURNBULL, J.E. and H.E. BELLMAN. 1977.Foundation failure of a silo on varved clay. 4th Guelph Symp. on Geomorphology, Reinforced extended ring foundations.Eng. J. 45(9): 54-57. Geological Survey of Canada Library, MGF Canadian Society of Agricultural

FRANSHAM, P.B. and N.R. GADD. 1977. 9S98 4th. Engineering, Annual Meeting, 14 to 18 Aug.Geological and geomorphological controls of SOWERS, G.B. and G.F. SOWERS. 1970. Guelph, Ont.

CANADIAN AGRICULTURAL ENGINEERING, VOL. 21, NO. 2, DECEMBER 1979 89