Tasarea Pamantului La Silozuri

SETTLEMENT AND LOAD TRANSFER OFRING FOUNDATION FOR TOWER SILOS

D.E. Becker and K.Y. Lo

Faculty of Engineering Science, University of Western Ontario, London, Ontario N6A 5B9

Received 16 February 1979

Becker, D.E. and K.Y. Lo. 1979. Settlement and load transfer of ring foundation for tower silos. Can. Agric. Eng. 21: 97-110.

A research program on the foundation behavior of tower silos on clay deposits in Southwestern Ontario was initiated in 1976.The program consists of detailed instrumentation for one new silo and settlement observations for several other existing andnewly-constructed silos. The results of field observations on settlement and tilt, silage density and pressure distribution beneaththe silage and the ring footing are reported in this paper. The phenomenon of load transfer of the silage to the silage wall andthence to the ring footing was also studied. It was found that in spite of the relatively low factor of safety against bearing capacityfailure and the considerable load transfer of 42% to the ring footing, the total maximum foundation settlement up to date wassmall (78 mm) and the differential settlement along the circumference of the ring footing was 9 mm. It may be concluded that theperformance of the silo foundation is entirely satisfactory, in spite of the relatively low factor of safety compared to thatcommonly used in foundation design. The main reason for this is the beneficial effect of the stiff surficial crust and the relativegeometry of the ring foundation layout with respect to the thickness of the crust below the foundation level. The results ofsettlement observations of other silos are also briefly reported and some significant observations are discussed.

N3.0 m x 12 2 in

CONCRETE SILO

6.1 m x 15.8 m

CONCRETE SILO

CONCRETE PAD

9.1 m x 21.9 m

CONCRETE SILO

{INSTRUMENTED SILO)

BARN

FEED LOT AREA

FOR CATTLE

INTRODUCTION

Current practice in construction of towersilo foundations is usually based on localexperience without the benefit of a soilsinvestigation. The trend towards theerection of larger and larger tower silos hasled to serious foundation problems withthese structures. A few failures of tower silos

have occurred in Ontario (Bozozuk 1972;Eden and Bozozuk 1962) attributable toinadequate bearing capacity of the subsoil tosupport the superimposed load. Manyothers have tilted or settled appreciably,affecting their performance. On the otherhand, silos up to 37 m in height have beenconstructed on soft clays of theWallaceburg-Chatham-Windsor areawithout any serious distress.

In Southwestern Ontario alone, 7600silos were constructed in the 5-yr periodbetween 1972 and 1976. This represents anannual expenditure of some $18 million.Thus, a better understanding of thefoundation behavior of tower silos leadingultimately to economical and adequatefoundation design appears to be ofeconomic significance.

As part of a comprehensive investigationon the behavior of tower silo foundations, afull scale concrete silo, 9.1 m in diameter and21.9 m high, with a ring footing 0.9 m deepand I.I m wide was fully instrumented forpressure distribution, pore pressureresponse and settlements duringconstruction, and the subsequent loadingand unloading cycles. In addition, a generalperformance survey, with respect tosettlement and tilt, of seven other silos in the

Chatham-Wallaceburg area was conductedover the three loading cycles since 1976. Thispaper describes the subsoil conditions, theinstrumentation, and the results of fieldobservations. An analysis of these results,together with the favorable performance ofthe instrumented silo is also included.

NOTE:

DETAILS OF INSTRUMENTATIONSHOWN ON FIG. 3

TRENCH LEADING '

TUBING TO

INSTRUMENT SHACK \'\

LEGEND :

O OUTSIDE REFERENCEPIEZOMETERS

® BOREHOLESCALE : METRES

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-INSTRUMENT SHACK

CORN FIELD

Figure I. General layout of site, borings and reference instrumentation.

CANADIAN AGRICULTURAL ENGINEERING, VOL. 21. NO. 2, DECEMBER 1979 97

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The

principalstratigraphy

ofthe

subsoilconsists

of

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of

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row

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esiccatedsilty

claycrust,

followed

byan

18.5-mthick

depositof

softto

firmgray

siltyclay.

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graysilty

clayin

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overliesa

thinveneer

ofhard

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he

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raph

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wn

inFig.

2alo

ng

with

asum

mary

of

pertinentgeotechnical

data.

Details

of

these

resultsm

aybe

foundelsew

here(L

oan

dB

ecker1978).

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euse

ofgeotechnical

data

insilo

foundationdesign

has

beeno

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Bo

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k(1974).

LA

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OF

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TA

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Th

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strum

ented

silois

ap

ou

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type

of

silo9.15

min

diam

eterand

21.94m

high.T

he

silow

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plied

with

ad

om

eor

asilage

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uto

r.T

he

fou

nd

atio

nis

a1.07-m

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eb

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.92

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foundedat

0.92m

belowthe

groundsurface.

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tethat

inthe

southernquarter

of

the

circumference,

the

depthis

ab

ou

t1.22

mdue

tothe

existenceo

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me

loo

sefill.)

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thick

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cated

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on

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ently,

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6m

.

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silow

all,a

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150m

min

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atio

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Fig.

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NOTE

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FOR DETAILS OF SECTIONREFER TO SECTION VIEWFIGURE 4

SCALE METRES

O PRESSURE CELLS

A PIEZOMETERS

• SETTLEMENT CELLS

H SETTLEMENT PINS

Figure 3. Plan of instrumentation of 9.1 x 21.9-m silo.

pressure cells below the ring foundation andsilage, piezometers at different depths in thesubsoil, settlement cells along twoperpendicular diameters beneath the centerslab and settlement pins along the wallcircumference on both the new and adjacentsilos. Details of installation and operationprinciples may be found elsewhere (Lo andBecker 1978).

LOADING CONDITIONS ANDSILAGE DENSITY

Silage LoadTo determine the weight of the whole-

plant corn silage, it had been originallyplanned to use portable electronic weigh

scales. The weigh scales, however,malfunctioned about a week prior to fillingof the silo. It therefore became necessary toadopt a new weighing-in program bymeasuring the average density of silage in awagon and the volume of each wagon load.The volume of the silage was determined bymeasurements with a 3-m pocket tape. Thedensity of the silage was routinelydetermined by the following method: A boxof known volume and mass was filled withsilage and compacted to roughly simulatethe effect of the wagon travelling across thefield to the silo. The mass of the silage inthe box was measured using a beam balanceand the density of the silage calculated. The

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

moisture content of the silage was alsodetermined for each density measurementby oven-drying for 24 h at 100°C. Usingthisprocedure, it was calculated that the totalmass of the 56% moisture silage in the silo inthe first loading cycle (1977) after tworefillings was about 1170 t (tonnes). Thetotal silage weight determined from themeasured vertical pressures beneath thesilage and foundation was calculated to be9.9 MN. The National Silo Association(NSA 1974) reports capacity values of 8501to 1220 t for silage at 50 and 65% moisture,respectively, for 9.1 x 21.3-m silos.Graphical interpolation of this data alongwith other published NSA capacity charts of

99

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SECTION A-A

• PRESSURE CELLS

& PIEZOMETERS

D SETTLEMENT CELLS

k SETTLEMENT PINS

NOTES :

1 NUMBER BESIDE SYMBOL REFERSTO THE DISTINCTION NUMBER OF

THE INSTRUMENT.

2 INSTRUMENTS LOCATED BY RADIALPROJECTION TO SECTION LINE.

SCALE: METRES

Figure 4. Typical section view of instrumentation of 9.1 x 21.9-m silo.

9.1-m diameter silos of varying heightsyielded a capacity of 10201 for silage at 56%moisture in a 9.1 m x 21.9 m silo. In 1978,only 12 m of fresh silage at an averagemoisture of 63% was put in the silo since thesilo still contained 9.4 m of silage from 1977.Based upon the described density-volumeprocedure it was determined that 540 t offresh silage was stored in the silo. Thepressure cell observations indicated that 5.1MN of fresh silage had been put in the siloin 1978. Graphical extrapolation of existingNSA capacity charts yielded a 1978capacityof 5301 of fresh silage. Consequently, duringthe first two loading cycles (1977 and 1978),the total silage weight determined from thepressure cell observations are within 13%and 4%, respectively, of the total silageweight determined from the describeddensity-volume procedure. In addition, thecalculated capacity values agree quitefavorably with NSA capacity charts. It isfelt, therefore, that the method of weighingthe silage into the silo has an accuracy within10%.

100

Silage DensityThe density of silage within a silo is not

constant. Due to its very compressiblenature, the density at a given locationincreases with height of silage above it.Consequently, only an average silagedensitymay be determined. To determine thisaverage silage density, the mass of silage andthe volume it occupied inside the silo wereroutinely measured and recorded everymorning and evening during filling of thesilo. The average density can then bedetermined.

The results of these measurements are

shown on Fig. 5 for the first two loadingcycles. It may be seen from the figure that thesilage density is highly dependent upon thesilage moisture content. In 1977, the cornwas harvested late in the year due to poorweather conditions. The corn therefore hadbeen subjected to several frost exposuresand was beginning to wither. The averagemoisture content was about 56% and, asconfirmed by the farmer, lower than usual;whereas in 1978 the average moisture

content was about 63% and increaseddensities, proportional to the increase inmoisture content, were observed. The curvefor 1978 spans over a silage height increaseof only 12 m. This represents the fresh silageput in the silo from 9.4- to 22-m height, asthe silo still contained 9.4 m of silage from1977 prior to refilling in 1978.

From the 1977 curve, it may be seen thatgenerally an increase of density of about 48to 80 kg/m3, depending upon silage height,was observed upon settling overnight. Theaverage density of silage increased fromabout 400 kg/ m3at a silage height of 5.5 m toan average density of 730 kg/m3 uponrefilling the silo and subsequent silagesettlement to 21 m; whereas in 1978 thecorresponding average density was about820 kg/m3 and agrees reasonably well withdensities reported by Bozozuk (1972).

The relationship between unloading andaverage density is also plotted along with therefilling portion in 1978 on Fig. 5. Duringunloading, the average silage density in thesilo increases. For example, when the silo


OOOr

900

800

700

E

START OFREFILLING

(1978)

START OFUNLOADING

(1978)-^

"1«0

600 W= 63% (1978)(HEIGHT REFERS TOFRESH SILAGE)-- ,

E

Z

8UJe><

<orUJ><

500

400

300

200

100

"OVERNIGHT

SETTLEMENT

AVERAGE MOISTURE CONTENT OF

SILAGE

AVERAGE DENSITY DURING LOADINGAND UNLOADING (1977)- FROM

PRESSURE CELLS

AVERAGE DENSITY AFTER SETTLING0VERNIGHT (1977) - FROMPRESSURE CELLS

AVERAGE DENSITY

OVERNIGHT ( 1978)

PRESSURE CELLS

AFTER SETTLING

- FROM

- 3

X

z

UJ

-J

J _L4 6 8 10 12

HEIGHT OF SILAGE14 16

(m)18 20 22 24

Figure 5. Relationship between average silage density and height of silage in a 9.1 x 21.9-m silo.

was full, the average density in the silo wasabout 730 kg/m3. However, when the silohad been unloaded to the 9.4-m height, theaverage silage density in the silo was 950kg/m3.

The variation of the average densityduring refilling in the second loading cycle isalso shown in Fig. 5. As fresh silage is putinto the silo from the 9.4-m height and up,the average density decreases markedly untilloading is interrupted or completed.Subsequently, as the silage in the silo settles,the average density once again increases andreaches a maximum value of 820 kg/m3prior to starting of unloading operations in

1978. This behavior of the average silagedensity is consistent with its compressiblenature.

LOADING CONDITIONS AND

PRESSURE DISTRIBUTION

General

A silo is an intermittently-loadedstructure subjected to cycles of loading andunloading. The components of loadingconsist of the dead weight of the structureand the varying load of the silage. The firstcomponent is known, but the load of thesilage must be established by some type of


direct weigh-in program, since the unitweight of silage varies with the moisturecontent and the superimposed load abovethe point under consideration. In addition,as the silage compresses, part of the load istransferred to the silo wall, thence to thefoundation. As a result, the applied pressureon the foundation soil is generally nonuniform.

During Silo ConstructionThe reliability of the pressure cell

measurements was first assessed bycomparing the pressures registered with thecomputed average pressure during the

101

8 10 12 14

HEIGHT OF SILAGE

Figure 6. Relationship between measured vertical pressure and height of silage during the first loading cycle (1977) beneath the ring foundation.

successive stages of construction of the silowall, taking the density of concrete to be2330 kg/m3. At full height of the wall, theaverage calculated pressure was 96.6 kPacompared to an average measured pressurefrom the six cells of 97.3 kPa. It was also

found that the response of the pressure cellswas practically instantaneous and thereadings remained essentially constant atconstant load. Based on the above

observations, the performance of thepressure cells, as installed, is considered tobe satisfactory. The total load of the

102

structure (foundation and wall) was foundto be 3.63 MN.

During the First Loading Cycle (1977)A typical set of results of measurements

from the pressure cells beneath the ringfooting (cells 1 - 6) are shown in Fig. 6. Theheight of silage plotted is the equivalentheight of the actual cone-shaped top of thesilage. From these results, the followingobservations may be made:(a) The rate of increase in pressure

increases with silage height.

(b) At high silage loading, the silage settleswhen filling is temporarily interrupted.The measured pressure increasesslightly under constant load with time,especially when silage was at full height.

(c) The measured pressures duringunloading are greater than loading forthe same silage height.

The results of the pressure cellmeasurements underneath the silage (cells 9- 12) are plotted against the equivalentheight of the silage in Fig. 7. It may beobserved that:


oa.

UJrr

COCOUJrra.

<o

h-rrUJ>

QUJrr3CO<UJ

UNLOADING o uj

HEIGHT OF SILAGE (m)

Figure 7. Relationship between measured vertical pressure and height ofsilage during the first loading cycle (1977) beneath the silage.

(a) The rate of increase in pressureincreases at high silage loading.

(b) The pressure cell nearest to center (R =0.46 m) recorded the highest pressurewhile the other cells at radial distances

of 1.8 - 2.7 m recorded lower pressures.(c) At constant silage load, the silage settles

and the pressure increases slightly. Thisis particularly noticeable at greatersilage height.

(d) The measured pressures duringunloading are greater than duringloading for the same height of silage.

Assuming that the pressures below thesilage at equal radial distances from thecenter are the same due to axi-symmetriccondition, the distribution of pressuresmeasured along the radial distances is shownin Fig. 8 for three stages of filling. It may beseen that the pressure distribution isessentially uniform at one-third height ofsilage. At two-thirds full height, a distinctnon-uniform distribution may be noticed, asa result of load transfer from silo wall to

foundation. At full height, the minimumpressure measured beneath the silage is only


half that of the foundation. Included in thefigure are the measured pressures at fullheight in the second loading cycle (1978). Itmay be seen that the behavior is almostidentical to that exhibited in the first loadingcycle (1977). The higher magnitude of themeasured pressures for the second loadingseason reflects the higher moisture contentof the silage in 1978. Although the numberof pressure cells is insufficient to defineprecisely the silage pressure distribution, itappears likely that the distribution ofpressure below the silage may be

103

o

Q.

a:3COCOUJrra.

<o

crUJ

>

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H= 21.9 m = H„

FIRST FILLING (1977)

SECOND FILLING (1978)

H=l5.2m*-f-Hmqx

H=7.8rr»*-5-Hn

RINGFOOTING

12 3 4 5

RADIAL DISTANCE FROM CENTRE OF SILO (m)

Figure 8. Relationship between measured vertical pressure and radial offset from center of silo.

approximately triangular.The average measured pressures beneath

the ring footing and that below the silage areplotted against the height of silage in Fig. 9.It may be seen that the average pressures areabout the same up to about one-third of thewall height. Subsequently, the increase inpressure becomes more rapid beneath thering footing, indicating that a greaterproportion of silage load is beingprogressively transferred to the foundation.At full height of silage (22.6 m), the averagepressure below the foundation is 148 kPa,

104

while the average pressure below the silage isonly 91 kPa. After refilling, thecorresponding values were 155 kPa and 95.2kPa.

The portion of the silage load carried bythe ring foundation may be determined. Thetotal silage load after refilling is 11.5 MN.The area of the ring foundation is 31.1 m2.With an average pressure of 155 kPa, theload carried by the foundation is 4.82 MN.The load transfer to the foundation is

thus 42% of the total silage load.A secondary effect may be noticed in Fig.

9. Subsequent to the completion of thefilling and refilling operations, the averagepressure below the foundation increases by10 kPa under constant load withaccompanying silage settlement of 1.2 m.This suggests that the load transfer may beslightly time-dependent related to thesettlement-time behavior of the silage.

During unloading, the average pressuresat any silage height are greater than thepressures measured during loading. Thisreflects the fact that the density of silagewithin a silo is not constant. Consequently,

CANADIAN AGRICULTURAL ENGINEERING, VOL. 21, NO. 2, DECEMBER J979

6 8 10 12

HEIGHT OF SILAGE

SILAGE HEAPEDABOVE SILO WALL

NOV 12, (11.4MN)OCT. 14, (II.5MN)OCT. 4,(II.5MN)OCT. I , (ll.l MN)SEPT. 30,( 10.8MN)

Figure 9. Relationship between average measured vertical pressure and height of silage.

during unloading, the average density at anygiven height will be greather than the densityat the corresponding height during filling.

During the Second Loading Cycle (1978)The relationships between measured

pressures and silage height during thesecond loading of the silo are very similar innature to that in the first loading cycle. Itwas observed, however, that the magnitudeof the pressures was slightly greater thanthat in the first loading cycle. The reasonsfor this have previously been outlined.

The portion of the silage load carried bythe ring foundation may be determined inthe same manner as before. The loadtransfer to the ring foundation is 44% of thetotal silage load. This is consistent with theamount of load transfer in first loading. Theslight difference probably reflects thedifferent silage moisture contents and the 9.4m of silage remaining in the silo from 1977.

Figure 10shows the relationship betweensilage wall friction and the height/diameterratio for concrete silos(Turnbull et al. 1978).

The Canadian Farm Building Code(Standing Committee on Farm BuildingStandards 1977) recommends use of thecurve associated with k - 4.72 based uponfield measurements of load transfer.Turnbull points out that the curve shouldreasonably estimate the wall friction load forsilage not over 70% moisture stored from 9-to 24-m depth. If the results of this caserecord are added to the above design curve,excellent agreement is obtained (Fig. 10).This agreement lends support to the validityof the design curve using k - 4.72 in the rangeof h/D of 2-3.

SETTLEMENTS AND TILT

Settlement of the Ring FoundationThe results of settlements measured by

the settlement pins during the constructionof the silo wall to the present date (19 Dec.1978) are shown in Fig. 11. In the upperdiagram, the loading history is shown. Thepins were installed and initial readingsobtained after the silo wall had reached one-


half of its maximum height of 21.9 m. Thus,the first part of the settlement curve duringconstruction of the silo was not recorded.However, from the settlement measured tothe end of construction of the silo, theamount of settlement missed may beassessed to be about 7 mm. Thus, thesettlement for the construction of the silo isabout 13 mm. It also appears that thesettlement is essentially "elastic."

In presenting the results ofmeasurements, only the maximum andminimum settlements are shown for the sakeof clarity. These two pins are diametricallyopposite to each other. Results from otherpins lie within these two bounding curves.From Fig. 11, the following observationsmay be made:(a) The settlement of the silo due to silage

loading (before unloading) in the firstloading cycle is 43 mm and themaximum differential settlement is 5mm.

(b) The maximum total settlementrecorded in the first loading cycle is 56mm.

105

I

2P

(-o

UJ

<_j

en

2 3

HEIGHT / DIAMETER RATIO (h/D)

Figure 10. Silage wall friction versus height/diameter ratio for concrete tower silos (after Turnbull et al. 1978).

(c) No rebound of settlement occurredduring unloading of the first cycle (i.e.time-dependent consolidationsettlement occurs throughout the firstloading cycle).

(d) In the second loading cycle, anadditional settlement of II mm

occurred prior to unloading.(e) The total maximum settlement of the

ring foundation to date is 78 mm with adifferential settlement of 9 mm.

The distribution of settlement (based onthe settlement pin measurements) aroundthe circumference of the silo is fairlyuniform. The tilt at the top of thesiloduringthe first loading cycle is about 13 mm(corresponding to 5 mm differentialsettlement) towards the adjacent silo,whereas in the second loading cycle, the tiltis about 23 mm (based upon the 9-mmdifferential settlement). The modest amountof tilting is generally considered quiteacceptable.

Influence of Settlement on Adjacent SilosDuring the construction and filling of the

new silo, the settlement of the closestadjacent silo was also monitored. It wasobserved that the consequent average

106

settlement of the adjacent silo was about 5mm. The greatest settlement, however,occurred immediately adjacent to the newsilo (Fig. 11). An additional "immediate"settlement of 5 mm occurred when the old

silo itself was filled. It may be further notedfrom Fig. 11 that the pin closest to the newsilo (pin #15) continued to settle to amaximum of 20 mm, whereas the furthestpin (#19) only settled 6 mm.

The reasons for the above interaction

effect being relatively small are:(i) The increase in vertical stress on the

ring foundation is largely carried by thestiff surficial crust and is therefore

localized,

(ii) The relatively short loading period ofthe new silo so that the influence of

consolidation settlement has not been

fully affected.

Settlement Beneath the SilageSettlements beneath the silage were

measured remotely by the two lines ofsettlement cells shown in Figs. 3 and 4.Results of these measurements show that the

surface settlement of the clay below thesilage is greatest in the central portion of theloaded area. The maximum settlement

during the first loading (1977) is 89 mm andis 51 mm greater than that of the ringfoundation. Detailed analysis of theseobserved settlements may be foundelsewhere (Lo and Becker 1978).

BEARING CAPACITY OF

SILO FOUNDATION

It has been observed (Bozozuk 1972) thatfailure of tower silos occurred as a unit. It

follows, therefore, that the factor of safetyagainst bearing capacity failure may bedetermined by the conventional methodusing the total area encompassed by theexternal circumference of the ring footing asthe bearing area.

The ultimate bearing capacity is given by

qult=Nccu + "jD (1)

where

N,

und rained shear strength, averagedover a depth of 2/3 diameter of thefoundation

bearing capacity factor for circularfoundation

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

LOADING OF 61 m by 15.2 m SILO (BUILT 1969) °-

LOADING OF 9 I m by 219 m SILO (BUILT 1977) *-

- REFILLING AFTER SETTLINGOF SILAGE

2nd LOADING CYCLE

Figure 11. Observed settlements of silo foundations with time.

y = total unit weight of the soilD = depth of embedment of the foundation

For shallow depth of embedment, theterm yD constitutes less than 5% of the totalcapacity and may therefore be neglected.The factor Nc is 6.6 for circular foundation.The weighted average undrained shearstrength from results of field vane tests overa depth of 2/3 diameter is 53 kPa, and over adepth equal to diameter is 45 kPa. Theultimate capacity is therefore 345 kPa and297 kPa, respectively.

While the undrained shear strength inEq. 1 is usually taken as the average over 2/3diameter (e.g. Skempton 1951), fieldobservation (Bozozuk 1972) showed thatsilo foundations failed as a monolithic unit.This suggests that it might be moreappropriate to take the average strengthover a depth equal to the external diameterof the ring foundation. The total appliedaverage pressure from the structure andsilage is 179 kPa. The factor of safety istherefore about 1.7.

FOUNDATION SETTLEMENTS AT

OTHER SITES

In addition to the fully instrumented siloat Wallaceburg, six other silos constructedand loaded in 1976 in the Chatham-

Wallaceburg area were monitored forsettlements of the ring foundation.Settlement measurements have been

continuing since 1976 or generally over threeconsecutive loading seasons up to thepresent date (December 1978). Whereverthere were other silos at a site, both the oldand new silos were monitored to ascertain

any interaction between two silos. Boringswere carried out at two of the sites to

determine the stratigraphy and subsoilconditions. For brevity, only theperformance of silos at these two sites will bediscussed.

Jack's Farm

This site is located about 6.4 km north ofChatham. The silos monitored at this siteinclude a 9.1 x 31.1-m poured concrete silo


built in August 1976 and an older 7.3 x 24.4-m concrete stave silo built in 1969. The new

silo was built approximately 5.5 m awayfrom the older silo in an open level area.

Based on information supplied by theowner and the contractor, it is understoodthat the foundation level of the new silo was

to be at 1.5 m. However, upon excavatingand encountering a wet sand layer, thecontractor deepened the foundation to 3.1m. The width of the ring footing was about1.8 m and its thickness about 1.2 m. Thefooting was heavily reinforced. The fullsilage load is estimated to be about 17.35MN and the weight of the structure itself tobe 4.80 MN. In total, therefore, the maximum load supported by the subsoil isapproximately 22.15 MN.

The sequence of soil strata consists of a1.2-m thick stiff brown silty clay crust, a0.46-m brown layer of silt, a 1.37-m compactwater-bearing brown silty sand, followed bythe main stratum of stiff to firm gray siltyclay which extends to a depth of 17.8 m

107

LOADING OF 91 m by 31 I m SILO (BUILT 1976)

LOADING OF 73m by 24.4m SILO (BUILT 1969)

3rd LOADING CYCLE

9th LOADING'. CYCLE

1979iQ iNI iDIJ i F |M iAiM i^ i J

900 1000

TIME (DAYS)

Figure 12. Observed settlements of silo foundation with time, Jack's farm.

below ground surface. A thin till layeroverlies the shale bedrock which occurs

generally at depths of 18.3 to 21.3 m in thisarea. Using the vane strength profile, thefactor of safety against bearing capacityfailure of the 9.1 x 31.1-m silo is 1.4.

Anderson's Farm

This site is located about 6 km

southwest of Thamesville. The silosmonitored include a 6.1 x 21.3-m pouredconcrete silo built in July 1976 and an older7.3 x 21.9-m poured concrete silo built in1969. The new silo was built adjacent to theolder silo and separated by a distance of 1.13m.

Based on information supplied by thecontractor, it is understood that the top ofthe ring footing is flush with the groundsurface, 1.1 m deep and 1.4 m wide. The

108

weight of the structure is calculated to be2.13 MN, and the weight of the ear corn to bestored in the silo is about 4.45 MN. In total,therefore, the maximum weight supportedby the subsoil is about 6.58 MN.

In general, the subsoil consists of surficiallayers of compact brown sandy silt to siltysand and stiff brown clay to 3.8-m depthbelow the ground surface. The silty sand iswater-bearing and overlies a very stiff grayclayey silt which extends to 6.1-m depth. Theclayey silt in turn is underlain by 1.8 m ofvery stiff gray silty clay. Below 8.1 m, thesubsoil is a hard gray silty clay till. This layerwas not fully penetrated when the boringwas terminated at 12.6 m. The factor ofsafety cannot be calculated from the vanestrength tests since the subsoil has a greaterstrength than the capacity of the vane. It isestimated, however, that the factor of safetyis greater than 3.

Summary of Settlement MonitoringThe results of settlements measured by

the settlement pins from September 1976 tothe present date (19 Dec. 1978) are shown onFigs 12 and 13. In presenting the results ofmeasurements, only the maximum andminimum settlements are shown for the sakeof clarity. These two pins are diametricallyopposite to each other. Results from theother pins lie within these two boundingcurves.

The results of all the silos monitored for

settlement are summarized in Table I. The

values of maximum and minimum settlement

are for the ring footing. It can be seen fromthe table that the maximum settlement ofsilos, subjected to first cycle of loading,varies from 30 to 109 mm; and thedifferential settlement generally increaseswith the absolute settlement. In addition, themajority of the total observed settlement (to


lOr-

<Q

<=! 2

EE

v_^ 20

LU

UJ

30-

UJC/)

40-

50-

LOADING OF 7.3m by 21.9m SILO (BUILT 1969)

LOADING OF 61 m by 21.3m SILO (BUILT 1976)

\ n9

BARN ^

/~^\§py \7 6.1 mby Tlj/ 7.3m by 21.9 m\I 21.3 m SILO P \ silo he

* * 1$ ">

9 th LOADING CYCLE

Figure 13. Observed settlements of silo foundations with time, B. Anderson's farm.

date) occurs in the first loading cycle.Furthermore, the differential settlement,once set up by first loading, appears toremain relatively constant thereafter. Thetilting at the top of the silo wall computedfrom differential settlements varies from 23to 101 mm and is generally consistent withtransit measurements wherever theconstruction tilt is negligible. Furthermore,the tilt of two adjacent silos is towards eachother.

CONCLUSIONS

The research program conductedconsists of the instrumentation andobservation of a new silo, and theperformance survey of other silos. Insummarizing the results of the investigation,those pertaining to the detailed instrumentation will be presented first, followed byobservations from the other silos. Finally,

the practical implications of the results ofthis study are discussed.(1) The results of observations from the

pressure cells show that average silagedensity increases with height of silage.The average silage density in the fullsilo, subsequent to silage settlementvaried from 730 to 820kg/m3 for silagemoisture contents of 56 and 63%,respectively.

(2) The results of observations from thepressure cells show that the averagestresses below the footing and the silageincrease linearly by the sameamount upto one-third of the full height.Thereafter, the increase in stress belowthe ring foundation occurred at agreater rate with silage loading. At fullheight, the average contact stressbetween soil and foundation due tosilage load is 155 kPa which is 1.6 timesthat below the silage area, indicating


important load transfer from thesilageto the silo wall and thence to thefoundation. The load transfer to thefoundation is 42% of the total silageload which amounts to 11.5 MN in thefirst loading cycle. This behavior alsotook place in the second loading cyclewith the load transfer amounting to44% of the total silage weight. Nomeasurable effect of wind load on thecontact pressure has been observed.

(3) With respect to the settlement of thering foundation measured by thesettlement pins, the settlement,essentially elastic, due to the dead loadof the silo wall and foundation is about13 mm under an averagepressure of 121kPa. The total maximum settlementdue to silage load observed to date isabout 78 mm. The maximumdifferential settlementisonly9 mm andcorresponds to a slight tilt of 23 mm

109

TABLE 1. SUMMARY OF RESULTS FROM SETTLEMENT MONITORING

Silo Dimensions

Pmax(mm)

Pdif

(mm)Rebound (mm)

at end of

Tilt at top Factorof silo wall of

(mm) safety

1st 2nd 3rdt 1st 2nd 3rd 1st 2nd

cycle cycle cycle cycle cycle cycle cycle cycleCalculated Measured

Burm's 9.1 x 21.9 m

(1977)||56 78

— 5 9— 0

— 23—

1.7

Jack's 9.1 x 31.1 m

(1976)11109 112 124 28 28 31 10 8 101 102 1.4

Jack's5 7.3 x 24.4 m

(1969)1121 26 28 7 7 10 9 6 32 0

—

Anderson's 6.1 x 21.3 m

(1976)1130 42 38 9 14 17 5 8 59 114 >3

Anderson's § 7.3 x 21.9 m

(1969)1110 14 10 3 0 0 3 9 6 89

—

t Maximum settlement generallyoccurs at some stageduring unloading,not at end of cycle.* As of 14 Nov. 1978.

§ Reported values are for 7th, 8th and 9th loading cycles.II Refers to year of construction.

towards the existing silos.(4) Results of measurements by settlement

cells below the silage show that amaximum settlement of 89 mm

occurred at the center of the silo in the

first loading cycle (1977). In addition,the near surface settlement of the

subsoil below the silage area isapproximately dish-shaped and thecenter settlement is 51 mm greater thanthat of the ring foundation.

(5) The factor of safety for bearing capacityis computed to be about 1.7.

(6) In spite of this relatively low factor ofsafety when compared to thatcommonly used for foundation designof other structures, the foundationperformance has proved to be entirelysatisfactory as indicated by the modestamount of settlement, differentialsettlement and tilt. The main factor for

this is the beneficial effect of the

surficial crust in relation to the

geometry of the foundation.From the performance survey of four

other silos in the Chatham-Wallaceburgarea, the following observations may bemade:

(a) For silos subjected to first cycle offilling, the maximum settlement of thering foundation varies from 30 to 109mm. The differential settlement

generally increases with the absolutesettlement. The upper values of thesequantities were measured for a 9.1 x31.1-m silo. The computed factor ofsafety for bearing capacity of thisstructure is 1.4.

(b) The differential settlement, once set upby first loading, appears to remainrelatively constant on unloading andreloading.

(c) The maximum tilting at the top of thesilo wall measured by a transit is 114

110

mm and the range of values is generallyconsistent with those computed fromdifferential settlement wherever theconstruction tilt is negligible. Inaddition, the tilt of two adjacent silos istowards each other.

Based on the above conclusions obtained

from the detail instrumentation and analysisof the new silo and complemented byperformance survey of other silos, an item ofconsiderable importance in the design oftower silo foundations appears to emerge.Whenever a stiff surficial crust exists, andthe depth of the ring foundation is so locatedthat the depth of the crust below the bottomof the foundation is at least twice the

foundation width, it is possible to design thefoundation with a relatively low factor ofsafety of 1.7, with satisfactory performancewith respect to settlement, differentialsettlement and tilting. The effect on adjacentsilos will also be slight. The ring foundation,appropriately reinforced with the silo wallconcentrically located above it, is aneffective design to optimize the utilization ofthe beneficial effect of the crust which

possesses high strength, low compressibilityand a high coefficient of consolidation. Theeconomic significance of using a low yetadequate factor ofsafety is evident in view ofthe large number of silos constructedannually in the general area ofSouthwesternOntario.

ACKNOWLEDGMENTS

The research performed is supported bySupply and Services Canada under Contract No.0SU76-00139. The Ontario and National Silo

Associations also contributed to the financial

support. Dr. M. Bozozuk, National ResearchCouncil, acted as scientific authority for theproject and contributed many helpful discussions.Thanks are due to the following personnel fortheir valuable suggestions particularly regardingselection of sites for instrumentation: H. Bellman,

Ontario Ministry of Agriculture and Food, R.Clayton, Agricultural Engineer, Ridgetown,Ontario Ministry of Agriculture and Food and J.Abercrombie and D. Rhoades, Ontario SiloAssociation. Appreciation is also expressed to F.Davidson of McCormick Silo Ltd. for his co

operation in installation of the instrumentation ofthe silo. Several University of Western Ontariopersonnel participated in the field and laboratorywork involved in the project, including M.Johnson, G. Lusk, L. DiNardo, G. Fisher, S.Eccles, R. Wai and N. King. In addition, R.Carrington of Solinst Canada Limited helpedwith the pressure cell installations. Many thanksare also due to the farmers who allowed their silos

to be instrumented and monitored.

BOZOZUK, M. 1972. Foundation failure ofthe Vankleek Hill tower silo. Proceedings ofthe Specialty Conference on Earth and Earth-Supported Structures, ASCE, Soil Mechanicsand Foundations Division 1(2): 885-902.

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

EDEN, W.J. and M. BOZOZUK. 1962.Foundation failure of a silo on varved clay.Eng. J. 45: 54-57.

LO, K.Y. and D. BECKER. 1978. Tower silofoundations in Southern Ontario. Universityof Western Ontario, Faculty of EngineeringScience Research Report, GEOT-1-78.

NATIONAL SILO ASSOCIATION. 1974. Silooperator's manual. The National Silo Association Inc., Cedar Falls, Iowa.

NEGI, S.C., J.R. OGILVIE, and E.R. NORRIS.1977. Silage pressures in tower silos. Part 3.Experimental model studies and comparisonwith some silo theories. Can. Agric. Eng.19(2): 107-110.

SKEMPTON, A.W. 1951. The bearing capacityof clays. Proceedings Building ResearchCongress, London, pp. 180-189.

TURNBULL, J.E., H.A. JACKSON, and D.LOWE. 1978. Reinforced extended ringfoundations for top-unloading concrete towersilos. Contribution No. 529, Engineering andStatistical Research Institute, AgricultureCanada, Ottawa, Ont.


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