=ounca:ions anc ear: swor ~s 'or

cy inc rica s:ee s:orace:an~sby GRAHAM M. HARRIS+

IntroductionFOR MANY YEARS cylindrical steel stor-age tanks have been in common use for thestorage of many industrial raw materialsand products. One major industry that hasup to the present been heavily committedto using steel storage tanks has been thepetroleum industry.

Nowadays there is an increasing ten-dency to store raw materials, such as crudeoil and liquid petroleum gas below groundsurface as much as possible whereeconomically feasible, either utilising dis-used mines or chambers specifically con-structed for this purpose. This has arisendue to aesthetic requirements or as the re-sult of strategic planning.

Refined petroleum products which arenumerous and are normally handled in

fairly small quantities cannot be so easilystored underground as bulk raw materials,such as crude oil, and it is expected thatthese will continue to be stored in conven-tional steel storage tanks for the foresee-able future.

It is the purpose of this article to reviewgeneral design and construction techniquesof the foundations for steel storage tanks-not only for those in use by the oil Industrybut also for those in use for storing bulkcommodities generally.

Apart from the underwriting and safetyaspects which will be commented uponbriefly, there is little difference in the plan-ning and design of steel storage tanks tohold aviation fuel or to hold calcium chlo-ride, except that one commodity is con-siderably lighter in density than the other.From a geotechnical viewpoint the require-ment for foundation performance is moreor less exactly the same, i.e. to provide asafe, economical support which will pre-clude the development of stresses andstrains within the steel tank plates thatcould either lead to rupture of the platesor interfere with the flow of the particularcommodity into or out of the tank. Pres-surised or refrigerated storage tanks willnot be considered.

Nature of storage tanksCylindrical steel tanks that are used for theretention of bulk materials at normal at-mospheric pressure are of two basic types,either fixed roof or floating roof. Whenempty the tanks are very light structuresand can be moved if necessary by flotationin a shallow depth of water. For exampletwo 100000 ton oil tanks were floated inless than 1m of water after the foundationfor one tank had failed during water testingand the foundation for the second tank hadbeen condemned at Fawley Refinery, Eng-land'.

Because of the thin steel wall and roofsections involved, the design, construction,maintenance and servicing of steel storagetanks give rise to some unique problemsespecially when soft ground conditions

*Senior Civil Engineer, Edward L. Bateman Ltd.,Bokshurg North, Transvaal, South Africa

24 Ground Engineering

have to be overcome during construction.Further problems are generated becauseunlike the majority of civil engineeringstructures, the dead load is relatively smallin proportion to the considerable variationin storage loads that is usually involved.

Storage tanks can vary in diameter froma minimum of about 10m to almost 100mfor the largest diameter floating roof tanksused for crude oil storage.

Fixed roof tanks usually have a conicalroof shape and are normally chosen forstorage of bulk commodities that are eithervolatile and inflammable, such as gasolinewhere the prevention of leakage is of para-mount importance, or where the volume ofstorage required is small. They are oftenchosen in situations where regular mainte-nance and inspection is difficult to carryout, and where ground conditions affectthe satisfactory foundation performance offloating roof tanks.

Floating roof tanks, as the name implies,possess roofs which float on the surface ofthe retained fluid, the roof being supportedon one or more pontoons that are guidedon columns. Of necessity floating rooftanks incorporate a number of seals roundthe periphery to prevent leakage and al-though these seals normally have a long lifefloating roof tanks are not normally consid-ered for retention of volatile inflammablefluids or fluids having a slurry type of com-position, such as calcium chloride.

Steel storage tanks have been used inCanada for the storage of grain; these arehowever, of fixed roof design.

Tank performanceAlthough a steel storage tank is a rela-

tively flexible structure and can tolerategreater settlements, either total or differen-tial, than most engineering structures,there is of course a limit to the settlementthat a tank can be expected to take with-out distress. Some of the effects of settle-ment which it is desirable to avoid in thedesign of a tank foundation are as follows:(a) Differential settlement across the dia-

meter which may affect gauging ac-curacy, jam floating roofs and over-stress internal piping connections.

(b) Differential settlement along the peri-phery which may jam floating roofmechanisms and overstress or warpthe shell plates.

(c) Differential settlement between thetank bottom and the shell plateswhich may overstress the shell weldsand cause loss of drainage facilitiesfor tank cleaning. In addition columnsupported roofs may undergo severewarping as a result of bottom settle-ments.

(d) Differential settlement between tankand external connecting pipeworkwhich may overstress the piping.

(e) Overall settlement of the tank whichmay lead to loss of superelevation ofthe tank pad above external tank com-pound grade or ground surface, even-tually resulting in the ponding of water

around the shell base plates and a con-sequent increase in corrosion of theseplates.

The magnitude of stresses and strainswithin a steel storage tank caused by thedegree of differential settlement experi-enced by the foundation are dependent ona number of factors. These are:

(i) the diameter and height of the tank,(ii) the uniformity of soil conditions under-

lying the tank,(r'ii) the loading intensity, and(iv) the type of tank involved.

Generally fixed cone roof tanks are moretolerant of differential settlement effectsthan floating roof tanks.

Theoretically, when soil conditions areuniform below a tank the settlement thattakes place at the centre will be abouttwice that at the circumference and thisexpected settlement may be allowed forby constructing the tank bottom coned up-wards the requisite amount. Opinions varyas to the maximum amount of tank bottomdeflection that can be tolerated. Howeverduring the flotation of two large diameter100000 ton oil storage tanks'he bottomsare reported to have deflected upwardsabout 1m.

From a practical standpoint tank bot-toms are usually quite irregular as the thinplates warp during welding. Due to the pre-sence of drainage sumps which are usuallylocated close to one edge of the tank, aswell as the provision of internal stiffeningcolumns in the case of fixed roof tanks, theactual settlement performance of a tankmay vary considerably from that predictedtheoretically.

Settlements which do take place resultfrom two separate types of soil behaviour.In most cases the soil consolidates underthe tank loading, to a magnitude and at arate that is dependent on the type of soilunderlying the tank. The soil and groundwater conditions will also control the time,after full loading has been realised, whentank settlement becomes negligible. For in-stance an impermeable clay soil will takeconsiderably longer to consolidate than afree draining sand under the same load. Thedepth of soil involved in this consolidationprocess is theoretically approximatelythree times the tank diameter.

The second type of settlement phenome-non which is often encountered is that aris-ing when the soil is overstressed by thetank loads. In this situation the soil flowsout from under the tank edge often in anunpredictable manner leading to large,non-uniform and rapid settlements takingplace. Such settlements can result in over-stressing of welds or rivetted connectionsand catastrophic failures from this causeare not unknown.

Actual tolerances to settlement are de-pendent to a great extent on the manufac-turer's specification for continued satisfac-tory performance of the tank. A commontolerance during erection is ~ Smm on thefinished tank pad surface around the cir-cumference. When in use, however, con-

siderable settlements can be tolerated de-pending on circumstances, and it is notunknown for tanks to have experiencedtotal settlements of up to 600mm withoutbeing seriously impaired.

As far as differential settlement is con-cerned a common criterion for performanceof a tank shell is 25mm per 30m measuredalong the tank perimeter. Tighter toler-ances than these are however more com-monly specified and for large (i.e. over60m) diameter floating roof tanks the fol-lowing performance requirements are con-sidered more desirable:

(i) 5mm maximum differential settlementin 10m of perimeter length as a com-bined result of pad construction, tankerection, water testing and ultimatelikely soil settlement,

(ii) 25mm maximum differential tiltingacross the tank diameter, and

(iii) 25mm maximum differential settlementfor every 10m of tank bottom mea-sured lineally in any direction.

A table indicating the desirable form ofbottom plate construction dependent onthe predicted settlements around the tankshell and across the bottom is given inTable I. No special requirements are neces-sary for tanks less than 50ft (15m) dia-meter. Where settlements larger than thoseindicated are predicted then some form ofsite improvement or provision of structuralsupport to the tank is necessary.

A special case of tank settlement forfixed roof tanks arises when planar tilttakes place through the points of maximumand minimum settlement. With regard tothe design of bottom plates, only non-planar differential settlement of the shellbecomes of consequence. For floating rooftanks this type of tilting may or may notaffect the performance of tanks dependingon the circumstances and the degree of tilt.

Tank loadingsShell bearing plates are normally used for

fixed roof tanks where shell bearing pres-sures are in excess of about 100 kilopascals

or where floating roof tanks are involved.These reduce the tendency for "punching-in" of the shell or localised edge bearingfailure taking place, and also assist in re-ducing perimeter differential settlements.

Large size cone roof tanks have theirroofs supported on columns and gen-erally a centre column; up to three ringsof interior columns can be involved de-pending on the tank diameter. The deadload on columns normally ranges from 50to 100kN whereas live load can add afurther 100 to 150kN per column. This isespecially important in regions wherelarge accumulations of snow can be ex-pected. Maximum column load can thusrange up to 250kN.

Such columns are usually carried onsquare base plates 20 to 25mm thickwhose dimensions are dependent on per-missible soil loading. The soil pressurebeneath such column loading can be esti-mated either on the net base plate areaprovided or alternatively based on the as-sumption that the effect of the tank bot-tom plates is to enlarge the column baseplates by a certain amount dependent onthe thickness of the tank bottom plates.

Column loading is additive to that im-posed by the tank fluid contents and is op-erative in the tank empty condition.Actual stress distributions within the soilare affected by the tank bottom platethickness and type of construction andwhether these are crowned up or down.This also dictates the pattern of tanksettlement which subsequently develops.

FoundationsGeneral

The cost of a tank foundation can insome circumstances, depending on thesoil conditions, exceed the cost of thetank itself. Because of the practice of loca-ting tanks in areas which are either remoteor undesirable for normal civil engineeringstructures (often indicative of poor subsoilconditions) it is particularly important thatan adequate soil investigation be carried

TABLE I. TANK BOTTOM DESIGN REQUIREMENTS+

Predicted settlementfi Tank diameter

Maximumat sheff

< 2in

(( 50mm)

< 6in

(< 150mm)

< 12in

(( 150mm)

Differentialin bottomf

< ~tin per30ft

(( 12mmper 10m)

( 1in per30ft

(( 25mmper 10m)

< 2in per30ftt

(( 50mmper 10m)

50ft to 150ft(15m to 50m)

Per API Specifica-tions.

Annular plates of 2ft(600mm) minimumwidth. Bottom platestwo-pass weldedwith 70 per centjoint efficiency.

Annular plates of 2ft(600mm) minimumwidth. Bottom platestwo-pass weldedwith 70 per centjoint efficiency.

Over 15lt (50mm JAnnular plates of 2ft(600mm) minimum widthwhen trimmed per APISpecification. Bottomplates two-pass weldedwith 70 per cent efffciency.

Annular plates of 3ft(1 000mm) minimumwidth when trimmed perAPI Specifications. Bottomplates two-pass weldedwith 70 per cent jointefficiency.

Annular plates of 6ft(2 000mm) minimumwidth when trimmed perAPI Specifications. Bottomplates minimum two-passwelded with 80 per centjoint efficiency.

'No special requirements for tanks under 50ft (15m) diameter,I)Predicted settlements are based upon

(aI inclusion of loading intensity from proposed water testing,(b maximum settlement being uniform around circumference,c) differential settlement refers to deviation from anticipated behaviour on uniform soils, andd) planar tilting of the bottom not detrimental to the tank bottom.

tAlong circumference as well as radially.tlf these settlements are exceeded then some farm of site improvement is required prior to tank erection.

26 Ground Engineering

out for all tanks to provide sufficient infor-mation for proper design and subsequentsafe operation of the tank. This is despitethe fact that such tanks can normallytolerate considerably greater settlementscompared with other structures.

Relatively large cost savings and theavoidance of distress from settlement canresult from adequate advance knowledgeof soil conditions. For instance, the authorwas involved in a case where an 85m dia.oil storage tank was relocated after softsoil conditions were encountered below asuperficial covering of dense glacial till ina region of comparative "safe" geology.The soft soil conditions were due to thecollapse and infilling of subsurface chan-nels in a minor but extensive stratum ofgypsum within parent shale bedrock. Cata-strophic failure of the tank during testingor initial loading would undoubtedly havetaken place if the tank had been erected inthe originally intended location.

Tanks are normally constructed on araised pad of free-draining granular mat-erial such as crushed stone to provide fordrainage and to deter tank bottom corro-sion. An asphalt or oil-sand seal is some-times provided to the surface of the padthus formed to prevent infiltration of waterwhich could increase the rate of corrosionof the bottom plates. If a tank pad is notconstructed out of free-draining material,then precautions to limit corrosion can beeffected by providing the tank with a100mm thick layer of oil-sand.

Some foundation solutions for varyingsoil conditions are shown in Fig. 1. Theseare referred to in the following sections.Shallow foundations in competent ground

Where tanks are to be located in com-petent soil conditions which provide ade-quate structural support, then the tankmay be wholly supported by a raised padof compacted soil. Before the pad is con-structed however, all superficially weakmaterials should be removed from the planlimits of the tanks, see Fig. 1a.

Tank pads may be constructed out ofany soil or locally available material thaton compaction will produce a strong reli-able, non-corrosive surface which willsafely support tank construction and whichon the basis of available precedent willstand up to the effects of weather, etc.

In this connection it would, for example,be advisable to use a free-draining, non-frost susceptible, granular fill for tank con-struction in areas subject to deep frostpenetration in order to preclude the pos-sibility of frost heave taking place withconsequent ice lensing, leading to failureof the foundation at time of thaw. Inregions where more temperate climaticconditions prevail an impermeable fillmight otherwise be quite suitable. The useof artificial materials such as blast furnaceslag should be carefully investigated priorto use, since such materials often exhibitundesirable swelling and chemical effectsover the long term. Similarly the potentialswelling characteristics of clays should beinvestigated when considered for use astank pad fill.

Tanks are often supported on ringwallsconstructed either of crushed stone or con-crete (see Fig. 1(b)). The ringwalls trans-fer the tank shell loadings to stronger soilsat shallow depth thus eliminating the pos-sibility of shear edge failure around theperiphery of the tanks.

Where concrete ringwalls are used thesehave the advantage of confining the soilwithin the ringwall thus preventing lateral

movement of the soil under full tank load.With this type of support system, how-ever, it is important that the soil be wellcompacted within the ringwall otherwisehigh shearing stresses can develop in thetank bottom above the point of contact be-tween the retained soil and the concreteringwall.

An alternative to seating the tank shellon a concrete ringwall is to locate it withinthe ringwall directly on the retained soil.In this situation the ringwall has to bedesigned to resist hoop tension that candevelop. This alternative approach has ad-vantages in situations where compactionof fill within the ringwall cannot be carriedout effectively or where natural soil is leftin place.Foundation alternatives for poor groundconditions

Where soil conditions are encounteredbeneath tank locations which are unsuit-able for direct tank support there are three

(o) RAISED PAD OF COMPACTED FILL

ON COMPETEN1 GROUND

WEAK SOILS REMOVED AND

REP(aCED WITH ENGINEERED FILL

LOAD TO BE EQUAL TO

1,5 TO 2,0 TIMES FULL

TANK LOAD

SAND DRAINS MAY

BE PROVIDED TO

ACCELERATE

C0N S0 L I D A 1 I 0 I4

NOTE — SAND DRAINS EXTEND THROUGH SOFT SOILS

TaNK ERECTION

AFTER FULL

CONSOLIDATION

TAKEN PLaCE

j-il.l JJJ Lj Ljl I I Li>"

(e) PRELOADING WITH EARTH FILL

Fig. 1. Various foundations for storage tanks

(I>) REINFORCED CONCRETE OR CRUSHED

STONE RINGWALL ON COMPETENT

GROUND

(BI) PILED FOUNDATION WITH

END-BEARING PILES

TANK ERECTED

ON RAISED

PAD OF FILL

SOIL CONSOLIDaTED

UNDER TaNK

PLUS V ATER

LOADINGW

DEFORMATION OF TANK BOT1OM

CORRECTED AFTER

C OM PL E I ION

OF LOADING

If) PRELOADING BY wa1ER- TESTING

general techniques for providing an ade-quate tank foundation. These are (a) re-moval of unsuitable soils and replacementwith engineered fill, (b) use of piles orother deep foundations to transfer tankloadings to a suitably competent soil orrock stratum at depth, or (c) strengthen-ing the soil by preloading, vibration orcompaction methods to render it suitablefor tank support. These approaches are dis-cussed in the following sections.Fill replacement technique

Where unsuitable soils are present todepths of 2-3m below surface, belowwhich a competent soil or rock is present,then the best solution to adopt, andusually the most economical (Fig. 1(c)),is to excavate and replace with engineeredfill, dewatering the excavation if neces-sary. The replacement fill chosen shouldpreferably be a clean granular soil becauseof its ease in placement, handling charac-teristics, good drainage qualities, etc.

However this does not preclude the useof other materials being considered if moreeasily available.

The final choice of material to be usedwill also depend on likely weather condi-tions to be experienced at the time ofconstruction and when in service. A fill

replacement programme must of necessitytake into account water conditions to bedealt with during excavation and if indica-tions are that expensive well-point de-watering is necessary then it might bemore economical to excavate in the wetby clamshell or dragline and use relativelymore expensive end-dumped crushed rockor stone, without recourse to pumping.

The main drawback to this approach isthat all unsuitable compressible soils maynot be removed from within the tank exca-vation and may even remain in a disturbedand thus more compressible conditionthan originally. The result can be exces-sive non-uniform settlements taking placewhich can be detrimental to tank perfor-mance unless observed at an early stageof loading, such as during water testing,and corrected in an appropriate manner.

The fill replacement technique of con-structing tank pads has been extended tovery large diameter oil storage tanks'nwhich up to 10m of alluvium has beendredged out by suction dredges, and re-placed by well-graded gravel which hasbeen vibro-compacted. It has been claimedthat the technique could possibly be ex-tended to replace poor soils up to 20mdepth.Piled foundations

Although the use of a piled foundationfor storage tanks (Fig. 1(d)) is the mostpositive method of dealing with weak sur-ficial soils strata it is frequently overall themost expensive solution and it is not un-knoiwn for the cost of a piled foundationto exceed the cost of the tank which itmust suppoi't.

Piled foundations for storage tanks arenot without problems and failures havetaken place during water testing'. Be-cause of downdrag, or negative skin fric-tion, which develops in weak superficialsoil strata under the combined effects ofsurcharge, tank and fill loadings, individualpile design loads normally have to bemaintained sufficiently low to allow for theadditional forces that come onto the pilesas the soil consolidates.

These additional forces can represent aconsiderable percentage of the pile carry-ing capacity especially where high sur-charge and fill loadings are involved. Re-cently however, bitumen coatings havebeen applied extensively to piles subjectto downdrag forces with the object of cre-ating a coating to the pile surface whichcan shear without transfer of the down-drag forces to the piles'. Thus pile sec-tions can be relied upon to carry a greaterproportion of their working loads and con-sequently become more economicallyeffective.

For very deep deposits of weak soilswhich cannot be improved by othermethods a piled alternative may be theonly viable foundation solution. The typeof piling chosen will depend to a great ex-tent on the soil strata through which pene-tration will have to be effected. For ex-ample the use of displacement piles drivenat close centres through a stratum of verysoft clay may have an undesirable netoverall effect in which the strength of theclay is reduced considerably by the re-moulding caused by driving.

July, 1976 27

Fig. 2 (above). Vibroflotation of sand in progress to provide adensified foundation for a large steel storage tank

(photo, Cementation (Africa) (Contracts) Pty Ltd., Durban)

Fig. 3 (right). Rig operated by Frankipile Ltd. at Canvey Islandfor installing vertical cardboard wick drains

On the other hand, the use of similartype piles where deep deposits of loosesand are to be penetrated can lead to anoverall increase in soil strength resultingfrom the vibratory effects of pile drivingon the sand causing an increase in com-pacted density. Although general rules can-not be laid down for all specific situationsthat may arise it is stressed that the choiceof a pile type for a given set of conditionsis very important, to ensure that a situationis not created whereby existing soil condi-tions are made worse by the proposedpiling technique. Otherwise a consequentoverall increase in the cost of the founda-tion work ensues over and above thatwhich is unavoidable.

One method of "piling" which is differentfrom conventional piling techniques is theuse of sand piles or rock piers. These canbe installed relatively cheaply and areoften used in soil conditions which arereasonably homogeneous. The techniqueis to create a hole, either by driving aclosed ended tube, or by augering, andthen filling the hole with compacted orvibro-compacted sand or graded refill. Fig.2 shows vibroflotation in'rogress todensify sand for the foundation of a largediameter storage tank.

There are disadvantages in using aclosed ended tube (usually closed with aplug of crushed stone or a disposable tip)in soft clay strata. The remoulding effectson the clay produced by this method ofcreating a hole can lead ultimately to somelateral instability of the sides of the sandpile or rock pier thus formed. The tech-nique of creating a hole by this punchingmethod can however be beneficial whereloose sand strata have to be penetrated.

Augered pile holes are generally impos-sible to put down in sand below groundwater level without the use of casing, but

can be the most suitable method wheredeposits of uniform clay are encountered,provided the clay is sufficiently firm tostand up without support for the depthpenetrated.

In this connection mixed ground condi-tions where clay, silt and sand strata areinterlayered with one another are often themost difficult soil conditions to be dealtwith using this technique. One other fac-tor which needs consideration in the useof a sand-pile or rock-pier solution is thatthe sand or rock cannot be fully compac-ted close to ground surface where com-paction is by dynamic means. In this caseuse should be made of an appropriate soilsurcharge through which the piles are com-pacted, or alternatively the top of the pilesmaintained at a minimum depth of 2m orso below surface to ensure adequate com-paction of the sand at the top of the pile.If this is not carried out either the groundsurface fails around the top of the pile orthe sand or rock is not fully compacted.

There are a number of variations of typesof sand-piles and rock-piers which havebeen used successfully depending on thesoil conditions. One such application onreclaimed land'nvolved seven tanks upto 45m dia. where rock piers up to 5mlong were created by excavation with agrab and rapid filling of the holes withquarried steelworks slag compacted by alarge vibrating poker.Foundation preloading

One solution to the construction of tankfoundations on poor ground which can becheap from a construction standpoint is topreload either with soil or, immediatelyfollowing tank erection, consolidate thesoil during the process of water testing(Fig. 1(e) and (f)). Preloading, however,requires, firstly, sufficient time before thetanks have to be put into service to

ensure that preloading is effective in re-ducing settlemens in the post-constructionperiod. Secondly, adequate prior know-ledge of the soil conditions is essential sothat the preloading programme can beproperly planned and executed effectivelywithin the estimated time period available.

A preloading programme also requiresthe co-operation and willingness of theowner to tolerate some post-constructionsettlements should it not have proved fullyeffective by the time the tanks are to enterservice. Despite the drawbacks of the pre-load technique with respect to the estima-tion of the uncertainties of time involved,the method is comparatively cheap whenconstruction costs are compared withthose of a piled alternative in a situationwhere deep deposits of weak unsuitablesoil are encountered. However the overalleconomics of a situation where a tank iserected and cannot enter service immedi-ately can detract from this approach.

The preload technique permits load tobe applied to weak soil conditions in acontrolled manner, permitting consolida-tion to take place with consequent in-crease in soil density and shear strength.The larger the area covered and the greaterthe magnitude of loading ultimately ap-plied the greater the consolidation and in-crease in shear strength.

When the preload is removed some elas-tic rebound of the soil system will takeplace but essentially a major permanentincrease in the strength of the soil is effec-ted. The technique requires a very care-ful engineering analysis to be made todetermine the amount of preload and thetime required to achieve a desirable in-crease in soil shear strength to restrictpost-construction settlements to withinallowable limits.

Where soils are extremely soft several

July, 1976 29

Where tanks are designed to store pet-roleum products, which have a specificgravity less than unity, water testing willindicate a factor of safety greater than 1.0if the tank is filled to the design storagelevel. Where tanks are designed to storeproducts with a specific gravity in excessof unity then water-testing of the tank willonly check its watertightness and the ade-quacy of the foundation can only be deter-mined properly by testing with the stor-age product involved or bulk material witha greater bulk density.

Irrespective of the purpose of water-testing for the particular circumstance in-volved, it is considered important that acheck be made on the settlement perfor-mance of tanks in the early stages of theirlives. Where the water-testing or con-trolled filling of the tank is intended to pre-load the soil as discussed in the sectionon foundation preloading, a number of geo-technical instruments, such as piezometers,settlement points and slope indicator holeswill be necessary to provide adequatecontrol on the preloading programme.Many of these may be installed under-neath the tank and read remotely; otherswill be situated around the periphery ofthe tank or within a distance which maybe affected by tank loading on the soilconcerned.

1

II

(b) that the foundation provided is ade-quate to carry the tank loadings with-out distressful settlements takingplace.

loading stages may be required in orderto avoid shear failure of the soil, each load-ing stage being maintained for an appro-priate length of time before additional loadis added. During preloading it is importantthat sufficient instrumentation of the soilbe carried out so that a continuous checkon the effectiveness of the preload tech-nique is available at all times. In this wayappropriate adjustments can be made tothe preloading programme so that the de-sired end result is effected as rapidly aspossible with appropriate safety at allstages.

If a surcharge loading is placed toorapidly there could be a failure resultingin loss of fill and remoulding of the soilwhich may not easily be strengthened. Forinstance, a clay which contains a consid-erable number of thin sand and silt part-ings may consolidate fairly rapidly but ondisturbance with consequent remouldingand loss of its "structure" may consoli-date at a considerably slower rate.

This preloading technique was used byPenman and Watson on their Teessidesite'ith considerable economic advan-tages compared with the use of piles androck piers. To facilitate consolidation andto increase the rate at which drainage waseffected from the soil a number of sanddrains were installed around the peripheryof the tanks involved.

A development which has taken placeover the past few years with regard toaccelerating the process of consolidationof soft soils under load is in the use ofpaper, or cardboard drains (the Kjellman-Franki method) as an alternative to sanddrains. These paper drains consist of stripsof high permeability impregnated paperwhich contains longitudinal drainage chan-nels through which pore-water may escapeafter passing through the paper from theconsolidating soil. The drains are installedat predetermined spacings using a specialmandrel (Figs. 3 and 4). This method ofaccelerating the consolidation of soft soilshas been widely used throughout theworld, but especially in Europe, Japan andin North America".

SETTLEMENT

RING -DATE I

SETTLEMENT ON INITIAL LOADING —DATE I

/[/(/[/[g/[gJSETTLEMENT BETWEEN DATES I IL 2

SETTLEMENT BETWEEN DATES 2 B 3Water testing and settlement recordsIt is usual to test a storage tank after

erection to ensure:(a) that it is water-tight, and Fig. 5. Typical settlement rings around tank periphery

30 Ground Engineering

Fig. 4. Cardboard drains of 300mm'- cross-sectional area being installed at close spacingbeneath an oil storage tank

taken place to any extent outside theimmediate environs of the refinery. Re-covery of the previously lost productsmade a useful addition to refinery stocksafter re-treatment.

Is

Object of the instrumentation is to en-able a comprehensive picture of the soilbehaviour to be obtained during the pro-cess of tank loading. Where soil condi-tions are relatively simple as for a raisedpad of compacted granular soil on compe-tent ground as shown in Fig. 1(a), therequired instrumentation can correspond-ingly be simple and may for example onlyconsist of settlement lugs welded on theside of the tank around its periphery, sayat the eighth positions. This is consideredto be the minimum requirement to be pro-vided during initial water-testing and eva-luation of tank performance.

Where only peripheral tank settlementsare to be recorded these are measured dur-ing initial filling and at regular intervalsthereafter. The readings thus taken maybe plotted in a variety of ways, one of themost useful being indicated on Fig. 5 wheresettlements are plotted on a radial basisaround the tank perimeter. Where settle-ment rings are relatively close togetherdifferential settlement is small comparedwith when the rings are relatively far apart.

This method of plotting settlement sur-veys has the advantage that a visual im-pression of peripheral tank settlement isimmediately available. Fig. 5 gives a typi-cal settlement diagram that can arise. It in-dicates that the northern side of the tankis settling considerably more than thesouthern side and that the greater differen-tial settlement between any pair of mea-surement points is to the south-south-eastof the tank centre.

Dyke compound areasThe purpose of a dyke compound, or

bund wall area, is to retain spillage fromtanks and to prevent flooding and pollu-tions of the region in the event of a tankfailure releasing vast quantities of product.The dyking precautions to be adopted in

any instance will depend on the nature ofthe commodity being stored.

For example underwriting requirements,which will be briefly discussed in a subse-quent section, lay down more stringentrules for the storage of liquified petroleumgases than they do for less volatile mat-erials such as crude oil. Nevertheless dykecompound areas should be properly de-signed to ensure that they effectively re-tain any spilled product. To this end peri-pheral dykes and the compound floorshould be leak-proof.

Depending on the foundation solution

adopted it may be necessary to ensurethat the foundations, where these consistof permeable material, are also leak-proof.

At Dalmeny, Scotland'he main founda-tions for some 78m diameter floating rooftanks consisted of compacted shale. Toprevent leakage of crude oil via the founda-tions into underlying shale bedrock a 3mmlayer of glass reinforced plastic wassprayed onto a polythene sheet coveringthe tank pad areas. Fig. 6 gives a generalview of the site during the earthworks andinitial construction phase of the tanks.

Dykes should be provided with an im-permeable core or surface seal where thebulk of soil being used is not impermeable,otherwise reinforced concrete dyke wallsshould be employed. Care should be takenin the choice of materials, where dykes areto be built out of soil, to ensure that theydo not alter their properties with time.For example where dykes are constructedof compacted shales these can breakdown under the influence of weathering tovirtually a clay material and dyke insta-bility can occur where the dykes are high.Also in freezing climates slope instabilitycan be induced by frost action on the sur-face of clay slopes necessitating additionalprotection measures.

Compound floor areas may easily berendered impermeable in the majority ofcases by providing up to 400mm of claysoil fully remoulded and compacted. How-ever, as with dyke walls, protection isnecessary, usually with granular fill to limitshrinkage taking place in dry weather. Thiscould result in leakage in the event of spil-lage, and softening and difficult traffic-ability ensuing in wet weather.

The choice of materials for use in dykeconstruction and the planning of measuresto prevent leakage from the compoundareas, such as around pipes passingthrough compound walls, is often giventoo little thought. Only trouble can ensuewhere dykes are built of random material,often in a non-compacted state, the com-pound floor left untreated, and no provi-sion made to seal off leakage that maytake place through pipe culverts.

The author is aware of one oil refinerywhere over a period of fifty years vastquantities of petroleum products had es-caped from tank compounds and storedthemselves above the ground water table.Fortunately the area was flat-lying and theground water virtually static and so nomovement of the petroleum products had

Fig. 6. Earthworks, and oil storage tank construction at Dalmeny, Scotland

Underwriting considerationsThe underwriting of steel storage tanks

varies considerably depending on (a) thenature of the product, (b) the pollutionand fire risk in the event of tank failure,(c) the country involved and its existinglegislation, and (d) the insurer. Thus nogeneral rules may be laid down on this im-portant aspect relating to storage tankplanning, design and construction. From anunderwriting point of view as far as tankcompounds are concerned an area of riskis any tank storage compound containingone or more tanks which are bounded by acommon dyke system.

The following comments relate to thestorage of hydrocarbons. A prime require-ment from an underwriting aspect is thatonly one class of hydrocarbon shall bestored in any one compound. Suggestedmaximum storage capacity in any one areaof risk might be as follows:Pressure tank storage 150 000 barrelsRefrigerated storage 300 000 barrelsRefined products 600 000 barrelsBoil-over products 900 000 barrelsCrude oil 900 000 barrels

A second requirement that is often madeis with respect to dyke height and limita-tions are usually laid down to the maxi-mum height that may be used. Howeverthere is no reason why, if dykes are prop-erly designed, the maximum height that isacceptable from an underwriting aspectshould not be raised. At the present time alimitation of 3-4m in height is often ineffect. Under special conditions, topo-graphy may be taken into consideration in

spacing requirements of tanks and dykes.With regard to storage capacity pro-

vided in a dyked enclosure, to safeguardagainst the likelihood of spillage, the mini-mum capacity normally laid down is equalto that of the largest tank plus 10 per centof the capacity of all other tanks in thesame enclosure. Spacing and number oftanks that may be incorporated into anyone enclosure is dependent on the productto be stored and the tank capacities.

Detailed requirements of this aspect ofcompound layout are dependent on theunderwriter. Similarly, minimum require-ments for the layout of spill dykes, firehydrants, drainage, piping and fittings aresimilarly specified.

AcknowledgementsThe author wishes to thank his col-

leagues at Edward L. Bateman Ltd., fortheir helpful suggestions and constructivecomments during the preparation of thisarticle.

References1. Legatt, A. J. and Bratchall, G. E.: "Submerged

foundations for 100 000 ton oil tanks". Pro-ceedings of the Institution of Civil EngineersPart 1, May, 1973 and Discussion, November,1973.

2. "Esso's giant oil tanks —a question of morehaste, less speed". Ivew Civil Engineer, 28thFebruary, 1974.

3. "Bitumen slip layers for bearing piles".Ground Engineering, November, 1971.

4. Penman, A. D. M. and Watson, G. H.:"Foundations for torage tanks on reclaimedland at Teesmouth." Proceedings of the Institu-tion of Civil Engineers, May, 1967 and discus-sion, April, 1968.

5. "Canvey settles on cardboard". ContractJournal, February 7th, 1974.

6. "Paper drains go in fast on Quebec project".Heavy Construction News, March 5th, 1973.

7. "Dalmeny tank farm gets dug in". ContractJournal, March 28th, 1974.

July, 1976 31