=ounca:ions anc ear: swor ~s 'or cy inc rica s:ee s:orace:an~s by GRAHAM M. HARRIS+

Introduction have to be overcome during construction. around the shell base plates and a con- FOR MANY YEARS cylindrical steel stor- Further problems are generated because sequent increase in corrosion of these age tanks have been in common use for the unlike the majority of civil plates. storage of many industrial raw materials structures, the dead load is relatively small The magnitude of stresses and strains and products. One major that has in proportion to the considerable variation within a steel storage tank caused by the up to the present been heavily committed in storage loads that is usually involved. degree of differential settlement experi- to using steel storage tanks has been the Storage tanks can vary in diameter from enced by the are dependent on . a minimum of about 10m to almost 100m a number of factors. These are: Nowadays there is an increasing ten- for the largest diameter floating roof tanks (i) the diameter and height of the tank, dency to store raw materials, such as crude used for crude oil storage. (ii) the uniformity of conditions under- oil and liquid petroleum gas below ground Fixed roof tanks usually have a conical lying the tank, surface as much as possible where roof shape and are normally chosen for (r'ii) the loading intensity, and economically feasible, either utilising dis- storage of bulk commodities that are either (iv) the type of tank involved. used mines or chambers specifically con- volatile and inflammable, such as gasoline Generally fixed cone roof tanks are more structed for this purpose. This has arisen where the prevention of leakage is of para- tolerant of differential settlement effects due to aesthetic requirements or as the re- mount importance, or where the volume of than floating roof tanks. sult of strategic planning. storage required is small. They are often Theoretically, when soil conditions are Refined petroleum products which are chosen in situations where regular mainte- uniform below a tank the settlement that numerous and are normally handled in nance and inspection is difficult to carry takes place at the centre will be about fairly small quantities cannot be so easily out, and where ground conditions affect twice that at the circumference and this stored underground as bulk raw materials, the satisfactory foundation performance of expected settlement may be allowed for such as crude oil, and it is expected that floating roof tanks. by constructing the tank bottom coned up- these will continue to be stored in conven- Floating roof tanks, as the name implies, wards the requisite amount. Opinions vary tional steel storage tanks for the foresee- possess roofs which float on the surface of as to the maximum amount of tank bottom able future. the retained fluid, the roof being supported deflection that can be tolerated. However It is the purpose of this article to review on one or more pontoons that are guided during the flotation of two large diameter general design and construction techniques on columns. Of necessity floating roof 100000 ton oil storage tanks'he bottoms of the foundations for steel storage tanks- tanks incorporate a number of seals round are reported to have deflected upwards not only for those in use by the oil Industry the periphery to prevent leakage and al- about 1m. but also for those in use for storing bulk though these seals normally have a long life From a practical standpoint tank bot- commodities generally. floating roof tanks are not normally consid- toms are usually quite irregular as the thin Apart from the underwriting and safety ered for retention of volatile inflammable plates warp during welding. Due to the pre- aspects which will be commented upon fluids or fluids having a slurry type of com- sence of sumps which are usually briefly, there is little difference in the plan- position, such as calcium chloride. located close to one edge of the tank, as ning and design of steel storage tanks to Steel storage tanks have been used in as the provision of internal stiffening hold aviation fuel or to hold calcium chlo- Canada for the storage of grain; these are columns in the case of fixed roof tanks, the ride, except that one commodity is con- however, of fixed roof design. actual settlement performance of a tank siderably lighter in density than the other. may vary considerably from that predicted From a geotechnical viewpoint the require- Tank performance theoretically. ment for foundation performance is more Although a steel storage tank is a rela- Settlements which do take place result or less exactly the same, i.e. to provide a tively flexible structure and can tolerate from two separate types of soil behaviour. safe, economical support which will pre- greater settlements, either total or differen- In most cases the soil consolidates under clude the development of stresses and tial, than most engineering structures, the tank loading, to a magnitude and at a strains within the steel tank plates that there is of course a limit to the settlement rate that is dependent on the type of soil could either lead to rupture of the plates that a tank can be expected to take with- underlying the tank. The soil and ground or interfere with the flow of the particular out distress. Some of the effects of settle- water conditions will also control the time, commodity into or out of the tank. Pres- ment which it is desirable to avoid in the after full loading has been realised, when surised or refrigerated storage tanks will design of a tank foundation are as follows: tank settlement becomes negligible. For in- not be considered. (a) Differential settlement across the dia- stance an impermeable soil will take meter which may affect gauging ac- considerably longer to consolidate than a Nature of storage tanks curacy, jam floating roofs and over- free draining under the same load. The Cylindrical steel tanks that are used for the stress internal piping connections. depth of soil involved in this consolidation retention of bulk materials at normal at- (b) Differential settlement along the peri- process is theoretically approximately mospheric pressure are of two basic types, phery which may jam floating roof three times the tank diameter. either fixed roof or floating roof. When mechanisms and overstress or warp The second type of settlement phenome- empty the tanks are very light structures the shell plates. non which is often encountered is that aris- and can be moved if necessary by flotation (c) Differential settlement between the ing when the soil is overstressed by the in a shallow depth of water. For example tank bottom and the shell plates tank loads. In this situation the soil flows two 100000 ton oil tanks were floated in which may overstress the shell welds out from under the tank edge often in an less than 1m of water after the foundation and cause loss of drainage facilities unpredictable manner leading to large, for one tank had failed during water testing for tank cleaning. In addition column non-uniform and rapid settlements taking and the foundation for the second tank had supported roofs may undergo severe place. Such settlements can result in over- been condemned at Fawley Refinery, Eng- warping as a result of bottom settle- stressing of welds or rivetted connections land'. ments. and catastrophic failures from this cause Because of the thin steel wall and roof (d) Differential settlement between tank are not unknown. sections involved, the design, construction, and external connecting pipework Actual tolerances to settlement are de- maintenance and servicing of steel storage which may overstress the piping. pendent to a great extent on the manufac- tanks give rise to some unique problems (e) Overall settlement of the tank which turer's specification for continued satisfac- especially when soft ground conditions may lead to loss of superelevation of tory performance of the tank. A common the tank pad above external tank com- tolerance during erection is ~ Smm on the pound or ground surface, even- finished tank *Senior Civil , Edward L. Bateman Ltd., pad surface around the cir- Bokshurg North, Transvaal, South Africa tually resulting in the ponding of water cumference. When in use, however, con- 24 Ground Engineering siderable settlements can be tolerated de- or where floating roof tanks are involved. out for all tanks to provide sufficient infor- pending on circumstances, and it is not These reduce the tendency for "punching- mation for proper design and subsequent unknown for tanks to have experienced in" of the shell or localised edge bearing safe operation of the tank. This is despite total settlements of up to 600mm without failure taking place, and also assist in re- the fact that such tanks can normally being seriously impaired. ducing perimeter differential settlements. tolerate considerably greater settlements As far as differential settlement is con- Large size cone roof tanks have their compared with other structures. cerned a common criterion for performance roofs supported on columns and gen- Relatively large cost savings and the of a tank shell is 25mm per 30m measured erally a centre column; up to three rings avoidance of distress from settlement can along the tank perimeter. Tighter toler- of interior columns can be involved de- result from adequate advance knowledge ances than these are however more com- pending on the tank diameter. The dead of soil conditions. For instance, the author monly specified and for large (i.e. over load on columns normally ranges from 50 was involved in a case where an 85m dia. 60m) diameter floating roof tanks the fol- to 100kN whereas live load can add a oil storage tank was relocated after soft lowing performance requirements are con- further 100 to 150kN per column. This is soil conditions were encountered below a sidered more desirable: especially important in regions where superficial covering of dense glacial till in (i) 5mm maximum differential settlement large accumulations of snow can be ex- a region of comparative "safe" . in 10m of perimeter length as a com- pected. Maximum column load can thus The soft soil conditions were due to the bined result of pad construction, tank range up to 250kN. collapse and infilling of subsurface chan- erection, water testing and ultimate Such columns are usually carried on nels in a minor but extensive stratum of likely soil settlement, square base plates 20 to 25mm thick gypsum within parent shale . Cata- (ii) 25mm maximum differential tilting whose dimensions are dependent on per- strophic failure of the tank during testing across the tank diameter, and missible soil loading. The soil pressure or initial loading would undoubtedly have (iii) 25mm maximum differential settlement beneath such column loading can be esti- taken place if the tank had been erected in for every 10m of tank bottom mea- mated either on the net base plate area the originally intended location. sured lineally in any direction. provided or alternatively based on the as- Tanks are normally constructed on a A table indicating the desirable form of sumption that the effect of the tank bot- raised pad of free-draining granular mat- bottom plate construction dependent on tom plates is to enlarge the column base erial such as crushed stone to provide for the predicted settlements around the tank plates by a certain amount dependent on drainage and to deter tank bottom corro- shell and across the bottom is given in the thickness of the tank bottom plates. sion. An asphalt or oil-sand seal is some- Table I. No special requirements are neces- Column loading is additive to that im- times provided to the surface of the pad sary for tanks less than 50ft (15m) dia- posed by the tank fluid contents and is op- thus formed to prevent of water meter. Where settlements larger than those erative in the tank empty condition. which could increase the rate of corrosion indicated are predicted then some form of Actual stress distributions within the soil of the bottom plates. If a tank pad is not site improvement or provision of structural are affected by the tank bottom plate constructed out of free-draining material, support to the tank is necessary. thickness and type of construction and then precautions to limit corrosion can be A special case of tank settlement for whether these are crowned up or down. effected by providing the tank with a fixed roof tanks arises when planar tilt This also dictates the pattern of tank 100mm thick layer of oil-sand. takes place through the points of maximum settlement which subsequently develops. Some foundation solutions for varying and minimum settlement. With regard to soil conditions are shown in Fig. 1. These the design of bottom plates, only non- Foundations are referred to in the following sections. planar differential settlement of the shell General Shallow foundations in competent ground becomes of consequence. For floating roof of a tank in The cost foundation can Where tanks are to be located in com- tanks this of type tilting may or may not some circumstances, depending on the petent soil conditions which provide ade- affect the performance of tanks depending soil conditions, exceed the cost of the quate structural support, then the tank on the circumstances and the degree of tilt. tank itself. Because of the practice of loca- may be wholly supported by a raised pad tanks in which are ting areas either remote of compacted soil. Before the pad is con- or undesirable Tank loadings for normal civil engineering structed however, all superficially weak Shell bearing plates are normally used for structures (often indicative of poor materials should be removed from the plan fixed roof tanks where shell pres- bearing conditions) it is particularly important that limits of the tanks, see Fig. 1a. sures are in excess of about 100 kilopascals an adequate soil investigation be carried Tank pads may be constructed out of any soil or locally available material that TABLE I. TANK BOTTOM DESIGN REQUIREMENTS+ on compaction will produce a strong reli- able, non-corrosive surface which will Predicted settlementfi Tank diameter safely support tank construction and which on the basis of available precedent will Maximum Differential 50ft to 150ft stand up to the effects of weather, etc. at sheff in bottomf (15m to 50m) Over 15lt (50mm J In this connection it would, for example, < 2in < ~tin per Per API Specifica- Annular plates of 2ft be advisable to use a free-draining, non- 30ft tions. (600mm) minimum width frost susceptible, granular fill for tank con- when trimmed per API struction in areas subject to deep frost (( 50mm) (( 12mm Specification. Bottom penetration in order to preclude the pos- per 10m) plates two-pass welded sibility of frost heave taking place with with 70 per cent efffciency. consequent ice lensing, leading to failure of the foundation at time of thaw. In < 6in ( 1in per Annular plates of 2ft Annular plates of 3ft regions where more temperate climatic 30ft (600mm) minimum (1 000mm) minimum conditions prevail an impermeable fill width. Bottom plates width when trimmed per might otherwise be quite suitable. The use (< 150mm) (( 25mm two-pass welded API Specifications. Bottom of artificial materials such as blast furnace per 10m) with 70 per cent plates two-pass welded slag should be carefully investigated prior joint efficiency. with 70 per cent joint to use, since such materials often exhibit efficiency. undesirable swelling and chemical effects over the long term. Similarly 12in 2in per Annular plates of 2ft Annular plates of 6ft the potential < < swelling characteristics of should 30ftt (600mm) minimum 000mm) minimum clays be (2 investigated when considered width. Bottom plates width when trimmed for use as per tank fill. (( 150mm) (( 50mm two-pass welded API Specifications. Bottom pad per 10m) with 70 per cent plates minimum two-pass Tanks are often supported on ringwalls joint efficiency. welded with 80 per cent constructed either of crushed stone or con- joint efficiency. crete (see Fig. 1(b)). The ringwalls trans- fer the tank shell loadings to stronger 'No special requirements for tanks under 50ft (15m) diameter, at shallow depth thus eliminating the pos- I)Predicted settlements are based upon sibility of shear edge failure around the (aI inclusion of loading intensity from proposed water testing, (b maximum settlement being uniform around circumference, periphery of the tanks. c) differential settlement refers to deviation from anticipated behaviour on uniform soils, and Where concrete ringwalls are used these d) planar tilting of the bottom not detrimental to the tank bottom. tAlong circumference as well as radially. have the advantage of confining the soil tlf these settlements are exceeded then some farm of site improvement is required prior to tank erection. within the ringwall thus preventing lateral 26 Ground Engineering movement of the soil under full tank load. general techniques for providing an ade- However this does not preclude the use With this type of support system, how- quate tank foundation. These are (a) re- of other materials being considered if more ever, it is important that the soil be well moval of unsuitable soils and replacement easily available. compacted within the ringwall otherwise with engineered fill, (b) use of piles or The final choice of material to be used high shearing stresses can develop in the other deep foundations to transfer tank will also depend on likely weather condi- tank bottom above the point of contact be- loadings to a suitably competent soil or tions to be experienced at the time of tween the retained soil and the concrete rock stratum at depth, or (c) strengthen- construction and when in service. A fill ringwall. ing the soil by preloading, vibration or replacement programme must of necessity An alternative to seating the tank shell compaction methods to render it suitable take into account water conditions to be on a concrete ringwall is to locate it within for tank support. These approaches are dis- dealt with during excavation and if indica- the ringwall directly on the retained soil. cussed in the following sections. tions are that expensive well-point de- In this situation the ringwall has to be Fill replacement technique watering is necessary then it might be designed to resist hoop tension that can Where unsuitable soils are present to more economical to excavate in the wet develop. This alternative approach has ad- depths of 2-3m below surface, below by clamshell or dragline and use relatively vantages in situations where compaction which a competent soil or rock is present, more expensive end-dumped crushed rock of fill within the ringwall cannot be carried then the best solution to adopt, and or stone, without recourse to pumping. out effectively or where natural soil is left usually the most economical (Fig. 1(c)), The main drawback to this approach is in place. is to excavate and replace with engineered that all unsuitable compressible soils may Foundation alternatives for poor ground fill, dewatering the excavation if neces- not be removed from within the tank exca- conditions sary. The replacement fill chosen should vation and may even remain in a disturbed Where soil conditions are encountered preferably be a clean granular soil because and thus more compressible condition beneath tank locations which are unsuit- of its ease in placement, handling charac- than originally. The result can be exces- able for direct tank support there are three teristics, good drainage qualities, etc. sive non-uniform settlements taking place which can be detrimental to tank perfor- mance unless observed at an early stage of loading, such as during water testing, and corrected in an appropriate manner. The fill replacement technique of con- structing tank pads has been extended to very large diameter oil storage tanks'n which up to 10m of alluvium has been dredged out by suction dredges, and re- (o) RAISED PAD OF COMPACTED FILL (I>) REINFORCED CONCRETE OR CRUSHED placed by well-graded which has ON COMPETEN1 GROUND STONE RINGWALL ON COMPETENT been vibro-compacted. It has been claimed GROUND that the technique could possibly be ex- tended to replace poor soils up to 20m depth. Piled foundations Although the use of a piled foundation for storage tanks (Fig. 1(d)) is the most positive method of dealing with weak sur- ficial soils strata it is frequently overall the most expensive solution and it is not un- knoiwn for the cost of a piled foundation to exceed the cost of the tank which it must suppoi't. WEAK SOILS REMOVED AND (BI) PILED FOUNDATION WITH REP(aCED WITH ENGINEERED FILL END-BEARING PILES Piled foundations for storage tanks are not without problems and failures have LOAD TO BE EQUAL TO TANK ERECTED taken place during water testing'. Be- 1,5 TO 2,0 TIMES FULL ON RAISED of downdrag, or skin fric- TANK LOAD cause negative PAD OF FILL tion, which develops in weak superficial soil strata under the combined effects of surcharge, tank and fill loadings, individual pile design loads normally have to be maintained sufficiently low to allow for the additional forces that come onto the piles as the soil consolidates. These additional forces can represent a considerable percentage of the pile carry-

SAND DRAINS MAY SOIL CONSOLIDaTED ing capacity especially where high sur- BE PROVIDED TO UNDER TaNK charge and fill loadings are involved. Re- ACCELERATE PLUS V ATER cently however, bitumen have W coatings LOADING C 0 N S0 L I D A 1 I 0 I4 been applied extensively to piles subject to downdrag forces with the object of cre- ating a coating to the pile surface which can shear without transfer of the down- drag forces to the piles'. Thus pile sec- tions can be relied upon to carry a greater proportion of their working loads and con- sequently become more economically — THROUGH SOFT SOILS NOTE SAND DRAINS EXTEND DEFORMATION OF TANK BOT1OM effective. AFTER TaNK ERECTION CORRECTED For very deep deposits of weak soils OM PL E I ION AFTER FULL C which cannot be improved by other LOADING CONSOLIDATION OF methods a piled alternative may be the TAKEN PLaCE only viable foundation solution. The type of piling chosen will depend to a great ex- tent on the soil strata through which pene- j-il.l Li>" tration will have to be effected. For ex- JJJ Lj Ljl I I ample the use of displacement piles driven at close centres through a stratum of very soft clay may have an undesirable net overall effect in which the strength of the (e) PRELOADING WITH EARTH FILL If) PRELOADING BY wa1ER- TESTING clay is reduced considerably by the re- Fig. 1. Various foundations for storage tanks moulding caused by driving. July, 1976 27 Fig. 2 (above). Vibroflotation of sand in progress to provide a densified foundation for a large steel storage tank (photo, Cementation (Africa) (Contracts) Pty Ltd., Durban) Fig. 3 (right). Rig operated by Frankipile Ltd. at Canvey Island for installing vertical cardboard wick drains

On the other hand, the use of similar can be the most suitable method where ensure that preloading is effective in re- type piles where deep deposits of loose deposits of uniform clay are encountered, ducing settlemens in the post-construction sand are to be penetrated can lead to an provided the clay is sufficiently firm to period. Secondly, adequate prior know- overall increase in soil strength resulting stand up without support for the depth ledge of the soil conditions is essential so from the vibratory effects of pile driving penetrated. that the preloading programme can be on the sand causing an increase in com- In this connection mixed ground condi- properly planned and executed effectively pacted density. Although general rules can- tions where clay, and sand strata are within the estimated time period available. not be laid down for all specific situations interlayered with one another are often the A preloading programme also requires that may arise it is stressed that the choice most difficult soil conditions to be dealt the co-operation and willingness of the of a pile type for a given set of conditions with using this technique. One other fac- owner to tolerate some post-construction is very important, to ensure that a situation tor which needs consideration in the use settlements should it not have proved fully is not created whereby existing soil condi- of a sand-pile or rock-pier solution is that effective by the time the tanks are to enter tions are made worse by the proposed the sand or rock cannot be fully compac- service. Despite the drawbacks of the pre- piling technique. Otherwise a consequent ted close to ground surface where com- load technique with respect to the estima- overall increase in the cost of the founda- paction is by dynamic means. In this case tion of the uncertainties of time involved, tion work ensues over and above that use should be made of an appropriate soil the method is comparatively cheap when which is unavoidable. surcharge through which the piles are com- construction costs are compared with One method of "piling" which is different pacted, or alternatively the top of the piles those of a piled alternative in a situation from conventional piling techniques is the maintained at a minimum depth of 2m or where deep deposits of weak unsuitable use of sand piles or rock piers. These can so below surface to ensure adequate com- soil are encountered. However the overall be installed relatively cheaply and are paction of the sand at the top of the pile. economics of a situation where a tank is often used in soil conditions which are If this is not carried out either the ground erected and cannot enter service immedi- reasonably homogeneous. The technique surface fails around the top of the pile or ately can detract from this approach. is to create a hole, either by driving a the sand or rock is not fully compacted. The preload technique permits load to closed ended tube, or by augering, and There are a number of variations of types be applied to weak soil conditions in a then filling the hole with compacted or of sand-piles and rock-piers which have controlled manner, permitting consolida- vibro-compacted sand or graded refill. Fig. been used successfully depending on the tion to take place with consequent in- 2 shows vibroflotation in'rogress to soil conditions. One such application on crease in soil density and . densify sand for the foundation of a large reclaimed land'nvolved seven tanks up The larger the area covered and the greater diameter storage tank. to 45m dia. where rock piers up to 5m the magnitude of loading ultimately ap- There are disadvantages in using a long were created by excavation with a plied the greater the consolidation and in- closed ended tube (usually closed with a grab and rapid filling of the holes with crease in shear strength. plug of crushed stone or a disposable tip) quarried steelworks slag compacted by a When the preload is removed some elas- in soft clay strata. The remoulding effects large vibrating poker. tic rebound of the soil system will take on the clay produced by this method of Foundation preloading place but essentially a major permanent creating a hole can lead ultimately to some One solution to the construction of tank increase in the strength of the soil is effec- lateral instability of the sides of the sand foundations on poor ground which can be ted. The technique requires a very care- pile or rock pier thus formed. The tech- cheap from a construction standpoint is to ful engineering analysis to be made to nique of creating a hole by this punching preload either with soil or, immediately determine the amount of preload and the method can however be beneficial where following tank erection, consolidate the time required to achieve a desirable in- loose sand strata have to be penetrated. soil during the process of water testing crease in soil shear strength to restrict Augered pile holes are generally impos- (Fig. 1(e) and (f)). Preloading, however, post-construction settlements to within sible to put down in sand below ground requires, firstly, sufficient time before the allowable limits. water level without the use of casing, but tanks have to be put into service to Where soils are extremely soft several July, 1976 29 1 Where tanks are designed to store pet- roleum products, which have a specific gravity less than unity, water testing will indicate a factor of safety greater than 1.0 II if the tank is filled to the design storage level. Where tanks are designed to store products with a specific gravity in excess of unity then water-testing of the tank will only 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 with a greater . Irrespective of the purpose of water- testing for the particular circumstance in- volved, it is considered important that a check be made on the settlement perfor- mance of tanks in the early stages of their lives. Where the water-testing or con- trolled filling of the tank is intended to pre- load the soil as discussed in the section on foundation preloading, a number of geo- technical instruments, such as , settlement points and slope indicator holes will be necessary to provide adequate Fig. 4. Cardboard drains of 300mm'- cross-sectional area being installed at close spacing control on the preloading programme. beneath an oil storage tank Many of these may be installed under- neath the tank and read remotely; others loading stages may be required in order (b) that the foundation provided is ade- will be situated around the periphery of to avoid shear failure of the soil, each load- quate to carry the tank loadings with- the tank or within a distance which may ing stage being maintained for an appro- out distressful settlements taking be affected by tank loading on the soil priate length of time before additional load place. concerned. is added. During preloading it is important that sufficient instrumentation of the soil SETTLEMENT be carried out so that a continuous check RING -DATE I on the effectiveness of the preload tech- nique is available at all times. In this way appropriate adjustments can be made to the preloading programme so that the de- sired end result is effected as rapidly as possible with appropriate safety at all stages. If a surcharge loading is placed too rapidly there could be a failure resulting in loss of fill and remoulding of the soil which may not easily be strengthened. For instance, a clay which contains a consid- erable number of thin sand and silt part- ings may consolidate fairly rapidly but on disturbance with consequent remoulding and loss of its "structure" may consoli- date at a considerably slower rate. This preloading technique was used by Penman and Watson on their Teesside site'ith considerable economic advan- tages compared with the use of piles and rock piers. To facilitate consolidation and to increase the rate at which drainage was effected from the soil a number of sand drains were installed around the periphery of the tanks involved. A development which has taken place over the past few years with regard to accelerating the process of consolidation of soft soils under load is in the use of paper, or cardboard drains (the Kjellman- Franki method) as an alternative to sand drains. These paper drains consist of strips of high permeability impregnated paper which contains longitudinal drainage chan- nels through which pore-water may escape after passing through the paper from the consolidating soil. The drains are installed at predetermined spacings using a special mandrel (Figs. 3 and 4). This method of accelerating the consolidation of soft soils — has been widely used throughout the SETTLEMENT ON INITIAL LOADING DATE I world, but especially in Europe, Japan and in North America". SETTLEMENT BETWEEN DATES I IL 2 /[/(/[/[g/[gJ

Water testing and settlement records SETTLEMENT BETWEEN DATES 2 B 3 It 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 taken place to any extent outside the immediate environs of the refinery. Re- covery of the previously lost products made a useful addition to refinery stocks after re-treatment.

Underwriting considerations The underwriting of steel storage tanks varies considerably depending on (a) the nature of the product, (b) the pollution and fire risk in the event of tank failure, (c) the country involved and its existing legislation, and (d) the insurer. Thus no general rules may be laid down on this im- portant aspect relating to storage tank planning, design and construction. From an underwriting point of view as far as tank compounds are concerned an area of risk is any tank storage compound containing one or more tanks which are bounded by a Is common dyke system. The following comments relate to the storage of hydrocarbons. A prime require- from an underwriting is that Fig. 6. Earthworks, and oil storage tank construction at Dalmeny, Scotland ment aspect only one class of hydrocarbon shall be stored in any one compound. Suggested Object of the instrumentation is to en- adopted it may be necessary to ensure maximum storage capacity in any one area able a comprehensive picture of the soil that the foundations, where these consist of risk might be as follows: behaviour to be obtained during the pro- of permeable material, are also leak-proof. Pressure tank storage 150 000 barrels cess of tank loading. Where soil condi- At Dalmeny, Scotland'he main founda- Refrigerated storage 300 000 barrels tions are relatively simple as for a raised tions for some 78m diameter floating roof Refined products 600 000 barrels pad of compacted granular soil on compe- tanks consisted of compacted shale. To Boil-over products 900 000 barrels tent ground as shown in Fig. 1(a), the prevent leakage of crude oil via the founda- Crude oil 900 000 barrels required instrumentation can correspond- tions into underlying shale bedrock a 3mm A second requirement that is often made ingly be simple and may for example only layer of glass reinforced plastic was is with respect to dyke height and limita- consist of settlement lugs welded on the sprayed onto a polythene sheet covering tions are usually laid down to the maxi- side of the tank around its periphery, say the tank pad areas. Fig. 6 gives a general mum height that may be used. However at the eighth positions. This is considered view of the site during the earthworks and there is no reason why, if dykes are prop- to be the minimum requirement to be pro- initial construction phase of the tanks. erly designed, the maximum height that is vided during initial water-testing and eva- Dykes should be provided with an im- acceptable from an underwriting aspect luation of tank performance. permeable core or surface seal where the should not be raised. At the present time a Where only peripheral tank settlements bulk of soil being used is not impermeable, limitation of 3-4m in height is often in are to be recorded these are measured dur- otherwise reinforced concrete dyke walls effect. Under special conditions, topo- ing initial filling and at regular intervals should be employed. Care should be taken graphy may be taken into consideration in thereafter. The readings thus taken may in the choice of materials, where dykes are spacing requirements of tanks and dykes. be plotted in a variety of ways, one of the to be built out of soil, to ensure that they With regard to storage capacity pro- most useful being indicated on Fig. 5 where do not alter their properties with time. vided in a dyked enclosure, to safeguard settlements are plotted on a radial basis For example where dykes are constructed against the likelihood of spillage, the mini- around the tank perimeter. Where settle- of compacted shales these can break mum capacity normally laid down is equal ment rings are relatively close together down under the influence of weathering to to that of the largest tank plus 10 per cent differential settlement is small compared virtually a clay material and dyke insta- of the capacity of all other tanks in the with when the rings are relatively far apart. bility can occur where the dykes are high. same enclosure. Spacing and number of This method of plotting settlement sur- Also in freezing climates slope instability tanks that may be incorporated into any veys has the advantage that a visual im- can be induced by frost action on the sur- one enclosure is dependent on the product pression of peripheral tank settlement is face of clay slopes necessitating additional to be stored and the tank capacities. immediately available. Fig. 5 gives a typi- protection measures. Detailed requirements of this aspect of cal settlement diagram that can arise. It in- Compound floor areas may easily be compound layout are dependent on the dicates that the northern side of the tank rendered impermeable in the majority of underwriter. Similarly, minimum require- is settling considerably more than the cases by providing up to 400mm of clay ments for the layout of spill dykes, fire southern side and that the greater differen- soil fully remoulded and compacted. How- hydrants, drainage, piping and fittings are tial settlement between any pair of mea- ever, as with dyke walls, protection is similarly specified. surement points is to the south-south-east necessary, usually with granular fill to limit of the tank centre. shrinkage taking place in dry weather. This Acknowledgements could result in leakage in the event of spil- The author wishes to thank his col- Dyke compound areas lage, and softening and difficult traffic- leagues at Edward L. Bateman Ltd., for The purpose of a dyke compound, or ability ensuing in wet weather. their helpful suggestions and constructive bund wall area, is to retain spillage from The choice of materials for use in dyke comments during the preparation of this tanks and to prevent flooding and pollu- construction and the planning of measures article. tions of the region in the event of a tank to prevent leakage from the compound failure releasing vast quantities of product. areas, such as around pipes passing References 1. Legatt, A. J. and Bratchall, G. E.: "Submerged The dyking precautions to be adopted in through compound walls, is often given foundations for 100 000 ton oil tanks". Pro- any instance will depend on the nature of too little thought. Only trouble can ensue ceedings of the Institution of Civil Part 1, May, 1973 and Discussion, November, the commodity being stored. where dykes are built of random material, 1973. For example underwriting requirements, often in a non-compacted state, the com- 2. "Esso's giant oil tanks —a question of more which will be briefly discussed in a subse- pound floor left untreated, and no provi- haste, less speed". Ivew Civil Engineer, 28th more made off that February, 1974. quent section, lay down stringent sion to seal leakage may 3. "Bitumen slip layers for bearing piles". rules for the storage of liquified petroleum take place through pipe culverts. Ground Engineering, November, 1971. gases than they do for less volatile mat- The author is aware of one oil refinery 4. Penman, A. D. M. and Watson, G. H.: "Foundations for torage tanks on reclaimed erials such as crude oil. Nevertheless dyke where over a period of fifty years vast land at Teesmouth." Proceedings of the Institu- compound areas should be properly de- quantities of petroleum products had es- tion of Civil Engineers, May, 1967 and discus- re- from tank compounds and stored sion, April, 1968. signed to ensure that they effectively caped 5. "Canvey settles on cardboard". Contract tain any spilled product. To this end peri- themselves above the ground water table. Journal, February 7th, 1974. pheral dykes and the compound floor Fortunately the area was flat-lying and the 6. "Paper drains go in fast on Quebec project". should leak-proof. ground water virtually static and so no Heavy Construction News, March 5th, 1973. be 7. "Dalmeny tank farm gets dug in". Contract Depending on the foundation solution movement of the petroleum products had Journal, March 28th, 1974. July, 1976 31