P 0288- Foundations Performance of Large Diameter Tanks

8/12/2019 P 0288- Foundations Performance of Large Diameter Tanks

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265Proceedings: Fourth International Conference on Case Histories in Geotechnical Engineering, St. Louis, MissourMarch 9--12 1998.

FOUNDATION PERFORMANCE OF LARGE DIAMETER TANKSL. M. Clementethersburg, Maryland - USA - 20878

A. Desai, H. Senapathy L. W. Young, Jr.Bechtel CorporationGaithersburg, Maryland - USA - 20878

Paper No

paper presents a detailed case history of foundation performance of six 60-m diameter, 15-m high, floating roof fuel oil tanby co

footings. General subsurface conditions at the site are discussed, along with proposed site grading and the rationale foselection. Because vibro-replacement improvement of site soils had been used beneath settlement-sensitive struto support the tanks on unimproved soils. To allay doubts about the adequacy o

of the tanks.

roof

six floating roof fuel oil tanksin con

of similar size and layout. Vibroof soils had been used for support of

a1so needed fornew plant structures. Thus, vibro-replacementof the soils beneath these 12 new tanks was

to be required as well. However, careful charof subsurface conditions beneath the tanks and

ground improvement. Available experience with tankher supported this conclusion. A compromise

The tanks were then erected and hydrod excellent settlement performance was observed. Itof subsurface

conditions, extensive analysis, and the more settletolerant nature of these tanks.The following sections provide summary descriptionstanks, site, and subsurface conditions; the tank founselection strategy; development of the hydrotesting/settmonitoring program; and the results of tank hydrotesting

TANKS AND TANK FARM LAYOUTThe six floating roof fuel oil tanks are 60 rn in diameter m high and are located immediately south of the epower plant. The six fixed roof process water tanks are in diameter and 20 m high and are located east of thpower plant, several hundred meters north of the fuel oiand immediately north of the existing power plant. Thouts of these two tank farms are shown on Fig. l whicincludes information to be referenced in subsequent seIt should be noted that Fig. I shows the layout of the tw

farms together to save space in this paper. The tank faractually several hundred meters apart, as indicated above


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T1'11_. _ _ BH 22

(a) Process Water Tank Farm

NOTES:. ... T : w f . . n l ~ShoNI ~ il/1 DIII'II'N'II from

L.ccallorw al t t . ......l.ll:.,.,,....,. ... l lgor. A l l dfDr SUl.urfKI l rOfllll.LEGEND:eSptT ORING T ST PIT PLATE LOAO TEST

- r - - -

P

IIIII

l=i l J (b) Fuel Oil Tank Farm

Fig Process water and fuel oil tank farms

2SITE AND SUBSURFACE CONDITIONSThe site is located on a coastline where the topography iserally flat and virtually no vegetation is present. The exgrade is about El. -2m with respect to plant datum (-2mat both the process water and fuel oil tank farm areas. A4.4 m of structural fill was placed in the process waterfarm area to reach final grade at El. +2.4 m PD. Mgrading was required in the fuel oil tank farm area to final grade at El -1.8 m PD.Subsurface conditions disclosed by SPT borings drilled fuel oil tank farm area are illustrated by the typical subsuprofile shown on Fig. 2. Also included on Fig. 2 are tyfuel oil tanks and final grade information. The stratigincludes a 2-m thick upper layer of generally loose to medense, fine, silty sand underlain by about 2 m of generallto medium stiff silts/clays. Another 4 m of silty sandencountered beneath the clay layer on the western portithe fuel oil tank farm. Intermittent ledges of coralline stone are encountered in a generally dense sand matrix bethe silty sand 8 m depth) and silts/clays (4 m depth.) sand layer with coralline limestone is identified as the layer on Fig. 2. SPT refusal was often encountered icoralline limestone, which was then cored. Ground wateencountered at a depth of about 2 m below existing grathe time of drilling. Laboratory consolidation tests on sentative, undisturbed samples of the silt/clay layer discthe following typical values: OCR = 2.3, CR = 0.21, 0.03 and cv = 4.2 m2/yr.Subsurface conditions disclosed by SPT borings drilled process water tank farm area are illustrated by the typicalsurface profile shown on Fig. 3. Also included on Fig. typical process water tanks and final grade information. stratigraphy is similar to that encountered by the SPT bodrilled at the fuel oil tank farm area, except that the coralis consistently encountered at a depth of about 8 mgrade. Ground water was encountered at a depth of aboubelow existing grade at the time of drilling. Laboratorysolidation tests on representative, undisturbed samples osilt/clay layer disclosed resu1ts similar to those at the futank farm area.

TANK FOUNDATION SELECTIONWhen the existing plant and tanks were buill, the soils beall plant structures and tanks were improved with columns installed to the top of the coral layer. A sground improvement program was developed for the newstructures, but ground improvement beneath the new generally was not deemed necessary. The case for notground improvement beneath the more settlement-totanks was made based on the careful characterization osurface conditions (summarized above), settlement cations, available tank settlement criteria, experience


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~ ~ : _ . . : . : : . : . ; : : ~ ..FOT7 . . . I .FDT 6 F015RAD:EI. -I.Bm

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Finish Grade [1. 2.4r1BH-12 siR.UCtURi\L FILL. . BH:_13 BH' -H.... ~ Y SAND - .....G 98 SILTS AND CLAYS '~ 15 132 - - - - 38. f - - - . . . . . . . . . --. 23 .. . 2727,, .. 22 26J5 2B . . . 20 . . .57 CORAL 3346 44. . 7f 46 - 39=- 50J7 27. . . .. .0 ... 0 10 . 32

HO.,IZONTAL SCALE- METERSQ .. ' . . . .. 46.--


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erecting and hydrotcsting tanks, and the development of acomprehensive staged hydrotesting program with extensivesettlement monitoring.

Tank Settlement CriteriaThe following tolerable settlement criteria were adopted as abasis for evaluating tank performance during hydrotcsting.These criteria are based on published literature Rosenbergand Journeaux 1982) and have been used extensively for tanksettlement performance evaluation.Shell Settlement. Uniform settlement of the concrete ringwallfooting is generally not used s a tolerable settlement criterion,because uniform settlements do not cause detrimental effectsto either the tank shell or bottom. Uniform settlement of theconcrete ringwall footing can generally be accommodated byproviding flexible tanklpipe connections.Planar tilt is defined as the difference in measured settlementbetween two diametrically opposed points on the tank shelldivided by the diameter of the tank, i.e.,Planar tilt= S 1 - S2 /D = ui+l ; out-of-plane differential movememm, for three neighboring, equall

spaced tank shell pointsL = distance, in mm, between equallyspaced points i, i-1, i+ 1

The maximum tolerable out of plane distortion is 1/4~ 2 2 percent. For a 96.8-m 96,800-mm) diameter tan

eight equally-spaced settlement markers, such as the pwater tanks, the maximum tolerable value of S o p is 84and for a 60-m 60,000-mm) diameter tank and eight eqspaced settlement markers, such as the fuel oil tankmaximum tolerable value of S o p is 52 mm.Bottom Plate Settlement. Edge to center distortion is das the maximum difference in measured settlement bethe center of the tank bottom over the radius of the tank,Edge to center distortion = Sc - SE)/R = S E c f Rwhere: SE

ScRll.SEC

==

settlement under the edge of the tank, mmsettlement under the center of the tankmmtank radius, in mrndifference between Sc and SE = differsettlement between the edge and centethe tank, in mm

The maximum tolerable edge to center distortion is 1/50percent. For a 48.4-m 48,400-mrn) radius tank, such process water tanks, the maximum tolerable valudifferential settlement ll.SEcl is 968 nun, and for a 30,000-mm) radius tank, such as the fuel oil tankmaximum tolerable value of differential settlement llS600 mm.

Settlement AnalysesSettlement analyses were performed using the typical c odation parameters previously described for the silt/clay and elastic parameters for the granular soils including tural fill to be placed in the process water tank area). Bathe SPT N-values, an elastic modulus of 17,500 kPselected for the natural granular soils in the process watearea, and a value of 13,500 kPa was selected in the fustorage area. The elastic modulus of granular structurwas selected to be 22,500 kPa, based on previous experieConsolidation settlement analysis of the silt/clay layeperformed using the TCON Version 4.99 software paTAGA 1993) that allows the simulation of load appli

with time.


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269Actual Maximum Elevation I Minimum Settlement

Settlement MeasurementPoint Typical) ---..........-------..8 2

31 Reference Radius

5lan V ew of Top of Shell _Showing D stort on Due toOut-of-Plane Tilt TowardsSettlement Measurement Actual Minimum Elevation I Maximum SettlementPoint No. 6

/ o


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and one set of readings50 percent.

4- Hold the 50 percent load and monitor settlement daily.of hold was to be determined based on the

of the tank It was estimated that the

5 and 6 - Similar to Steps 3 and 4, but for 75 percent

7- Similar to Step 3, but for 100 percent load (full tank.)8 - Hold the full load and monitor settlement daily for 2

and then twice a week thereafter. The duration of holdto be determined based on the settlement performance of

t was estimated that the load would have to be held

ofirst fuel oil tank (FOT 4) and the first process water tank

4) to be hydrotested.

MONITORING RESULTSresult ;; arc presented for FOTPOT 4, i.e., the first fuel oil tank and the first process

to be hydrotested. Changes to the hydrotesting proof FOT 4 and PWT

re discussed. Remarks are offered regarding the settleof the remaining tanks.

0

-20

-40

es -60E-80

'

1 00

-120

5

- --- 113--:r 114-6 - 115- e - 116- -+- 161- - - - 162_.._ 163- - 1 6 4_._Center

Fuel Oil Tanks 2

The time vs. settlement curves for the eight settlement mlocated along the sides and at the center of FOT 4 arein Fig. 5The data in Fig. indicate that the maximum settlementedge of FOT 4 was I l l mm and the minimum settleme38 mm. The average settlement along the edge of the tanabout 71 mm, which is larger than the predicted 55 mwell within tolerable limits. The data in Fig. 5 also shoquickly the settlements stabilized after loading stagesreached. Based on these results, the hydrotesting prowas changed to allow holding the 100 percent load for a of no more than 2 weeks for the remaining fuel oil tanks.Figure 6 shows a plot of settlements for equally smarkers located along the perimeter of FOT 4 and PWthe end of hydrotcsting under 100 percent load. The includes a continuous cosine-shaped curve thatrepresent a perfect tilt of the tank and actual settmeasurements that are represented by hollow squares.vertical distances between the hollow squares ancontinuous curve represent out-of-plane diffesettlements at the settlement marker locations.The data in Fig. 6 indicate that the maximum out-ofdifferential settlement for FOT 4 was about 23 mm.maximum out-of-plane distortion was about 1/2,100, orpercent. This value is about five times smaller than thpercent allowable.

Time days)

30 35 40 45

to 75

Holdat 75 Load I

0 ~ ~ ~ d a t 1 tl 1 4 0 L ~ ~ ~ ~ ~ ~ ~Fig 5 Time vs settlement curve or fuel oil tank No 4


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c F 0 T t 4 '0 ' co B in e c u .. 6 P W T f 4 w ' eosin c u r . ,_.i

1 0 Q

v- ~ i'1--- ~ ; ; /w>

'2 0 ' -. ~ - - ~ - . ; _ _ - - I I _ _ _ _ _ ...,~ ~ 4 i . ;;1 . 0 IA '-1 eo ' 13S tao 225 ' " ,ANGLA i l D I STANCE de g r ooo

Fig. 6 Out of plane distortionplanar tilt for FOT 4 was 69 mm, whicht 1/870 or 0.12 percent. This value is

of settlements of two points on theas the center of tank settlement for

4 and PWT 4. The data in Fig. 7 indicate that the4

87 mm. The maximum edge-to-center distortion was11870 or 0.29 percent. This value is more than 6 times

of the remaining fuel oil tanks wasof FOT 4.

of PWT 4 are shownFig. 8. It can be observed that the settlement marker placed

_, 1w;;

1 0 050 1---0, l 50w>

at the center of the tank was damaged while filling the 50 percent capacity. Also, the holding period at 50loading did not fully stabilize before the tank was filledpercent capacity.The data in Fig. 8 indicate that the maximum settlemenedge of PWT 4 was 154 mm and the minimum settwas 119 mm. The average settlement along the edgetank was about 136 mm, which is almost identical predicted 140 mm and well within tolerable limits. The Fig. 8 also show how quickly the settlements stabilizethe 75 percent and 100 percent loading stages were reBased on these results, the hydrotesting procedurchanged to allow holding the 100 percent load for a peno more than 2 weeks for the remaining process water taThe data in Fig. 6 indicate that the maximum out-odifferential settlement for PWT 4 was 13 mm at the soside of the concrete ringwall footing. The maximum plane distortion was about 11400 or 0.025 percent. Thiis more than 8 times smaller than the allowable.

TF 0 T 4

--------2 0 0 1--- - - - - - - - - - ~ - + - - - - - - - ' , _ _ , _ _ ___ - - - - - - - ~ ~ - - = - ' = = - - - - + - - - - - - ~ - - - 1- -2 50

0.2 5 0 5 0 '7 5RATIO OF i D I A M E T A I C D I S T A N C E A L ONG T A N K B A S E ) / O I A M E T E R

Fig. 7 Centerto edge distortion


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272Time days)

020 30 40

-20

-40

E -60 Load to 50%..E ~ . -8 0E -Q- 2

----tl.-- 31 00 --e--.4--5

120 --.--.7--a-140 C en Ia r

-1 6 0

Fig. 8 Time vs settlement curve or process water tank No 4planar tilt for PWT 4 was 23 mm, which

or 0.023 percent. This value is

on the settlement analysis results and edgeat the centerthe bottom plate was damaged, as shown by readings on

The data in Fig. 7 indicate that the maximum edge-towas 94 mm. The

or 0.19This value is more than 10 times smaller than the

of the remaining process water tanksof PWT #4, except that edge settlements of200 mm were observed for PWTs #7, #8

#9. However, maximum planar tilts, out-of-planelimits.

of subsurface conditions, detailedof an acceptable staged hydrotesting procedure

served as the basis for the foundation selection strategy folarge diameter tanks. The available data and rationale icated that the tanks could be built without the then-percenotion that ground improvement would be required. The twere erected and hydrotcsted without using ground imprment, and excellent settlement performance was observedwas confirmed that vibro-replacement ground improvemwas not needed, due to the more settlement-tolerant naturthese tanks.REFERENCES

Rosenberg, P_ and N. L Journcaux [1982]. "Settlemlimitations for cylindrical steel storage tanks." CanaGeotechnical Journal, Ottawa, Canada, 19:232-238.Senapathy, H., J_ Davie, and L. W. Young, Jr. [19"Performance of a steel tank founded on a marine clay." Po Settlement 94, ASCE Geotechnical Special Publication40, New York, Vol . I, pp. 830-84 LTAGA [1993]_ TCON Version 4.99 - Consolidation sement analysis software, Lafayette, California.

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P 0288- Foundations Performance of Large Diameter Tanks