Analysis and Performance of Piled Rafts Designed Using Innovative Criteria

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Analysis and Performance of Piled Rafts Designed UsingInnovative Criteria

Luca de Sanctis1 and Gianpiero Russo2

Abstract: In this paper the main criteria adopted for the design and some aspects of the observed behavior of the piled foundations ofa cluster of circular steel tanks are reported. They were designed to store sodium hydroxide, a toxic liquid with a unit weight of15.1 kN /m3. Shallow foundations would have been safe against a bearing capacity failure, while the predicted settlement was beyond theallowed limit. Accordingly piles were designed to reduce the settlement and improve the overall performance of the foundations. Whileconventional capacity based design approach led to a total of 160 piles to support the five tanks the settlement based design approach ledto a total of 65 piles achieving significant savings on the cost of the project. The settlements of four out of the five tanks were measuredand for two out of the five tanks the load sharing among the raft and the piles was also observed. Both the analyses carried out at thedesign stage and the back-analyses of the observed behavior were based on the interaction factors method as implemented in the computercode NAPRA �Russo �1998�, Int. J. Numer. Anal. Methods Geomech., 22�6�, 477–493�.

DOI: 10.1061/�ASCE�1090-0241�2008�134:8�1118�

CE Database subject headings: Raft foundations; Pile foundations; Foundation design; Foundation settlement.

Introduction

Piles as settlement reducers have been discussed for over a quar-ter of a century �Burland et al. 1977� as the basis for a morerational design approach to piled foundations, and some signifi-cant applications have been reported �Hansbo and Kallstrom1983; Burland and Kalra 1986; Sommer et al. 1991; Viggiani1995; Katzenbach et al. 1997�. Nevertheless the traditional capac-ity based design approach for pile foundations is still dominant inengineering practice. This is mainly due to the impositions madeby codes and regulations in many countries. In recent years boththeoretical and experimental research has shown that the capacitybased design approach is often too conservative, mainly becauseit does not allow one to take advantage of the load sharing be-tween the piles and the raft. At the time the authors designed thefoundations of the tanks the Italian code, like many other nationalcodes, was based on this conservative principle.

A new code on geotechnical design has recently been ap-proved in Italy which will coexist with the old code which hasexisted for almost 2 years. Among the innovative concepts in-cluded in the code there is the possibility of taking advantage ofthe load sharing between piles and raft. It is stated that if the raftprovides adequate bearing capacity, the piles can be used as a

1Research Assistant, Dept. of Civil Engineering, II Univ. of Napoli,Aversa �Caserta� 81031, Italy �corresponding author�. E-mail: [email protected]

2Associate Professor, Dept. of Geotechnical Engineering, Univ. ofNapoli Federico II, Napoli 80125, Italy.

Note. Discussion open until January 1, 2009. Separate discussionsmust be submitted for individual papers. To extend the closing date byone month, a written request must be filed with the ASCE ManagingEditor. The manuscript for this paper was submitted for review and pos-sible publication on August 2, 2006; approved on December 7, 2007. Thispaper is part of the Journal of Geotechnical and GeoenvironmentalEngineering, Vol. 134, No. 8, August 1, 2008. ©ASCE, ISSN 1090-
0241/2008/8-1118–1128/$25.00.
1118 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGIN

J. Geotech. Geoenviron. Eng.

means to reduce the settlement and no safety factors are pre-scribed to their ultimate capacity. Despite the limitations existingwhen the foundations were designed it was decided to explore thepotential of this innovative approach by considering the piles onlyas a means to reduce the settlement.

Traditional and Innovative Design for Piled Rafts

The traditional design approach for piled foundations is mainlyfocused on adjusting diameter, length, and number of piles tocarry the vertical component of the total load transmitted by thesuperstructure with an adequate safety factor. The piles are gen-erally uniformly spread underneath the foundation at a spacing ofabout 3 diameters and the contact between the raft and the soil isneglected. The experimental evidence collected by Mandolini etal. �1997� shows how unnecessarily small the settlement of piledfoundations is and provides a clear perception of the room exist-ing for less conservative but still adequate design solutions.

Innovative design approaches should allow one to take advan-tage of such a contact relying upon the load sharing between thepiles and the soil directly loaded by the raft. In recent years sci-entific literature has adopted the term piled raft to identify foun-dations where such load sharing is appropriately considered.

Burland et al. �1977� first introduced the basis for more ratio-nal and efficient design approaches with reference to situationswhere the raft-soil system had been determined to have a suffi-cient margin of safety against a bearing capacity failure but settle-ment higher than tolerable was expected. According to Burlandet al. �1977� in such a case the designer should only introducepiles as needed to reduce the settlement within a tolerable limit.By doing this the authors were assuming that the introduction ofthe piles was not going to deteriorate the safety margin of the raftalone and were pushing the designers toward the adoption ofsettlement based design criteria. This last issue was recalled andstrongly put in evidence by Randolph �1994� in his state of the
art. After the publication of this state of the art the amount of
EERING © ASCE / AUGUST 2008

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scientific papers on improved design criteria for piled foundationshas largely increased. There is a widely diffused opinion thatsignificant savings in the design of piled foundations can be ob-tained if the raft connecting the head of the pile is properly con-sidered with its capacity to transfer load directly from thesuperstructure to the soil. Furthermore once the decision to intro-duce piles underneath a raft is taken the designer should logicallyestablish the “amount” of piles needed to obtain a satisfactoryperformance, focusing on the reason that led to the initial deci-sion. For instance if the bearing capacity of the raft-soil system issufficient to carry the total load with an adequate safety factor, butthe settlement of the raft is larger than an allowable value appro-priately fixed, then the best design solution should determine theminimum “amount” of piles to be added to reduce the settlementwithin the tolerated values. Nevertheless following the traditionalcapacity based design approach the minimum amount of piles tocarry the total load with adequate safety factors is first determinedand a settlement calculation is carried out only to verify the ad-equacy of the design solutions. The lack of logic of such a pro-cedure is evident and it typically produces unduly conservativedesign solutions even if such a procedure can also lead to designsolutions which are more expensive and perform less. This pos-sibility was clearly demonstrated by de Sanctis et al. �2002�. Theycompared different design solutions obtained either via the tradi-tional capacity based approach or via an alternative and moreappropriate approach for a case history of a large piled raft wherethe differential settlement control was an important design issue.

However the variety of conditions which can lead to thechoice of piled foundations instead of shallow ones is such thatany attempt to provide general criteria to search for the optimumdesign solution is destined to fail.

The load sharing between the piles and the raft is a fundamen-tal quantity in most of the recent scientific papers which propose

Table 1. Case Histories with Observation of Load Sharing between Pile

Case Name Referen

1 Multispan bridge Van Impe and De

2 Building Urawa Yamashita et

3 Stonebridge park Cooke et al.

4 Messe Turm Sommer et a

5 Apartment block Joustra et al

6 Dashwood house Hight and Gre

7 House 1 Jendeby �

8 House 2 Jendeby �

9 Uppsala house Jendeby �

10 Garigliano bridge Russo �1

11 Messe Torhaus Katzenbach et

12 Westend 1 - DG Bank Katzenbach et

13 Japan Centre Katzenbach et

14 Forum Katzenbach et

15 Congress Centre Katzenbach et

16 Main Tower Katzenbach et

17 Eurotheum Katzenbach et

18 Treptowers Katzenbach et

19 National Westiminster Bank Hooper �1

20 Hide Park Cavalry Barracks Hooper �1

21 Serbat 12 Porto di Napoli Russo et al.

22 Serbat14 Porto di Napoli Russo et al.

s and Raft

ce s /d AG /A L /BQR /QT

�%�

Clerq �1994� 3.8 0.70 1.00 27

al. �1993� 7.8 0.90 0.64 51

�1981� 3.6 0.90 0.65 23

l. �1991� 6.4 0.83 0.52 45

. �1977� 5.2 0.90 0.70 22

en �1976� 3.0 0.90 0.50 19

1986� 6.5 0.90 2.10 8

1986� 10.5 0.90 2.20 66

1986� 11.2 0.90 2.20 64

996� 3.0 0.88 4.50 20

al. �2000� 3.5 0.80 1.14 20

al. �2000� 6.0 0.52 0.63 50

al. �2000� 5.5 0.45 0.60 60

al. �2000� 6.0 0.55 0.70 62

al. �2000� 5.8 0.62 1.00 60

al. �2000� 3.3 0.70 0.50 15

al. �2000� 5.2 0.55 0.80 70

al. �2000� 6.5 0.86 0.38 52

979� 3.8 0.91 0.50 29

979� 4.3 0.72 0.90 39

�2004� 5.8 0.82 0.92 48

�2004� 5.0 0.82 1.10 48

new and advanced design approaches. Available experimental

JOURNAL OF GEOTECHNICAL AND GE


Fig. 1. Load sharing versus filling factor �AG /A� / �s /d�

OENVIRONMENTAL ENGINEERING © ASCE / AUGUST 2008 / 1119

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data have recently been collected by Mandolini et al. �2005�.They reported the data of 22 case histories where the interactionamong the piles and the raft was observed �Table 1�. In Fig. 1 theload taken by the raft QR as a percentage of the total load appliedto the foundation QT is plotted against the dimensionless quantity�AG /A� / �s /d�, where AG is the area occupied by the piles; A is thetotal area of the raft; s is the spacing among the piles; and d is thepile diameter. This quantity can be defined as the filling factor ofthe raft.

Some of the case histories refer to advanced piled rafts, de-signed according to some innovative criteria, and have beenmarked with full dots. In such a case the definition of advanced orinnovative designed piled rafts was given by the authors of thepapers from which the data were collected. It would take toomuch space to attempt to summarize the innovative design con-cepts applied by the authors and in some papers this issue is alsomissing. However the plot of Fig. 1 is useful to outline somecommon key features of the geometry and of the behavior of

Fig. 2. Plan of exi

Fig. 3. Soil pr



these advanced foundations: the raft load is always very signifi-cant and higher than 40% while their geometry is characterizedby a value of the filling factor typically less than 0.15. It can alsobe noted that the raft load is never negligible even for tradition-ally designed piled foundations.

Case History of Five Storage Tanks in Port of Napoli

A cluster of steel tanks for the storage of sodium hydroxide wasbuilt some decades ago in the area of the Port of Napoli. Due tothe increasing need for storage, some more tanks were recentlyadded �Fig. 2�.

The existing tanks have diameters ranging from 7 to 10 m,heights from 10 to 12 m, and volume from 400 to 800 m3; theyare on shallow foundations consisting of thick reinforced concretecircular rafts. The new Tanks �numbers 11, 12, 13, 14, and 15� are

anks and new ones

t site of tanks

sting t

ofile a


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significantly larger and higher than the existing ones having di-ameters ranging between 10 and 12 m and a constant height of15 m. Their volume capacity thus ranges within 1,200–1,700 m3.

The subsoil of the area consists of a deep silty sand depositcovered by a few meters of made ground; the water table is 2.5 mbelow the ground surface.

The soil profile at the site of the tanks, including standardpenetration test’s �SPT’s� blow counts and two cone penetrationtest’s �CPT’s� profiles is sketched in Fig. 3. The made groundlocated below the surface paving is 5.25 m thick and consists ofsand incorporating fragments of bricks and rubble. The silty sandlayer �upper sand� is approximately 20 m thick and is followed byanother silty sand layer located between 25 and 30 m below theground surface. The lower sand is both finer and looser than theupper sand.

Both in the made ground and in the upper sand the resistanceto the penetration generally increases with the depth and exhibitsa pronounced scatter. The average value of NSPT increases fromnearly zero at the surface to about 35–40 at a depth of 25 m; theqC profiles are more scattered and an average value of15–20 MPa is reached at a depth of 20 m. In the lower sand onlySPTs were carried out and NSPT ranges from 11 to 13. For designpurposes, the average penetration resistances listed in Table 2were fixed. The area where the tanks are located is a seismic zonewhere, according to Italian regulations, earthquakes are expectedto occur. For this reason in the design stage the liquefaction po-tential of the sandy layer was evaluated and it was concluded thatliquefaction was not going to occur.

Design Strategy

As previously mentioned the existing tanks are on shallow foun-dations; this was also the first design option explored for the newtanks.

The evaluation of the bearing capacity of a shallow circularfoundation with a diameter of 10–12 m was preliminarily done.A conservative and constant value of the friction angle �=35°

Table 2. Average Properties of Various Layers

LayerMax. depth

�m�qC

�MPa� NSPT

Made ground 5.25 7.2 8

Slightly silty sand 25 11.7 29

Silty sand 30 — 12

Fig. 4. Layout of piles as



was selected on the basis of the penetration tests. Sufficiently highsafety factors, ranging between 8 and 9 under static load andbetween 5 and 6 including pseudostatic earthquake induced loads,were calculated.

Subsequently the settlements of the tanks were calculatedusing both the method by Schmertmann et al. �1978�, based onCPT results, and the method by Burland and Burbidge �1984�,based on SPT results. Both methods consider the increase ofsettlement due to the creep of granular soils. Furthermore themethod by Burland and Burbidge allows also one to calculate theadditional settlement caused by cyclic loads. The results of thecalculations are reported in Table 3. First of all the higher valuesobtained by the Burland and Burbidge method are mainly dueto the cyclic nature of the applied loads. A second comment is thatin such calculations the interaction between adjacent tanks wasneglected.

The computed average settlements were rather high and differ-ential settlement of the same magnitude was expected, due to theheterogeneity of the subsoil and to the mutual influence of adja-cent tanks. Two main aspects were considered and analyzed toestablish whether the calculated settlement could be tolerated bythe tanks. The tilt �inclination� and the average settlement of thetanks were compared with limiting values that have been fixed bythe client to avoid damages to the hydraulic connections betweenthe tanks. The tolerable value for the tilt was 0.15% while theallowable average settlement was 1 /500 H, with H being theheight of the tanks. The comparison led to the conclusion that theexpected settlements were not compatible with the safe operationof the tanks.

The decision of using piled foundations was therefore taken.According to the Italian regulations existing at the time of thedesign the whole external load had to be supported by the piles,neglecting any contribution of the raft, i.e., the raft had to beconsidered clear of the soil. Despite this limitation, it was decidedto explore design solutions considering the piles only as a meansto reduce the settlement.

The local market of the piling companies and the subsoil con-ditions led to the selection of continuous flight auger piles as the

Table 3. Calculated Settlement of Unpiled Rafts after 30 Years

Diameter of the tank �m� 10 12

Settlement computedfollowing Schmertman �mm�

90 105

Settlement computedfollowing Burland and Burbidge �mm�

157 180

ned by traditional design
obtai

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most suitable technology. The subsoil consists in fact of cohesion-less pyroclastic deposits and the groundwater table is found at adepth of 2.5 m. Under this circumstance the CFA piles eliminatethe need for casing or bentonite slurry. The typical pile diametersavailable in the Italian market vary in the range 400–800 m andconsequently an intermediate value �d=600 mm� has been cho-sen. During the installation of a CFA pile one of the most criticalsteps is the placement of the reinforcement, especially in the caseof a long reinforcement cage that can pose considerable problemsand contaminate the shaft with eroded soil. This is the reason whyCFA piles are usually no longer than 20–25 m. Because a singlereinforcement cage element does not exceed 12 m a pile lengthof 11.3 m was assumed, thus avoiding the need to link differentcage elements. At this point the only parameter needed to definethe foundations was the number of piles. The bearing capacity ofthe CFA piles was estimated by a combination of both end bear-ing and side friction �Qult=2,020 kN�; adopting a safety factor�FS��2.5, an allowable load of 810 kN was then obtained. As-suming a group efficiency factor equal to 1, as usual in cohesion-less soil, the conventional capacity based design approach led to atotal of 128 piles �Fig. 4� for the four adjacent tanks.

The axial stiffness of the single pile was evaluated by theempirical relationship suggested by Viggiani �2003�: w=�d /FS,with � being a coefficient depending on the pile technology. As-suming �=70 �auger piles in cohesionless soil� this gives a pilestiffness ks=236 kN /mm. The expected settlement of the piledtanks was evaluated by the empirical method suggested by Man-dolini et al. �1997, 2005� obtaining the values reported in Table 4.No interaction between adjacent tanks nor cyclic loading effectswere considered at this stage. The computed settlements werevery small, and no significant additional settlements were ex-pected due to the mutual influence of the tanks and to the cyclicload effects.

These preliminary design evaluations led to the conclusionthat there was the possibility of a more efficient design solution,giving the piles only the role of settlement reducers. A numberof trail evaluations have been performed by taking the pilesuniformly spread and gradually reducing their number. The solu-tion reported in Fig. 5 was finally adopted; it includes 52 pilesinstead of 128. For practical reasons, 13 piles have been placedbelow each tank, irrespective of its diameter; accordingly, the

Table 4. Calculated Settlement of Piled Rafts Designed UsingTraditional Approach

Diameter of the tank �m� 10 12

Estimated settlement �mm� 11.6 13.4

Fig. 5. Final lay



larger is the tank diameter, the higher is the average load perpile. The same final design was adopted for the isolated Tanknumber 15.

In the late design stage a test pile was also installed and loadtested to about 2,100 kN; the results obtained are reported inFig. 6. The experimental load-settlement curve of the single pilewas used for more refined design analyses. They were carried outby the program NAPRA �Russo 1998� following the procedureNon Linear suggested by Mandolini and Viggiani �1997�. In sucha procedure the experimental load-settlement relationship for thesingle pile plays a major role. The results of all the available siteand laboratory investigations are first used to develop a model ofthe subsoil, in which the geometry is adapted to a scheme ofhorizontal layering. The relative stiffness of the layers is alsoevaluated; such an evaluation is rather straightforward on thebasis of the results of laboratory tests or site tests such as CPT,SPT, and dilatometer test �DMT�. The initial tangent stiffnessexhibited by the pile is then used to back-figure the absolute valueof the stiffness of the different layers. Once the subsoil model isfixed and the stiffness of each layer established, the same model isused by NAPRA. In such a code the full pile-soil-raft interactionis modeled by the interaction factors method �Puolos 1968;Banerjee and Driscoll 1978�. Nonlinearity is concentrated at thepile-soil interface and is accounted for by a stepwise linear incre-mental analysis.

The Young’s moduli of the soil layers evaluated by the back-analysis of the load test are resumed in Table 5. The nonlinearityinvolved in the predicted load-settlement relationship for the piled

opted in design

Fig. 6. Results of load test on single pile

out ad


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raft is quantitatively the same as that observed in the single pileload test.

Among the various loading histories considered in the designstage, two cases are briefly reported here: �1� complete filling ofTank number 12 only; and �2� simultaneous filling of the fouradjacent tanks.

The results obtained, in terms of settlement, rotation, and loadsharing between the raft and the piles, are summarized in Table 6.The predicted settlement and rotations were considered as toler-able and the final design solution was then accepted.

Monitoring of Behavior

The settlements of a number of targets installed on the rafts ofboth the existing and the new tanks were monitored by an opticalsurvey. The loads directly transmitted by the raft to many piles ofTanks 12 and 14 were also measured. Three vibrating wire loadcells were installed on each pile. The layout of the instrumentedpiles and the targets installed on Tanks 11, 13, 14, and 12 areillustrated in Fig. 7. The load cells and the installation procedurewere similar to the ones adopted by Mandolini et al. �1992� inmonitoring the foundation of the main pier of a cable stayedbridge over the Garigliano River. More details on the installationtechnique are also reported by Russo et al. �2004�.

Settlement and load sharing among the raft and piles weremonitored during the construction and the subsequent filling ofthe tanks. The construction stage was not significant because ofthe small entity of the applied load due to the low self-weight ofthe steel tanks. The subsequent first filling was intended as a prooftest.

The applied load versus time is plotted in the upper part of Fig.8 while the average settlements of the tanks are plotted in thelower part. Settlement observations started after the completion ofthe rafts, so the measurements cannot provide any information onthe ground movements due to the self-weight of the rafts. Further-

Table 5. Young’s Moduli Back-Figured from Load Test on Single Pile

LayerMax. depth

�m�E

�MPa�

Made ground 5.25 65

Slightly silty sand 25 138

Silty sand 50 58

Fig. 7. Targets for optical survey and in



more, the low weight of the steel tanks can be neglected andtherefore the load considered in the diagrams of Fig. 8 is only theweight of the liquid filling the tanks.

The plant was kept in service by the owners during the firstfilling and loads could not be increased monotonically, dependingon the storage and supply needs of sodium hydroxide; particularlyfor Tank number 12 two partial unloading were observed. Neithera complete monotonic filling of a single tank, nor a contemporaryfilling of the four Tanks 11, 13, 14, and 12 was indeed performed.One out of the four adjacent tanks �Tank number 13� was notincluded in this initial loading program, while Tank number 15was submitted to a complete and quick proof test in mid August2003.

Differential settlement and load distribution among the pilesfor Tank numbers 12 and 14 are presented with reference to fourkey dates. In Table 7 the sodium hydroxide levels achieved inthese two tanks and the corresponding loads at the selected datesare reported while the settlement profiles observed along a sectionof the two tanks are plotted in Fig. 9. They both experiencedsignificant rotations due to the mutual interaction. The maximumaverage and differential settlements were recorded at date �3��June 25, 2003� and are reported in Table 8.

The load carried by some of the piles of the two tanks isplotted in Fig. 10. On date �1� the Tank number 14 was empty andthe piles below it were only slightly loaded by the raft and thetank weight. It is interesting to point out that the load on pile 39,belonging to Tank number 14, decreased when the adjacent Tanknumber 12 was further loaded �date �2��, probably due to theeffects of induced negative skin friction. At the maximum appliedload �date �3�� a significant edge effect is evident on the pile loaddistribution for both the tanks; i.e., the edge piles of both thetanks carried more load than the internal ones.

Table 6. Results of Design Analyses: Cases 1 and 2

Quantity Case 1 Case 2

Average settlement w �mm� 14.1 21.2

Maximum differential settlement � �mm� — 2

Rotation �� /D� — 1.67�10−4

Load sharing by piles �Pi /Q �%� 46 51

Pper / Pave 1.22 1.32

Pcenter / Pave 0.75 0.82

Note: D�diameter of the raft; Pi�load on the ith pile; Q�total appliedload�30 MN; Pave=�Pi /n; n�number of piles�13; Pper�load on theperipheral piles; and Pcenter�load on center pile.

nted piles for Tanks 11, 13, 14, and 12
strume

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Table 7. Levels of Sodium Hydroxide in Tank Numbers 12 and 14 atSome Reference Dates

DateQ14

�MN�Q12

�MN�H14

�m�H12

�m�

�1� May 28, 2003 0 12.5 0 7.

�2� June 19, 2003 0 15.9 0 9.3

�3� June 25, 2003 15.9 22.9 13.4 13.4

�4� July 17, 2003 8.9 0 7.5 0



Table 8. Average and Differential Settlement Recorded at Date �3� �June25, 2003�

Tanknumber

H�m�

wavg

�mm��

�mm�

14 13.4 9.7 14.8

12 13.4 28.4 14

Fig. 8. Load history and average settlements for Tanks 11, 14, and 12

Fig. 9. Settlement profiles along Tanks 14 and 12 at selected dates
Fig. 10. Load distributions among piles along alignment defined bydiameters of Tanks 14 and 12

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In Fig. 11 the total load taken by the piles is plotted against thetime. The pile load is estimated multiplying the average measuredload per pile times the total number of piles. In the same figurethe total applied load including the weight of the raft and of thesteel tank is also plotted. For both tanks, the piles initially sup-ported about 40% of the weight of the raft plus the steel tank.During the filling step the pile load increased up to about 50% ofthe total applied load. The unloading of the tanks produced asubstantial change in the load sharing with the piles supportingalmost 100% of the weight of the raft plus the steel tank. In otherwords the weight of the raft is partially transferred from the soilbelow the raft to the piles at the end of a complete loading cycle.

The load percentage taken by the pile at the maximum appliedload �date �3�� is also numerically reported in Table 9.

Prediction versus Performance

In this section some results of the back-analyses carried out by thecomputer code NAPRA, using the NL procedure suggested by

Table 9. Measurements of Load Sharing between Raft at Date �3��June 25, 2003�

TankH

�m� �PR

12 13.4 0.52

14 13.4 0.53

Fig. 11. Load sharing between piles and raft for tanks



Mandolini and Viggiani �1997� are reported and commented on. Afirst set of analyses were carried out for each separate tank mod-eled as an isolated piled raft. Further analyses with the foundationof the tanks modeled as interacting piled rafts were also per-formed. As the original code NAPRA �Russo 1998� could onlysimulate the loading process of an isolated piled raft, a new ver-sion of this code was appropriately developed �NAPRA 7.0�, stillbased on the interaction factors method. The new version is basi-cally a simple extension of the original code, obtained by consid-ering the mutual interaction factors between piles, even ifbelonging to different raft foundations. The same approach is ap-plied to the soil directly loaded by the raft; i.e., a loaded areabelow a raft is allowed to interact with a loaded area below anadjacent and independent raft.

The actual loading sequence of the cluster of adjacent tankswas neither a complete filling of a single tank nor a contemporaryfilling of the new tanks. It was controlled by the storage andsupply needs of the sodium hydroxide as specified in “Monitoringof Behavior.” Only for Tank number 15 was the first fillingenough to achieve the full tank capacity.

Foundation of Tanks Modeled as Isolated Piled Rafts

The load-settlement relationships observed for all the tanks areplotted in Fig. 12. For each tank, the maximum load was keptconstant for several days and some increase of the settlement wasobserved. The immediate settlement wmeas, at maximum load, thesettlement increase �wcreep at constant loading, and the time

Fig. 12. Measured and predicted load-settlement curves

spent, are resumed in Table 10. In Fig. 12 all the comparisons


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between the experimental load-settlement curves and those calcu-
lated by modeling the tanks as isolated foundations are plotted.The total load applied to each tank in the analyses was the maxi-mum load achieved at the end of the first filling. As the measure-ments started after the raft completion the analyses were carriedout considering only the load due to the weight of the fillingliquid. The model implemented in NAPRA and the proceduresuggested by Mandolini and Viggiani �1997� are such that creepeffects cannot be predicted. Thus the immediate settlements haveto be compared to the calculated ones. The agreement is rathersatisfactory, with the exception of Tank number 12 where thedifference between wmeas and wcal is 32% �Table 10�. However forTank number 12 a partial unloading and reloading cycle occurredeven during the first filling with a significant influence on theresponse of the tank. Fig. 12 clearly shows that the initial ob-served load-settlement response was rather similar to the calcu-lated one, while after the unloading-reloading cycle, a significantsoftening occurred in the observed behavior. This could be par-tially due to the observed phenomenon that the self-weight of theraft, neglected in these back-analyses, is entirely transferred fromthe raft-soil contact to the pile after an unloading cycle.

In Table 11 the average loads per pile as computed under themaximum applied load are reported. They are slightly differentfor each tank and range between 909 and 1,113 kN. Tank number

Table 10. Comparison between Measured and Computed Settlements

Tanknumber Datea

Q�MN�

H�m�

11 September 2, 2003 17.19 12.9

12 June 13, 2003 22.46 13.15

14 June 20, 2003 15.89 13.4

15 August 12, 2003 14.41 15.0aDate at the end of the first filling.

Table 11. Computed Average Load per Pile for Different Tanks

Tanknumber Date

Q�MN�

H�m�

D�m�

Pavea

�kN�

11 September 2, 2003 17.19 12.90 10.6 978

12 June 13, 2003 22.46 13.15 12.0 1,113

14 June 20, 2003 15.89 13.40 10.0 944

15 August 12, 2003 14.41 15.00 9.0 909aAverage load per pile.

Fig. 13. Comparison between measured and predicted displacementsof benchmarks belonging to Tank numbers 12 and 14



12 is the one where this load has the highest value followed byTanks 11 and 14. The average load per pile has been also com-puted for Tank number 15. As it was subjected to a completefilling test the average load per pile is very close to that computedfor Tank number 14.

Interacting Piled Rafts

The interaction effects between the different foundations havebeen simulated with the new version of the program NAPRA.

On date �2� Tank number 14 was empty while the height ofsodium hydroxide in Tank number 12 was 9.3 m. Later on Tanks12 and 14 were hydraulically connected until date �3� when theheight of the sodium hydroxide achieved 13.4 m in both thetanks. On the same date the adjacent Tanks 13 and 11 were empty.The interaction analyses described in this section were carried outby applying the load combination corresponding to reference date�3� �see Table 6�.

In Fig. 13 the calculated and the measured settlement for alongitudinal section crossing both the loaded tanks are compared.If the average settlement is compared the agreement is rathersatisfactory while major discrepancies arise for the rotations ofthe two tanks, particularly for Tank number 14 where the ob-served rotation is significantly higher than the calculated one.

The load sharing among the piles belonging to the same crosssection is compared in Fig. 14 while the global load sharing be-

D�m�

wmeas

�mm�wcal

�mm��wcreep

�mm�t

�days�

10.6 9.6 10.6 5.9 12

12.0 19.9 13.5 3.1 9

10.0 8.1 10.0 2.1 19

9.0 9.8 9.6 2.0 16

Table 12. Measured and Predicted Load Sharing at Reference Date �3�for Tanks 12 and 14

Tanknumber

Q�MN�

H�m�

D�m� �PRmeas

�PRcalc

12 22.46 13.15 12 0.52 0.55

14 15.89 13.40 10 0.53 0.63

Fig. 14. Load sharing between piles for Tank numbers 12 and 14


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tween raft and piles is compared in Table 12. The general agree-ment between the calculations and the observations can beconsidered satisfactory for engineering purposes.

Concluding Remarks

Some of the key points of the design steps for the foundations ofsteel tanks are reported; it is a typical example of the advantagesthat can be obtained with the innovative design approaches, usingpiles to control average and differential settlement.

The design analyses and the back-analyses have been per-formed with relatively simple procedures as the one suggested byMandolini and Viggiani �1997� based on the availability of a pileload test and using the code NAPRA �Russo 1998�. The generalagreement between the analyses and the experimental results israther satisfactory confirming the validity of both the computercode and the procedure of analysis. It should also increase the lowconfidence the engineers typically have when faced with the cal-culations of the settlement of piled foundations.

The substantial contribution of the raft in supporting almosthalf of the total applied load is an indirect result of a designapproach where the piles have only the role of settlement reduc-ers. This approach led to a substantial saving in the total numberof piles without significantly affecting the overall performance ofthe piled rafts. This remark is immediately evident if the averagesettlement calculated for the traditional design solution �Table 4�is compared with the observed and the calculated settlement ofthe innovative design solution adopted in practice.

It is believed that the time is ripe for a wider consideration ofthe contribution of the raft in the design. To this aim, the devel-opment of suitable recommendations and regulations is of pri-mary importance.

The results of the measurements also allowed us to show thatthe cyclic nature of loading, the time effects for piled foundationsin granular soils, and the interaction between adjacent piled foun-dations appear to play a significant role. Even if the simple back-analyses carried out considering only the third issue andneglecting both the first and the second issue resulted in a quitesatisfactory interpretation of the overall observed behavior, moreexperimental results on the above listed topics could be fruitfullyemployed to better elucidate their relevance.

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