deep foundation practice_barrettes

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5th International Conference on

DEEP FOUNDATION PRACTICEincorporatingPILETALK International 20014 6 April 2001, Singapore

RECORD LOAD TEST ON A LARGE BARRETTE AND ITSPERFORMANCE IN THE LAYERED SOILS OF BANGKOK

Narong Thasnanipan, Aung Win Maung, Zaw Zaw AyeSEAFCO Co., Ltd., Bangkok, Thailand

Pornpot TansengSuranaree University of Technology, Nakornratchasima, Thailand

CI-PREMIER CONFERENCE ORGANIZATION

ISBN: 981-04-2512-0


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Record load test on a large barrette and its performance

in the layered soils of Bangkok

Narong Thasnanipan, Aung Win Maung, Zaw Zaw AyeSEAFCO Co., Ltd., Bangkok, Thailand

Pornpot TansengSuranaree University of Technology, Nakornratchasima, Thailand

ABSTRACT

This paper presents the load transfer characteristics of fully instrumented barrette of 1.5x3.0m in sizeseated about 57m below the ground level. The test results were compared with those from theinstrumented load test on bored pile of diameter 1.5m with the same length, located 30m away. Nosignificant difference in load transfer has been observed between barrette and pile despite theconsiderable difference in construction method applied and time consumed.

Keywords : Barrette, static load test, load transfer characteristics

1. INTRODUCTION

Barrettes have been used as deep foundations for various structures in Bangkok for a number ofyears. The large load carrying capacity achievable by flexible dimension and length of barrettesprovides a major advantage in prevailing subsoil condition of Bangkok. In addition to extensive bearingcapacity requirement, the demand for barrettes is necessitated mainly by site constraints, applicableconstruction method and equipment. Barrettes with dimension ranging from 0.80mx2.7m to1.5mx3.0m for safe working load capacity from 1100 to 2300 ton have been used in some majorprojects. To assess the performance of large-capacity foundation element in layered subsoil ofBangkok, instrumented load testing is compulsory. This paper presents the static load test results ofinstrumented barrette tested up to 5290 ton for foundation of a fifty-storey building in Bangkok. Thetest results, particularly load transfer characteristics, and shaft friction capacity were compared with

those of bored pile diameter 1.50m with the same length located 30m away.

2. OVERVIEW OF THE PROJECT

Foundation of the fifty-storey tower called for 560 bored piles of 1.2m and 1.5m diameter and 24number of barrettes having cross section size of 1.5m x 3.0m. Bored piles and barrettes were seatedat approximate depth of 57m in the second sand layer. Barrettes were designed to support the largeload for towers lift shafts as bored piles were not feasible to utilize in such case. Base grouting wasapplied for barrettes and bored piles at the locations of high column load mainly in the central towerarea. Instrumented static pile load test was proposed for one barrette and one bored pile of diameter1.5m. The design safe working loads for the base grouted barrettes and bored piles are 2,300 ton and1,175 ton respectively. Foundation plan of the project is shown in Figure 1.

5th

International Conference on Deep Foundation Practice incorporating Piletalk : 4 6 April 2001, Singapore

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Figure 1 Layout of foundation showing location of test barrette and bored pile

3. SUBSOIL CONDITION

Soil investigation from five boreholes at different location reveals that subsoil layers along the siteare relatively consistent. Similar to other localities in Bangkok a typical subsoil profile at the site ischaracterized by the alternating layers of clay and sand deposits as soil succession shown in Figure 7.

Soft, highly compressible dark gray marine clay lies beneath weathered crust layers of 2m thick andextends up to 13.5m. Stiff Clay layer occurs directly underneath Soft Clay and its depth goes up to26m. Below Stiff Clay layer, First Sand layer of 10m in thickness can be found. Hard Clay layerunderlies First Sand and it is found to be about 12m thick. Second Sand layer occurs at depthsbetween 50 to 72m. Undrained shear strength (Su) obtained from unconfined compression test andStandard Penetration Test (SPT) are shown in Figure 7.

4. CONSTRUCTION METHODS

Both barrette and bored pile were constructed by wet process under bentonite slurry. Bentoniteslurry conforming to the widely accepted specification was used. Properties of the bentonite slurryused are given in Table 1. Single stage base grouting was applied 24 hours after concreting for both

barrette and bored pile. The detailed procedures utilized in the construction of two different foundationstructures are outlined below.

Table 1. Comparison of bentonite slurry propertiesBarrette Bored Pile

Properties Before feedingto the borehole

After Recycling &Before Concreting(near trench base)

Before feeding tothe borehole

After Recycling & Beforeconcreting

(near borehole base)

Density (g/cc) 1.10 1.17 1.08 1.10

Viscosity (sec) 36 49 33 36

Sand Content (%) 1.0 1.1 0.1 0.8

pH value 8 9 8 8

Mechanical rope-grab was used to excavate the trench. A guide wall cast with inside cleardimensions slightly larger than the nominal size of the barrette was used to guide the grab during

132.10

132.68

Test-Barrette

96.20

94.00

Test

Pile

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initial bites. Since time consumed in the preparation of instrumentation was relatively long, desandingwas continuously done to keep the bentonite slurry agitated, which also helps to alleviate the growth offilter cake by minimizing the actual exposure time. As another measure, trench was once againoccupied by grab to scrap the trench walls to remove, if any, filter cake formed on the walls. Thisattempt is in line with the recommendation made by Reese and ONeill (1988) [1]. It is authors opinionthat, if due to some unforeseen reasons, reinforcement cage lowering have to be delayed for

considerable period of time, it is a good practice to use the grab again to scrap the trench walls. Thismeasure eliminates any foreseeable negative impacts caused by unexpected delays. After loweringthe rebar cage, tremie concreting was done.

Rotary drilling was employed for bored pile excavation. Different from barrette excavation,temporary casing of 15 m length was used as a support in Soft Clay layer for bored pile drilling toassure the stability of the borehole. Firstly, auger was used to drill within the temporary casing,followed by rotary bucket with bentonite slurry down to final depth of excavation. The base of theborehole was cleaned by recycling technique to minimize any congregated sediments. Before loweringthe reinforcement cage special cleaning bucket was used to scrap of the borehole walls and the base.Reinforcement cages were then lowered inside the borehole while attaching the instrumentationsimultaneously at specified locations. Soon after lowering the rebar cage tremie concreting wascommenced. Polystyrene grains plug was used before the first charge of concrete to avoid the mixing

of bentonite with concrete. Time consumed in different construction activities is plotted in Figure 2.

Figure 2 Time consumed in different construction activities of barrette and bored pile( after Thasnanipan, 1999 [2] )

5. LOAD TEST PROGRAM AND INSTRUMENTATION

5.1 Test Pile Layout

Test pile layout of barrette is presented in Figure 3. With overall height of 8m, reaction frameutilized for static load testing on barrette in this project was claimed to be one of the biggest of its kindin the region. Four barrettes were used as anchoring system. Five numbers of built-up steel girders

supported on each side by two 1st level cross beams were used as main beams to achieve themaximum capacity of 6000 tons. First level beams were supported against the second level crossbeams. Second level cross beams were anchored against surrounding barrettes using anchor blocksat the top. Specially fabricated rigid transfer girders were used to distribute the tension force comingfrom the tie-bars to dowel bars above the anchor barrette heads. Sixteen numbers of hydraulic jackseach having 500 ton capacity were placed between the test barrette cap and the main beams of thereaction frame. General view of the barrette load test set up is presented in Figure 4.

Test pile layout of bored pile was similar to those of other static bored pile load tests in Thailand.Steel test frame anchored against four bored piles was used in bored pile load test. Different from thetest frame of barrette, only 1 layer of cross beams was required for that of bored pile.

0 10 20 30 40 50 60 70 80

Bored Pile

Barrette

Time Consumed (hours)

Drilling

DeSanding

Cage Lowering

Tremie Pipe Preparation

Concreting

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Figure 3 Barrette test pile layout showing the configuration of 6000 ton capacity reaction frame

5.2 Monitoring System and Instrumentation

Direct measurement from four dial gauges placed in diametrically opposite positions havingequidistance from the test pile axis was used as a main monitoring system of test pile headmovement. Precise leveling and piano wire were also utilized as backup for pile head movementmeasurement. Additional two dial gauges were also used to monitor the lateral movement of the pile.Eight number of SINCO load cells of 500 ton capacity each were installed on top of the hydraulic jacksto evaluate the actual applied load of the first load cycle . Vibrating wire strain gauges (VWSGs) and

Mechanical Extensometers (ME) were fixed at five levels along the shafts at the known interfaceboundaries of different soil layers. At each level, four sets of VWSGs and one set of ME were installedfor the pile whereas six sets of VWSGs and two sets of ME were installed for the barrette.

Figure 4 General view of the reaction frame for static load test on barrette

2nd Cross

Beam

1st Cross Beam

Main Beam

Hydraulic

Jacks

5.00 5.00

5.0

0

5.0

0

ANCHOR

BARRETTE

1st Cross

Beam

2nd Cross

Beam

TEST

BARRETTE

2nd Cross

Beam

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5.3 Load Test

Pile load tests were carried out in accordance with ASTM D 1143-81. Pile and barrette weretested under three and four cycles of loading and unloading respectively.

6. LOAD TEST RESULTS AND INTERPRETATIONS

6.1 Pile Head Movement at Applied Load

Load vs pile head movement graph of barrette and bored pile is illustrated in Figure 5. Measuredpile head movements at design load, double of design load and maximum test load are presented inTable 2. As can be seen in the table, both barrette and bored pile experienced only negligible pilehead movement at relevant design load.

Table 2 Predicted and measured pile head movement at specific applied load

Measured gross pile head movement at applied loadTest Pile Design Load (DL) Max. Test Load

At DL At 2 x DL At Max. Test Load

Barrette 2300 ton 5290 ton -5 mm -12 mm -61 mm

Bored Pile 1175 ton 2700 ton -6 mm -19 mm -67 mm

Table 3 shows the estimated ultimate load capacity of barrette and bored pile from the load vs pilehead movement using different method.

Table 3 Estimated ultimate capacity of barrette and bored pileUltimate capacity (Qult) and pile head movement

at Qult estimated by different methodTest Pile Parameter

Davissons Mazurkiewiczs Butler and Hoys

Qult (ton) 5180 ton 5156 ton 5100 tonBarrette

Pile head movement at Qult 18 mm 18 mm 17 mm

Qult (ton) 2675 ton 2734 ton 2620 tonBored Pile

Pile head movement at Qult 30 mm 38 mm 22 mm

Figure 5 Pile head movement at the last load cycle (maximum loading) of barrette and bored pile

0

20

40

60

80

0 1000 2000 3000 4000 5000 6000

Applied Load (Tons)

PileHeadMovement(mm)

Barrette

Pile

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Figure 6 Ratio of mobilized skin friction to mobilized maximum skin friction against ratio ofpile head movement to equivalent pile diameter

BarretteBored

Pile

1.50 m 3.00 m

1.5

0m VWSG

Mechanical

Extensometer

60

BUTT LOAD (TON)

Tip Level

-57.5 m

0

Tip Level

-57.5 m

0 100 0 20 00 30 00 4000 50000 1000 2000 3 000

Weathered

Crust

First Stiff

C lay

First

Sand

Second

Sand

0

-1 0

-2 0

-3 0

-4 0

-5 0

DEPTH (m)

-6 0

0 5 1 0

Soft

C lay

Hard

C lay

0 50 100

SPT-N (blows/30cm)

Su (UC) t /m2

0 .0

0 .2

0 .4

0 .6

0 .8

1 .0

1 .2

0 .0 0.5 1.0 1.5 2 .0 2 .5

s / D (%)

(fsAs)mobilized/(fsAs)maximum

0

0 .2

0 .4

0 .6

0 .8

1

1 .2

0 .0 1 .0 2.0 3 .0 4 .0 5 .0

s / D (%)

(fsAs)mobilized/(fsAs)maximum

6.2 Mobilized Skin Friction vs Pile Head Movement

The graphs showing the ratio of mobilized skin friction to maximum mobilized skin friction againstratio of pile head movement to pile diameter (s/D) are presented in Figure 6a and 6b respectively.Symbol D shown in Figure 6a represents the barrette dimension in equivalent diameter. According tothe figures, in general, skin friction mobilized to the maximum values at pile head movement of 0.6%

and 1.6% of equivalent shaft diameter for barrette and diameter of bored pile respectively.

6.3 Load Transfer Characteristics of Barrette and Bored Pile

Load transfer curves along the shaft of barrette and bored pile at various applied load are shownin Figure 7. The values of unit skin friction developed at the different soil layers along the shaft of

barrette and bored pile in comparison with those of calculated ultimate unit skin friction aredemonstrated in Figure 8. This comparison suggests that mobilized skin frictions of barrette and boredpile are higher than those of calculated values using empirical formulas. It proves the findings ofvarious researchers on shaft friction improvement of base grouted piles ( eg. Teparaksa et al 1999,[3] ). It is also evident that overall shaft resistance of both pile was not fully developed at design load.

Figure 7 Load transfer curves of test barrette and bored pile with typical soil profile at the site

6a (Barrette) 6b (Bored Pile)

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Figure 8 Comparison of calculated and mobilized unit skin friction of barrette and bored pile

6. 4 Effect of Construction Time on Shaft Capacity Reduction

Excessive construction time is one of the main parameters claimed to be responsible for shaft frictioncapacity reduction of drilled shaft foundation constructed with wet process under bentonite slurry asreported by various researchers. Total construction time of barrette and pile were 75 hours and 27hours respectively. Though total construction time consumed for test barrette was almost 3 times morethan that of bored pile, there is no significant difference in shaft load transfer between them. Themeasures adopted in construction of barrette to minimize the excessive filter cake formation along thesand layers by proper desanding (continuous agitation), and retrenching with grab are considered themain reasons contributed to this achievement. Comparison of developed unit skin friction values ofbarrette and pile proves that difference in utilized bentonite viscosity as shown in Table 1 does nothave significant effect on the shaft capacity. These findings are in line with conclusions made byThasnanipan et al (1999) [4].

6.5 Effect of Shape on Shaft Load Transfer

Hosoi et al (1994) [5] concluded, from the results of numerical analysis that earth pressure acting onthe flat surface of diaphragm wall panel is larger than that of circular bored pile. Thasnanipan et al(1999) [2] attempted to assess the effect of different aspect ratios (L/B) on the earth pressuredeveloped around the trenches by using finite element program and reported that no significantdifference in earth pressure was observed between the borehole (L/B=1) and the barrette (L/B=2).Further research is necessary to evaluate this finding. No significant difference is found between themaximum mobilized skin friction against displacement of rectangular-shape barrette and circular-shape bored pile according to Figure 6a and 6b respectively. Displacements at maximum mobilizedskin frictions of barrette and bored pile fall within the range (0.5 % - 2 % of shaft diameter) reported by

Reese (1978) [6].

0

10

20

30

40

50

60

0 10 20 30

Max. Mobilized Unit

Skin Friction (ton/m2)

Depth(m)

Bored Pile

Barrette

Calculated

0

10

20

30

40

50

60

0 10 20 30

Mobilized Unit Skin Friction

at Design Load (ton/m2)

Depth(m)

Bored Pile

BarretteCalculated

Weathered

C rus t

First Stiff

C l ay

F i rs t

S an d

Second

S an d

0

- 10

- 20

- 30

- 40

- 50

DEPTH (m)

- 6 0

0 5 10

So f t

C lay

H a r dC lay

0 50 10 0

SPT-N (b lows/30cm)

Su (UC) t/m2

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7. CONCLUSIONS

(1) Elastic deformation of both barrette and bored pile at relevant design load was found to benegligible. Shaft resistance of barrette and bored pile was not fully mobilized at relevant designload. These observations suggested that the selected size and length of barrette and bored pilefor specified design loads are sufficient for acting as friction piles.

(2) Mobilized shaft frictions of barrette and bored pile at maximum test load particularly in the firstsand layer are considerably higher than calculated values. Shaft friction capacity improvementwas considered to be contributed by base grouting which is in line with the findings reported byvarious researchers.

(3) Despite the considerable difference in construction method applied and construction timeconsumed (75 and 27 hours for barrette and bored pile respectively), similar characteristics ofshaft load transfer were observed between barrette and bored pile. This achievement indicatedthat adopted measures (proper desanding and scraping of trench wall by grab prior toreinforcement lowering) to alleviate the excessive growth of bentonite filter cake on the barrettewall proved to be effective.

(4) Static load test up to 5290 ton conducted on barrette set the record as the highest load evertested for a single cast-in-situ deep foundation in Thailand.

ACKNOWLEDGEMENTS

The authors wish to express their appreciation to Dr. Wanchai Teparaksa and Mr. Kamol Singtogawfor providing invaluable suggestions in the initial stage of the load test program. The authors also wish

to thank Mr. Chanchai Submaneewong for his assistance in preparing this paper.

REFERENCES

[1] Reese L.C., & ONeill M. W., Drilled Shafts : Construction Procedures and Design Methods,ADSC: The International Association of Foundation Drilling, Dallas, Texas, USA, 1988

[2] Thasnanipan N., Anwar M. A., Maung A.W., Tanseng P., Performance Comparison of Bored andExcavated Piles in the Layered Soils of Bangkok, Symposium on Innovative Solutions inStructural and Geotechnical Engineering in Honor of Professor Seng-Lip Lee, Asian Institute ofTechnology & National University of Singapore, 1999, p.p. 345-353.

[3] Teparaksa W., Thasananipan N., and Anwar M. A., Base Grouting of Wet Process Bored Piles inBangkok Subsoils, The Eleventh Asian Regional Conference on Soil Mechanics andGeotechnical Engineering, Seoul, Korea, 1999, p.p. 269-272.

[4] Thasnanipan N., Anwar M. A. & Maung A. W., Review of the Shaft Capacity Degradation ofBored Piles Constructed with Bentonite Slurry, Civil and Environmental Engineering Conference,Asian Institute of Technology, Bangkok, Thailand, 1999, p.p. V-59 to V-68.

[5] Hosoi T., Yagi N., & Enoki M., Consideration to the Skin Friction of Diaphragm Wall Foundation,3

rdIntl. Conf. on Deep Foundation Practice incorporating PILETALK, Singapore, 1994

[6] Reese L. C., Design and Construction of Drilled Shafts, 12th

Terzaghi Lecture, Proc. ASCE, Vol.104, No. GT1, 1978, p.p. 95-116

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