PROCEEDINGS OF THE SYMPOSIUM IN HONOR OF PROFESSOR SENG-LIP LEE

May 14-15, 1999 Asian Institute of Technology, Pathumthani, Thailand

Innovative Solutions in Structural and

Geotechnical Engineering

Edited by

Pisidhi Karasudhi and Anil C. Wijeyewickrema Asian Institute of Technology, Thailand

K. Y. Yong and T. Balendra The National Institute of Singapore

Performance comparison of bored and excavated piles in the layered soils of Bangkok

N. Thasnanipan, M.A. Anwar, Maung A. W. and P. Tangseng

Seafco Company Limited Bangkok, Thailand

Asian Institute of Technology

National University of Singapore

Symposium on Innovative Solutions in Structural and Geotechnical Engineering in Honor of Professor Seng-Lip Lee, May 14-15, 1999

PERFORMANCE COMPARISON OF BORED AND EXCAVATED PILES, IN THE LAYERED SOILS OF BANGKOK

Narong Thasnanipan, Muhammad Ashfaq Anwar, Aung Win Maung, Pornpot Tanseng

SEAFCO Co., Ltd., Bangkok, Thailand.

ABSTRACT: Installation techniques are considered to effect the performance of cast in situ piles. This paper compares the load transfer characteristics of a fully instrumented bored pile (Dia. 1.5 m) and barrette (1.5 x 3.0 m) having same length equal to 57 m, embedded in identical ground conditions (on same site, 30m apart). Total time consumed to construct the pile and barrette differ considerably, 27 and 75 hours, respectively. No significant difference in load transfer has been observed between the pile and the barrette. It has been concluded that, by observing good engineering practices, any conjectured degradation effects on load capacity of such piles can be eliminated. Additionally, the load applied on barrette was one of the highest test load in the region with a reaction frame “tower” capacity of 6000 tons. Reaction frame and loading system arrangement, to achieve such a high load capacity is also discussed. 1. INTRODUCTION Barrettes become a common foundation element in combination with bored piles and diaphragm walls in Bangkok (Refer: Thasnanipan et al, 1998-b). Foundations for the BECM tower project located at Rama IX road Bangkok, Thailand, required 560 bored piles of 1.2 m and 1.5 m diameter and 24 number of barrettes having cross section size of 1.5 m x 3.0 m. Foundation plan of the tower is shown in Figure 1. Barrettes were necessary to transfer the very high loads coming from the central 52 story tower’s lift shafts. Piles were approximately 57.0 m long with tips embedded into the second sand layer. Depending upon the load requirements piles were base grouted in the central tower area and at the locations of high column loads. Load testing on working piles was required to verify the design parameters, and it was proposed to test one bored pile of 1.5 m diameter and one barrette with full instrumentation. Since, pile and barrette were constructed using two different techniques and the parameters like construction time, bentonite properties etc, were considerably different, results from the load tests have provided a unique chance to assess the extent of difference in behavior, which have been speculated to effect considerably in the past by some theoreticians. 2. SOIL PROFILE

Soil profile at the site is no different than the typical soil profiles of plain of Bangkok and is shown in Figure 2. Below the top 2 m of weathered crust 11.5 m thick Bangkok Soft Clay is present. First stiff clay starts from 13.5 m depth and extends to 26 m, below is the First sand layer and is about 10 m thick. Below the First sand layer a thick layer of Hard clay which extends down to 54 m, is present, and then starts the second sand layer which extends down to about 80 m where the next hard clay layer starts. Undrained shear strength, Su obtained from unconfined compression test and Standard Penetration Test (SPT) are shown in Figure 2. Ground water table depth at the site generally varies from 1 to 1.5 m, piezometeric conditions of the site are also quite similar to the other parts of the Bangkok with piezometric draw down of about 22 m near the bottom of the first stiff clay layer.

Figure 1. Foundation footprint of BECM tower project, showing the location of test pile and barrette. 3. CONSTRUCTION PROCEDURE 3.1 Bored Pile Wet process method with rotary drilling bucket was employed. Bentonite slurry conforming to the widely accepted specifications were used as supporting fluid. Top 15 m soft clay was temporarily cased to assure 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. Airlift technique was applied to clean the borehole base of any congregated sediments. Before lowering the reinforcement cage special cleaning bucket was used to scrap of the borehole walls and the base. Reinforcement cages were lowered inside the borehole by simultaneously attaching the instrumentation at specified locations. Soon after lowering the rebar cage tremie concreting was started. Properties of the bentonite slurry used are given in Table 1. 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 3. 3.2 Barrette Mechanical rope-grab in conjunction with bentonite slurry was used to excavate the trench. A guide wall cast with inside clear dimensions slightly more than the nominal size of the barrette was used to guide the grab during initial bites. Since time consumed in the preparation of instrumentation was quite long, refer Figure 3, desanding was continuously done to keep the bentonite slurry agitated, which also helps to minimize the growth of filter cake by actually reducing the exposure time.

132.10

132.68

Test-Barrette96.20

94.00

TestPile

346

Figure 2. Comparison of bored pile and barrette, with soil profile at the site.

As another measure, trench was once again occupied by grab to scour the trench walls and to remove, if any, filter cake formed on the walls (Refer: Drilled Shaft Inspector’s Manual, 1989). It is authors opinion that, if due to some unforeseen reasons, reinforcement cage lowering have to be delayed for considerable period of time or for overnight. It is a good practice to use the grab again to scrap the trench walls which eliminates any foreseeable bad effects due to such unexpected delays. After lowering the rebar cage, tremie concreting was done. Table 1. Comparison of bentonite slurry properties.

Pile Barrette Before feeding to the borehole After Recycling & Before

concreting (near borehole base)

Before feeding to the trench

After Recycling & Before concreting (near trench base)

Viscosity (sec) 33 36 36 49 Density (g/cc) 1.08 1.10 1.10 1.17 pH value 8 8 8 9 Sand Content (%) 0.1 0.8 1.0 1.1

60

0

-10

-20

-30

-40

-50

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DEPTH (m)

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WeatheredCrust

0 5.0 10.0

SoftClay

First StiffClay

HardClay

SecondSand

FirstSand

Tip Level-57.5 m

0 50 100SPT-N (blows/30cm)

0

60

Tip Level-57.5 m

BarretteBored Pile

1.50 m 3.00 m

1.50 m

Su (UC) t/m2

0.0 10.0 20.0 30.0

Bored Pile

Barrette

Max. Mobilized Skin Friction(ton/m2)

0 1000 2000 3000 4000 50000 1000 2000 3000

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Figure 3. Comparison of time consumed in different construction activities. 4. INSTRUMENTATION

Both, pile and barrette were instrumented with Vibrating Wire Strain Gauges (VWSGs) and Mechanical Extensometers (ME) at five levels along the pile shaft at the known interface boundaries of different soil layers. Pile has four sets of VWSGs and one set of ME at each level. While the Barrette has six sets of VWSGs and two sets of ME at each level, as depicted in Figure 2.

5. LOAD TEST ARRANGEMENT

Reaction frame anchored against the four working piles was used to load the piles. Reaction

frame used to load the barrette was one of the biggest of its kind used in the region with overall height equal to about 8 m as shown in Figures. 4a and 4b. Five numbers of built-up steel girders supported on each side by two 1st level cross girders were used to achieve the required maximum frame capacity of 6000 tons. 1st level crossbeams were supported against the 2nd level cross girders. 2nd level cross girders were then anchored against the surrounding barrettes using anchor blocks at the top. Especially fabricated rigid transfer girders as shown in Figure 5 were used to distribute the tension force coming from the Tie-bars to the dowel bars above the anchor barrette heads. Sixteen number of hydraulic jacks each having 500 ton capacity, were installed between the barrette cap and main girder of the reaction frame. Jacks were arranged in a symmetrical fashion as shown in Figure 4b, to avoid any possible eccentricity of loading. Ball bearings were used between the main girders and hydraulic jacks to keep the line of loading in the true vertical direction.

6. EFFECT OF SHAPE ON SHAFT LOAD TRANSFER Assessment of shape effect on the shaft load transfer has been attempted by various researchers

in the past. Hosoi et al (1994), concluded that the earth pressure acting on the flat surface of a rectangular diaphragm wall panel is larger than that of circular bored piles. Numerical analysis performed by them showed that the earth pressure on the flat surface of a trench with L/B = 1000 (plain strain conditions), where, L and B are the length and width of the trench in plan respectively, is higher than that of circular borehole. In order to assess the effect of different aspect ratios (L/B) on the earth pressure developed around the trench, attempt has been made to model the behavior of trench using finite element computer program. Figure 7 shows the orientation of principal stresses and stress contours developed around the trenches with different L/B ratios. It has been observed that sufficient hoop compression stresses develop around the trench and if L/B <3. But if L/B is increased above 5, longer sides of the trench start yielding and it can be concluded that under such conditions hoop stresses around the trench become less effective which give rise to the increased

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0 10 20 30 40 50 60 70 80

Bored Pile

Barrette

Tim e Consum e d (hours)

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DeSanding���Cage Lowering

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Concreting

348

Figure 4a. View of the test frame having maximum capacity of 6000 tons.

Figure 4b. Barrette load test plan.

5.00

m

2 x I 600 x 2700 x 10000 mm.

B9

Reference Beam

1st Cross Beam

B

2nd Cross Beam

5.00 m

I 350 x 2000 x 8000 mm.

5 x I 400 x 2000 x 9000 mm.

2 x I 600 x 2700 x 10000 mm.

1st Cross Beam (R)

B13

B14

B20

5.00 m

Main Beam

B15

A

5.00

m

I 350 x 400 x 7500 mm.2nd Cross Beam

I 350 x 2000 x 8000 mm.

2nd Cross Beam

Hydraulic Jacks

349

Figure 5. Stiffening girder used to distribute force to dowel bars.

Figure 6. A view of the instrumented barrette cap with jacks.

350

Figure 7. Orientation of principal stresses and stress contours around the excavated trenches with different aspect ratios.

L/B = 1

L/B = 3

L/B = 5

351

earth pressure on flat surface of trenches with large aspect ratios. Although the exact determination of earth pressure developed on the surface of trench depends on the soil properties and the over burden pressure at the level of consideration, it can be concluded that no significant difference in earth pressure is observed between the bore hole (L/B=1) and the barrette (L/B =2). 7. RESULTS OF LOAD TEST

Values of unit skin friction developed at the interface of different soil horizons along the pile and barrette shafts are given in Table 2 and are also plotted in Figure 2. Comparison of unit skin friction values prove that there is no significant difference in shaft load transfer between pile and barrette, even though the construction methodology adopted is different. These findings are in line with the conclusions made by Thasnanipan et al (1998-a). Load settlement curves of the pile and barrette are shown in Figure 8. Table 2. Comparison of unit skin friction and end bearing mobilized for the test pile and barrette.

Mobilized Skin Friction (ton/m2)

Depth Soil Type Pile Barrette

(m) At Design Load Max. Mobilized At Design Load Max. Mobilized

0 - 13.5 Bangkok Soft Clay (CH) 0.20 2.63 0.26 2.24 13.5 - 25.0 Stiff - V. Stiff Clay (CH) 5.90 7.01 3.58 8.86 25.0 - 35.0 Med.-Dense Silty Sand (SM-SP) 11.46 22.76 13.25 25.28 35.0 - 50.0 Hard Silty Clay (CH) 3.09 11.11 3.55 9.61 50.0 - 55.0 3.5m Hard Clay + 1.5m Clayey Sand (SC) 3.43 14.84 3.19 7.49

Maximum Mobilized End Bearing (Tom/m2) 270 101

Figure 8. Comparison of Load Settlement Curves of Barrette and Pile.

0

20

40

60

80

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500

A p p lie d L o a d (T o n s)

Pil

e H

ea

d M

ove

me

nt

(mm

Barrette

Pile

352

8. CONCLUSIONS 1) No significant difference in shaft load transfer has been observed between the wet process bored

pile and barrette, in spite of the presence of considerable difference in construction techniques followed and other relevant parameters like bentonite slurry viscosity and construction time.

2) All foreseeable negative effects attributed to construction parameters can be eliminated by

observing good engineering practices. 3) Reaction frame can be designed and erected to apply very high compression loads of the order of

6000 tons for pile load testing. ACKNOWLEDGEMENTS

The authors wish to thank Dr. Wanchai Teparaksa and their colleague Mr. Kamol Singtogaw for their invaluable suggestions. They also wish to thank Mr. Natamon of STS Engineering Consultants Co., Ltd. for providing the necessary data. REFERENCES Drilled Shaft Inspector’s Manual (1989), Prepared by The Joint caisson – Drilled Shaft Committee of

the ADSC: The International Association of Foundation Drilling and DFI: Deep Foundation Institute, NJ. USA.

Hosoi, T, Yagi N & Enoki, M. (1994), Consideration to the Skin Friction of Diaphragm Wall Foundation, 3rd Intl. Conf. on Deep Foundation Practice incorporating PILETALK, Singapore.

Static Load Testing Report of Barrette Pile for BECM Tower Project, Rama IX Road, Prepared by EDE-STS Engineering Joint Venture, Bangkok, Thailand.

Static Load Testing Report of Bored Pile for BECM Tower Project, Rama IX Road, Prepared by EDE-STS Engineering Joint Venture, Bangkok, Thailand.

Thasnanipan, N, Baskaran G. & Anwar, M.A. (1998-a), Effect of Construction Time and Slurry Viscosity on Shaft Capacity of Bored Piles, Proc. of the 3rd Intl. Geotechnical Seminar on Deep Foundations on Bored and Auger Piles, Ghent, Belgium, Balkema.

Thasnanipan, N., Maung, A.W. & Tanseng, P (1998-b), Barrettes Founded in Bangkok Subsoils, Construction and Performance, Proc. of the 13th Southeast Asian Geotechnical Conf. Taipie, Taiwan, R.O.C.

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