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An Experimental Study on Seismic Performance

of Battered Pile Foundation in Soft Ground

K. Tomisawa

Civil Engineering Research Institute for Cold Region, Japan

S. MiuraUniversity of Hokkaido, Japan

L. LiColumbia University, USA

SUMMARY:

In comparison to vertical pile foundations, battered pile foundations are considered to provide a higherhorizontal bearing capacity and reduce construction costs as a result of fewer piles needed. However, since the

seismic behaviour of battered piles in soft ground has not been fully verified from a technical point of view,

centrifuge model tests were performed in this study to examine the related mechanism and clarify the dynamicmechanical behaviour involved. The results confirmed that battered pile foundations generally show a higherseismic horizontal bearing capacity in soft peat ground than vertical pile foundations. It was also verified thatbattered pile foundations provide the required seismic performance in Level 1 and 2 earthquake motions. Theresults confirmed the practical suitability of battered pile foundations for soft ground.

Keywords: battered pile, soft ground, centrifuge model test, seismic performance

1. INTRODUCTION

A number of studies have examined the mechanism of battered pile foundations in light of their

potential to provide higher horizontal resistance and maximum bearing capacity than vertical pilefoundations. In previous studies conducted in Japan (Satoh et. al., 1969a and 1969b), the mechanical

behaviour and usefulness of battered pile foundations were verified through theoretical analysis.

Design methods based on these studies (e.g. Japan Road Association, 2007) are currently applied.

However, as the seismic behaviour of battered pile foundations in soft ground under Level 1 and 2

earthquake motions has not been fully verified, centrifuge model tests were conducted to confirm the

suitability of such foundations for soft peat ground where consolidation settlement may occur. In the

tests, seismic performance in soft ground for both Level 1 and 2 earthquake motions was examined.

2. OVERVIEW OF CENTRIFUGE MODEL TESTS

Centrifuge model tests were performed to clarify the static and dynamic behaviour of battered pile

foundations. In the tests, dynamic excitations were applied to the vertical pile foundation and the

battered pile foundation in a 50 G centrifugal field. The model setups and instrumentations of the pile

foundations are shown in Figure 1. The model pile was made of steel (JIS SS400) with an outer

diameter of 10 mm, a thickness of 0.2 mm and a length of 400 mm to simulate a prototype scale steel

pipe pile with a diameter of 500 mm, a wall thickness of 10 mm and a length of 20 m. In both

foundation models, the piles were fixed at the bottom to represent supported pile boundary conditions.

As the main purpose of this study was to use a battered pile foundation as an abutment foundation of

road bridges, an arrangement involving four coupled piles (2 2) was adopted, and a weight of 800

g which is equivalent to 980 kN in prototype scale was installed to the pile heads to represent the mass

of the superstructure. To simulate soft peat ground as a typical soft medium, model ground was

prepared by mixing peat moss and kaolin clay at a dry-weight ratio of 1:1 to represent saturatedground with an initial water content of w = 300%.

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Mass 800gACC: Accelerometer

P: Strain gauge

d: Laser transducerHorizontal load

Vertical pile

Peat ground

Base ground

Instrumented pile

(a) Vertical pile foundation

(b) Battered pile foundationFigure 1. Model setup and instrumentation

To confirm the effects of the pile spacing (distance between pile centre =D, D: pile diameter) and the

inclination angle () on the seismic behaviour of the battered pile foundations. The combination of

three pile spacing (3D, 4.5D and 6D) and three inclination angle (9, 12 and 15 degrees) was adopted

for tests with battered pile foundation. Together with the test for vertical pile foundation, a total

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number of 6 test cases were carried out. Table 1 shows the test conditions.

The procedure of centrifuge model tests is summarized as follows.

1) Static horizontal loading test on coupled piles2) Level 1 dynamic excitation3) Static horizontal loading test on coupled piles4) Level 2 dynamic excitation5) Static horizontal loading test on coupled piles

In dynamic excitation tests, a sine wave with 20 cycles and a frequency of 1.5 Hz was used as the

input wave form to simulate the earthquake motion in Type III grounds including soft peat ground,

and the acceleration amplitudes of 150 gal and 750 gal were set for Level 1 and 2 earthquake motions

respectively (Japan Road Association, 2002a and Japan Society of Civil Engineers, 2002).

Table 1. Test condition

Test No. Pile type Pile spacing Pile inclining angle

Case 1 Vertical 3D -

Case 2 Battered 3D 12 degrees

Case 3 Battered 4.5D 12 degrees




3. SEISMIC PERFORMANCE OF BATTERED PILE FOUNDATION

The seismic performance of the battered pile foundation in soft peat ground was clarified through

centrifuge excitation tests. Focus was placed on its seismic deformation performance in relation to

that of the vertical pile foundation, and on its soundness under Level 1 and 2 earthquake motions.

3.1. Horizontal displacement response of pile head

The acceleration inputs for Level 1 and 2 earthquake motions in the centrifuge excitation tests are

shown in Figure 2. The wave form was input from the base ground.

Figure 3 shows the horizontal displacement response of the pile head for the vertical pile foundation

and the battered pile foundation in Level 1 and 2 excitations which was measured using a laser

displacement transducer. Since there was almost no difference in static horizontal resistance for

battered pile foundation when the pile spacing was changed from 3D to 6D, focus was mainly placed

on the inclination angle (Cases 2, 5 and 6). As shown in Figures 3 (a), the pile head of the battered

pile foundation showed reducing trend in horizontal displacement response in comparison to that of

the vertical pile foundation (Case 1) under Level 1 earthquake motion. The decrease value was almostproportional to the increase in the inclination angle. The maximum horizontal displacement was

approximately 2 cm for vertical pile foundation, whereas those for battered pile foundations were

approximately 1.5 cm, 1.0 cm and 0.5 cm with the inclination angles of 9 degrees, 12 degrees and 15

degrees respectively. As shown in Figures 3 (b), the maximum horizontal displacement of the pile

head under Level 2 earthquake motion was approximately 6 cm for the case with an inclination angle

of 9 degrees, which was slightly higher than that for the vertical pile foundation (approximately 5 cm).

However, the horizontal displacements were significantly lower for the cases with inclination angles

of 12 degrees and 15 degrees (approximately 2cm and 1 cm).

Based on these test results, it can be concluded that the seismic performance of battered pile

foundations improves with increased inclination angle in comparison to that of vertical pile

foundation.

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(a) Level 1

(b) Level 2Figure 2. Input earthquake motion (in prototype scale)

3.2. Seismic resistance of batter piles before and after vibration

To examine changes in the horizontal resistance of battered pile foundations under Level 1 and 2

earthquake motions, static horizontal loading tests were performed on coupled battered piles after the

dynamic excitation with each level. Figure 4 shows the static loading curves (horizontal loadHversus

horizontal displacement at loading-point y) before and after the dynamic excitation for the all 6 test

cases (Case 1 (Figure 4 (a) for vertical pile foundation and Case 2 - 5 (Figure 4 (b) - Figure 4 (f) for

battered pile foundation). From the figure, it is confirmed that there almost no difference in the loadingcurves before and after the dynamic excitation for all the test cases.

Same as the results of the static horizontal loading tests, the results of the dynamic excitation tests

indicate that the battered pile foundation in soft ground has higher horizontal resistance to both Level

1 and 2 earthquake motions than the vertical pile foundation and provides sufficient seismic

performance. A similar trend was observed for both cases under Level 1 and 2 earthquake motions.

Battered pile foundation with larger pile inclination angle generates potential restraining ability to the

earthquake induced horizontal displacement. There was no difference in the horizontal resistance of

battered pile foundation before and after dynamic excitation under Level 1 and 2 earthquake motions,

and there was no damage to the piles. It can therefore be judged that battered pile foundations provide

required earthquake resistance of the pile structure as specified in the design standard (Japan Road

association, 2002b).

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(a) Excitation under Level 1 earthquake motion (b) Excitation under Level 2 earthquake motion

Figure 3. Horizontal displacement response of pile head under Level 1 and 2 earthquake motions

(in prototype scale)

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(a) Case 1 (b) Case 2

(c) Case 3 (d) Case 4

(e) Case 5 (f) Case 6

Figure 4. Static horizontal loading curves of pile before and after Level 1 and 2 dynamic excitations

(in prototype scale)

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4. CONCLUSIONS

The results of centrifuge model tests performed to clarify the mechanism of battered pile foundations

in soft ground confirmed the following findings in regard to dynamic behaviour:

1) In the centrifuge excitation tests, the battered pile foundation in soft ground showed a trend ofreducing pile head horizontal displacement response under both Level 1 and 2 earthquake

motions in comparison to the vertical pile foundation. The trend was similar to that confirmed

for static horizontal resistance. Seismic performance was therefore considered generally to be

better in the battered pile foundation. It was also found that a larger inclination angle of the

battered pile resulted in a greater pile displacement restraining ability.

2) Based on the results of the static horizontal loading tests on coupled piles conducted after thedynamic excitation tests under Level 1 and 2 earthquake motions, it is considered that battered

pile foundation provide the required seismic structural resistance, as the horizontal resistance

of the foundation in soft ground was generally the same as that confirmed in the horizontal

loading tests performed before dynamic excitation was applied.

Accordingly, it can be concluded that battered pile foundation in soft ground provide the required

seismic structural performance, and that they are suitable and useful in practical application.

REFERENCES

Japan Road Association. (2002a). Specifications for Highway Bridges, Part V: Seismic Design. 210-228.Japan Road Association. (2002b). Specification for Highway Bridges, Part IV: Substructures. 243-265.Japan Society of Civil Engineers. (2002). Standard Specifications for Concrete Seismic Performance

Verification. 107-112.Japan Road Association. (2007). Pile Foundation Design handbook (2006 Edition). 414-429.

Satoh, A., Akai, K. and Funasaki, T. (1969a). Study on Calculation Method for Negative Skin Friction and

Bending of Batter Piles Part 1: Calculation Method for Negative Skin Friction and Bending of Batter Piles.Report of the Japan Highway Public Corporation Laboratory. 76-82.

Satoh, A., Akai, K. and Funasaki, T. (1969b). Negative Skin Friction and Bending Moment of Batter Piles Strain Measurement of Steel Pipe Piles in the Fukuroi District. Proceedings of the Japan Highway PublicCorporation Laboratory Conference. 357-379.

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