Development and Performance Study of a New

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The 12 International Conference of International Association for Computer Methods and Advances in Geomechanics (IACMAG) 1-6 October, 2008 Goa, India

Development and Performance Study of a New Apparatus to Impart Lateral Cyclic Load on Model Piles S. Basack Assistant Professor in Applied Mechanics, Bengal Engineering and Science University, Shibpur, Howrah-711103, India. Keywords: Pile foundations, offshore structures,cyclic loading ABSTRACT: The environment prevalent in ocean necessitates the pile foundations supporting offshore structures to be designed against lateral cyclic loading initiated by wave action. Such quasi-static load reversal induces progressive deterioration in the strength and stiffness of the soil-pile system. To understand the effect of lateral cyclic load on model pile foundation, a new apparatus was designed and fabricated. For performance study of the set up, a series of trial tests were performed. This paper presents detailed description of the apparatus, calibration and operating principle of each of its components and the observations made from the experiments performed.

1 Introduction Offshore structure, namely, oil drilling platforms, jetties, tension leg platform etc. is supported on pile foundation. Apart from the usual super structure load, wave loading is an important parameter for design of such piles. Ocean waves are though random, periodic in nature. This random periodicity causes the cyclic loading on the offshore pile foundation, which may be under load-controlled mode and displacement controlled mode. In case of load-controlled mode of cyclic loading, the load applied at pile head varies cyclically with time such that the maximum and minimum remains constant for all cycles. In case of displacement controlled mode of cyclic loading, on the other hand, the lateral displacement at pile head varies cyclically with time such that the maximum and minimum remains constant for all cycles. The quasi-static nature of this type of loading induces progressive degradation of foundation associated with increased pile head displacement. The offshore pile foundations should be designed considering two phenomenon, viz. adequate factor of safety against ultimate failure and acceptable deflection at pile head. In accordance with Basack (1999), the alteration in pile capacity due to transient loading depends upon the following parameters (generally known as cyclic load parameters) : number of cycles, frequency and amplitude. The work reported herein is aiming towards development and performance study of a new apparatus for imparting lateral cyclic loading on model pile foundation under controlled condition. The set-up consists of both mechanically controlled as well as electrically controlled components. This paper includes a detailed description of the apparatus developed, calibration and operating principle of each of its components, the observations made from trial experiments and the relevant conclusions drawn therefrom.

2 The Apparatus Since no standard apparatus for imparting lateral cyclic load on piles is available, a new set up was designed and fabricated in the Applied Mechanics Department, B.E.S.University, India. A schematic diagram and a photographic view of this apparatus are shown in Figs.1 & 2 respectively.

2.1

General description

The important components of the apparatus were : I) II) III)

The test tank The loading device and Ancillary equipments.

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A brief description of these equipments is given in the following sections. 2.1.1 Test tank A special stainless steel tank was designed and manufactured for preparing the soil bed. The tank

To mains Central Motor and Gear System

Chain Load Controlled Unit

Detachable Sproket and Sprocket

Detachable Sproket

Displacement Controlled Unit

Static Loading Device

Through Connecting Rod Through Rollers

Through Sliding Unit (Male & Female) Pile Head Connector

Figure 1. Basic operating principle of the Cyclic Loading Device.

Figure 2. Photographic view of the Cyclic Loading Device. consisted of three flanged segments each having 200 mm height, 400 mm internal diameter and 5 mm wall thickness. Flanges of each of the segments were provided with holes for bolting purpose. Rubber gaskets were sandwitched between the flanges of the adjacent segments for water-tightness. Provision was kept at the bottom of the tank to allow drainage of water from the soil bed, whenever required.

The loading device 2.1.2 The loading device consists of two major units, viz. the static loading unit and the cyclic loading units. Both of them were connected in parallel with a central motor and gear system, such that one unit could be operated at a time. By chain and sprocket arrangements, each unit could be engaged separately with the motor-gear system.

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2.1.2.1 Central motor and gear system The central motor and gear system consisted of a 2 H.P., 3 Phase, reversible, induction type of motor which rotates at 1200 r.p.m. By means of a 1:2 reduction gear box, this speed was reduced. This reduced speed was fed to a PIV (Positive Infinitely Variable Drive) Drive where by means of a slotted chain, further speed reduction from an input speed of 600 r.p.m. to the output speeds of minimum 10 r.p.m. and maximum 100 r.p.m. were obtained. To transmit the power from the motor to the reduction gear box, a two-step belt and pulley arrangement was used. 2.1.2.2 Static loading device For static load test, the apparatus was designed in a manner that the strain controlled loading can be applied at the pile head, where the pile is to be pushed unidirectionally at a constant rate of horizontal displacement. By measuring the applied lateral load and the corresponding horizontal deflection of pile cap, the lateral loaddeflection curve could be plotted. The static loading device consisted of three shafts connected in series between the output point of the central motor and gear system and the pile head by means of bevel gears (gear ratio = 1:1). The end shaft was threaded throughout its length to provide the steady unidirectional motion with the aid of worm gear mechanism. The holder of the worm gear was connected to the pile head connector through a load cell. The photographic view of the static loading device is shown in Fig.3.

Figure 3. The static loading device. 2.1.2.3 Cyclic loading device The set up was made in such a way that the cyclic loading tests could be performed under the two modes at different amplitudes. Both the displacement controlled and load controlled units were connected in parallel between the pile head and the motor gear system such that one unit can be operated at a time. The displacement-controlled tests were performed by means of an adjustable differential cam mechanism to convert the rotation into horizontal sinusoidal translation which was finally be applied to the pile head. The differential camshaft was uniquely designed to get various amplitudes of cyclic displacement. It consisted of an eccentric shaft attached to the main shaft. A slider was being fixed to the eccentric shaft by means of which the eccentricity and hence the displacement amplitude could be adjusted by rotating the two bolts fixed at the opposite ends of the slider. The load-controlled unit, on the other hand, was capable of providing a symmetrical two-way lateral cyclic load. It consisted of an oscillating arm supported on a single point joint. At the bottom of the arm, a semi-circular pinion was fixed which was engaged to a rack. The other end of the rack is connected to the pile head. A movable weight was allowed to slide over the oscillating arm keeping the pin joint at the middle. This weight was provided over the oscillating arm by means of a cylindrical stainless steel container in which different weight blocks could be placed. The container was capable to move smoothly along the rail welded on the top of the oscillating arm. The container was connected to a crankshaft arm. The motion from the main shaft to the crank was provided by means of chain and sprocket arrangement. The photographic views of the two units are presented in Figs.4(a & b).

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2.1.3 Ancillary equipments A number of ancilliary equipments are attached with the apparatus, as described below: 1)

Load Cell: To measure the axial load applied on the model pile during static test in progress, a load cell with ± 500 kg. capacity is attached between spindle and the pile cap.

2)

Dial Gauge: To measure the pile head deflection in the axial direction a dial gauge with 0.01 mm. least count is used.

3)

LVDT: A Linear Variable Differential Transducer having used.

± 30 mm displacement measurement capacity is

(a)

(b) Figure 4. The cyclic loading device :

(a) load-controlled unit.

(b) displacement-controlled unit.

4)

Strain Gauge: Electrical Resistance Strain gauges are used in the Instrumented Pile to measure the strains acting on various points on pile.

5)

Digital Indicator: A digital indicator is used to display the Load Cell reading, LVDT reading and especially the Strain gauge reading digitally.

6)

Pile Head Connector: To attach loading frame with the pile a detachable mild steel plate is used, which can be rigidly fixed with the pile head by threads.

7)

Pile Driving Unit: To insert the pile into the soil bed a screw-jack type arrangement is fabricated. It can be operated by a driving wheel. The photographic view of the Pile Driving Unit is shown in Fig.9.

8)

Mechanical Counter: To measure the applied number of cycles, a mechanical counter is attached to the main shaft.

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3 Calibration of Various Components 3.1

Cyclic loading device

The basic operating principle of the cyclic loading device is illustrated in Fig.5, which represents the free body diagram of the oscillating member. Cyclic loading device of the experimental set up was calibrated by placing various loads of different values into the load container, setting the container at the two extreme ends of the rail track and taking the corresponding load cell reading for each case, the average of which indicated the amplitude of applied cyclic load on the model pile foundation. A linear variation was observed from the graph plotted by the calibration data. Fig.6 shows the variation of transmitted load (W) against amplitude of lateral cyclic load.

x

h

W

R

H V H= Wx /h = (Wx0/h) Sin 2πft where, x0 = Amplitude of ‘x’ f = Frequency. Figure 5. Free body diagram of the oscillating beam of the Cyclic Loading Device.

3.2

Mechanical frequency controller

The main component of the Mechanical Frequency Controller was the PIV Drive by which can regulate the speed of the shaft between 15 c.p.m to 85 c.p.m. The knob of the PIV Drive was marked against different speeds of the shaft counted by a Tachometer.

3.3

Time variation of lateral cyclic load

The cyclic loading device consists of an oscillating arm supported on a single point joint. A movable weight slides over the oscillating arm sinusoidally keeping the point joint at mean. A known movable weight was placed at specified locations on the arm-rail and the corresponding load cell reading in each case was recoded. The actual variation of load was obtained by plotting the experimentally obtained lateral cyclic load with respect to time. Fig.7 shows the variation of cyclic load on pile with time, which

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Transmitted Load (kg)

30 25 20 15 10 5 0 0

10

20

30

Imposed Load (kg)

Figure 6. Variation of imposed load with cyclic load amplitude.

1.5 Load Normalized Cyclic L

Sinusoidal variation

1.0

Measured variation

0.5 0.0 0.0

0.2

0.4

0.6

0.8

1.0

-0.5 -1.0 -1.5 Normalized Time

Figure 7. Time variation of lateral cyclic load generated.

was observed to be approximately sinusoidal. The slight deviation from the true sinusoidal variation is due to various losses i.e. frictional loss, mechanical losses, etc.

3.4

Rate of horizontal translation of the spindle in static loading device

A graph showing the rate of spindle movement of the static loading device was plotted against the output speed of the PIV gear box. The variation was linear in nature, as shown in Fig.8.

3.5

Displacement control unit

The Displacement Controlled Unit consists of an adjustable cam device which was calibrated by adjusting the bolts and marking the effective eccentricity of the cam.

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Spindle Movement Rate (mm/mt)

60 50 40 30 20 10 0 0

10

20

30

40

Speed (r.p.m.)

Figure 8. Variation of the rate of spindle movement with output speed

4 Test Procedure The testing should be done following a definite sequential procedure as described below : 1)

First of all, the soil bed was prepared in the test tank following a definite procedure described in the following section.

2)

Then the pile group was inserted into the soil bed by means of the pile-driving unit, the rate of insertion was as low as 1mm/min. The pile cap was then fixed rigidly with the pile head connector by means of bolts.

3)

The pile head connector was connected with either the load controlled or displacement controlled unit depending on the mode of cyclic loading desired in the experiment.

4)

Thereafter, the desired load amplitude or displacement amplitude are set as per requirement. The frequency was also set to desired magnitude.

5)

The motor was started. As soon as the desired number of cycle was attained, it was stopped.

6)

Now the cyclic loading unit was dis-engaged from the pile head connector and the static loading unit is engaged to the power shaft through an electronic load cell.

7)

The dial gauge or the linearly variable differential transducer (LVDT) was placed with the static loading shaft

8)

The machine was started once again. The load and displacements were recorded at regular intervalzs upto about 4 mm lateral deflection of pile cap (about 20% of external pile diameter).

9)

For each test, separate soil bed is to be prepared.

5 Materials 5.1

Soil

Uniformly graded yellow sand (particle size ranging from 0.1 mm to 0.2 mm) having specific gravity of 2.58 was used for preparing the test bed. The tank was filled with the sand in six equal layers, each layer being filled up initially by rainfall technique form a uniform height of 1m and thereafter by compaction with a special rammer (60 N x 450 mm). The relative density of the prepared sand bed was found to be 65%. As obtained from direct shear test, the values of c and φ of the sand bed were 0 and 32o and the initial tangent and the secant modulii were 2 2 obtained respectively as 605 N/mm and 327 N/mm .

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5.2

Pile

Experiments were carried out with 2 x 2 pile group, each pile being hollow circular stainless steel bar having 20 mm outer diameter, 5 mm wall thickness and 600 mm overall length. The depth of embedment was 500 mm (L/d = 20) and the lateral load was applied from a height of 90 mm above the soil surface. In order to push the piles easily through the soil medium, the tip of the piles were pointed in shape. The piles were threaded at the top to attach with a common pile cap which was a 16mm thick square steel plate. The c/c distances between the piles in the group was 60 mm. (= 3d). The photographic view of the piles with the cap are shown in Fig.9.

Figure 9. The piles used for experimentation.

6 Test Programme In order to study the performance of the apparatus, a series of experiments were performed under load-controlled and displacement-controlled modes of lateral cyclic loading. The tests conducted are presented in Table-1. The frequency was kept at 15 cycles per minute (c.p.m.), the minimum value that may be incorporated in the apparatus. This value is close enough to the Indian coasts where the period of sea waves is about 10-12 seconds. Table.1. The experimental programme. Mode of Cyclic Loading

Amplitude Number of Cycles (%) 14* 100 250 500 Load-controlled 20 100 250 500 27 100 250 500 $ Displacement16 100 250 500 Controlled 32 100 250 500 Frequency : 15 cycles per minute (c.p.m.) * Normalized with static lateral capacity of pile-group. $ Normalized with external pile diameter.

7 Results and Discussion 7.1

General observations

During experiments in progress, a basin-like depression was observed to form around the pile group in the vicinity of soil surface. This is understandably due to gradual shifting of soil particle away from the pile surface as well as simultaneous compaction of the sand mass surrounding the pile group due to cyclic stresses generated. The plan of this conical depression was observed to be elliptical. The diameters of the ellipse were noted to increase with increment of the cyclic loading parameters.

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7.2

Load displacement curve

The recorded values of lateral static load applied on piles were plotted against the corresponding horizontal deflection of the pile cap to obtain the load-deflection curves. These curves are observed to be hyperbolic in nature. A representative load deflection curves [pre-cyclic static] is shown in Fig.10 The ultimate capacities are estimated by double-tangent method. A set of typical post-cyclic static load-displacement curves is presented in Fig.11.

Degradation factor 7.2.1 The deterioration of the pile-soil interactive performance was expressed by a non-dimensional term called degradation factor defined as the ratio of post-cyclic to pre-cyclic ultimate lateral pile capacities [Purkayastha and Dey (1981)]. The experimental degradation factors are presented in Table-2. It was observed that for the majority of the test conducted, the values of the degradation factor are less than unity which indicates deterioration of pile capacity. In other cases, the degradation factor is marginally above unity implying improvement of pile capacity.

250

Ultimate capacity : 99 N. Ultimate capacity : 50 N.

Lateral Load N) (N)

200

150

100

50 Pile Group

Single Pile

0 0

2

4

6

8

10

12

Lateral Deflection (m m )

Figure 10. Pre-cyclic load-deflection curves.

Lateral Load (N)

.

100

Displacement-controlled test. Amplitude = 16 %.

80 60 40

N=100 20

N=250

No. of cycles (N) : N=500

0 0

2

4

6

8

10

12

Lateral Deflection (mm)

Figure 11. Post-cyclic load-deflection curves.

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Table-2 : Values of experimental degradation factors. No. of cycles

Load-controlled mode

Displacementcontrolled mode Amplitude (%) : 16 32 0.74 1.007

Amplitude (%) : 20 27 0.83 0.98

100

14 0.78

250

0.71

0.85

1.02

0.91

1.07

500

0.85

1.00

0.87

0.92

1.05

7.2.3 Variation of degradation factor with cyclic loading parameters The degdaration factor was found to vary with cyclic load parameters. A set of representative curves indicating the variation of degradation factor with the various cyclic loading parameters are shown in Figs.12 & 13. However, no definite pattern of variation could be concluded. 1.2

Load-controlled tests.

Degradation factor

1

0.8

Amplitude (A) :

0.6

A=14% A=20% A=27%

0.4 0

100

200

300

400

500

600

Number of cycles

Figure 12. Variation of degradation factor with number of cycles.

Degradation factor

1.2

Load-controlled test.

1

No. of cycles (N) : 0.8 N=100

0.6

N=250 N=500

0.4 10

15

20

25

30

Am plitude (%)

Figure 13. Variation of degradation factor with cyclic load amplitude.

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8 Conclusion In order to carry out model tests on piles under lateral cyclic loading, a mechanically operated apparatus has been developed. The apparatus mainly consists of motor and gear arrangements associated with crank shaft mechanism and other mechanical components to provide symmetrical two-way lateral cyclic loading on piles under load-controlled and displacement-controlled modes. For performance study of this apparatus, a series of tests have been conducted on 2 x 2 pile group embedded vertically in a bed of well graded fairly dense yellow sand. The entire work may be concluded as follows: •

The Multi Purpose Cyclic Loading Device developed was observed to perform quite satisfactorily. The cyclic loading parameters, i.e., frequency and amplitude can be varied in a wide range.

•

It was observed that the time variation of cyclic lateral load applied on the pile cap, in case of loadcontrolled mode, was approximately sinusoidal in nature. On the other hand, the time variation of cyclic lateral displacement applied on the pile cap, in case of displacement-controlled mode, was purely sinusoidal in nature because of differential cam mechanism.

•

Under the effect of lateral cyclic loading, the ultimate lateral capacity of pile foundation altered. In few limited cases, some improvement in the pile capacity was observed as well.

•

During cyclic loading in progress, basin-like depression was observed to form around the piles in the vicinity of soil surface. In plan view, the shape of this depression was elliptical.

•

Both pre-cyclic and post-cyclic load-displacement responses were observed to be hyperbolic in nature.

•

It was observed that for the majority of the test conducted, the degradation factor varied with cyclic load parameters. However, no definite pattern could be concluded.

9 Acknowledgement The author gratefully acknowledges the financial assistant received in form of a major research project under the University Grants Commission, India, to carry out the entire investigation.

10 References Basack, S. “Behaviour of Pile under Lateral Cyclic Load in Marine Clay”, Ph.D. thesis submitted to Jadavpur University, Calcutta, India, August 1999. Babu Rao, D., and Ahmed, M., “Lateral Load Analysis of a Single Pile”, Proceedings of Indian Geotechnical Conference, Vol.1, December-2004, pp 372-375 Gupta, S.S. and Datta, M., “Degradation of Static Pull Out Capacity of Cyclically Loaded Piles in Soft Clay”, Indian Geotechnical Journal, Vol. 24, No. 7, August 1996, pp 364-385. Purkayastha, R.D., and Dey, S., “Behavior of Cyclically Loaded Model Piles in Soft Clay”, Proceedings of Second International Conference on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics, University of Missouri – Rolla, December 1991, pp 1-9.

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