Proceedings of Indian Geotechnical Conference December 15-17, 2011, Kochi(D-177)

BEHAVIOUR OF SINGLE PILE IN SLOPING GROUND UNDER STATIC LATERAL LOAD

Sivapriya S.V, Research Scholar, Department of Civil Engineering, IITM, Chennai-36,sivapriyavijay@gmail.com

S.R.Gandhi, Professor, Department of Civil Engineering, IITM, Chennai-36, srgandhi@iitm.ac.in

ABSTRACT: Earth slopes are either man made or natural on which either structure are placed or construction activities are

carried out. Typical examples of earth slopes are rail embankment, road embankments, river training bunds,excavation for

mines, dredging for berthing structures results in slope over which certain facilities are required to be built. These facilities

are often tall and subjected to large lateral loads and may require use of pile foundation for stable support on slopes. The

behaviour of soil slope and pile foundation passing through it is a complex soil-structure interaction problem. Though the

behaviour of pile under lateral load is well known for horizontal ground surface, its behaviour on sloping ground is not

studied in detail. The lateral capacity of pile gets considerably reduced when the applied horizontal force is in the direction

of slope. No well defined guideline is available to estimate the lateral capacity of such piles on slope. This also depends on

the relative position of the pile with respect to the slope. An attempt is made by conducting experiments to study the

behaviour of pile in sloping ground. The parameters varied are position of piles and variation of slopes. The results are

compared with the pile in normal ground condition.

INTRODUCTION

Pile can be subjected to axial (tension or compression) load

and lateral load or combination of these loads. The loads

are from super structure, earth pressure on retaining wall,

wind load, wave and current actions in case of off- shore

structures and slope movement. The soil-pile interaction

mechanism in a sloping ground is different from that in a

horizontal ground. Pile subjected to lateral load at the head

and transmitting that load to the soil occur in active pile;

whereas Pile subjected to load from moving soil which

induces forces and bending moment to the pile is termed as

passive pile [1]. In passive piles the soil is pushed beneath

the embankment and between groups of piles. The

generation of passive lateral pressure in pile cause local soil

shear as the soil `squeezes' between and around piles [2].

In active pile pile-displacement y is relatively rigid to boundaries, which is also termed as relative soil

displacement [3]. The active length of the pile depends

upon the head conditions, loading conditions and stiffness

of the pile [4]. This active length is different for soil in

horizontal ground and sloped ground. Inclination in ground

significantly affects the ultimate load carrying capacity of

the pile, where the loading is in downslope direction. But

the stiffness of the p-y curves is not much affected at small

deflection [5].It is observed [6] from the limit equilibrium

model that the effect of slope for pure cohesive soil was

significantly less than the effect predicted for c- soils. Using upper bound plastic solution [7] a chart for undrained

lateral capacity of soil in which reduction factors are

included. When a pile is rested in a sloping ground, the

additional force due to instability of the slope should be

considered for design purpose [8].Whereas for a short pile

group near the crest of a slope, a significant reduction in the

group efficiency is observed as the displacement increases.

In short pile groups located near the crest of the slope, the

rotation point of the leading piles occurs at a slightly

greater depth than that of the single pile, with no variation

of its depth irrespective of the pile spacing [9].

A graph is proposed for non- vertical ground [10]. The

study illustrates the effect of slope on the strength of the

pile-soil system could be neglected for piles located beyond

5 pile diameters (5D) from the crest of the slope. The effect

of slope on ultimate load capacity for slope angles less than

45 becomes less than 10% for distances greater than 6D,

and therefore the slope effect beyond that can be

neglected[11]. From small-scale models and mathematical

models underestimate the effects of slope on maximum

bending moments developed in piles. The locations of

maximum and minimum bending moments in the pile were

not affected by the presence of a slope [12].

Here the study is mainly focused on active pile where the

tendency to drag the soil with the piles is ignored.

SOIL PROPERTY

The soil is taken from Siruseri, Chennai. The soil is then air

dried and soil properties were found as per Indian

standards. The basic soil properties are listed in Table 1.

Table 1 Soil Property

Property Values

Liquid Limit 66% Plastic Limit 27% Plasticity Index 39% Specific Gravity 2.68

0.5%

23% &76.5%

Grain Size Distribution Sand Silt & Clay IS classification CH

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Sivapriya.S.V & S.R.Gandhi

MATERIAL PROPERTY PILE

The diameter of the pile is fixed as 16mm with 1mm

thickness, so that it is free from side wall effects. The

material opted for the experiment is a hollow Aluminium

pipe. Pile length is taken as 450mm, which is termed as

semi-infinite. From the lateral load behaviour, it is observed

that the pile behaves as long flexible pile. The flexural

stiffness of the pile is 93.1x 106 Nmm2 which is found using

simple beam bending test.

EXPERIMENTAL WORK

Preparation of Clay bed

The soil bed is prepared for two different shear strengths

say 30kPa and 50kPa with unit weight of 1.8g/cc and

1.98g/cc. The soil is filled in the tank of dimension 1m x

0.8m x 0.6m by kneading compaction method (Rao [13]).

Instrumentation of Pile

Strain gauges of 120 resistance and 10mm gauge factor are used for the experiments. The gauges are pasted in 0.1L,

0.25L, 0.5L and 0.7L from the top in the tension side of the

modelled pile; where L is the length of the pile. These

strain gauges are then connected to the data acquisition

system by means of quarter bridge connection. The data

acquisition system, Spider 8 with Carman professional is

used; the deflection is measured using LVDT.

Calibration Calibration of strain gauges is performed using simple

beam bending test in which the model pile is placed simply

supported on a knife edge. The calibration constant is found

to be 0.0278 N-m/ strains. The calibration graph is shown

in Fig 1.

Parameters to be varied

In present, the study is focused on single pile. The Position

of piles and slope angle are varied for the two different soil

strengths. The varied parameters are listed in Table 2.

By conducting tests pile subjected to lateral load in

horizontal ground, the relative stiffness factor( R) is found

to be 94 mm for 30kPa soil and 76mm for 50kPa soil.

Fig 1. Calibration of Strain Gauges

Table 2 Parametric Study

Parameter Variations

Soil Strength 30 kPa and 50kPa Ground Condition 1V:2H, 1V:2.5H ,1V:3H

and Horizontal ground Position of pile Crest ,1R and 2R

Experimental Procedure

The soil is filled in the tank to posses the desired strength.

The desired strength is obtained by determined unit weight

and water content. This is obtained by means of proctor

compaction and UCC tests. The pile used is of in-situ semi

infinite pile, which is initially placed in position followed

by soil. The known weight is compacted by means of

kneading compaction technique. The load applied is of

static lateral load which is applied towards the slope. High

tension wire is used for the application of load. The

schematic representation of the set-up is shown in Fig 2.

Fig 2. Schematic diagram of Loading Set-up

RESULTS AND DISCUSSION

Load displacement curve

The dead load is applied and allowed for fifteen minutes to

ensure the displacement occurred due to the loading. LVDT

is used to read the displacement. Initial test was carried out

for the horizontal ground condition and followed by the

sloping ground (1V:2H, 1V:2.5H and 1V:3H) and different

position of piles (crest of the slope, 1R and 2R). The pile at

1R and 2R positions has eccentricity as the load is applied

at same level as that of the horizontal ground. The

maximum load is calculated by eliminating the eccentricity

(e).

(1)

The load corresponds to 3mm deformation (y) is taken

initially taken when a pile is at crest, where e = 0. Using the

Eq. 1 the value of yh is found and the value of Mh is found

by the chart given by Davission and Gill [14] for free pile

head in soil of constant modulus. It was then back

calculated to find the load corresponding to the deformation

obtained through the Eq. 1.

Figures 3 and 4 show the typical load- displacement curve

for the slope 1V:2H for soil strength of 30kPa and 50kPa.

DAS-

Spider 8

PC with Catman

Professional

LVDT +

Read out

unit

Instrume

nted Pile

Clay

Tank dim

1x0.8x0.6

(m)

Loading

set-up 0.45

1.00m

0.3m

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Behaviour of single pile in sloping ground under static lateral load

The load carrying capacity of pile to the deformation

corresponding to the obtained value through Eq. 1for the

slope 1V:2H, 1V:2.5H and 1V:3H are listed in Table 3 and

4.

Fig 3. Load-Displacemnt curve for single pile(1:2) - 30kPa

Fig 4. Load-Displacemnt curve for single pile (1:2) - 50kPa

Table 3 Load corresponding to zero eccentricity-30kPa

Capacity of the pile ,N Slope/Distance

Horizontal Crest 1R 2R

1V:2H 215 140 85

1V:2.5H 250 170 125

1V:3H

300

280 210 135

Table 4 Load corresponding to zero eccentricity 50kPa

Capacity of the pile ,N Slope/Distance

Horizontal Crest 1R 2R

1V:2H 270 215 165

1V:2.5H 350 240 215

1V:3H

420

400 260 225

The reduction in capacity for horizontal ground and 1V:3H

is less compared to other slopes. And the capacity decreases

with increase in slope. The reduction in capacity is

significantly less when compared to 1R and 2R position.

From the Fig 5 and 6 shows a comparative graph drawn for

the two soil strengths and for three slopes. For 50 kPa soil,

the load carrying capacity of the pile for slope 1V:2.5H is

similar to 1V:3H.

Fig 5. Lateral load capacity of pile 30kPa

Fig 6. Lateral load capacity of pile 50kPa

Bending Moment

The bending moments at different sections along the pile

were obtained using the bending strains measured at

various points along the length of the model pile by the

strain gauges and bending moment is calculated using the

bending moment constant 0.0278 N m/ strains.

The maximum bending moment is obtained about 0.4L

along the length of the pile for horizontal ground. The

bending moment increases with increase in load and the

pattern satisfies the condition of long flexible pile. The

variation of maximum bending moment with applied lateral

load shows the linear relation between the maximum BM

and the applied load. As the load increases, the soil in front

of the pile fails leaving the top of the pile unsupported and

thus altering the depth to maximum bending moment.

A comparison graph is drawn in Fig. 7 for a slope of

1V:2H.

The study is made with the pile at different locations of

slope (ground level; crest, 1R and 2R towards the slope) for

201

Sivapriya.S.V & S.R.Gandhi

a soil condition 0f 30kPa for a similar load. There is no

significant change in bending moment pattern in the pile at

horizontal ground and pile at crest towards the slope.

The maximum bending moment is observed about 0.4L in

three conditions (ground level; crest and 1R towards the

slope); whereas when the pile is kept in 2R position the

maximum bending is observed at 0.5L.

CONCLUSION

The experimental results of the behaviour of laterally

loaded pile in sloping ground founded in clay shows that

the load carrying capacity of the pile decreases with

increase in slope and distance away from the slope. The

following observations are made,

1. The load carrying capacity decreases as the slope

increases (1V:3H to 1V:2H) as the pile move away

from the crest when compared to that of pile in

horizontal ground.

2. For a given slope angle, compared to the pile capacity at

the crest, the capacity decreases as the pile move

towards the slope.

3. The bending moment increases with the increase in

applied load and when the pile move away from the

slope and decreases as the soil strength increases.

REFERENCES

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loads, State ofthe Art Report, Proc., 9th ICSMFE,

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1018.

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S.R, (2002), Behaviour of sloping ground under

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Bending Moment, N.m

Fig 7. Bending moment of single pile at different location in a slope of 1V:2H

Depth,m

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