Behavior of a model shallow foundation on

reinforced sandy sloped fill under cyclic loading

Md. Jahid Iftekhar Alam

School of Engineering & Information Technology (SEIT)

The University of New South Wales (UNSW)

Canberra, Australia

Presentation outline

Introduction

Limitations of the previous experimental studies

Objectives

Experimentation

Test results

Conclusions

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Introduction

Loading from structures is one of the most important design factors for

foundation. In addition to static loads, the foundation is often subjected to

live loads of different types such as cyclic loading.

Examples of such foundations are:

• Bridge abutments

• Road embankments

• Machine foundations

• Foundations of oil reservoirs

• Coastal structures

• Wind turbine foundations

• Overhead water tank etc.

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Fig. 1: Pictorial view of a bridge abutment

The presence of slopes in its vicinity is another factor that affects the

stability of foundation. In many practical conditions the shallow foundation

may need to be constructed on or near the crest of a sloped soil mass.

Fig. 2: Schematic diagram of a typical bridge abutment on embankment slope

V

Original ground

Road pavement

Common fill

Granular fill

Bridge deck

Bridge girder

Bridge abutment

Sloping surface

H

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In practice, the design of a shallow foundation is often based on the

approximations of the bearing capacity under monotonic loading conditions

which lead to the use of large factor of safety and excessive cost.

Main reasons behind this practice

• Lack of adequate experimental and theoretical studies for

understanding the actual behaviour under cyclic loading conditions.

• Experimental studies require special equipment, and they are time

consuming and costly.

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Fig. 3: Typical load-deformation response of granular soil under a particular loading cycle

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The materials used to support foundations may vary but, generally, well-

graded granular materials such as sand and gravel are used due to their high

bearing capacity, and good drainage and frictional characteristics.

Limitations of the previous experimental studies

Majority of them are related to the foundation on the flat ground and

involvement of slopes is very limited.

Few recent studies can be found in the literature however those are

performed for a particular soil type.

Investigations on the behavior of foundation on geogrid reinforced sloped

granular fill under cyclic loading conditions are also limited.

The effect of any load interruptions during the cyclic loading period which

experience a shallow foundation in practical conditions was not

investigated.

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Objectives

To experimentally investigate the effects of loading amplitude and

number of load cycles on the behavior of a shallow foundation on geogrid

reinforced sandy sloped fill under cyclic loading.

To investigate permanent deformation behaviour of the footing under

cyclic loading.

To investigate the behaviour of residual soil stress at different depth of

the soil mass subjected to cyclic loading.

To investigate the effect of any load interruption on the deformation and

stress behaviour.

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Tested soil

• Well-graded sand (5% non plastic fines)

• Maximum dry density (MDD)=1819.5 kg/m3

• Optimum moisture content (OMC)=4.75%

• From triaxial test ϕ=440, ψ=130, c=8.2 kPa

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Experimentation

Fig. Particle size distribution curve

Geogrid reinforcement

• Polyester geogrid (Miragrid 8XT)

• Longitudinal member width=8 mm @ 20 mm

• Transverse member width=4 mm @ 30 mm

Fig. Pictorial view of the geogrid reinforcement

Model foundation test configuration

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Fig. Schematic diagram of the model shallow foundation testing system

1

0.2 m

Compacted soil

Footing

1 m

2 m

0.5

m

Surcharge

Geogrid

reinforcement

0.2

m

EPCs

LVDTHydraulic actuator

2

Applied cyclic loading paths

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Fig. Loading path-1

Fig. Loading path-2

Time, t

Applie

d load (

kN

)

Stage-1 Hold-1 Stage-2 Hold-2 Stage-3 Hold-3 Stage-4

Time, t

Applie

d load (

kN

)

Stage-1

Hold-1

Stage-2

Hold-2

Stage-3

Test resultsPermanent deformation behaviour

Fig. Cumulative vertical permanent deformations vs. N Fig. Cumulative horizontal permanent deformations vs. N

Permanent deformations increased with the increase of N.

The curves did not show any deflection during the hold periods which was indicative

of a negligible effect of hold periods on the permanent deformations.

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Fig. Cumulative permanent deformations vs. N for F-0.5-27&45*

The hold period had a significant effect on the permanent deformations where

sudden increases of both the vertical and horizontal permanent deformations were

observed after each hold period.

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Residual soil stress behavior

Fig. Residual soil stresses vs. N for F-0.5-27&45 Fig. Residual soil stresses vs. N for F-0.5-27&45*

Residual soil stress was maximum at a depth of 200 mm and reduced with the

increment of depth. Hold periods had almost negligible effect for loading path-1.

The reloading after each hold period showed significant increase of residual soil

stress at every EPC level for loading path-2.

Conclusions

The cumulative vertical and horizontal deformations increased with the

increase of N and the majority of the permanent deformations occurred

within first few thousand loading cycles.

The hold periods showed negligible effect on permanent deformations

when the load was held at the minimum value of the cyclic loading

amplitude.

Significant increases of vertical and horizontal permanent deformations

were observed in each stage when the load was entirely released during

hold periods.

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The residual soil stresses at different depth of the soil mass increased

with the increase of N for first few thousand loading cycles and after that

the stress remained almost constant up to the end of the test.

A negligible effect of hold periods on residual soil stresses was observed

when the load was held at the minimum value of the cyclic loading

amplitude. However, significant increase of residual soil stresses was

evident for the test where the load was entirely released during hold

periods.

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Thank you!