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!