Thursday 16 January 2020
Thursday 10 May 2012
FAILURE OF FOUNDATION DUE TO EARTH QUAKE-LIQUEFACTION
INTRODUCTION
Earth
quake
An earthquake is the
result of a sudden release of energy in the earth’s crust that creates seismic
waves. The seismicity, seismism or seismic activity of an area refers to the
frequency, type and size of earthquakes experienced over a period of time. Earthquakes
are measured using observation from seismometers. The moment magnitude is the
most common scale on which earthquakes larger than approximately 5 are reported
for the entire globe.
The most recent large
earthquake of magnitude 9.0 or large was a 9.0 magnitude earthquake in japan In
2011(as of march 2011),and it was the Japanese earthquake since records began.
One of the most devastating
earthquake in recorded history occurred on 23 January 1556 in the Shaanxi
province, China,Killing more than 830,000 people
Fault or fault pane : the surface where when two block of the earth
suddenly slip past one another
hypocenter : The
location below the earth’s surface where the earthquake starts
.Epicenter
: The location on the surface of the earth directly
above the hypocenter
TYPES
OF EARTHQUAKES
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Tectonic earthquakes
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Tectonic
earthquakes are the most common type of earthquake.
It may be of small or of extremely high
magnitude
.
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Volcanic
earthquake
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occur usually after a volcanic
activity has taken place. The magma that erupts
during an earthquake leaves a space, to fill the space left by the magma the
rocks move towards the space to fill it in, causing severe earthquakes.
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Collapse
earth quakes are comparatively small earthquakes
and
they take place around underground mines.
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Collapse
earthquakes
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Explosion
earthquakes
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The explosion earthquakes are caused due
to the nuclear explosions
EARTHQUAKES
FORM
Stress in the earth’s
outer layer cause a pushing effect against the sides of the fault. Due to this motion, rocks slip or collide
against each other releasing energy.
This released energy travels in waves through the earth’s crust and
causes the shaking that we fell during an earthquake.
Under the surface of
the earth, the two sides of a fault are constantly moving, relative to one
another. This movement is known as a
fault slip. The movement of these two
sides is not smooth and is accompanied by a gradual build-up of elastic strain
energy within the rocks along the fault.
The location on a fault
where the slip first occurs is called the hypocenter, whereas the position
directly above it on the ground surface is called the epicenter.
EARTHQUAKES
MEASURES
The strength of an
earthquake can be measured by magnitude and intensity.
It is commonly measured
on the Richter Scale which is an open-ended logarithmic scale.
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Date
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Location
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Name
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Magnitude
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March 11, 2011
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|||
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February 27, 2010
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8.8
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December 26,2004
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Indian
ocean,Sumatra,indonesia
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Indian ocean
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9.1-9.3
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January,12,2010
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Aleppo, syria
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Aleppo
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Unknown
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March 28, 2005
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8.6
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September 12, 2007
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8.5
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EFFECTS
OF EARTHQUAKES
Direct Effects:
1.
Ground failures(or instabilities due to
ground failures) surface faulting surface or fault rupture, or effects of
seismic waves,
Ground cracking,
Liquefaction.
2.
Vibrations transmitted from the ground to the
structure
Indirect Effects:
1.
Tsunamis
2.
Landslides
3.
Floods
|
High
frequency body waves shake low buildings more. Low frequency
surface waves shake high buildings more.
Intensity of shaking also
depends
on type of subsurface material.
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4. Fires
GROUND
SHAKING EFFECT ON STRUCTURES|
Column
failure on interstate highway
overpass,
Northridge earthquake
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Column
failure, Loma Prieta earthquake
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Landslides
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describes a wide variety of processes
that result in the downward and outward
movement of slope-forming materials including rock,
soil, artificial fill, or a
combination of these. The materials
may
move by falling, toppling, sliding, spreading, or flowing.
|
 La Conchita, coastal area
of southern
California
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Liquefaction
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Liquefaction is a physical
process that takes place during some earthquakes
that may lead to ground failure. liquefaction takes
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place
when seismic shear waves pass through a saturated granular soil
layer
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EARTHQUAKE
EFFECTS
(Shaking,
Landslides, Liquefaction, and Tsunamis)
Geologic
Effects on Shaking
The level of damage
done to a structure depends on the amplitude and the duration of shaking. The
amplitudes are largest close to large earthquakes and the duration generally
increases with the size of the earthquake (larger quakes shake longer because
they rupture larger areas). Regional geology can affect the level and duration
of shaking but more important are local site conditions.
EARTHQUAKE
EFFECTS
Ground
shaking
The principal cause of
earthquake-induced damage is ground shaking. As the earth vibrates, all
buildings on the ground surface will respond to that vibration in varying degrees.
the effect of ground shaking on buildings is a principal area of consideration in
the design of earthquake resistant buildings.
Ground failure
Earthquake-induced
ground failure has been observed in the form of ground rupture along the fault
zone, landslides, settlement and soil liquefaction. can occur in low
density saturated sands of relatively uniform size. The phenomenon of liquefaction
is particularly important for dams, bridges, underground pipelines, and
buildings standing on such ground
GROUND
SHAKING ON STRUCTURES
Inertia forces
As the base of a
building moves the superstructure including its contents tends to shake and vibrate
from the position of rest, in a very irregular manner due to the inertia of the
masses.
Factors affecting seismic load
The soil-foundation factor Fs
depends upon the ratio of fundamental elastic period of vibration of a building
in the direction under consideration and the characteristic site period.
Therefore, Fs is a numerical coefficient for site-building resonance
Effect of site conditions on
building damage
Past earthquakes show that site
condition significantly affects the building damage. Earthquake studies have
almost invariably shown that the intensity of a shock is directly related to
the type of soil layers supporting the building. Lessons learned from recent earthquake show
that the topography of a building site can also have an effect on damage.
OTHER
FACTORS AFFECTING DAMAGE
Building configuration
A building shaped like a box, as
rectangular both in plan and elevation, is inherently stronger than one that is
L-shaped or Ushaped, such as a building with wings. An irregularly shaped
building will twist as it shakes, increasing the damage.
Earthquake Ground
Motion
.
The complexity of
earthquake ground motion is due to three factors:
·
The seismic waves
generated at the time of earthquake fault movement were not all of a uniform
character
·
As these waves pass
through the earth on their way from the fault to the building site, they are
modified by the soil and rock media through which they pass
·
Once the seismic waves
reach the building site they undergo further modifications that are dependent
upon the characteristics of the ground and soil beneath the building. We refer
to these three factors as source effects, path effects, and local site effects.
The response of the
building to ground motion is as complex as the ground motion itself, yet
typically quite different. It also begins to vibrate in a complex manner, and
because it is now a vibratory system, it also possesses a frequency content.
However, the building's vibrations tend to center around one particular
frequency that is known as its natural or fundamental frequency. Generally, the
shorter a building is the higher its natural frequency, and the taller the
building is, the lower its natural frequency.
Building Frequency and Period
Another way to
understand this is to think of the building's response in terms of another
important quantity, the building's natural period. The building period is
simply the inverse of the frequency: Whereas the frequency is the number of
times per second that the building will vibrate back and forth, the period is
the time it takes for the building to make one complete vibration. The
relationship between frequency f and period T is
thus very simple math:
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Building Heights & Natural
Frequency
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Building Height
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Typical Natural Period
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2
story
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.2
seconds
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5
story
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.5
seconds
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10
story
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1.0
second
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20
story
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2.0
second
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30
story
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3.0
second
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50
story
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5.0
seconds
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Rigidity
distribution
The rigidity of a building along
the vertical direction should be distributed uniformly Columns or shear walls
should run continuously from foundation to the roof, without interruptions or
changes in material.
FOUNDATION FAILURE MECHANISMS OF
EARTHQUAKES
Buildings, which are structurally
strong to withstand earthquakes sometimes fail due to inadequate foundation
design. Tilting, cracking and failure of superstructures may result from soil
liquefaction and differential settlement of footing. isolated footings of columns are
likely to be subjected to
differential settlement particularly where the supporting ground consists of
different or soft types of soil Very shallow foundations deteriorate because of
weathering, particularly when exposed to freezing and thawing in the regions of
cold climate.
Free standing masonary wall
The free standing wall B fixed on
the ground in Fig 2.3(b) is subjected to ground motion in its own plane.
Wall enclosure without roof
Now consider the combination of
walls A and B as an enclosure shown in Fig 2.4. For the X direction of force as
shown, walls B bact as shear walls and, besides taking their own inertia, they
offer resistance against
the collapse of wall A as well. As
a result walls A now act as vertical slabs supported on two vertical sides and
the bottom plinth. The walls A are subjected to the inertia force on their own
mass. Near the vertical edges, the wall will carry reversible bending moments
in the horizontal plane for which the masonry has little strength. Consequently
cracking and separation of the walls may occur along these edges shown in the
figure
Fig 2.4 Failure mechanism of wall enclosure without roof
Fig 2.5
Roof on two walls
Roof on two
walls
In Fig 2.5 (a) roof slab is shown
to be resting on two parallel walls B and the earthquake force is acting in the
plane of the walls. Assuming that there is enough adhesion between the slab and
the walls, the slab will transfer its inertia force at the top of walls B,
causing shearing and overturning action in them.
Roofs and
floors
The roofs and floors, which are
rigid and flat and are bonded or tied to the masonry, have a positive effect on
the wall, such as the slab or slab and beam construction be directly cast
over the walls or jack arch floors or
roofs provided with horizontal ties and laid over the masonry walls In the case
of a floor consisting of timber joists placed at center to center spacing of 20
to 25 cm with brick tiles placed in directly over the joists and covered with clayey earth, the
brick tiles have no binding effect on the joists.
Earthquake And Their Effect On
Foundation
The violent
shaking of an earthquake can quickly damage homes, building and bridges. The
most noticeable damage appears in the wall or roof of building, but building
foundation are also effected by the earth’s sudden movement. How an earthquake
effect foundation? When an earthquake begins, the earth can lunch with sudden
jolts or roll with several waves. A sudden lunch does little to the foundation
itself, however, the walls of a building can quickly shift off the foundation.
The foundation moves with the earth back and forth, but the wall are slow to
follow and literally break apart at the seams. Even after the ground and also
roll during an earthquake. Major damage to a foundation is often identified and
fixed soon after an earthquake, but attention should also be given to small
cracks that can develop into larger problem over time. Small cracks can shift
time due to soil conditions or small earthquakes. So crack should be measured
every couple of months. Cracks can also allow water into the foundation, enlarging
cracks and washing away soil.
Building Foundation Movement due to Earthquake,
Flood, or Storm Damage
Earthquakes shake building structures in different
patterns, sometimes unique to a particular quake. The basic movements are side
to side, up and down, or a combination of these. Depending on which forces are
exerted, buildings shift and structural components fail in different patterns.
In
the photo just below, lateral movement caused the failure of supporting columns
columns that failed were hollow steel posts while
others that remained standing at the same building project were ones that had
been filled with concrete to resist bending.
Liquefaction
Definition: liquefaction
Liquefaction is the process that
leads to a soil suddenly losing strength, most commonly as a result of ground
shaking during a large
earthquake. Not all soils however, will liquefy in an earthquake. The following
are particular
features of soils that
potentially can liquefy:
• They are sands and silts and
quite loose in the ground. Such soils do not stick together the way clay soils
do.
• They are below the water table,
so all the space between the grains of sand and silt are filled with water.
Dry soils above the water table
won’t liquefy.
When an earthquake occurs the
shaking is so rapid and violent that the sand and silt grains try to compress
the
spaces filled with water, but the
water pushes back and pressure builds up until the grains ‘float’ in the water.
Once that
happens the soil loses its strength – it has liquefied. Soil that was once
solid now behaves like a fluid.
Liquefaction
occurrence
Liquefaction is
more likely to occur in loose to moderately saturated granular soils with poor
drainage, such as silty sands or sands and gravels capped or containing seams
of impermeable sediments.
Depending on the
initial void ratio, the soil material can respond to loading either
strain-softening or strain-hardening. Strain-softened soils, e.g. loose sands,
can be triggered to collapse, either monotonically or cyclically, if the static
shear stress is greater than the ultimate or steady-state shear strength of the
soil. In this case flow liquefaction occurs, where the soil deforms at a low
constant residual shear stress.
If stress
reversal does not occur, zero effective stress is not possible to occur, then
cyclic mobility takes place.[10]
Earthquake
liquefaction
The pressures
generated during large earthquakes with many cycles of shaking can cause the
liquefied sand and excess water to force its way to the ground surface from
several metres below the ground. This is often observed as "sand
boils" also called "sand blows" or "sand volcanoes" at
the ground surface.
The other common
observation is land instability - cracking and movement of the ground down
slope or towards unsupported margins of rivers, streams, or the coast. The
failure of ground in this manner is called 'lateral spreading', and may occur
on very shallow slopes of angles of only 1 or 2 degrees from the horizontal.
More is discussed on this aspect under the section 'Effects'.
Sand .boils.
formed during the 1934 magnitude 6.6 Hansel Valley earthquake (courtesy
University of Utah Marriott Library Special Collections.
Condition
for liquefaction
Three critical
factors must be present for sediments to be prone to
liquefaction.
The sediment must be
1) saturated with ground water,
2) composed of
sand or silt-sized particles,
3) compacted fairly loose. For liquefaction to
occur, all three factors must be
present at the same time.
Ground Water -
Sediments must be saturated with ground water in order to liquefy during an
earthquake.
Grain Size - The
size of the sediment particles controls the size of the pore spaces. This is
critical in clay and fine silt grains (those less than 1/32 of an inch in
diameter) because, although water can fill the small pore spaces, the flow of
water between pores becomes so restricted that liquefaction becomes difficult.
The sands and
silts must also be relatively “clean” for liquefaction to occur. This means
that liquefaction is most likely to occur in sands and coarse silts with a
uniform grain size.
Soil Density
Soil density
generally increases with the age and depth of deposits.
Sediments tend
to compact over time and with burial, increasing their density. Historically,
liquefaction has been observed mainly in sediments less than 45 feet below the
ground surface.
Some other
consequences of the soil liquefying are:
• Settlement of
the ground surface due to the loss of soil from underground.
• Loss of
support to building foundations.
• Floating of
manholes, buried tanks and pipes in the liquefied soil - but only if the tanks
and pipes are mostly empty.
• Near streams
and rivers, the dry surface soil layers can slide sideways on the liquefied
soil towards the streams. This is called lateral spreading and can severely
damage a building.
It typically results in long tears and rips in
the ground surface that look like a classic fault line. Not all of a building’s
foundations might be affected by liquefaction. The affected part may subside
(settle) or
be pulled
sideways by lateral spreading, which can severely damage the building. Buried
services such as sewer pipes can be damaged as they are warped by lateral
spreading, ground settlement or floatation.
After the
earthquake
After the
earthquake shaking has ceased, and liquefaction effects have diminished (which
may take several hours), the permanent effects include:
• Lowering of
ground levels where liquefaction and soil ejection has occurred. Ground
lowering may be sufficient to make the surface close to or below the
watertable, creating ponds.
• Disruption of
ground due to lateral spreading. The liquefied soil that is not ejected onto
the ground surface re-densifies and regains strength, in some cases
re-densified soil is stronger than before the earthquake. Careful engineer
Effects
The effects of
soil liquefaction on the built environment can be extremely damaging. Buildings
whose foundations bear directly on sand which liquefies will experience a
sudden loss of support, which will result in drastic and irregular settlement
of the building causing structural damage, including cracking of foundations
and damage to the building structure itself, or may leave the structure
unserviceable afterwards, even without structural damage
.
The irregular
settlement of ground may also break underground utility lines. The upward
pressure applied by the movement of liquefied soil through the crust layer can
crack weak foundation slabs and enter buildings through service ducts, and may
allow water to damage the building contents and electrical services.
Bridges and
large buildings constructed on pile foundations may lose support from the
adjacent soil and buckle, or come to rest at a tilt after shaking.
Diagram
illustrating a lateral spread landslide. Arrows indicate direction of flow.
Drawing modified from Youd (1984).
LIQUIFACTION CAN BE REDUCED BY ;
1) Avoid
liquefaction-prone areas.
Perhaps the
simplest method of dealing with liquefaction is to locate
new development
in areas that do not have liquefiable soils.
The liquefaction
map is a very useful tool for developers seeking sites
for future
development as well as for individual home buyers.
2) Soil
mitigation.
Problems with
liquefaction may be mitigated by altering the site soil
characteristics.
Examples include lowering the ground water table with
drains or pumps,
densification of the soils by dynamic compaction or
vibration,
installation of stone columns, and grouting.
3) Structural
mitigation.
The damaging
effects of liquefaction is most frequently reduced using
structural
techniques. Strengthening the structure using additional
foundation,
wall, and roof ties is common. Foundation support
redistributed
through the use of piles or caissons which extend through the liquefiable
layers can help reduce liquefaction induced damage. Specially designed mat
foundations have also been used in some buildings in Salt Lake County
Assessment
of building damage due to liquefaction
Liquefaction is
induced by strong ground motion due to a large earthquake nearby mainly in alluvial
plain, and destroys buildings, for example, by leaning and subsidence. In this
chapter, we introduce some assessment methods of building damage due to
liquefaction.
The liquefaction
index (PL value) for the assessment of liquefaction potential is adopted in
earthquake damage assessment by many local governments in Japan. The potential
of liquefaction is connected with the liquefaction index (PL value) as follows.
:
Liquefaction potential is quite low. 
:
Liquefaction potential is low.
:
Liquefaction potential is high.
:
Liquefaction potential is very high.
In the method
introduced in this chapter, PL value is used as the index which is connected to
the building damage due to liquefaction.
FOUNDATION
FAILURE-LIQEUFACTION
Pad and strip foundations: Failure modes
In addition to transferring vertical loads safely into the
soil, shallow foundations in the form of pads or strips must also transfer the
horizontal forces and overturning moments arising during an earthquake.
(a) Sliding failure:
Resistance to sliding in shallow footings will usually be
mobilized from the shear strength of the soil interfacing with the footing. In
granular materials, the minimum vertical load which could occur concurrently
with the maximum horizontal force must be considered, since this condition will
minimize shear resistance. The maximum seismic uplift should be assessed as the
sum of components due to overturning and vertical seismic accelerations,
combined by the SRSS method.
(b) Bearing capacity failure’s
Static bearing capacity can be determined from formulae
which allow for the inclination and eccentricity of the applied load.
(c) Rotational failure (overturning):
Where the soil is strong, the foundation may start to rotate
before a bearing capacity failure occurs, particularly if the vertical load is
small. In the case of pad foundations supporting a moment-resisting frame, such
a rotation
may be acceptable, since a frame with pinned column bases
still retains
lateral stability.
However, the associated redistribution of moments would lead
to increased moments at the top of the lower lift of columns, which would need
to be designed for.
In contrast, an isolated cantilever shear wall is not
statically stable with a pinned base. Rocking should, therefore, be prevented
under design forces in most circumstances.
Uplift can be prevented by provision of additional weight or
by piles or anchors to resist the transient vertical loads, or by a wider
foundation.
Modes of
failure in pad foundations: (a) sliding failure; (b) bearing capacity failure;
(c) overturning; and (d) structural failures, where (i) shows shear failure in
footing, (ii) shows shear failure in stub column, (iii) shows bending failure
in footing, and (iv) shows bending failure in ground beam.
Structural failure in the foundation:
Sufficient strength must be provided to prevent brittle
failure modes in the foundation structure, such as shear failure in footings or
stub columns.
Ties between footings:
Some form of connection is usually needed at ground level to
link isolated footings supporting a moment-resisting frame. The ties prevent
excessive lateral deflection in individual footings, caused by locally soft
material or local differences in seismic motion. Where the footings are founded
on rock or very stiff soil, however, the tendency for relative movement is much
less and the ties are generally not required.
The connection can take the form of a ground beam, which
will also assist in providing additional fixity to the column bases and will
help to resist overturning.
Alternatively, the ground-floor slab can be specially
reinforced to provide the Restraint.
Raft foundations:
All of the soil failure modes illustrated in Fig below may
apply to raft foundations. The analysis would assume a uniform soil pressure
distribution in equilibrium with the peak and moments within the raft near its
edge, since the soil, being poorly restrained, has low bearing capacity there.
More complex analysis would allow for soil nonlinearity and dynamic effects.
applied loads. Figure below shows that this may lead to an underestimate of
shears.
Pressure distribution near the edge of a raft under seismic
loading the effect of the uplift on internal forces within the raft foundation
and superstructure must be accounted for.
Piled foundations:
Vertical and horizontal effects:
Vertical loading on pile groups during an earthquake arises
from gravity loads, seismic overturning moments and vertical seismic
accelerations. Since the two latter effects are not correlated, they can be
combined by the SRSS method, and added to the gravity load. The procedures are
straightforward, and the design of end-bearing piles is similar to that for
static vertical loads. Friction piles may be less effective under earthquake conditions
and require special consideration. Flexible piles may be able to conform to the
deflected soil profile without distress, but large-diameter piles are
relatively much stiffer than the soil and large forces may be generated.
Inertial and kinematic loading on piles
Usually, locations of plastic hinges other than at the tops
of the piles are not considered acceptable. Further considerations for
detailing of concrete piles are given in the next section.
Particular regions where special detailing measures may be
required are as follows.
(a) The junction between pile and pile cap is a highly
stressed region where large curvatures may occur in the pile. Unless adequate
confinement and good connection details are present, brittle failure may occur.
(b) Junctions between soft and hard soil strata may also
impose large curvatures on piles; such junctions are likely to be potential
points for formation of plastic hinges.
(c) Piling through soil which may liquefy can pose special
problems. In this case the pile may have a large unsupported length through the
liquefied soil and should be reinforced as though it were an unsupported
column. A reliable ductile behaiour will also be necessary in this situation.
DAMAGE
PATTERNS AND FAILURE MECHANISMS OF BRIDGE PILE FOUNDATION UNDER EARTHQUAKE
Lots
of factors will cause damage of bridge pile foundation under earthquake, such
as soil conditions, excessive inertia force caused by superstructure and
incorrect design of piles. According to the damage statistics of pile foundation,
its failure modes are complex, but soil displacement and sandy soil
liquefaction are the most common ones
Crushing failure , Hyogoken-Nanbu
earthquake
Gaps with differential wide opened
around the piles, Tangshan earthquake
detachment failures between pile
head and pile cap, Higashi bridge, Hyogoken-Nanbu earthquake
FAILURE
PATTERNS OF PILE FOUNDATION WITH LIQUEFACTION
Soil liquefaction was
primary cause of bridge foundation distress. Pile damage with
liquefaction-induced phenomena can be classified as damage without soil lateral
spreading and with soil lateral spreading.
Pile
damage without soil lateral spreading[2]
In the case of liquefiable but no soil lateral
spreading condition, there appears various phenomena when earthquake occurs,
such as sand erupting, water oozing, detachment between pile cap and soil etc.
If the distributions of load, quality of liquefied soil and thickness of
liquefied soil are non-uniform, the bridge foundations often produce a quite
large uneven settlement under earthquake action. In the case of uniform
distributions, pile maybe failed at the interface between liquefied soil and
un-liquefied soil or at pile head with little uneven settlement.
Failure
mechanisms of pile with soil lateral spreading
Bridges are often located
at the impact band of rivers, where there exist a lot of liquefiable sand and
silty layers with gentle slope (00~50), where lateral spreading easily happen
under earthquake loading. When soil liquefies under earthquake loading, its
shear resistance will decrease,
Damage
to bridge pile with lateral soil spreading was attributed to several
mechanisms:
3)
Due to uneven settlement of superstructure,
great horizontal displacement and additional bending moment are generated in tall
structure. With this additional bending moment, the interior side pile bears
tensile stress, so earthquake damage of piles can be relieved and there maybe
exist only one plastic hinge for side pile
Force of pile
shaft under earthquake loading
When soil produces
lateral movement under earthquake loading, pile will produce lateral
deformation due to the lateral thrust of soil.
When soil doesn‟t
produce lateral movement under earthquake loading, piles only bear additional
dynamic stress that is induced by soil-pile-superstructure interaction. If soil
produces lateral movement under earthquake loading, piles bear additional dynamic
stress and additional static stress that is produced by soil lateral movement.
pile is connected with
superstructure through pile cap. Pile movement must be coordinated with soil
movement, which results in dynamic stress in pile. On the other hand, the
seismic inertial force of superstructure is transmitted to pile through pile
cap, which also results in dynamic stress in pile. Therefore, the dynamic
stresses that the pile bears under earthquake loading include the two parts
above.
Failure mechanism of pile
The failure mechanism of pile is related to
the force conditions of pile under earthquake loading,
1)
The failure caused by additional dynamic
stress that is induced by vibration.
2)
The failure caused by additional static stress
that is induced by soil lateral movement.
3)
The length of pile penetrating into
steady soil layer is not enough or pile tip don‟t arrive at steady soil layer,
so pile foundation is easy to lose bearing capacity due to liquefaction of
sandy soil under earthquake loading.
COUNTERMEASURES
AND SUGGESTIONS ON BRIDGE PILE DESIGN
Liquefaction-induced
large deformation of soil is the most main reason for bridge pile damage, so
prevention measures to pile must be taken in seismic design.
Site selection
and survey of engineering geology[7]
1) In bridge route design, the destruction to
natural balance condition of site should be avoided because it causes high and
steep free face during construction.
(2) The purpose of survey is to ascertain the
thickness and buried depth of liquefiable soil layers, grade of soil interface,
ground slope, history of river channel, and retaining structures of bank etc..
(3) With regards to the
region with large-area liquefiable soil, the principle, which is “bypass rather
than pass
FAILURE OF RAFT FOUNDATIONS DURING
EARTHQUAKE
LIQUEFACTION
In this paper dynamic centrifuge modeling has been used to
understand the mechanism of raft settlement and failure in liquefiable soils
Usually bearing capacity, settlement and uplift pressure are
the factors that have to be considered for foundation design under ordinary
conditions. However, when the ground is subjected to cyclic motion due to
earthquake loading saturated sands lose their shear strength and behave like a
liquid for a short period of time. This is termed as liquefaction and upon
liquefaction the bearing capacity of the soil is sharply reduced and the
building foundation may suffer excessive settlement and rotation. Most of the
earthquake induced ground failures involving soil liquefaction have resulted in
the tilting and collapsing of buildings with the superstructure remaining
intact. This is mostly due to the loss of stiffness of the underlying soil
through excess pore pressure generation in undrained conditions or large
strains induced due to high level of seismic shaking. In many reported case
histories subsidence has continued to occur after the end of strong
The figure shows that settlement ratio decreases with
increasing width ratio.
Building that toppled due to
liquefaction-induced loss of bearing strength beneath shallow mat foundation. (Photo: Youd)
FAILURE OF SPREAD
FOUNDATION
Foundation
soil failure
Failure of spread foundations is usually
the result of failure of thesupporting soil, which is often associated with
liquefaction the``se
failures result in gross settlements, but
the failing soil is unable to transmit strong shaking to the structures which survive.
Eurocode 8 Part 5
lists the following instances where SSI (Soil-Structure Interaction) should be
allowed for.
(a) Structures where P–Δ effects play a significant role.
(b) Structures with massive or deep-seated foundations, such
as bridge
piers, caissons and silos.
(c) Tall and slender structures such as towers and chimneys.
(d ) Structures supported on very soft soils.
(e) The effect of the interaction between piles and the
surrounding soils
during earthquakes needs to be considered when the piles
pass through
interfaces between very soft soils and much stiffer soils.
Some factors that should be considered in connection with
seismic
resistance are as follows.
(b) Superstructure systems that involve large uplift forces
(e.g. shear walls with a high height-to-width ratio) are only suitable if
foundations can be built economically to resist these tension forces.
(c) Piles have loads imposed upon them due to lateral
deflection of the
upper layers of softer soil during earthquakes. Small driven
piles of less than 0.5m diameter are generally sufficiently flexible to accept
this movement without suffering large bending stresses. Large-diameter piles,
however, may experience significant lateral forces as they are relatively stiff
compared with the soil.
(d ) Raking piles are generally to be avoided because they
add greatly to the lateral stiffness of the pile group. Their stiffness means
that they will not be able to conform to the deformations of the soft soil
strata, but will receive very large lateral loads, arising from the mass of the
soft soils attempting to move past the stiffened pile group. Raking piles have
been found to be prone to failure during earthquakes.
( f ) Raft foundation support via a basement may be an
alternative solution when founding on potentially liquefiable layers. The main
features to consider in the seismic design of foundations are as follows.
(a) A primary design requirement is that the soil–foundation
system must be able to maintain the overall vertical and horizontal stability
of the superstructure in the event of the largest credible earthquake.
(b) The foundation should be able to transmit the static and
dynamic forces developed between the superstructure and soils during the design
earthquake without inducing excessive movement.
(c) The possibility of soil strength being reduced during an
earthquake needs to be considered.
(d ) It is not sensible to design a perfectly detailed
ductile superstructure supported by a foundation which fails before the
superstructure can develop its yield capacity.
( f ) Special measures are needed if liquefaction is a
possibility.
(g) Special considerations apply to piled foundations.
Causes and Remedies of Earthquakes
An Earthquake is a
sudden movement of the ground that releases the elastic energy stored within
the rocks, creating destructive seismic waves. The word "seismic"
comes from the Greek word "seismos" meaning an earthquake. These
quakes are not isolated events. They come with smaller shocks, called aftershocks,
with smaller effects.
An earthquake is caused
when two sides of a large fracture in the rocks within the earth slide past
each other.
Small earth quakes are caused by small faults or
small parts of big faults. These last only for a fraction of a second and
usually unnoticed, as the rocks on either side of the faults are not displaced
much. The larger ones are caused due to faults which are tens to thousands of
kilometers long, taking minutes and the displacement of the rocks is around
tens of meters.
Tectonic earth quakes
are the most devastating, and unfortunately the most unpredictable. The
volcanic quakes are seldom important or devastative, but they help predict the
eruption of volcanoes. The artificial ones are caused due to human activities,
like pumping fuels deep into the earth or due to explosives, and several other
reasons.
liquefaction occurs
only under ideal conditions as a result of an earthshaking event and is
controlled by the following variables
1.
grain size of the soil
2.
duration of the earthquake and amplitude
and frequency of shaking
3.
distance from yhe epicenter
4.
location of the water table
5.
cohesiveness of the soil
6.
permeability of the layer
Earthquake Resistant Techniques
Base
Isolation fig 1
It is easiest to see
this principle at work by referring directly to the most widely used of these
advanced techniques, which is known as base isolation. A base isolated
structure is supported by a series of bearing pads which are placed between the
building and the building's foundation.
A lead–rubber bearing
is made from layers of rubber sandwiched together with layers of steel. In the
middle of the bearing is a solid lead "plug." On top and bottom, the
bearing is fitted with steel plates which are used to attach the bearing to the
building and foundation. The bearing is very stiff and strong in the vertical
direction, but flexible in the horizontal direction.
Earthquake Generated Forces
Fig;2
As a result of an
earthquake, the ground beneath each building begins to move.
The complex nature of
earthquake ground motion, the building actually tends to vibrate back and forth
in varying directions. Figure 3 is really a kind of "snapshot" of the
building at only one particular point of its earthquake response.
Damping
devices and bracing system
CONCLUDING
REMARKS
According to the
failure patterns and failure mechanisms of bridge pile foundation described
above, the following conclusions can be obtained:
(1) Failure modes of
bridge piles are complex, but soil expanding and sand liquefaction are the most
common ones, while the amplification effect of ground motion and excessive
deformation of pile are also included.
(2) The analysis on failure patterns and
failure mechanisms of bridge piles indicate that, the failure probability of
bridge piles in the slope or bank-side site is higher than that of pile with
large displacement induced by earthquake liquefaction.
(3) When soil large displacement with
liquefaction-induced occurs, bridge piles bear horizontal stresses, which
include dynamic stress induced by inertial forces of superstructure as well as
additional stress induced by soil lateral movement. In most cases, the latter
plays a more important role than the former.
(4) With regards to
site selection for engineering, it is necessary to keep the site clear of the
region with large-area liquefiable soil. Even if not, the location of piles
must be far from bank slope, especially far from abrupt slope.
(5) With regards to the pile foundations in
the liquefiable soil, it is necessary to take active preventive measures such
as enhancing horizontal bearing capacity of piles, strengthening the fixity
between pile head and pile cap, foundation consolidation etc.
REFERENCE
MANIKANDAN.S VIBINLAL.A
DIPLOMA IN CIVIL ENGINEERING, DIPLOMA IN
CIVIL ENGINEERING
CONTACT NO:8870622855,7639469326
SURENDIRAN.G VINOTH
.K
DIPLOMA IN CIVIL ENGINEERING DIPLOMA IN
CIVIL ENGINEERING
CONTACT NO:
8870909723 CONTACT
NO: 8870826041
RAJAKUMARAN.K
HARISH .C
DIPLIOMA IN CIVIL ENGINEERING DIPLOMA IN CIVIL
ENGINEERING EMAIL:rajbadboy1993@gmail.com CONTACT NO: 9994737040
CONTACT NO: 9943107884
MORE
REFERENCE:
www.eartquake resistant technics.com
by : surenduran.g
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