failure of foundation due to earth quake-liquefaction
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FAILURE OF FOUNDATION DUE TO EARTH QUAKE-LIQUEFACTION
INTRODUCTIONEarth 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 Tectonic earthquakes
Tectonic earthquakes are the most common type of earthquake. It may be of small or of extremely high magnitude.
Volcanic earthquake
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.
Collapse earth quakes are comparatively small earthquakesand they take place around underground mines.
Collapse earthquakes
Explosion earthquakes
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.Date Location Name Magnitude
March 11, 2011 Pacific Ocean, Tōhoku region, Japan
2011 Tōhoku earthquake 9.0[3][4][5]
February 27, 2010 Maule, Chile 2010 Chile earthquake 8.8
December 26,2004 Indian ocean,Sumatra,indonesia Indian ocean 9.1-9.3
January,12,2010 Aleppo, syria Aleppo Unknown
March 28, 2005 Sumatra, Indonesia 2005 Sumatra earthquake 8.6
Date Location Name Magnitude
September 12, 2007 Sumatra, Indonesia 2007 Sumatra
earthquakes 8.5
EFFECTS OF EARTHQUAKESDirect 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 structureIndirect Effects:
1. Tsunamis2. Landslides3. Floods
High frequency body waves shake low buildings more. Low frequency surface waves shake high buildings more. Intensity of shaking alsodepends on type of subsurface material.
4. Fires
GROUND SHAKING EFFECT ON STRUCTURES
Column failure on interstate highwayoverpass, Northridge earthquake
Column failure, Loma Prieta earthquake
Landslides
describes a wide variety of processes that result in the downward and outwardmovement of slope-forming materials including rock, soil, artificial fill, or acombination of these. The materials may move by falling, toppling, sliding, spreading, or flowing.
La Conchita, coastal area of southern CaliforniaLiquefaction
Liquefaction is a physical process that takes place during some earthquakes that may lead to ground failure. liquefaction takesplace when seismic shear waves pass through a saturated granular soil layer
EARTHQUAKE EFFECTS(Shaking, Landslides, Liquefaction, and Tsunamis)
Geologic Effects on ShakingThe 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 EFFECTSGround shakingThe 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 STRUCTURESInertia forcesAs 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 loadThe 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 damagePast 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 DAMAGEBuilding configurationA 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:
Building Heights & Natural Frequency
Building Height Typical Natural Period
2 story .2 seconds
5 story .5 seconds
10 story 1.0 second
20 story 2.0 second
30 story 3.0 second
50 story 5.0 seconds
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 arelikely 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 againstthe 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 wallsRoof on two wallsIn 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 DAMAGEEarthquakes 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 groundshaking 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 thespaces 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 liquefactionThe 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 toliquefaction. 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) orbe 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 locatenew development in areas that do not have liquefiable soils.
The liquefaction map is a very useful tool for developers seeking sitesfor future development as well as for individual home buyers.
2) Soil mitigation.Problems with liquefaction may be mitigated by altering the site soilcharacteristics. Examples include lowering the ground water table withdrains or pumps, densification of the soils by dynamic compaction orvibration, installation of stone columns, and grouting.
3) Structural mitigation.The damaging effects of liquefaction is most frequently reduced usingstructural techniques. Strengthening the structure using additionalfoundation, wall, and roof ties is common. Foundation supportredistributed 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 modesIn 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’sStatic 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 retainslateral 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 pilesUsually, 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 earthquakedetachment 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 loadingWhen 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 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 failureFailure 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 bridgepiers, 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 soilsduring earthquakes needs to be considered when the piles pass throughinterfaces between very soft soils and much stiffer soils.Some factors that should be considered in connection with seismicresistance 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 theupper 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 soil2. duration of the earthquake and amplitude and frequency of shaking3. distance from yhe epicenter4. location of the water table5. cohesiveness of the soil6. 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.