Seismic soil
structure
interaction and
soil liquefaction
By
Dr. Amey D. Katdare
Ph. D. (IIT Bombay)
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Part I
Soil Structure Interaction
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Soil
• Soil is a material that is formed by disintegration of
rock.
• Soil is purely a heterogeneous material.
• Properties of same soil changes from every place like
water content, voids ratio, porosity etc.
• Soil is a material which supports structures in many
cases (where hard rock is not available)
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Types of soil
– Cohesive soil (c soil)
• Posses particle to particle attraction
• This attraction is a major resisting force as
shear strength is derived from the same
• Particles cannot be separated with simple
techniques
Fig. Cohesive soil
• Examples are clay which is used to make
Ganesh idols etc.
• Classified based on Atterberg’s limits (LL,
PL and SL)
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Types of soil
– Cohesion-less soil (ϕ soil )
• do not have any particles to particle
attraction, have friction between particles
• All particles can be separated with simple
measures like sieving
• Examples are sand (Rangoli)
• Classified as fine grained and coarse
grained soil
• Tests
performed
are
sieve
analysis,
relative density etc
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Types of soil
– c – ϕ soil
• These soils do have some
part of particles to particle
attraction and also angle
of internal friction
• This is type of soil, which
occurs most in nature.
• Since, first two are
generally considered as
theoretical cases of c – ϕ
soil
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Important soil properties
•
•
•
Physical properties
•
Density and Unit weight
•
Specific gravity of soil solids
Index properties
•
Water content, Void ration and porosity
•
Degree of saturation
Engineering properties
•
Compaction
•
Consolidation
•
Permeability
•
Shear Strength
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Structure
Where these structures are resting?
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What is relation between soil and structure?
• During
load
transfer
mechanism,
soil
and
structure act as one part.
• However, in most of the conventional design
process, structure is considered as a separate
part and soil is treated as different
• However, there is a need to consider, soil and
structure together, to make the structure safe.
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Soil Structure Interaction (SSI)
• The study of soil and structure together, is termed
as ‘soil structure interaction (SSI)’.
• This includes study of
– Soil as foundation material
– Building superstructure
• The
above
two
components
are
studied,
considered together to make a stable structure.
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Soil Structure Interaction
• When
an
earthquake
occurs, the building and
structure
vibrate
by
influencing each other.
• This phenomena is called
as ‘seismic soil structure
interaction’.
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Soil Structure Interaction
Soil
Site effects
Soil Structure
interaction
Kinematic
effect
Inertial effect
Seismic performance
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Dynamic Soil Structure Interaction
• The important dynamic properties of soil
considered in soil structure interaction are:
– Shear modulus of soil (G)
– Poisson’s ratio for soil (ν)
– Shear, Primary and Rayleigh wave velocity
through soil medium
– Damping ratio (D)
– Dynamic spring constant (KA)
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Dynamic Soil Structure Interaction
•
The seismic excitation experienced by structures is a function of
– earthquake source
– travel path effects
– local site effects
– soil-structure interaction (SSI) effects.
•
Result of the first three of these factors is a ‘‘free-field’’ ground motion.
•
Structural response to free-field motion is influenced by SSI.
Fig. Travel path effects, Kramer (1996)
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Vertical
direction of
shaking
should also be
considered….
14
Need to study SSI
• Soil-structure interaction topics are generally not taught in
graduate earthquake engineering courses.
• Unfortunately, practice is hindered by a literature that is
often difficult to understand, and codes and standards that
contain limited guidance.
• Most articles rely heavily on the use of wave equations in
several dimensions.
• Many times inconsistent nomenclature is used.
• Practical examples of SSI applications are sparse.
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Questions in SSI
1. When is the use of foundation springs and dashpots
important, and which structural response parameters
are affected?
2. Under
what
conditions
is
consideration
of
the
differences between foundation input motions and freefield ground motions important?
3. What field and laboratory investigations are necessary
to develop foundations springs and dashpots for SSI
analysis?
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Continued…..
• Once the decision to implement SSI has been made, a
basic level of understanding of the physical phenomenon
and a practical analysis methodology for simulating its
effects are needed.
• This presentation describes the principal components of
SSI in a clear and concise way, and consistent
nomenclature is used throughout.
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IMPORTANT
• Implementation of SSI within a design setting
requires close collaboration between structural
and geotechnical engineers.
• Neither discipline alone is likely to have sufficient
knowledge of structural, foundation, and site
considerations necessary to properly complete a
meaningful analysis considering SSI effects.
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Remarks
• In particular, accelerations within structures are
affected by the flexibility of foundation support and
variations
between
foundation
and
free-field
motions.
• Consequently, an accurate assessment of inertial
forces and displacements in structures can require
a rational treatment of SSI effects.
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SSI Analysis Procedures
• Inertial Interaction
– Inertia developed in the structure due to its
own vibrations gives rise to base shear and
moment, which in turn cause displacements of
the foundation relative to the free-field.
– Inertia effects are considered and equations
are developed.
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SSI DESIGN PROCEDURES
• Kinematic Interaction
– The presence of stiff foundation elements on or in soil
cause foundation motions to deviate from free-field
motions as a result of ground motion in- coherence,
wave inclination, or foundation embedment.
– Kinematic effects are described by a frequency
dependent transfer function relating the free-field
motion to the motion that would occur on the base
slab if the slab and structure were massless.
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Methods of SSI
– Direct methods
• In a direct analysis, the soil and structure are include
within the same model and analyzed as a complete
system.
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Methods of SSI
Subsurface approach
– an evaluation of free-field soil motions and corresponding
soil material properties
– an evaluation of transfer functions to convert free-field
motions to foundation input motions
– incorporation of springs and dashpots (or more complex
nonlinear elements) to represent the stiffness and damping
at the soil-foundation interface; and
– a
response
analysis
of
the
combined
structure-
spring/dashpot system with the foundation input motion
applied.
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Steps in Subsurface approach
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Steps in Subsurface approach
• Specification of a foundation input motion (FIM)
• The stiffness and damping characteristics of the soilfoundation
interaction
are
characterized
using
relatively simple impedance function models or a
series of distributed springs and dashpots.
• The superstructure is modeled above the foundation
and the system is excited through the foundation by
displacing the ends of the springs and dashpots using
the rocking and translational components of the FIM.
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Soil-Structure System Behavior
• A rigid base refers to soil supports with infinite
stiffness (i.e., without soil springs).
• A fixed base refers to a combination of a rigid
foundation elements on a rigid base.
• A
flexible
base
analysis
considers
the
compliance (i.e., deformability) of both the
foundation elements and the soil.
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Introductory example on SSI
Fig. Schematic illustration of deflections caused by force
applied to (a) Fixed base structure (b) Flexible base structure
(NEHRP, NIST GCR 12-917-21)
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Introductory example
Without SSI
We can write for, a single degree-of-freedom structure with stiffness, k, and mass, m,
resting on a fixed base, as depicted in Figure. A static force, F, causes deflection, Δ:
From structural dynamics, the undamped natural vibration frequency, ω and
period, T, of the structure are given by Clough and Penzien (1993) as:
At static case, ∆ =
F
k
From this, ω =
T 2 = (2π ) 2
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k
2π
m
,T=
= 2π
ω
m
k
m
m∆
= (2π ) 2
F
( F / ∆)
Dr. A. D. Katdare
(1)
(2)
28
Introductory example
With SSI
If a force, F, is applied to the mass in the x direction, the structure
deflects, as it does in the fixed-base system, but the base shear (F)
deflects the horizontal spring by uf , and the base moment (Fxh),
deflects the rotational spring by θ. Accordingly, the total deflection with
respect to the free-field at the top of the structure, ∆ is:
F
+ u f + θ .h
k
F F  Fh 
∆ = + +
 h

k k x  k yy 
∆=
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(3)
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Introductory example for SSI
From equation (2) in (3), we get,
 1 1 h2 
m∆
2
= (2π ) m  + +
T = (2π )


F
 k k x k yy 
2
2
2
T 
k  1 1 h2 
 From this, we get,
  = m  + +
T
m
k
k
k
x
yy 
 

2
T 
k kh 2
  = 1+ +
k x k yy
T 
•
Above equation represents, equation by considering SSI.
•
It is in simplest form (SDOF, without damping)
•
Based only on first mode of vibration
•
Based on elastic behaviour of structure
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(4)
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Introductory example of SSI
• In initial work on SSI, Veletsos and Nair (1975) and Bielak
(1975), showed that, the period lengthening depends on,
h h B
m
, , ,
,ν
VsT B L ρ s 4 BLh
where h is the structure height (or height to the center of
mass of the first mode shape), B and L refer to the half-width
and half-length of the foundation, m is the mass (or effective
modal mass), ρs is the soil mass density, and ν is the
Poisson’s ratio of the soil.
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Introductory example of SSI
• To the extent that h/T quantifies the stiffness of the
superstructure
• the term h/(VsT) represents the structure-to-soil stiffness ratio.
• The term h/T has units of velocity, and will be larger for stiff
lateral force resisting systems, such as shear walls, and smaller
for flexible systems, such as moment frames.
• For typical building structures on soil and weathered rock sites,
h/(VsT) is less than 0.1 for moment frame structures, and
between approximately 0.1 and 0.5 for shear wall and braced
frame structures (Stewart et al., 1999)
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Introductory example with damping
• Damping factor is calculated using
β0 = β f +
1
(T T )
n
βi
where
βi is the structural damping in the superstructure assuming a
fixed base,
βf is foundation damping (for soil),
The exponent, n, is taken as 3 for linearly viscous structural
damping, and 2 otherwise (e.g., for hysteretic damping)
(Givens, 2013).
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More examples on SSI
Computational model of bridge
bent including pile foundation by
Liam Finn, 2010
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Dynamic soil-shallow foundation-structure
model for horizontal and rocking
foundation motions by Moghaddasi et al.,
2010
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Typical results of SSI for Rayleigh waves
After Betti et al. (1998)
Remarks on the example
• From the example, it is clear that,
– In SSI both soil and structure are considered
together
– Soil is modelled as springs and stiffness of the
spring depends on the type of soil and its dynamic
properties
– In SSI, ultimately we develop theories, models or
equations which involves properties of soil and
structure both.
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General observations on SSI
• The dialogue between structural and geotechnical engineers
varies widely, both in extent and sophistication.
• It appears that an increase in the amount of collaboration
would be beneficial, as would better understanding of what
each discipline does, and needs, and why?
• Many geotechnical engineers, are not sure how their
recommendations are ultimately being used, and often do not
know whether or not their recommendations are being
properly implemented.
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General observations on SSI
• In many cases, static and dynamic springs for
modeling soil properties are not being consistently
or properly developed by geotechnical engineers,
nor
are
they
being
consistently
or
properly
implemented by structural engineers.
• For typical foundation situations, there is no
consensus among structural engineers on the best
modeling approaches to use.
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General observations on SSI
• Understanding of SSI principles is fairly limited
among structural engineers, and is usually
limited to application of vertical foundation
springs.
• A broader implementation of SSI techniques is
rare, and there is virtually no use of foundation
damping in any explicit way.
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Collaboration Between Design Professionals
• On most projects, structural engineers are engaged
by the architect, but geotechnical engineers are
engaged by the building owner.
• This arrangement is primarily the result of a
perceived increase in liability for geotechnical
engineering, and the reluctance of architects, and
their professional liability insurers, to engage
geotechnical engineers as subconsultants.
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Requirements for SSI
• Collaboration between design engineers
– Geotechnical
engineers
are
not
directly
managed by the architect as lead design
professional, and geotechnical engineers are
typically not part of formal design team
meetings arranged by the architect.
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Information Needed by Structural Engineer
• Structural engineers routinely seek a common
set of information from geotechnical engineers
on most projects.
• This includes a description of the soil and rock
characteristics at the site, geotechnical hazards
that need to be mitigated, and recommendations
on appropriate foundation systems.
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Information Needed by Structural Engineer
• Design bearing pressures under footings
• Estimates of allowable settlements
• At rest, active and passive lateral pressures
• Vertical and lateral capacities for deep foundation
• Expected site seismicity
• Soil profile type
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Information Needed by Structural Engineer
(For SSI)
• Depending
sophistication
on
the
of
the
size,
nature
project,
and
additional
information is often needed, including forcedisplacement
relationships
or
springs
to
represent vertical and horizontal soil properties,
site specific spectra, and response histories etc.
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Geotechnical Engineer’s report
(For SSI)
• Geotechnical
engineers
recommendations
when
can
they
provide
better
more
detailed
have
information on soil
• Geotechnical engineer provide following data in his/her
report:
– Soil type and properties (static and dynamic)
– borings and other critical soil data
– Suitable type of foundation from given loading and site
investigation
– Any other information if needed at the site
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Significance of SSI
• Understanding of SSI principles varies widely across both
the structural and geotechnical engineering disciplines.
• SSI effects are more pronounced in soft soils, and many
are aware that foundation input motions can differ from
free-field ground motions.
• By including the soil in the modeling process, engineers
can gain a better understanding of the distribution of
forces and displacements in the structure, and additional
insight into the foundation design.
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Codes available on SSI
•
•
•
•
ATC 03 (1978)
NEHRP 2003 (FEMA 440)
ASCE/SE/7-05
Eurocode 7
Indian codes…….??
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Conclusions
• Soil and structure should be considered together
to build a safe structure during earthquake.
• There should be co-ordination between
structural and geotechnical consultant for a
better design.
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Part II
Effects of Earthquakes:
Geotechnical Engineering
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Total stress, Pore water pressure
and Effective stress
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Case
Total
Pressure
Pore
Pressure
Effective
Pressure
Figure- 1
475
150
325
Figure- 2
475
250
225
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Soil Liquefaction
•
The October 17, 1989 Loma Prieta earthquake was responsible for 62
deaths and 3,757 injuries. In addition, over $6 billion in damage was
reported.
•
This damage included damage to 18,306 houses and 2,575 businesses.
•
Approximately 12,053 persons were reported displaced. The most
intense damage was confined to areas where buildings and other
structure where situated on top of loosely consolidated, water saturated
soils.
•
Loosely consolidated soils tend to amplify shaking and increase
structural damage during earthquake.
•
Water saturated soils compound the problem due to their susceptibility to
liquefaction. Consequently, there is loss of bearing strength.
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Soil Liquefaction
• The failure was termed as ‘soil liquefaction’.
• The concept of liquefaction was first introduced by A.
Casagrande.
• Liquefaction is a physical process that takes place during
some earthquakes.
• Prior to an earthquake, the water pressure is relatively low.
• However, earthquake shaking can cause the water pressure
to increase to the point where the soil particles can readily
move with respect to each other.
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Soil Liquefaction
Fig. Indicating the cause of soil liquefaction
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Why soil liquefies?
• A soil deposit consists of an assemblage of
individual soil particles.
• Liquefaction occurs when the structure of a loose,
saturated sand breaks down due to some rapidly
applied loading.
• As the structure breaks down, the loosely-packed
individual soil particles attempt to move into a
denser configuration.
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Why to study Soil Liquefaction?
Fig. Failure due to Soil
liquefaction
In Niigata in 1964.
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Fig. Underwater slide during
the San Fernando earthquake,
1971.
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Why to study Soil Liquefaction?
Fig. Sand boils during Loma
Prieta earthquake (1989)
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Fig. Lateral spreading
during earthquake
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Why to study Soil Liquefaction?
Fig. Loss of bearing capacity during Caracas (1967) earthquake
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Where liquefaction can occur ?
• Liquefaction only occurs in loose saturated
sandy soils.
• Loose sandy saturated soil when subjected to
dynamic loading, looses all it’s strength.
• Its effects are most commonly observed in lowlying areas near bodies of water such as rivers,
lakes, bays, and oceans.
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Factors governing liquefaction in field
• In order to have liquefaction, there must be
ground shaking (dynamic loading).
• The condition most conducive to liquefaction
is near surface ground water table location.
• Soil types most susceptible to liquefaction
during earthquake is deposit consisting of fine
to medium sand.
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Factors governing liquefaction in field
• Uniformly grades, cohesionless soils in loose relative density
state are susceptible to liquefaction.
• Uniformly grades soil liquefy with relative ease (susceptible to
liquefaction)
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Factors governing liquefaction in field
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Factors governing liquefaction in field
• Greater the confining pressure, lesser the soil has
tendency to liquefy.
• Soils having rounded particles tend to densify more easily
than angular shaped soil particles.
• Uniformly grades soils are more susceptible than well
graded soil.
• New soil deposits are more susceptible to liquefaction
than old deposits.
• Construction of heavy building on top of sand deposit
decreases liquefaction resistance of soil.
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Soil Liquefaction
• Liquefaction results from the tendency of the soil
to
reduce
in
volume
to
strengthen
itself
(compression)
• When loose, saturated sands are subjected to
dynamic loads, the soil grains try to rearrange
into more dense packing, with less space in
voids as water in the voids is forced out
(dissipation of pore water pressure)
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How to avoid soil liquefaction?
•
Compaction of loose sand
–
Compaction by rollers
–
Compaction by pile driving
–
Compaction by vibrofloation
–
Blasting
•
Grouting and Chemical Stabilization
•
Application of Surcharge
•
Drainage Using Coarse Material Blanket and Drains
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Can Liquefaction be predicted?
• Occurrence of liquefaction can’t be predicted easily
• Possible to identify areas giving detailed information
that have the potential for liquefaction
• Mapping of liquefaction potential on a regional scale
• Maps exists for many regions in USA and Japan
• India also is developing in terms of these maps
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Can Liquefaction be predicted?
• Various research groups are doing the
working for preparing maps for India.
(liquefaction susceptibility map )
• liquefaction susceptibility: capacity of soil
to resist liquefaction
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Is it possible to prepare for liquefaction ?
• Possible to identify areas potentially subject to
liquefaction with hazard zone map
• Emphasis in terms of developing appropriate
public policy or selecting mitigation technique in
area of major concern
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Is it possible to prepare for liquefaction?
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Is it possible to prepare for liquefaction?
• Use of hazard map by public and private owners the
seriousness of expected damage and most vulnerable
structure
• Using
this
map
local
government
could
designate
liquefaction potential areas, and require by ordinance, site
investigation
and
possible
mitigation
techniques
for
properties in these area particularly underground pipes and
critical transportation routes
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Conclusions
• Liquefaction is loss of shear strength of soil and due
to it soil flows as a liquid.
• It takes place in saturated, loose sands.
• Though liquefaction cannot be predicted accurately,
it can be avoided with certain measures.
• Liquefaction susceptibility maps are prepared by
local governments for assessing safety of different
locations.
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The
author
wishes
to
gratefully
acknowledge the various sources used
during the preparation of this presentation
which have aided and enhanced the quality
either in the form of information, data,
figure, photo, graph or table.
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Karl von Terzaghi (October 2, 1883 – October 25, 1963) was
an Austrian civil engineer andgeologist known as the "father of soil
mechanics".
“Engineering is a noble sport . . . but occasional blundering is a part of the
game. Let it be your ambition to be the first one to discover and announce
your blunders. . . . Once you begin to feel tempted to deny your blunders in
the face of reasonable evidence you have ceased to be a good sport. You are
already a crank or a grouch.”
------- Karl Terzaghi’s advice to his students at Harvard
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