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Introduction to Earthquake Geotechnical Engineering and It

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Introduction to Earthquake Geotechnical Engineering and It s Practices by Dr. Deepankar Choudhury Assistant Professor, Department of Civil Engineering, – PowerPoint PPT presentation

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Title: Introduction to Earthquake Geotechnical Engineering and It


1
Introduction to Earthquake Geotechnical
Engineering and Its Practices
  • by
  • Dr. Deepankar Choudhury
  • Assistant Professor, Department of Civil
    Engineering,
  • IIT Bombay, Powai, Mumbai 400 076, India.
  • URL http//www.civil.iitb.ac.in/dc/

2
Earthquake Hazards related to Geotechnical
Engineering
D. Choudhury, IIT Bombay, India
3
  • Ground Shaking Shakes structures constructed on
    ground causing them to collapse
  • Liquefaction Conversion of formally stable
    cohesionless soils to a fluid mass, causing
    damage to the structures
  • Landslides Triggered by the vibrations
  • Retaining structure failure Damage of anchored
    wall, sheet pile, other retaining walls and sea
    walls
  • Fire Indirect result of earthquakes triggered by
    broken gas and power lines
  • Tsunamis large waves created by the
    instantaneous displacement of the sea floor
    during submarine faulting

D. Choudhury, IIT Bombay, India
4
Damage due to Earthquakes
Earthquakes have varied effects, including
changes in geologic features, damage to man-made
structures and impact on human and animal life.
Earthquake Damage depends on many factors
  • The size of the Earthquake
  • The distance from the focus of the earthquake
  • The properties of the materials at the site
  • The nature of the structures in the area

D. Choudhury, IIT Bombay, India
5
Ground Shaking
Frequency of shaking differs for different
seismic waves. 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. Unconsolidated materials amplify
shaking more than rocks do. Buildings respond
differently to shaking depending on construction
styles, materials Wood -- more flexible, holds
up well Earthen materials, unreinforced concrete
-- very vulnerable to shaking.
D. Choudhury, IIT Bombay, India
6
Collapse of Buildings
D. Choudhury, IIT Bombay, India
7
Soft first story
Loma Prieta earthquake damage in San Francisco. 
The soft first story is due to construction of
garages in the first story and resultant
reduction in shear strength. (Photo from
http//earthquake.usgs.gov/bytopic/photos.html) On
October 17, 1989, at 50415 p.m. (P.d.t.), a
magnitude 6.9 (moment magnitude surface-wave
magnitude, 7.1)
D. Choudhury, IIT Bombay, India
8
Inadequate attachment of building to foundation
House shifted off its foundation, Northridge
earthquake.  (Photo from Dewey, J.W.,
Intensities and isoseismals, Earthquakes and
Volcanoes, Vol. 25, No. 2, 85-93, 1994)
D. Choudhury, IIT Bombay, India
9
Image of Bachau in Kutch region of Gujarat after
earthquake
D. Choudhury, IIT Bombay, India
10
Failure of Bridge Abutment
Foundation and column of a dwelling at the
long-bean-shaped hill (Kashmir October 8, 2005)
D. Choudhury, IIT Bombay, India
11
Suspension Bridge in Balakot (Kashmir October 8,
2005) Right Abutment Moved Downstream
D. Choudhury, IIT Bombay, India
12
Building design Buildings that are not designed
for earthquake loads suffer more
D. Choudhury, IIT Bombay, India
13
Causes failure of lifelines
D. Choudhury, IIT Bombay, India
14
Earthquake Destruction Landslides
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15
Earthquake Destruction Liquefaction
Flow failures of structures - caused by loss of
strength of underlying soil
D. Choudhury, IIT Bombay, India
16
Sand Boil Ground water rushing to the surface
due to liquefaction
Sand blow in mud flats used for salt production
southwest of Kandla Port, Gujarat
D. Choudhury, IIT Bombay, India
17
Lateral Deformation and Spreading
D. Choudhury, IIT Bombay, India
18
Lateral Spreading Liquefaction related phenomenon
Upslope portion of lateral spread at Budharmora,
Gujarat
D. Choudhury, IIT Bombay, India
19
Lateral spreading in the soil beneath embankment
causes the embankment to be pulled apart,
producing the large crack down the center of the
road.
D. Choudhury, IIT Bombay, India
20
Lateral Deformation and Spreading
  • Down slope movement of soil, when loose sandy
    (liquefiable) soil is present, at slopes as
    gentle as 0.50
  • In situations where strengths (near or post
    liquefaction) are less than the driving static
    shear stresses, deformations can be large, and
    global instability often results

D. Choudhury, IIT Bombay, India
21
Estimation of Lateral Deformation
  • Estimates of large deformations are usually
    accurate within a factor of /- 2 it has been
    argued that accuracy is not an issue, because
    large demands mitigation, regardless of the
    exact figure
  • Approaches for estimating lateral displacements
  • Statically-derived empirical methods based on
    back-analysis of field case histories (Youd et
    al. 2002, Hamade et al. 1986)
  • Simple static limit equilibrium analysis, Newmark
    sliding block (with engineering judgement)
  • Fully non linear, time-domain finite element or
    finite difference analyses

D. Choudhury, IIT Bombay, India
22
Youd Empirical Approach
  • Based on earthquake case histories in U.S. and
    Japan
  • Accurate within a factor 2, generally, least
    accurate in the small displacement range
  • Two models sloping ground model and free face
    model

D. Choudhury, IIT Bombay, India
23
Youd Empirical Approach
  • Sloping ground model
  • Log Du -16.713 1.532 M 1.406 log R -
    0.012 R 0.592 log W
  • 0.540 log T15 3.413 log (100 F15) 0.795
    log (D5015 0.1 mm)
  • Free face model
  • Log Du -16.213 1.532 M 1.406 log R -
    0.012 R 0.338 log S
  • 0.540 log T15 3.413 log (100 F15) 0.795
    log (D5015 0.1 mm)
  • Where Du estimated lateral ground displacement,
    m
  • M moment magnitude of earthquake
  • R nearest horizontal or map distance from the
    site to the seismic energy source, km
  • R0 distance factor that is a function of
    magnitude, M R0 10(0.89M-5.64)
  • R modified source distance, R R R0
  • T15 cumulative thickness of saturated granular
    layers with corrected below counts (N1)60 lt 15, m
  • F15 average fines content (fraction passing no.
    200 sieves), , for granular materials within T15
  • D5015 average mean grain size for granular
    materials within T15
  • S ground slope,
  • W free face ratio defined as the height (H) of
    the free face divided by the distance (L) from
    the base of the free face to the point in question

D. Choudhury, IIT Bombay, India
24
  • Other Methods for Lateral Displacement
  • Newmark sliding block analysis, which assumes
    failure on well defined failure plane, sliding
    mass is a rigid block, and so on
  • Dynamic finite element programs with effective
    stress based soil constitutive models

D. Choudhury, IIT Bombay, India
25
Newmarks Sliding block analysis
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26
Liquefied soil exerts higher pressure on
retaining walls,which can cause them to tilt or
slide.
D. Choudhury, IIT Bombay, India
27
Increased water pressure causes collapse of dams
D. Choudhury, IIT Bombay, India
28
Earthquake Destruction Fire
Earthquakes sometimes cause fire due to broken
gas lines, contributing to the loss of life and
economy.
The destruction of lifelines and utilities make
impossible for firefighters to reach fires
started and make the situation worse eg. 1989
Loma Prieta 1906 San Francisco
D. Choudhury, IIT Bombay, India
29
Earthquake Destruction Tsunamis
  • Tsunamis can be generated when the sea floor
    abruptly deforms and vertically displaces the
    overlying water.
  • The water above the deformed area is displaced
    from its equilibrium position. Waves are formed
    as the displaced water mass, which acts under the
    influence of gravity, attempts to regain its
    equilibrium.
  • Tsunami travels at a speed that is related to the
    water depth - hence, as the water depth
    decreases, the tsunami slows.
  • The tsunami's energy flux, which is dependent on
    both its wave speed and wave height, remains
    nearly constant.
  • Consequently, as the tsunami's speed diminishes
    as it travels into shallower water, its height
    grows. Because of this effect, a tsunami,
    imperceptible at sea, may grow to be several
    meters or more in height near the coast and can
    flood a vast area.

D. Choudhury, IIT Bombay, India
30
Tsunami
Tsunami Movement 600 mph in deep water
250 mph in medium depth water
35 mph in shallow water
D. Choudhury, IIT Bombay, India
31
The tsunami of 3m height at Shikotan, Kuril
Islands, 1994 carried this vessel 70 m on-shore.
The waves have eroded the soil and deposited
debris.
D. Choudhury, IIT Bombay, India
32
Foundation failure in Kerala during Tsunami
(December 26th, 2004)
D. Choudhury, IIT Bombay, India
33
Geomorphological Changes
  • Geomorphological changes are often caused by an
    earthquake e.g., movements--either vertical or
    horizontal--along geological fault traces the
    raising, lowering, and tilting of the ground
    surface with related effects on the flow of
    groundwater
  • An earthquake produces a permanent displacement
    across the fault.
  • Once a fault has been produced, it is a weakness
    within the rock, and is the likely location for
    future earthquakes.
  • After many earthquakes, the total displacement on
    a large fault may build up to many kilometers,
    and the length of the fault may propagate for
    hundreds of kilometers.

D. Choudhury, IIT Bombay, India
34
Ground Improvement for Liquefaction Hazard
Mitigation
D. Choudhury, IIT Bombay, India
35
Ground Improvement in IS Code
In poor and weak subsoils, the design of
conventional shallow foundation for structures
and equipment may present problems with respect
to both sizing of foundations as well as control
of foundation settlements. Traditionally, pile
foundations have been employed often at enormous
costs. A more viable alternative in certain
solutions, developed over the recent years, is to
improve the subsoil itself to an extent such that
the subsoil improvement would have resultant
settlements within acceptable limits. The
techniques for ground improvement has developed
rapidly and has found large scale application in
industrial projects.
IS 13094 1992 (Reaffirmed 1997)
D. Choudhury, IIT Bombay, India
36
Ground Improvement in IS Code
  • Ground improvement is indicated if
  • Net loading intensity of the foundation exceeds
    the allowable bearing pressure as per IS
    64031981
  • Resultant settlement or differential settlement
    (per IS 8009 Part 1 or 2) exceeds acceptable
    limits for the structure
  • The subsoil is prone to liquefaction in seismic
    event

D. Choudhury, IIT Bombay, India
37
Types of Ground Improvement by Function
  1. Excavation, fill placement, groundwater table
    lowering
  2. Densification through vibration or compaction
  3. Drainage through dissipation of excess pore water
    pressure
  4. Resistant through inclusions
  5. Stiffening through cement or chemical addition

Note some method serve multiple functions
D. Choudhury, IIT Bombay, India
38
Densification through vibration and compaction
  • Vibrating probe/vibroflotation
  • Vibrations of probe cause grain structure to
    collapse densifying soil raised and lowered in
    grid pattern

Most Suitable Soil Type Saturated or dry clean sand
Max effective treatment depth 20 m, ineffective in upper 3-4 m.
Special materials required None
Special equipment required Vibratory pile driver or vibroflot equipment
Properties of treated material Can obtain up to Dr 80
Special advantages and limitations Rapid, simple, cheaper than VR stone columns, compaction piles less effective than methods that employ compaction as well as vibration, difficult to penetrate stiff overlayers, may be ineffective for layered systems
Relative Cost Moderate
D. Choudhury, IIT Bombay, India
39
  • Vibro-compaction/replacement stone/sand columns
  • Steel casing is driven in to the soil, gravel or
    sand is filled from the top and tamped with a
    drop hammer as the steel casing is successfully
    withdrawn, displacing the soil

Most Suitable Soil Type Cohesionless soil with less than 20 fines
Max effective treatment depth 30 m
Special materials required Granular Backfill
Special equipment required Vibrofolt equipment, steel casing, hopper for backfill
Properties of treated material Can obtain high relative density
Special advantages and limitations Rapid, useful for a wide range of soil types May require a large volume of backfill, noisy
Relative Cost Moderate
D. Choudhury, IIT Bombay, India
40
  • Dynamic Densification (heavy tamping)
  • A heavy weight is dropped in a grid pattern, for
    several passes

Most Suitable Soil Type Cohesionless soil, waste fills, partly saturated soils, soils with fines
Max effective treatment depth 30 m, less at the surface, degree of improvement usually decreases with depth
Special materials required None
Special equipment required Tamper and crane
Properties of treated material Good improvement and reasonable uniformity
Special advantages and limitations Rapid, simple, may be suitable for soils with fines lack of uniformity with depth, not possible near existing structures, may granular backfill surface layer
Relative Cost low
D. Choudhury, IIT Bombay, India
41
  • Other methods
  • Displacement piles densification by displacement
    of pile volume, usually precast concrete or
    timber piles
  • Compaction grouting densification by
    displacement of grout volume

D. Choudhury, IIT Bombay, India
42
Stiffening through cement or chemical addition
Permeation or penetrating grouting High
permeability grout is injected into the ground at
numerous points, results in solidified soil mass
Most Suitable Soil Type Saturated medium to coarse sand
Max effective treatment depth gt 30m
Special materials required Grout
Special equipment required Mixers, tanks, pumps, hoses, monitoring equipment
Properties of treated material Impervious, high strength where completely mixed
Special advantages and limitations Produces a hard, stiff mass of soil, useful for existing structures as it causes little or no settlement or disturbance, low noise Area of permeation can vary, can be blocked by pockets of soil with fines, difficult to determine the improved area, requires curing time
Relative Cost Least expensive of grout systems, but moderately expensive compared to vibro methods
D. Choudhury, IIT Bombay, India
43
Earthquake resistant design of geotechnical
structures
Geotechnical structures like, Retaining
wall/Sheet pile Slope Shallow
foundations Deep foundations Must be designed
to withstand the earthquake loading
D. Choudhury, IIT Bombay, India
44
Seismic Design of Retaining Wall
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45
Mononobe-Okabe (1926, 1929) Method
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46
Seismic Slope Stability
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47
Wedge Method of Analysis by Terzaghi (1950)
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48
Seismic Bearing Capacity of Shallow Foundations
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49
Seismic Bearing Capacity of Shallow Strip Footings
Choudhury, D. and Subba Rao, K. S. (2005),
Seismic bearing capacity of shallow strip
footings, Geotechnical and Geological
Engineering, An International Journal, Springer,
Netharlands, 23(4) 403-418.
D. Choudhury, IIT Bombay, India
50
Guideline as per Indian Code
  • According to IS 1893, isolated RCC footing
    without tie beams, or unreinforced strip
    foundation shall not be permitted in soft soils
  • Shallow foundation elements should be tied
    together so that they move uniformly, bridge over
    areas of local settlements, resist soil movements
    which ultimately reduces the level of shear
    forces induced in the elements resting on the
    foundation
  • Buried utilities, such as sewage and water pipes,
    should have ductile connections to the structure
    to accommodate the large movements and
    settlements that can occur under seismic loading

D. Choudhury, IIT Bombay, India
51
Questions?
D. Choudhury, IIT Bombay, India
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