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8/12/2019 Lecture40-Mitigation and Ground Improvement
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Mitigation of Earthquake Hazards &
Ground Improvement
Lecture-40
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Major Hazards of Earthquakes
Ground Motion: Shakes structures causing themto collapse
Liquefaction: Conversion of formally stable
cohesionless soils to a fluid mass, causing damage
to the structures
Landslides: Triggered by the vibrations
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 2
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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
Earthquakes have varied effects, including changes ingeologic features, damage to man-made structures and
impact on human and animal life. Earthquake Damage
depends on many factors:
Damage due to Earthquakes
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Mitigation Options
•Avoiding the hazard
•Building Earthquake resistant
structures
•Ground Improvement
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Mitigation Options: Avoiding hazard
Where the potential for failure is beyond the acceptable level and not
preventable by practical means, the locations of seismic threat can be
avoided and the structures should be relocated sufficiently far away
from the threat.
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Mitigation Options: Earthquake Resistant Structures
Methods to increase capacity/ Decrease demand:
•Special Construction materials
•Special Foundation Techniques
•Special Construction Techniques
Seismic demand should be less than the Computed capacity
‘Seismic demand’ is the effect of the earthquake on the structure.
‘Computed capacity’ is the structure’s ability to resist that effect
without failure.
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Special Construction Materials
Some of the special materials:
Rubber, lead, copper, brass, aluminum, stainless steel, fibre-
reinforced plastics and shape-memory alloys
These materials absorb a part of seismic energy and thereby reduce the
effect of earthquake on structure. These materials are strategically used tomodify the force –deformation response of structural components and/or
enhance their energy dissipation potential.
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Mitigation Options: Special Construction Techniques
•Base Isolation Systems
•Energy Dissipation Systems
•Active Control Systems
Special construction techniques are adapted to reduce the seismic
demand on the superstructure by sharing the earthquake loadsthrough non-conventional structural elements.
Some of these Techniques Include:
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Base Isolation Systems
In base-isolated systems, the superstructure is isolated
from the foundation by certain devices, which reduce the
ground motion transmitted to the structure. These devices
help decouple the superstructure from damaging
earthquake components and absorb seismic energy by
adding significant damping .
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Passive Energy Dissipation Systems
Various Energy Dissipating Devices (EDD) are used to dissipate
the seismic energy. These devices are like ‘add-ons’ to
conventional fixed-base system, to share the seismic demand
along with primary structural members.
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Active Control Systems
They control the seismic response through appropriate adjustments within
the structure, as the seismic excitation changes. In other words, active
control systems introduce elements of dynamism and adaptability into the
structure, thereby augmenting the capability to resist exceptional
earthquake loads.
A majority of these techniques
involve adjusting lateral strength,
stiffness and dynamic properties of
the structure during the earthquake
to reduce the structural response
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Mitigation Options: Ground Improvement
Earthquake damage is greater in poorer soil areas, and significant life and
property losses are often associated with soil-related failures.
Buildings and lifelines located in earthquake-prone regions, especially
structures founded upon loose saturated sands, reclaimed or otherwise
created lands, and deep deposits of soft clays, are vulnerable to a variety of
earthquake-induced ground damage such as liquefaction, landslides,
settlement, and ground cracking.
Recent experiences show that engineering techniques for ground
improvement can mitigate earthquake related damage and reduce losses.
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Mitigation Options: Ground Improvement
Fundamental approaches of Ground Improvement to mitigate earthquake
damages are either to increase capacity of soil or to decrease the
earthquake demand on the soil using several techniques.
Increasing Capacity Decreasing Demand
Soil Densification
Providing drains for rapid
dissipation of pore pressures
Grouting
Soil Reinforcement
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Ground Improvement: Soil Densification
Soil densification techniques:
•Compaction
•Vibro-replacement (Vibroflotation & Stone Columns)
•Blasting
•Grouting
•Compaction Piles
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Filed Compaction
• Pneumatic rubber tired roller
Different types of rollers (clockwise
from right):
Vibratory roller
Smooth-wheel roller
Sheepsfoot roller
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Dynamic Compaction
- pounding the ground by a heavy weight
Suitable for granular soils and landfilles
Crater created by the impact
Pounder (Tamper)
(to be backfilled)
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Dynamic Compaction
Pounder (Tamper)
Mass = 5-30 tonne
Drop = 10-30 m
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Dynamic Compaction
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Ground Densification: Vibro-Compaction
Vibro-Compaction also knows as VibroFlotation is used to
densify clean, cohesionless soils. The action of the
vibrator, usually accompanied by water jetting, reduces
the inter-granular forces between the soil particles,
allowing them to move into a denser configuration,typically achieving a relative density of 70 to 85 percent.
Compaction is achieved above and below the water table.
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vibrator makes a hole
in the weak groundhole backfilled ..and compacted Densely compacted stone
column
Vibroflotation
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Vibroflotation
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Ground Densification: Vibro-Compaction
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• Generally used to improve density of silty sands-sandy gravels(non-cohesive soils)
• Makes use of dynamic/undrained loading conditions to causeliquefaction-induced settlement
•
Sudden dynamic loading breaks cohesion and any cementation• Shockwave temporarily liquefies soil layer
• Settlement occurs as excess pore water pressure approaches zero.
• Typical vertical strain between 2% and 10%
Ground Densification: Blasting
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Aftermath of blasting
For densifying granular soils
Ground Densification: Blasting
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Grouting is is a technique whereby a slow-flowing water/sand/cement mix is
injected under pressure into a granular soil.
The grout forms a bulb that displaces and hence densifies, the surrounding
soil.
Compaction grouting is a good option if the foundation of an existing
building requires improvement, since it is possible to inject the grout from the
side or at an inclined angle to reach beneath the building.
Jet grouting involves the injection of low viscosity liquid grout into the pore
spaces of granular soils. This creates hardened soils to replace loose
liquefiable soils.
Soil Densification: Grouting
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Soil Densification: Compaction Grouting
Compaction Groutinguses displacement toimprove groundconditions.
A highly viscousaggregate grout ispumped in stages,forming grout bulbs,which displace and
densify the surroundingsoils.
Used for loose soils,liquefiable Soils andcollapsible Soils
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Soil Densification: Cement Grouting
Cement Grouting, also known asSlurry Grouting, is the intrusionof microfine cement slurry (fineportland cement mixed with adispersant and larger quantitiesof water) into fine sand andfinely cracked rock underpressure
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Jet Grouting
Grout is pumped through the rod and exits the horizontal nozzle(s) at high
velocity [approximately 200m/sec]. This energy breaks down the soil matrix
and replaces it with a mixture of grout slurry and in-situ soil (soilcrete). Jet
grouting is most effective in cohesionless soils.
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Jet Grouting
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Ground Densification: Deep soil mixing
Deep Mixing Method is the mechanical blending of the in situ soil with
cementitious materials using a hollow stem auger and paddlearrangement. These materials could be Cement or Fly ash or Ground Blast
Furnace Slag or Lime or Additives or Combination of these. Soil mixing has
the ability to strengthen soft and wet cohesive soils in a very short time
period to permit many types of construction projects.
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Ground Improvement: Vertical Drains
The installation of prefabricated
vertical drains provides shorteneddrainage paths for the water to exit
the soil. Drainage remediation
methods mitigate liquefaction
hazards by enhancing the rate of
excess pore pressure dissipation.The most common methods of
drainage remediation are through
the use of gravel, sand or wick
drains. Drains are suitable for silts
or clays.
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Ground Improvement: Vertical Drains
Primary consolidationsettlement will already beachieved during theconstruction period by usingvertical drains
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EARTHQUAKE DRAINS
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EARTHQUAKE DRAINSSM
Earthquake Drains are prefabricated in the field to project specifications. The drain is fitted with a sacrificial
endplate. The completed drains are fed into the installation mandrel and driven to treatment depth. When the
mandrel is withdrawn, the endplate anchors in the soil leaving the drain in-place. 34
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EARTHQUAKE DRAINS
• DISSIPATES EXCESS PORE PRESSURES AS THEY GENERATE DURING A
SEISMIC EVENT
• CAN BE USED TO RETROFIT EXISTING STRUCTURES
• APPROXIMATELY ONE THIRD THE COST OFTRADITIONAL STONE COLUMNS
• INSTALLATION TIMES APPROXIMATELY ONE THIRD TO
ONE HALF OF THAT FOR STONE COLUMNS.
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EARTHQUAKE DRAINS
FINS: Transmit vibratory
motion to the soil for
densification
STEEL CASING: Protects
drain from driving stresses
PREFABRICATED
DRAIN
Figure 2.1: Cross section of casing and prefabricated drain
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EARTHQUAKE DRAINS
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EARTHQUAKE DRAINS
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Geosynthetic Reinforced Soil Retaining walls
Seismic wave action in GRSWall
Geosynthetics allow for the movement of theearth to pass through the reinforced soil mass
similar to a wave passing through a body of
water.
Once the wave passes, the water returns to its
original state. As the wave of ground
movement passes through the soil mass, thegeosyntetic reinforcement flexes with the
movement of the earth but returns to its
prequake position
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Performance of GRS walls during earthquakes
The wall was completed in 1992 for a total length was about 300 m. It was deformed and moved only slightly
during the devastating earthquake the occurred in Japan, while more than half of the wooden houses in front
of the wall collapsed totally. This type of geogrid-reinforced soil retaining wall was broadly employed to
reconstruct the damaged conventi onal type retaining walls after the earthquake since it performed so well.
Kobe , Japan - MW6.9
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Mechanically stabilized earth wall within a few meters of the primary fault
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Turkey Earthquake of August 17, 1999
Mechanically stabilized earth wall within a few meters of the primary fault
rupture. Although subjected to differential settlement, it suffered only minor
damage.
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Rehabilitation of Koyna Bridge abutment in
Maharashtra, India, located on SH 78 inseismic Zone-IV was done using geosynthetic
reinforced wall technique encapsulating the
cracked return wall.
The project was completed in the year1996 and
its performance in seismic Zone-IV, which isvulnerable to frequent earthquake, is very
satisfactory in spite of repeated after shocks,
including recent ones
Koyna Bridge Abutment: GRS technique Employed
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Dowrick, D. J. (1987). Earthquake Resistant Design , John Wiley & Sons.
Das, B. M. (1993). Principles of Soil Dynamics , Brooks/Cole
Kramer, S.L. (1996) Geotechnical Earthquake Engineering, Prentice Hall.
Day, R.W. (2001) Geotechnical Earthquake Engineering Handbook, McGraw-Hill
http://www.geoforum.com/knowledge/texts/compaction/index.asp (Accessed
on 29 April 2012)
References