lecture40-mitigation and ground improvement

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Mitigation of Earthquake Hazards &

Ground Improvement

Lecture-40

1



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



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

3



Mitigation Options

•Avoiding the hazard

•Building Earthquake resistant

structures

•Ground Improvement

4



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.

5



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.

6



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.

7



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 .

9



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.

10



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

11



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.

12



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

13



Ground Improvement: Soil Densification

Soil densification techniques:

•Compaction

•Vibro-replacement (Vibroflotation & Stone Columns)

•Blasting

•Grouting

•Compaction Piles

14



Filed Compaction

• Pneumatic rubber tired roller

Different types of rollers (clockwise

from right):

Vibratory roller

Smooth-wheel roller

Sheepsfoot roller

15



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)

16

Source: http://www.geoforum.com



Dynamic Compaction

Pounder (Tamper)

Mass = 5-30 tonne

Drop = 10-30 m

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Dynamic Compaction

18




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.

19




vibrator makes a hole

in the weak groundhole backfilled ..and compacted Densely compacted stone

column

Vibroflotation

20




Vibroflotation

21




Ground Densification: Vibro-Compaction

22



• 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

23



Aftermath of blasting

For densifying granular soils

Ground Densification: Blasting

24



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

25



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

26



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

27



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.

28



Jet Grouting

29



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.

30




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.

31




Ground Improvement: Vertical Drains

Primary consolidationsettlement will already beachieved during theconstruction period by usingvertical drains

32



EARTHQUAKE DRAINS

33



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



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.

35



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

36



EARTHQUAKE DRAINS

37



EARTHQUAKE DRAINS

38Source: google images



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

39



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

40

Mechanically stabilized earth wall within a few meters of the primary fault



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.

41



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

42



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

http://www.geoforum.com/knowledge/texts/compaction/index.asp

http://www.geoforum.com/knowledge/texts/compaction/index.asp

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