“liquefaction in soils during strong...

UNIVERSITA’ DEGLI STUDI DI FIRENZEDIPARTIMENTO DI INGEGNERIA CIVILESezione geotecnica

“LIQUEFACTION IN SOILSDURING STRONG EARTHQUAKES ”

Dott. Ing. Johann [email protected]

International Doctoral Course on“risk management on the built environment”

Florence, 15 sep 2005


2/2/6464

Introduction

TECHNICAL UNIVERSITY “CAROLOTECHNICAL UNIVERSITY “CAROLO--WILHELMINA” WILHELMINA” -- BRAUNSCHWEIGBRAUNSCHWEIGInternational Doctoral Course on” risk management on the built eInternational Doctoral Course on” risk management on the built environment”nvironment”Liquefaction in soils during strong earthquakesLiquefaction in soils during strong earthquakes

Liquefaction and related phenomena have been responsible for tremendous amounts of damage in historical earthquakes around the world

British scientists think a natural cause, rather than God's anger, lay behind the destruction of Sodom and Gomorrah.A strong ancient earthquake may have liquefied the ground.

They may have been Bronze Age towns, that might have been builton the edge of the Dead Sea, where the ground is very unstable, lying on the joint between two of the Earth's tectonic plates which are moving in opposite directions. The area is vulnerable to earthquakes.

(Cambridge University)


TECHNICAL UNIVERSITY “CAROLOTECHNICAL UNIVERSITY “CAROLO--WILHELMINA” WILHELMINA” -- BRAUNSCHWEIGBRAUNSCHWEIGInternational Doctoral Course on” risk management on the built eInternational Doctoral Course on” risk management on the built environment”nvironment”Liquefaction in soils during strong earthquakesLiquefaction in soils during strong earthquakes 3/3/6464

Introduction

Liquefaction and related phenomena have been responsible for tremendous amounts of damage in historical earthquakes around the world

Liquefaction affected the built environment with spectacular evidence like bridge and buildings collapse, rotation of embedded structures, etc.

Liquefaction risk management is one the main problem in some seismic areas of the world (Japan, California, etc.) and, even if in a lesser way, is present in some part of Mediterranean area (also in Italy)

We can find somewhat liquefaction evidences from historical chronicles in past centuries; only since Anchorage earthquake (M = 9.2) and Niigata earthquake (M = 7.5) liquefaction effects were accurately reviewed and deeply investigated

UNIVERSITA’ DEGLI STUDI DI FIRENZEDIPARTIMENTO DI INGEGNERIA CIVILESezione geotecnica Index


WHAT (is soil liquefaction?)

HOW (does it occur?)

WHY (does it occur?)

WHEN (does it occur?)

WHERE (does it occur?)

HOW (to estimate liquefaction risk?)

HOW (to mitigate liquefaction effects on soils and structures?)

Description of physical phenomenon and observation of effects onbuilt environment

Liquefaction risk estimate : an application, mitigation of effects



What

Static Liquefaction

Dynamic (seismic) Liquefaction

“Liquefaction is a phenomenon (physical state) in which the strength and stiffness of a soil is reduced (to zero) to the point where the soil particles can readily move with respect to each other”

due to upward seepage phenomenon (hydraulic gradient greaterthan critical value)

due to earthquake shakingor other rapid loading which causecause the water pressure to increaseand accumulate inloose granular saturated soils

(σ’ = σ - u 0)

WHAT IS SOIL LIQUEFACTION?

UNIVERSITA’ DEGLI STUDI DI FIRENZEDIPARTIMENTO DI INGEGNERIA CIVILESezione geotecnica What

SEISMIC LIQUEFACTION

6/6/6464

Flow liquefaction (high static shear stresses, steep slopes or near field)


In this case the term “liquefaction” has actually been used to describe a number of related phenomena with similar effects but caused by differentmechanisms. These phenomena can be divided into three main categories:

Cyclic mobility (low static shear stresses, gentle slopes or near field)

Cyclic liquefaction (no static shear stresses, free field, flat ground)

effects: sand volcanoes, sand boils, rising up of soil and water

effects: limited and permanent deformations (lateral spreading),no movements after the earthquake

effects: large and rapid movements even after the earthquake,loss of bearing capacity for buildings, slope slides


7/7/6464

WhatFlow Liquefaction


Turnagain Heights landslide(Alaska, 1964)

Motagua River(Guatemala, 1976)

Cyclic mobility

Hwy 98(El Centro, 1979)

Cyclic liquefaction


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How


Sand boils, volcanoes, vents

Hwy 98(El Centro, 1979)

LIQUEFACTION EFFECTS ON SOILS AND BUILT ENVIRONMENT

sand volcano, Loma Prieta (California, 1989) sand boils , Anchorage (Alaska, 1964)


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How


Loss of bearing capacity

Kocaeli, Turkey (1999) Niigata, Japan (1964)


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How


Ground settlements

Kocaeli, Turkey (1999)


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How


Kobe, Japan (1995) Loma Prieta (California, 1989)

Lateral spreading


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How


Cracks



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How


Slope failure

Anchorage, Alaska (1964)


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How


Subsidence and tilting



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How


Lake forming

Kocaeli, Turchia (1999)



16/16/6464TECHNICAL UNIVERSITY “CAROLOTECHNICAL UNIVERSITY “CAROLO--WILHELMINA” WILHELMINA” -- BRAUNSCHWEIGBRAUNSCHWEIGInternational Doctoral Course on” risk management on the built eInternational Doctoral Course on” risk management on the built environment”nvironment”Liquefaction in soils during strong earthquakesLiquefaction in soils during strong earthquakes

A granular saturated soil consists of an assemblage of individual soil particles. Each particle is in contact with a number of neighboring particles. The weight of the overlying soil particles produce contact forces between the particles - these forces hold individual particles in place and give the soil its strength, which is expressed by theMohr-Coulomb law:τ = σ’·tg ϕ’= (σ -u) ·tg ϕ’ Le

vel o

f po

re w

ater

pre

ssu

re

The contact forces are large when the porewater pressure is low.

WhyWHY DOES LIQUEFACTION OCCUR?

To understand liquefaction, it is important to recognize the conditions that exist in a soil deposit before an earthquake. at a microscopic scale (soil particles)



The water is "trapped" and prevents the soil particles from moving closer together. This is accompanied by an increase in water pressure which reduces the contact forces between the individual soil particles, thereby softening and weakening the soil deposit. The pore water pressure may become so high that many of the soil particles lose contact with each other. The soil will behave more like a liquid than a solid -

Why

Liquefaction occurs when the structure of a loose, saturated sand breaks down due to some rapidly applied loading: the soil particles attempt to move into a denser configuration.

The contact forces are small when the porewater pressure is high



Why

WHY DOES LIQUEFACTION OCCUR?




at a macroscopic scale (soil deposit)

Sand volcanoes and boilsSettlements

Water and sand rises

before during after

Case 1: soil deposit (horizontally layered or homogenous granular soil) free field (no static shear stresses)

CYCLIC LIQUEFACTION

a) Homogeneus soil




Settlements

Water and sand rises

b) not liquefiable covering soilSand cones

before

Not liquefiable soil

during after



Why

WHY DOES LIQUEFACTION OCCUR?

At a near field site (under built environment) or in slope condition, the stress state acting on a volume element is characterised by the effective normal stresses σ'v e σ’0 and the shear stresses τD Granular soil

σ′0

σ′v τDShear strength is due to effective stress σ 'v e σ’0 and partially balances shear stresses τD (static equilibrium)

Stress state before the earthquakeCase 2: soil slopes/ near field



Why

WHY DOES LIQUEFACTION OCCUR?Stress state during the earthquake

τmax

σv

γ τDσ0

τcycσv

τcyc

τD

γ

During the earthquake, time variable cyclic shear stresses τcyc(t), being added to static shear stresses, τd, induce the shear strain, γ, to increase up to a threshold value, called volumetric shear strain, γv.σ0At this point pore water pressure starts to increase and shear strength to reduce.

t

τcyc

t As the shaking goes on, the soil reaches liquefaction condition behaving as a viscous fluid with even more larger strains

∆u

tt



Why

WHY DOES LIQUEFACTION OCCUR?Stress state after the earthquake

t

At the end of the shaking (post-seismic conditions) two different scenarios may happen whether the residual (post seismic) undrainedshear strength of soil, Sr, is greater or lower than the static shear stresses, τd.

τD < Sr

τ

γSr

SpτD >Sr

τD

FLOW LIQUEFACTION Undrained condition persists and static shear stresses are no more balanced (for flat ground: loss of bearing capacity and tilting up of buildings, for slopes: large and rapid movements of wide volume of soil)

t

Sp

Sr

CYCLIC MOBILITY τ

Soil rapidly returns to drained condition with low and permanent deformations

τD (limited displacements for slopes and low settlements under foundations)γ

UNIVERSITA’ DEGLI STUDI DI FIRENZEDIPARTIMENTO DI INGEGNERIA CIVILESezione geotecnica When

WHEN DOES LIQUEFACTION OCCUR?

Liquefaction only occurs in granular saturated soil deposits under the water level and when all the following factors are simultaneously present.

Many of them are related to composition, age, physical and stress state of soil (soil susceptibility factors) .

Some of them depend on the seismic parameters of the triggering event (triggering seismic factors) like duration, magnitude and peak ground acceleration.




When

SOIL SUSCEPTIBILITY FACTORS

Soils composed of particles that are all about the same size are more susceptible to liquefaction than soils with a wide range of particle sizes(where the small particles tend to fill in the voids between the bigger particles thereby reducing the tendency for densification and pore water pressure development when shaken).

Compositional factors

The friction between angular particles is higher than between rounded particles, hence a soil deposit with rounded particles is normally weaker and more susceptible to liquefaction.

Average diameter 0.02 mm < D50< 2 mmFine content (< 74 mm) FC < 15%

Shape factors

ATT. Liquefaction has also been observed in gravel and silt with specific properties. Strain-softening of fine grained soils can produce effects similar to those of liquefaction.



When

Geological factorsSaturated soil deposits that have been created by sedimentation in riversand lakes (fluvial or alluvial deposits), deposition of debris or eroded material (colluvial deposits), or deposits formed by wind action (aeoliandeposits) can be very liquefaction susceptible, especially if recent (Pleistocene and Holocene). Man-made soil deposits, particularly those created by the process of hydraulic filling, may also be susceptible to liquefaction.State factors

The initial "state" of a soil is defined by its density and effective stress at the time it is subjected to rapid loading. At a given effective stress level, looser soils are more susceptible to liquefaction than dense soils.

Relative density Dr < 60 %For a given density, soils at high effective stresses are generally more susceptible to liquefaction than soils at low effective stresses.



When

TRIGGERING SEISMIC FACTORS

The strongest and more recent earthquake where liquefaction effects have been so widespread, are:

Magnitude M > 6Peak Ground Acceleration PGA > 0.10 gDuration D > 15-20 s

Anchorage, Alaska (USA), 1964:M = 9.2 (2nd largest in the world), D = 3 min, tsunamiNiigata, Japan, 1964:M = 7.5, tsunamiLoma Prieta, California ( USA), 1989:M = 7.1 , D = 15-20 secKobe, Japan , 1995:M = 6.9 , D = 20 sec

Liquefaction can only occur in susceptible soils and in consequence of an earthquake strong enough to trigger the phenomenon:



WhereWHERE DOES LIQUEFACTION OCCUR?

Liquefaction effects are most commonly observed in:

low-lying areas near bodies ofwater such as rivers, lakes, bays,sea shores and oceans

port and wharf facilities

http://www.ce.washington.edu/~liquefaction/selectpiclique/kobe95/portgraben.jpg



WhereWHERE DOES LIQUEFACTION OCCUR?

soil embankments (highways, etc.), bridge piles

Holocene and Pleistocene loose sand with shallow water level (< 5 m)

before after



HowHOW TO ESTIMATE LIQUEFACTION RISK

There are many methods to estimate resistance of soil deposits to liquefaction:

EMPIRICAL METHODS

SIMPLIFIED METHODS

estimate of soil susceptibility from common static geotechnical tests, generally not related to triggering seismicity factors and based on qualitative data (historical, geological, state and compositional criteria)

evaluation of soil resistance to liquefaction in terms of safety factor comparing stresses induced by an expected earthquake (triggering factors) to critical stress state of deposit (susceptibility factors) using empirical chart based on in situ and laboratory geotechnical tests (cyclic stress approach)DYNAMIC METHODSevaluation of soil resistance to liquefaction by comparing stress and deformation time history calculated by complex codes that needs deep knowledge of seismicity (reference accelerogram) and dynamic properties of soil: uncoupled methods (total stresses) and coupled methods (effective stresses)



How

CYCLIC STRESS APPROACH

One of the most common approach followed to determine the risk of liquefaction is to compare, in terms of safety factor, FSL, the liquefaction resistance of the soil and the earthquake-induced loading , both expressed in terms of cyclic shear stress ratio, respectively Cyclic Resistance Ratio and Cyclic Shear Ratio:

FSL = Liquefaction resistance of the soil

Earthquake-induced loading

CRRCSR

=



How

CHARACTERISATION OF EARTHQUAKE LOADING1. Seismic loading acting on a soil during an earthquake is expressed

in terms of cyclic shear stresses induced by the earthquake.2. The beginning of liquefaction phenomenon is related to the level of

excess pore water pressure, which depends on the amplitude and the number of cycles of the shear stress time history (so that the magnitude and duration of the event).

3. The shear stress time history induced by an earthquake is generally irregular and transient and is converted to an equivalent series of Nequniform stress cycles of the same amplitude, τcyc, that would produce an increase in pore water pressure equivalent to that of the irregular time history.

O t

+ τ

−τ

Ot

+τ

−τ

τ cycl e q, Ν



How

CHARACTERISATION OF EARTHQUAKE LOADINGThe earthquake loading is characterised by the amplitude, τcyc, and the number of cycles, Neq, of the series of uniform stress cycles equivalent to the actual shear stress time history (that is which produces the same increase in pore water pressure) .

Neq, is related to the duration, that is the magnitude, of the earthquake by means of empirical relationships (Seed et al., 1975).

τcyc is generally assumed equal to 65% of the peak of cyclic shear stress, τmax

τcyc = 0.65 τmax



How

CHARACTERISATION OF EARTHQUAKE LOADING

1. by the time history of shear stresses predicted by a seismic response analysis;

2. by simplified procedures (Seed and Idriss, 1971),from which:

where amax is the maximum expected acceleration at the ground surface and σv0 the vertical total stress at the considered depth

dvomax

max rg

a⋅⋅= στ

The cyclic shear stress, τcyc, normalised to the effective vertical stress,σ’v0 , is called:

equivalent cyclic shear stress ratio (CSR)

The value of τmax, varying with the considered vertical and, for the same vertical, with depth, can be obtained:

dv

v

v

cyc rg

a⋅⋅= '

0

0max'0

65.0σσ

στ

z

am a x

τ τ = γ zam a x

g

am a x

g γ z

z (m

)

rd

0 0.2 0.4 0.6 0.8 1.00

20

40

60

Valori medi

Valori di dispersioneper depositi diversi



How

CHARACTERISATION OF LIQUEFACTION RESISTANCE

1. methods based on the results of laboratory tests (cyclic simple shear tests, cyclic triaxial tests); ;

2. methods based on in situ tests and observations of liquefaction evidences in past earthquakes

“The liquefaction resistance of an element of soil depends on how close the initial state of soil is close to the state of failure due to liquefaction and on the intensity of the loading required to move it from the initial state to the failure state”.In the cyclic stress approach the liquefaction resistance is determined without distinction among the different liquefaction phenomena, and by using different methods:

They use liquefaction case histories to characterise liquefaction resistance of the damaged soils in terms of measured in situ parameters



How

CHARACTERISATION OF LIQUEFACTION RESISTANCEEarthquake loading parameter, L, relative to the past earthquakes of the same magnitude, M, that stroke the observed sites and expressed in terms of equivalent cyclic stress ratio, CSRM, is compared to a liquefaction resistance parameter, R, expressed in terms of a parameter measured in-situ at the observed sites (SPT number from SPT test, tip resistance from CPT test, shear wave velocity from Down-hole test, etc.).

Liquefaction Resistance Curve

L (C

SRM)

R (qc, vS,NSPT, etc.)

Liquefaction observedNo liquefaction observed

CRRM

Cyclic Resistance RatioGenerally these plots are made with reference:

1. to earthquakes of 7.5 magnitude (M = 7.5)2. to soils with similar properties (grain-size distribution,fine content, etc.)3. to in situ measured parameters normalised to effective vertical stress

(to be compared with CSR)



How

ROBERTSON AND WRIDE METHOD (1997)This a CPT-based method, where the tip resistance, qc, normalised to the effective vertical stress, qc1n , and measured with CPT tests at sites stroken by earthquakes magnitude 7.5 and characterised by the same kind of soils (clean sands), (qc1n)cs, is compared to the equivalent cyclic stress ratio, CSR7.5, corresponding to considered earthquakes.

The boundary line between cases of liquefaction and not liquefaction, is given by:

with (qc1n)cs < 50( )0.05

1000q

833.0CRR csc1n7.5 +⎟⎟

⎠

⎞⎜⎜⎝

⎛⋅=

( )0.08

1000q

93CRR3

csc1n7.5 +⎟⎟

⎠

⎞⎜⎜⎝

⎛⋅= with 50 < (qc1n)cs < 160 if (qc1n)cs >160

soil is not liquefiable



How

Robertson and Wride method also allow to classify soils by means of two adimensional factors F and Q , and a classification index, Ic:

1000

×−

=vc

s

qfFσ 0

0

'vvcqQ

σσ−

=

22 )47.3(log)22.1(log1

−++=nc

qFIc

Classification index is used:

1. to determine normalised tip resistance:n

v

a

a

cNc

ppqq ⎟⎟

⎠

⎞⎜⎜⎝

⎛⋅⎟⎟

⎠

⎞⎜⎜⎝

⎛= '

01 σ

where pa = 1 atm and n = f(Ic)by iterative procedure

2. to determine equivalent normalised tip resistance for clean sands:(qc1n)cs = Kc qc1n where Kc = f(Ic)

3. to classify the soils and exclude from the liquefaction risk analysis all the layers not susceptible to liquefaction from a lithological point of view (Ic > 2.6)

Ic < 1.31

1.31 < Ic < 2.05

2.05 < Ic < 2.60

2.60 < Ic < 2.95

2.95 < Ic < 3.60

Ic > 3.60



HowCALCULATION OF SAFETY FACTOR (FSL)

1. CRR7.5 value is so calculated for every vertical investigated and at each depth where the in situ parameter is measured and attributed to the corresponding layer. Some of these layers can be at priori excluded in the following cases:

layers below the water table;layers at depth greater than 20 m

(where the liquefaction phenomena can be considered negligible);layers not susceptible to liquefaction from a mechanical point of view: (qc1n)cs>160;layers not susceptible to liquefaction from a lithological point of view (Ic > 2.6).

2. For the same layers the CSR value corresponding to the maximum expected event of magnitude M (with a certain return period) is calculated.

3. The resistance of soil corresponding to magnitude M, CRRM, is obtained by multiplying CRR7.5 to a magnitude scaling factor, MSF, determined by empirical relationships.

4. The safety factor against liquefaction is so determined:

MSFCSR

CRRCSRCRRFSL 5.7 ⋅==



How

CALCULATION OF LIQUEFACTION POTENTIAL INDEXFSL indicates whether or not liquefaction is expected to occur (greater or less than one) and the “intensity” of the phenomenon (much lesser or close to one) at each layer investigated, but since the effects of liquefaction phenomena at a certain site is the resultant of thecontributes of all the underlying layers, it is necessary to define for each explored profile, a final cumulative parameter of liquefaction, liquefaction potential index (Iwasaki et al., 1978):

∫ ⋅⋅=20

0L dz)z(w)z(FP with F(z) = 0 per FSL>1

F(z) = 1 – FSL per FSL≤1⎟⎠⎞

⎜⎝⎛⋅−=

20z1010)z(w

Zcrit = 20

PL Liquefaction risk PL < 1 Very low

1 < PL ≤ 5 low 5 < PL ≤ 15 High

PL > 15 Very high

5 classes of liquefaction risk are defined on the basis of the liquefaction potential index values:

0-



Example

APPLICATION

“ASSESSMENT OF LIQUEFACTION RISK IN THE HARBOUR AREA OF GIOIA TAURO ”



Example

GEOGRAPHICAL SETTING

“GIOIA TAURO, of very ancient origin, has always been a very little town given to fisher activities with the maximum growth and trade expansion in “Magna Grecia”times and during the Spanish rule of the XVI century.Nowadays Gioia Tauro has become a centre of special interest from an economic point of view because it hosts one of the most important trade port junction of Southern Europe”.



Example

SEISMOLOGICAL SETTINGGioia Tauro and the surrounding area were struck in the past by several seismic events with intensity superior to VIII MCS, and several phenomena attributable to liquefaction occurrences were observed, especially in the event of 1783 that completely destroyed the town.

Evidences of liquefaction during the 1783 earthquake

Oppido

Rosarno



Example

SEISMOLOGICAL SETTING

Earthquake recurrenceat Gioia tauro

Is = macroseismic intensity x 10

Instrumental data(1964-1992)

Macroseismic data(from 1900)

Map of observed epicentres(M > 4)

Map of maximum observedmacroseismic intensity (Imax)



Example

SEISMOLOGICAL SETTINGMap of predicted PGA (g) values

(475-year return period)

Map of predicted MCS intensity

(475-year return period)

UNIVERSITA’ DEGLI STUDI DI FIRENZEDIPARTIMENTO DI INGEGNERIA CIVILESezione geotecnica Example

SEISMOLOGICAL SETTING

Seismic zones

Gioia Tauro falls within 1st

seismic zone (highest).It was estimated a maximum expected event (with a return period of 475 years) of magnitude 7.3 and peak ground acceleration (at the ground surface), PGA, of 0.45 g

(1984)

Zone IZone IIZone IIINot class.

(2005)

Zone IZone IIZone IIINot class.Zone IV


UNIVERSITA’ DEGLI STUDI DI FIRENZEDIPARTIMENTO DI INGEGNERIA CIVILESezione geotecnica Example


(proposed, 1997)

FOCUSING ON THE HARBOUR AREA OF GIOIA TAURO



Example

TYRRHENIAN SEACANAL PORT

500 m0 m NS

W

E

CPT Tests

SPT Test

SPT TestsBoreholes

Section n.3

Area: 5.7 km2

Debris soil

LEGEND

GEOLOGICAL SETTING



Example

GEOLOGICAL SETTING

GEOLOGICAL SETTINGS 218 S 119 S 203 S 209 S 213 S 212 S 244

LEGENDA

Riporto

Formazione A

Formazione B

Formazione C

Formazione D

Formazione E

6 m

0 m s.l.m.

10 m

20 m

30 m

40 m

50 m

60 m

70 m

80 m

90 m

200 m0 m

*

Stratigraphical section (n.3)

Soil B

Soil C

Soil D

Soil E

LEGEND

*

*

Soil A

Debris soil



Example

GEOLOGICAL SETTING

S 203 S 209 S 213 S 212 S 244 LEGENDA

Riporto

Formazione A

Formazione B

Formazione C

6 m

0 m s.l.m.

5 m

10 m

15 m

200 m0 m

7

6

6 6

67

7

7

7

NC NC NC

NC

6

6666

6

55

5

6

7

NC

Miscele di sabbie:da sabbia limosa a limo sabbioso

Sabbie:da pulite a limose

Sabbia ghiaiosa

Non penetrabile

6 6

Stratigraphical section (n.3)

Debris soil

Soil B

Soil C

Sand mixtures – silty sand to sandy silty

LEGEND

Soil A

CPT

-bas

ed R

ober

tson

’s so

il cl

assi

ficat

ion

Sands – clean sand to silty sand

Gravelly sand to sand

Not classified



Example

1. High seismicity of this area makes the seismic hazard very high.2. The presence of one of the most important trade port junction of

Southern Europe, from a structural point of view, geological andphysical properties of soil, from a geotechnical point of view, yields high vulnerability values.

3. Then the liquefaction risk is considered very high and a liquefaction hazard zonation is opportune.

This is the reason why in the past years this area was the object of extensive geological, geotechnical and seismological surveys. To carry out this research, were utilized :

54 soundings (with maximum investigated depth between 16.95 m and91.30 m)

laboratory tests 115 mechanical CPT tests121 SPT tests

EVALUATION OF LIQUEFACTION POTENTIAL INDEX



Example

EVALUATION OF LIQUEFACTION POTENTIAL INDEX

0

5

10

15

20

0 10 20 30 40 50

qc (MPa)

z (m

)

0

5

10

15

20

0 0,25 0,5 0,75 1

fs (MPa)

0

5

10

15

20

0 0,5 1 1,5 2FSL

0

5

10

15

20

0 0,1 0,2 0,3 0,4 0,5

CRR

CSR

0

5

10

15

20

0 5 10 15 20∫ ⋅⋅−=z

L dzzwzFP20

)()(

PL

Low

liqu

efac

tion

risk

Hig

h liq

uefa

ctio

n ri

sk

Ver

y hi

gh li

quef

actio

n ri

sk



Example

SPATIAL INTERPOLATION OF CALCULATED PL VALUES

PL values so calculated from CPT tests at each explored profile were interpolated by using:

• DETERMINISTIC METHODS (inverse distance weighted)• GEOSTATISTICAL METHODS (kriging)

The latter are generally preferred because:1. the estimated parameter is calculated considering the spatial

variability of the whole data-set;2. they provide better estimate especially when locations of measure

points are not equispatially distributed in the area under examination3. they allow to calculate the most expected value and the

corresponding errors in order to test method reliability



Example

LIQUEFACTION RISK MAP (predicted values)TY

RR

HEN

IAN

SEA

PORT

CA

NA

L

0 500



Example

LIQUEFACTION RISK MAP (errors)TY

RR

HEN

IAN

SEA

PORT

CA

NA

L

0 500



How

HOW TO REDUCE LIQUEFACTION RISKThere are basically three ways to reduce liquefaction risk when designing and constructing new buildings or other structures as bridges, tunnels, and roads. :

AVOID LIQUEFACTION SUSCEPTIBLE SITES

BUILDING LIQUEFACTION RESISTANT STRUCTURES

by considering both seismicity of the area (hazard) and susceptibility of soils (vulnerability)

by designing the foundation elements to resist the effects of liquefaction (pile foundations driven to the shallowest layer of non liquefiable soil, shallow rigid foundations which can tolerate high settlements, etc.)

IMPROVE THE SOIL

by improving the strength, density, and/or drainage characteristics of the soil



How

LIQUEFACTION RESISTANT STRUCTURESA structure that possesses ductility, has the ability to accommodate large deformations, adjustable supports for correction of differentialsettlements, and can decrease the amount of damage a structure may suffer in case of liquefaction.Another important aspect to consider is the foundation:SHALLOW FOUNDATIONS

It is important that all foundation elements in a shallow foundation are tied together to make the foundation move or settle uniformly, thus decreasing the amount of shear forces induced in the structural elements resting upon the foundation. A stiff foundation mat is a good type of shallow foundation, which can transfer loads from locally liquefied zones to adjacent stronger ground.



How

LIQUEFACTION RESISTANT STRUCTURESDEEP FOUNDATIONSLiquefaction can cause large lateral loads on pile foundations.

Piles driven through a potentially liquefiable, soil layer to a stronger layer not only have to carry vertical loads from the superstructure, but must also be able to resist horizontal loads and bending moments induced by lateral movements if the weak layer liquefies.



How

LIQUEFACTION RESISTANT STRUCTURESDEEP FOUNDATIONS

Sufficient resistance can be achieved by piles of larger dimensions and/or more reinforcementIt is important that the piles are connected to the cap in a ductile manner that allows some rotation to occur without a failure of the connection.If the pile connections fail, the cap

cannot resist overturning moments from the superstructure by developing vertical loads in the piles.



How

LIQUEFACTION RESISTANT STRUCTURES

PIPELINES

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 due to liquefaction.The pipes in the photo connected the two buildings in a straight line before the earthquake



HowSOIL IMPROVEMENT

The main goal of most soil improvement techniques used for reducing liquefaction hazards is to avoid large increases in pore water pressure during earthquake shaking and to increase soil strength. This can be achieved by densification of the soil and/or improvement of its drainage capacity.

Mechanical energy is transferred to soil in different ways to densifyand reinforce its structure (vibroflotation, dynamic compaction, compaction piles, etc.)

Energy contribution



HowSOIL IMPROVEMENT

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 (good for existing buildings)

Chemical contribution

Drainage techniques

Drainage techniques include installation of drains of gravel, sand or synthetic materials. Synthetic wick drains can be installed at various angles, in contrast to gravel or sand drains that are usually installed vertically.

Soil replacementThe potentially liquefiable soil is partially or totally replaced



References

REFERENCES

Committee on Earthquake Engineering, Commission on Engineering andTechnical Systems, National Research Council 1985. ”Liquefaction of Soils DuringEarthquakes”.Committee on the Alaska Earthquake of the Div. of Earth Sciences, NationalResearch Council 1973. “The Great Alaska earthquake 1964”. Engineering,Geology, and Summary Volumes, National Academy of Sciences, Holtz, Robert D.; William, Kovacs D. 1981 . “An Introduction to GeotechnicalEngineering”. New Jersey, Prentice Hall.Kawasumi-Hirosi (editor) 1968. “General report on the Niigata earthquake of1964”.

. Kramer, S.L. 1996. “Geotechnical Earthquake engineering”. New Jersey, PrenticeHall, 654 p.Richart, F.E., Jr., Hall, J.R., Woods, R.D. 1970. Vibration of soils and foundation.Englewood Cliffs, New Jersey, Prentice Hall, 414 p.

On books:



References

On line:@ http://ceor.princeton.edu/~radu, Soil Dynamics and Geotechnical Earthquake Engineering at Princeton University @ http://geosystems.gatech.edu/Research/gpr.html, Georgia Institute of Technology, Liquefaction research. @ http://nisee.ce.berkeley.edu/, National Information Service for Earthquake Engineering @ http://peer.berkeley.edu/, Pacific Earthquake Engineering Research Center. @ http://quake.wr.usgs.gov/, United States Geological Survey Earthquake Information @ http://wrgis.wr.usgs.gov/, USGS, Western Region Geologic Information Server. @ http://www.abag.ca.gov/bayarea/eqmaps/liquefac/bayaliqs.gif, Hazard map for the bay area in San Francisco, ABAG 1983 @ http://www.ce.berkeley.edu/Programs/Geotech/

http://www.eerc.berkeley.edu/, Earthquake Engineering Research Center. @ http://www.geotechnics.com/, Geotechnics America, soil improvement contractor @ http://www.haywardbaker.com/, Hayward Baker, soil improvement contractor. @ http://www.liquefaction.com/, Web site with general liquefaction information and more specialized research information.

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