loss of soil resistance or soil rigidity during or after

8/14/2019 Loss of Soil Resistance or Soil Rigidity During or After

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Overturned buildings with no structural damages

Niigata, Japan (1964)


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Sand boils or sandy fine materials at the level ground

Kobe, Japan (1995)


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Generalised subsidences

Kobe, Japan (1995) Anchorage, Alaska (1964)


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Large lateral spreadings

Kobe, Japan (1995)


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Dry soils : movements with shearing densification

0=V Saturated soils : u

Reduction of contactforces between grains

Floating

grains in the

water

V


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uvv+

=

uv while

u profile at the instant t1

u

vdepth

Zone whereliquefaction can starts

uuustatic

+=with

u profile at the instant t1


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u

v

depth

Profil de u un instant t1

Liquefied zone

Sand boils

Dissipation of u

reconsolidation

settlement


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Sandy soils in a loose or medium dense state

alluvional or wind borne deposits

if FC >50 %, non plastic fine materials

Low permeabiliy

Recently deposited materials (no cimentation)


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Behaviour of sands

Drained triaxial tests (1/3)Stage 1 : Isotropic Consolidation

3 = 0 3 = 0

1=

0

Stage 2 : Deviatoric test

3 = 0 3 = 0

1

dv/d 1 = 0

Transformation

State

Critical State


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Dense sand (dilative)

(q-1)

- Rise of resistance

- Presence of a peak of resistance

- Softening

- A steady state is reached

(q-v

)

- Contratant phasis

- State of maximum contractancy

- Dilative phasis


Behaviour of sands

Drained triaxial tests (2/3)

Loose sand (contractive)

(q-1)

- Rise of resistance


(q-v

)

- Contrative phasis



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Behaviour of sands

Drained triaxial tests (3/3)

Loose sand

q

p

Failure : Mohr-Coulomb

0

3

1contractancy

0

Isotropic

consolidation

q

p

Failure : Mohr-Coulomb

0

3

1 contractancy

dilatancy

Dense sand

0

Critical state


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Behaviour of sands

Undrained triaxial tests

p

1%

1% a : essentially contractive sand

b : poorly dilative sand

c : dilative sand

0

0

Total stress path = LC

u

Material behaving like a fluid

Transient loss of resistance


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Contractive sands :

ru=100% is obtained and large induced deformations


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Poorly dilative sand (cyclic mobility) :

almost ru=100% while large transient large induced deformations

q


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In situ geotechnical tests (SPT, CPT) to forbid any building construction

Drainage devices (drains, drain wells, gravel columns)

Soil densification (dynamic compaction)

Devices with drainage and densification (vibroflottation, gravel columns)

Soil improvement (injections)


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Drain wells / gravel columns (digging and drain installation (PVC) or filling

with gravels)

Dynamic compaction (superficial soil improvement)

Mass of 8 - 50t

Falling mass heigth : < 40m

Several mass falling

Depth action < 10m


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Vibroflottation (or vibrocompaction)

Addition of densified sand/gravel with the vibration Addition of large gravel (gravel column)

Column diameter : < 4m


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Kobe harbour

PortIsland

RokkoIsland


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Subsidence during Kobe earthquake (1995)

Kobe harbour


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Simplified method (so-called Seeds method, (1971)) (2001)

Method valid for soil layers ubicated at a depth z < 20m

Answer to the seismic shaking (at a given depth z)

CSR : cyclic stress ratio

Resistance of the investigated layer (at a given depth z)

CRR : cyclic resistance ratio

If CRR/CSR


27/31

27High depths -> high scatter levels

: acclration maximale en surface (%g)maxa

dr : coefficient de rduction de contraintes

zrd

00765.01 = 15.9zavec m

zrd

0267.0174.1 = avec mzm 2315.9

d

v

v

v

av rgaCSR ..65.0

0

0max

0

==

CSR


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CRR

obtained after correlations with in situ tests (SPT)

= CSR value leading to liqufaction

Back analysis of sites where liquefaction

took place

Validitity : M=7.5 (magnitude)

clean sand

Computation of the seismic answer with

real seismic recordings CSR

In situ test to assess (N1)60

Identification of FC (Fine Content)


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Correction (MSF) for earthquakes which magnitudes differents than 7.5

56.2

24.210

wMMSF=


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Correction (K) for high overburden pressures


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KKMSF

CRS

CRRFS ...5.7

=

Correction (K) to take into account initial shearing stress field (near

slopes) Poorly reliable so far ( take K=1)

Liquefaction safety factor

loss of soil resistance or soil rigidity during or after

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