Loss Of Soil Resistance Or Soil Rigidity During Or After

1

Loss of soil resistance or soil rigidity during or after earthquakes due to an excessive pore pressure generation

2

•

Overturned buildings with no structural damages

Niigata, Japan (1964)

3

•

Sand boils or sandy fine materials at the level ground

Kobe, Japan (1995)

4

•

Generalised subsidences

Kobe, Japan (1995)

Anchorage, Alaska (1964) 5

•

Generalised subsidences

Izmit, Turkey (1999) 6

•

Large lateral spreadings

Kobe, Japan (1995)

7

•

Dry soils : movements with shearing
densification

• Saturated soils :

∆V = 0

∆V ⇓

u⇑

« Floating » grains in the water

Reduction of contact forces between grains

8

′

σv = σv + u

with

u = u static + ∆u u

′

σv ⇓

while

u⇑

depth

u profile at the instant t1

Zone where liquefaction can starts

σv

u profile at the instant t1

9

ru = u σ v

ru = 100%


Liquefaction for

′ σv = 0

Superstition Hills, USA (1987)

10

Sand boils

u Liquefied zone reconsolidation Dissipation of ∆u

settlement

σv depth Profil de u à un instant t1

11

• 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)

12

Behaviour of sands Drained triaxial tests (1/3) Stage 1 : Isotropic Consolidation σ1 = σ0

σ3 = σ0

Critical State

σ3 = σ0

Stage 2 : Deviatoric test σ1 dεv/d ε1 = 0 σ3 = σ0

σ3 = σ0

Transformation State 13

Behaviour of sands Drained triaxial tests (2/3) •

Loose sand (contractive)

•

Dense sand (dilative)

(q-ε1) - Rise of resistance - A steady state is reached

(q-ε1) - Rise of resistance - Presence of a peak of resistance - Softening - A steady state is reached

(q-εv) - Contrative phasis - A steady state is reached

(q-εv) - Contratant phasis - State of maximum contractancy - Dilative phasis - A steady state is reached 14

Behaviour of sands Drained triaxial tests (3/3) Loose sand

Dense sand Critical state

q

q

Failure : Mohr-Coulomb

Failure : Mohr-Coulomb

dilatancy

3

σσ00

3 contractancy

1

1

p

σ0

contractancy

p

Isotropic consolidation 15

Behaviour of sands Undrained triaxial tests Transient loss of resistance Material behaving like a fluid

Total stress path = LC ∆u ε1 %

σ0

p’

σ0 ε1 %

a : essentially contractive sand b : poorly dilative sand c : dilative sand

16

• Contractive sands :

ru=100% is obtained and large induced deformations

17

• Poorly dilative sand (cyclic mobility) :

almost ru=100% while large transient large induced deformations q

18

• Dilative sand (cyclic mobility) :

ru=100% never obtained and limited deformations extent of the dilative phasis similar to the extent of the dilative phasis

19

•

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)

20

•

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 21

•

Vibroflottation (or vibrocompaction) – Addition of densified sand/gravel with the vibration – Addition of large gravel (gravel column) – Column diameter : < 4m

22

•

Injection (grouting) Product : water+sand+cement Soil rearrangement around the bulb
densification of the surrounding soil


zones with good mechanical properties

23

Kobe harbour

Port Island

Rokko Island

24

Kobe harbour •

Subsidence during Kobe earthquake (1995)

25

Simplified method (so-called Seed’s 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 <1
liquefaction FS =CRR/CSR

Liquefaction safety factor 26

• CSR a CSR = = 0.65 max ′  g σv0 τ av

  σ v 0  . .rd ′     σv0 

amax

: accélération maximale en surface (%g)

rd

: coefficient de réduction de contraintes

rd = 1 − 0.00765z

avec

z ≤ 9.15 m

rd = 1.174 − 0.0267z

avec

9.15 m ≤ z ≤ 23 m

High depths -> high scatter levels

27

•

CRR obtained after correlations with in situ tests (SPT) = CSR value leading to liquéfaction

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)

28

•

Correction (MSF) for earthquakes which magnitudes differents than 7.5

10 2.24 MSF = 2.56 Mw

29

•

Correction (Kσ) for high overburden pressures

30

•

Correction (Kα) to take into account initial shearing stress field (near slopes)
Poorly reliable so far (
take Kα=1)

• Liquefaction safety factor

 CRR7.5  FS =  .MSF .Kσ .Kα  CRS 

31