Disaster Mitigation Geotechnology

10

Laboratory soil characterisation

What is laboratory soil characterisation?

Determining soil properties / soil characteristics by means of

laboratory tests

Field investigation

Soil sampling

Laboratory

characterisation

Modelling

Analysis

Design

Construction

Feedback

Soil characterisation Soil properties / characteristics

expressed as ‘soil parameters’

Soil Properties and Soil Parameters

e.g.

Soil parameter cu

(undrained shear

strength)

distribution

Watebe et al. (2009)

Peak strength

Critical State?

Residual strength

Strain softening

Strain

hardening

pln

e Yield stress

Compression

index

Swelling

index

Laboratory tests for soil characterisation

- Physical tests

- Index tests (PL, LL, PI)

- Physical properties (density, unit weight, etc.)

- XRD, SEM, MIP, etc.

- Mechanical tests

Today’s topic

- Other

- Hydraulic tests (permeability, water retention, etc.)

- Thermal tests (thermal conductivity, heat capacity, etc.)

- Chemical tests (pH, leaching, etc)

X-Ray Diffraction

Scanning Electron Microscope

Mercury Intrusion Porosimetry

Mechanical tests

Static / pseudo-dynamic tests

- Direct shear, simple shear, ring shear (直接せん断＝一面せん断、単純せん断、リングせん断試験)

- Unconfined compression (一軸圧縮試験)

- Triaxial compression/extension (三軸圧縮／伸張試験)

- Hollow cylinder shear (中空ねじり試験)

- Oedometer (1-D compression) (圧密試験)

- Plane strain shear (平面ひずみ試験) (for research)

- True triaxial (真の三軸) (for research)

Dynamic tests

- Resonant column (共振試験)

- Bender element (ベンダーエレメント)

Direct shear (box shear)

Sand (Ohshima et al., 1999)

vhv

?:h

(Potts & Zdravkovic, 2001)

Clay (Takada, 1993)

- Only controls vh and v

Constant pressure (for sand)

vs Constant volume (for clay)

- c, f determined.

- Strains cannot be measured.

Only displacements.

- Significant non-uniformity

within a specimen.

Incidentally…

vhv

?:h

Not knowing h, Mohr’s stress

circle cannot be drawn. The c, f

values cannot be rigorously

determined, unless the vh-plane

coincides with the plane of

maximum /.

13

),( vhv

),( vhh

f’ from direct shear test

f’ from triaxial test

=

?

=

?

=

?

cu from

triaxial test

cu from

direct sheartest

Ring shear

- Only controls vh and v

- c, f determined.

- Strains cannot be measured.

Only displacements.

- Significant non-uniformity

within a specimen.

(Bishop et al., 1971)

Similar to direct shear:

But,

- No displacement limit

Residual strength

- hv automatically

generated.

Unconfined compression

- Undrained shear strength, Su

- Young’s modulus, E (often E50)

Applicable only to clay or cemented soils

1

032 032 (Constant) (Constant)

z

yx

50E

f

uq5.0

uu Sq 2

0E

Triaxial compression / extension

- 2-DOF(degree of freedom)s each for

stress and strain

(v, h, v, h) all controlled & measured.

- Can change confining stress

- Requires more skill and time than

unconfined compression

Applicable to any soils

LVDTs

Suction capLoad cell

Bender elementsystem(also in other side of soil specimen)

Mid-height PWPtransducer

Soil specimen

Tie rod

Perspex wall Drainage

Ram

Porous stone

(Global)displacementtransducer

Radial belt

Ram pressure chamberfilled with oil

Bearing

To oil/air interfaceor CRS-pump

1

32 32

3

21 21 (Constant) (Constant)

(Increase) (Constant)

(Increase) (Increase)

Triaxial compression Triaxial extension

z

yx

z

yx

Anisotropy on shearing direction

su＝(suc+2sus+sue)／4

su=(suc+sue)／2 or su=sus

Strength anisotropy sue/suc≒0.7

Triaxial

Extension

sue

Triaxial

Compression

suc

Direct

Shear

sus

56m

56m

83m

83m

193m

193m

142m

142m

-1400

-700

0

700

1400

0 700

Effective mean stress

p' (kPa)

De

via

tor

str

ess

q (

kP

a)

(b)

56m

56m

83m

83m

193m

193m

142m

142m

-1400

-700

0

700

1400

0 4 8 12

Axial stress (%)

De

via

tor

str

ess

q (

kP

a)

56m: 'v0=344kPa

83m: 'v0=521kPa

142m: 'v0=935kPa

193m: 'v0=1302kPa

(a)

Osaka Bay clay

Hollow cylinder (HC) apparatus

- 4-DOFs each for stress and strain

(z, r, q, zq, z, r, q, zq) all controlled & measured.

- Can rotate principal stress axes Anisotropy

- Cyclic zq loading relevant to earthquake motions

- Very difficult to perform!

Applicable to most soils

z

q

r

z

q

r

Innercellpres-sure

Outer cellpressure

Axial force

Torque zq

1

2 3 1 3= +b( - )

3

(a)

(b)

(c)

qz

zq

z

(d)

Oedometer (1-D compression test)

- Confine the specimen in a rigid ring no horizontal strain

- 1-D compressibility (consolidation characteristics) is investigated.

Applicable to most soils

(Potts and Zdravkovic, 2001)

v

vh K 0

(Increase) (Increase)

(Increase)

vh K 0

Plane strain shear

- No normal strain allowed at least in one direction

(direct shear is thus a particular type of plane strain shear)

- More relevant to 2-D problems (such as in linear structures than

axi-symmetric triaxial tests

- A plane strain condition tends to give larger strength parameter

values than in triaxial tests.

Only direct shear is commonly adopted in practice.

1

2

3

02

1

2

3

02

1

23 02

Plane-strain extension

(PSE)

Direct simple shear

(DSS)

Plane-strain compression

(PSC)

True triaxial test Limited to research purposes only

- 3-DOFs each for stress and strain

(x, y, z, x, y, z) all controlled & measured.

- Influence of intermediate principal stress can be investigated.

- Requires very complex design to prevent platen interferences.

- Famous study by Lade (1975)

x

y

z“No one managed to make true triaxial

apparatus that really works – Lade

came closest to it, but not quite.

Anyway, how ingenious he was… to

use balsa!” (by Bruce Menzies)

Comparison of stresses & strains

There is no all-mighty apparatus – combine their use, at most.

Stresses Strains

Direct shear × × ◎ 0 0 ◎ 0 0 × 0 0 ×

Ring shear × × ◎ 0 0 ◎ 0 0 × 0 0 ×

Unconfined C. 0 0 ◎ 0 0 0 △ △ ◎ 0 0 0

Triaxial ○ ○ ◎ 0 0 0 ○ ○ ◎ 0 0 0

Hollow C. ◎ ◎ ◎ 0 0 ◎ ◎ ◎ ◎ 0 0 ◎

Oedometer × × ◎ 0 0 0 0 0 ◎ 0 0 0

Plane strain ◎ 0 ◎ 0 0 0 ◎ 0 ◎ 0 0 0

True triaxial ◎ ◎ ◎ 0 0 0 ◎ ◎ ◎ 0 0 0

zxyzxy

zyxzx

yzxyzyx

◎: Controlled or measured ○: Controlled or measured but not independently

△: Not controlled but measured ×: Neither controlled or measured

0: Meant to be zero (in theory)

Elastic wave velocity measurement

tanG

secG0G

Unloading

&reloading

x

)(xu

tx

u

x

uG

t

u

2

3

2

2

2

2

2

22

2

2

x

uV

t

us

sVG 2

2

2

2

x

uG

t

u

(Visco-plastic elasticity)

(Elasticity)

Wave propagation in

an elastic medium

Wave equation

Very small strain

amplitude

Bender elements

‘Transmitter’ & ‘Receiver’ BE

v (or z)

h (or r)

hh

hv

Bender elements

SoilSpecimen

-0.5 0 0.5 1 1.5 2

Time [mSec]

-100

-50

0

50

100

Am

plit

ude o

f sig

nals

in a

rbitra

ry u

nits

First arrivalt = 0.514 mSec

InputOutput

TE4: After consolidationf = 9 kHz, vh-direction

Beginningof signal

Function

Generator

Oscilloscope

Piezo-ceramic elements to

measure shear wave velocity

Resonant column

Apply excitation with

varying frequency, and

search for resonance

frequency

Shear modulus, G

or

Young’s modulus, E

- Amplitude (strain level)

can be changed

- Not as portable and

versatile as BE.

Various types of resonant column

Active Active

Active

Passive

F F

F

Ka

Ca

Ca

(a) Fixed-free (b) Fixed-base-spring –top (c) Free-free

Ka

(Hight et al., 1997)

Example of resonant column device incorporated in

hollow cylinder apparatus

Bellofram cylinder

Rotary table

Oscillator

Soil specimen

Load cell

Foundation

Oscilloscope

& function generator

for RC

Bellofram cylinder

Hardin oscillator

Specimen

Load cell

Proximity transducers

Tie rod

Acrylic chamber wall

Stepper motor for

torsion

Cam

Outer cell and pore water

pressure transducers

Sprocket and torque

transmission chain

Displacement

Transducer

Clamp

To foundation

Rotary tension cylinder

Ram

Examples of laboratory characterisation scheme

Example 1: Ground vibration analysis

(Iwasaki et al., 1978)

Soil

Base rock

Either of cyclic triaxial

cyclic HC

resonant column From cyclic HC

G & D (damping ratio) at different strains

Example 2: Long-term (consolidation) settlement in soft clay

K0-compression

(Ohta, 2009)

Compression with

small lateral strain

allowed

Compression with

large lateral strain

allowed

Along the centre line:

Case 1

Case 2

Case 3

Case 1: May involve significant shear deformation and shear failure

Long term: Oedometer (k, pc’, Cc, Cs) + Triaxial tests (c, f)

Short term: Unconfined compression or direct shear (Su)

Case 3: No worry for shear failure: Long term settlement only

Oedometer (k, pc’, Cc, Cs)

Example 3: Liquefaction resistance

0.0

0.1

0.2

0.3

0.4

1 10 100

Dr=50% Dr=70% FLIP K = 0.5 K = 1.0 K = 2.0

繰り返し回数 N

応力

比

/v0'

Number of cycles

Str

ess r

atio,

vh/

v’

20

Liquefaction resistance, R20

R20

Undrained cyclic triaxial

or

Undrained cyclic HC

Liquefaction of sand

(Ishihara, 1985; reproduced

after Iai et al., 1991)

Laboratory characterisation – Summary

- No single testing apparatus is all-mighty, unfortunately.

- Need to choose and combine an appropriate testing methods

according to a problem in interest

- Even if a single parameter (such as G and f) is determined from

different types of tests, its value may differ, due to anisotropy and

intermediate principal stress effects and non-uniformity within a

specimen.