Binh NGUYENDepartment of Civil EngineeringTokyo Institute of Technology

Numerical analyses on the failure of deep mixing columns reinforced by a shallow mixing layer

12th Nov., 2015

1. Introduction 1

Deep mixing method

Applications of DMM Deep mixing columns Mass mixing method

A soft ground improvement method, by mixing soil and additives to increase the strength and stiffness of soft ground.

High strength materialEfficient and economical methodApplicable for many soil types

1. Introduction 2

Failure of DM columns under embankment:

DM columns with shallow mixing:- Ishikura (2009) to estimate embankment settlement- Kitazume (2011) to observe the effect of shallow layer on

the failure of columns as well as embankment displacement

(Kitazume 2011)

Embankment supported by DM columns:- Prevent sliding- Increase stability- Reduce settlement

(Kitazume 2013)

(Kivelo & Broms 1999)

Internal failure patterns

(Kitazume et. al. 2007)

External failure patterns

- To understand the failure mechanisms of deep mixing columns reinforced by a shallow mixing layer

- Failure pattern on external stability- Failure pattern on internal stability

- To explore the effect of shallow mixing layer- On failure pressure of improved ground- On deformation of embankment

1. Introduction 3

Objectives

Methodology

- Numerical analyses: by Finite Element method with Plaxis 2011 Code (Plain strain condition)

3. Numerical Analysis 4

Ground condition

Materials

(Kitazume & Terashi, 2013)

SOIL unsat sat cu ' E kN/m3 kN/m3 kPa degree kPa

Embankment 16 18 1.0 30 20,000 0.3

Clay layer 14 16 25 0 2,500 0.45

Sand layer 16 18 0 35 20,000 0.3

Mixing material 14 16 Changed 0 Changed 0.45

qu (kPa) 250 ~ 5,000

SL thickness (m) 0 ~ 10

3. Numerical Analysis 5

Results - Without shallow layer

- qu value has small effect on horizontal displacement of embankment toe.

- High strength columns are tilted under embankment load at 170 kPa while low strength ones are bent (at 160 kPa).

-2 -1.5 -1 -0.5 00

2.5

5

7.5

10

Most rear

qu = 250 kPaqu = 500 kPa

Horizontal displacement (m)

Dept

h fro

m th

e cla

y su

r-

face

(m)

Most-rear col.

0 50 100 150 2000

200

400

600

800

1000

1200qu = 250 kPaqu = 500 kPaqu = 1000 kPa

Embankment pressure (kPa)

Horiz

onta

l disp

lace

men

t (m

m)

3. Numerical Analysis 6

Results - With shallow layer

0 50 100 150 200 2500

250

500qu = 250 kPa

SL 2 m

Embankment pressure (kPa)

Horiz

onta

l disp

lace

men

t (m

m)

0 50 100 150 200 2500

250

500

750

1000qu = 5000 kPa

SL 2 mSL 4 mSL 6 m

Embankment pressure (kPa)

Horiz

onta

l disp

lace

men

t (m

m)

Horizontal displacement at embankment toe was investigated for low and high strength columns with various shallow layer thickness.

3. Numerical Analysis 7

Results - With shallow layer

-2 -1.5 -1 -0.5 00

2.5

5

7.5

10

qu = 250 kPaat Pe =165 kPa

SL 2 m

Horizontal displacement (m)

Dept

h fro

m c

lay

laye

r sur

face

(m

)

-2 -1.5 -1 -0.5 00

2.5

5

7.5

10

qu = 5000 kPaat Pe =230 kPa

SL 2 m

Horizontal displacement (m)

Dept

h fro

m c

lay

laye

r sur

face

(m

)

3. Numerical Analysis 8

DiscussionsOn failure patterns

(qu = 250 kPa) - At Pe = 160 kPa

(qu = 250 kPa, SL = 2 m) - At Pe = 165 kPa

(For low strength columns)

3. Numerical Analysis 9

DiscussionsOn failure patterns

(qu = 5000 kPa, SL = 2 m) - At Pe = 230 kPa

(qu = 5000 kPa) - At Pe = 170 kPa

For high strength columns

Plastic points were not found in the tests for high strength columns without using the shallow layer.

Sliding failure pattern can be consider as the main failure pattern when the columns are reinforced by the shallow layer

3. Numerical Analysis 10

DiscussionsOn effect of shallow layer thickness

0 2 4 6 8 100

50

100

150

200

250

qu = 250 kPaqu = 500 kPa

Shallow layer's thickness (m)

Emba

nkm

ent p

ress

ure

at

yield

(kPa

)

Embankment pressure at yield

0 2 4 6 8 100%

25%

50%

75%

100%

Shallow layer's thickness (m)

Horiz

onta

l disp

lace

men

t ra-

tio

at Pe = 160 kPa

Horizontal displacement at toe

3. Numerical Analysis 11

Summary

On failure patterns

In terms of internal failure, columns without SL experience bending failure at middle of columns length, while with shallow layer, columns also bend but fail at connecting position between SL and DM columns.

In terns of external failure, columns without SL experience tilting failure while sliding failure is significant when SL is applied.

On the effect of shallow layer thickness

Horizontal displacement decreases with increment of shallow layer thickness irrespective of the strength of mixing materials.

Strong decrement in horizontal displacement is observed with high strength columns even with a very thin shallow layer.

Thank you for your kind attention!