Lecture No.7

Compaction of soils （土の締固め）

■ What is compaction? （締固めとは？） 1

Construction of embankments, river dykes, earth fill dams, etc

（盛土・堤防・アースダム）

Earth fill dam

Sumo mound

Compaction:

To densify （密実化） and stabilize （安定化） soil by applying

static or dynamic load

■ Changes in state by compaction （締固めによる状態の変化） 2

• Air

• Water

• Soil particles

rd

e

Air

Water

Soil

Compaction

vV

sV

V

Void ratio （間隙比）

Dry density （乾燥密度）

rd

e

■ Compaction curve （締固め曲線） 3

Water content, w (%)

Dry density, rd (kg/m3)

Zero air void curve

(Saturation curve)

（ゼロ空気間隙曲線） Sand

Clay

wopt wopt

rd,max

rd,max

Proctor (1933)

When a given soil is compacted at a given energy with varying water

content, the dry density – water content relationship shows;

•

•

w = 10%

w = 30%

■ How to obtain compaction curves – Compaction test 4

① Prepare soil at a given w ② Add soil to mould ③ Compact with rammer

④ Complete each layer ⑤ Trim the surface and weigh

➡ Total density, rt

Add water and repeat for a different w.

Adjust water content

and make it uniform Use a collar for the top layer

2.5 kg or

4.5 kg

Drop from

30cm or 45cm

Repeat ②～④.

Fill with 3 or 5 layers

Typically, 25 blows

per layer

⑥ Measure w

➡ Calculate rd from w and rt

■ Reference curves in compaction diagram 5

Water content, w (%)

Dry density, rd (kg/m3)

Sr =100% 80%

60%

■ Equations for reference curves 6

Saturation curve Constant degree of

saturation curve

Constant air content

curve

Air

Water

Soil

vV

sV

V wV

aVDegree of saturation （飽和度）

[%] 100V

Vv a

a

vwr VVS / [%] 100s

w

W

Ww

s

wsG

r

re

wGS s

r w

ssG

r

r

Water content （含水比）

Air content （空気間隙率）

Specific gravity （比重）

■ Optimum water content （最適含水比） 7

Water content, w (%)

Dry density, rd (kg/m3)

wopt

rd,max

Increasing w:

rd bound by zero air void curve

Decreasing w:

Increase of suction

Increase of stiffness

➡ harder to deform

(harder to be compacted)

wopt: Important parameter in controlling soil compaction

■ Characteristics of compaction curve （締固め曲線の特性） 8

Effect of compaction energy

WR： Rammer weight [kN]

H： Fall height [m]

NB： Blow number per layer

NC： Number of layers

V ： Mould volume [m3]

Compaction energy （締固めエネルギー）

= Work done to soil （土に対する仕事量）

Expected field compaction energy

Compaction energy in laboratory test

[kJ/m3]

■ Characteristics of compaction curve （締固め曲線の特性） 9

Effect of compaction energy

Greater energy: (i.e. Larger rammer weight or

more blows)

wopt ➡

rd,max ➡

However… Over-compaction （過転圧、オーバーコンパクション） Decrease of strength due to too much compaction energy in some soils

Water content, w (%)

Dry density, rd (kg/m3)

wopt

rd,max

■ Characteristics of compaction curve （締固め曲線の特性） 10

Effect of soil type

Location & shape of compaction curves depends on soil types

Particle size (mm)

Perc

ent finer

(%)

Water content, w (%)

Dry

density,

rd (

g/c

m3)

■ Engineering properties of compacted soils 11

Strength

After compaction

After wetting

wopt

Max

w < wopt

Large

decrease w > wopt

No significant change

Dry

de

nsity,

rd (

kg

/m3)

Water content, w (%)

Water content, w (%)

Un

co

nfin

ed

co

mp

ressio

n

str

en

gth

, q

u (

kN

/m2)

Maximum when compacted

at slightly dry states

(Unconfined compression test)

■ Engineering properties of compacted soils 12

Permeability

wopt

Dry

de

nsity,

rd (

kg

/m3)

Water content, w (%)

Min

Co

effic

ien

t o

f p

erm

ea

bili

ty

aft

er

sa

tura

tion

, k

(m/s

ec)

Water content, w (%)

(at compaction)

Saturated permeability

And unsaturated permeability:

To be distinguished

■ Compaction in field 13

For given soil, the thinner each layer is,

the denser it gets compacted.

1-unit energy for 15-cm lift

2-unit energy for 30-cm lift

3-unit energy for 30-cm lift Which is best?

Roller

Depth

Energy

Control variables

• Compaction machine (weight, mechanism)

• Layer thickness (“lift”)

• Number of load application

Compaction of earth fill dam

Tamping roller

Vibration roller

Vibrating plate

compactor

■ Quality control of compacted soils 14

Degree of compaction, Dc（締固め度）

and air content, va（空気間隙率）, as common indices

Dry

de

nsity,

rd

Dry

density,

rd

Water content, w Water content, w

Generally …

• Embankment： Dc≧85～95%

➡

• Subgrade （路床）： Dc≧90～95%

■ Quality control of compacted soils 15

Example: River dyke building with wet sand – clay mixed soil

0 10 20 30 -10 -20 -30 (m)

2015年築堤

2016年築堤

1.0

1.1

1.2

1.3

1.4

1.5

10 15 20 25 30 35 40 45

乾燥密度

ρd（g/

cm

3）

含水比 w（%）

SP2210 2層目

SP2210 4層目

SP2210 6層目

SP2275 2層目

SP2275 4層目

SP2275 6層目

Sr100% ρ s=2.611g/cm3

Sr90%Sr80% １次盛土（H27施工）

最大乾燥密度の90%= 1.27g/cm3

Water content, w (%)

Dry

de

nsity,

rd (

g/c

m3)