INFLUENCE OF INTERNAL EROSION ON DEFORMATION AND STRENGTH OF GAP-GRADED NON-COHESIVE SOIL

L. KE, PhD Candidate

A. TAKAHASHI, Associate Prof.

Department of Civil Engineering

Tokyo Institute of Technology

August 30th 2012

Motivation

The gradual migration of fine particles through a coarse matrix leaving the coarse skeleton alone.

It cause changes in soil porosity and hydraulic conductivity.

Suffusion

Seepage

flow

Page 3

Methodology Experimentally evaluate the strength of

cohesionless soil after suffusion

Experimental investigations Parametric study

Upward seepage test:

create certain soil state induced by suffusion

Cone Penetration Test:

obtain the soil strength change by interpreting CPT data

Variable parameters:

Fine particle content

Maximum imposed hydraulic gradient

Certain relationship between hydraulic properties and strength of a soil specimen may exist

Motivation (Cont’d) Influence on soil strength

Coarse particles Fine particles

Soil structure change Suffusion Soil strength change ?

Test material

Silica No.3 is coarse material working as skeleton while Silica No.8 is fine material which could be washed away by seepage flow

0.001 0.01 0.1 1 100

20

40

60

80

100

Grain size (mm)Per

centa

ge

pas

sing b

y w

eight

(%)

Silica No.3 sand

Silica No.8 sand

Test specimens

Fine content: 25%, 20%, 16.7% and 14.3%

0.001 0.01 0.1 1 100

20

40

60

80

100

Grain size (mm)

Perc

enta

ge p

ass

ing b

y w

eig

ht

(%)

SpecimenA (25% fine content)

SpecimenB (20% fine content)

SpecimenC (16.7% fine content)

SpecimenD (14.3% fine content)

Imposed hydraulic gradient: 0~0.5 (All specimens)

Tested specimen

4

3

2

1

Upward flow Seepage Test

Constant head tank

Saturated Soil

Standpipes

2mm single-sized glass marbles

Inlet port

Zone A

Zone B

Zone C

Filter

Cylinder

Cone Tip

0 0.1 0.2 0.3 0.4 0.50

0.05

0.1

0.15

Imposed hydraulic gradient, i

Aver

age

flow

vel

oci

ty (

cm/s

)Seepage test results

Specimen: 25% fine content and 60% relative density

Critical hydraulic gradient of suffusion

(Onset of suffusion)

Critical hydraulic gradient of stability

(Soil specimen reaches zero effective stress)

The hydraulic conductivity increases with the process of suffusion

0 0.1 0.2 0.3 0.4 0.50

0.1

0.2

0.3

Imposed hydraulic gradient, i

Hy

dra

uli

c co

nd

uct

ivit

y (

cm/s

)

Between Nos.3 & 4 pipes

Between Nos.2 & 3 pipes

Average hydraulic conductivity

Seepage test results

---Hydraulic conductivity

No.4

NO.3

No.2

No.1

Standpipes

Onset of suffusion

Specimen: 25% fine content and 60% relative density

Seepage test results Fine particle migration (25% fine content; 60% relative density)

Before suffusion i=0.17

i=0.20 i=0.23

Seepage test results

By observation, the maximum settlement was approximately 10 mm, which is equal to 5.8% in volumetric strain

Specimen void ratio and volume change

Specimen: 25% fine content, 60% relative density

---Influence of internal erosion on soil strength

CPT Test results

Specimen: 25% fine content, 60% relative density

Cone tip resistance decreases on the internally eroded soil

The extent of decrease relates with the imposed hydraulic gradient

0

50

100

150

0 50 100 150

Cone tip resistance (N)

Dep

th (

mm

)

Before Internal Erosion

After Erosion imax=0.46

After Erosion imax=0.51

---Influence of internal erosion on soil strength

Bearing capacity number =Cone resistance/Vertical effective stress

Soil strength decrease with the imposed hydraulic gradient

A sharp decrease

Bearing capacity theory

Sample A-60 : 25% fine content, 60% relative density

Sample B-60 : 20% fine content, 60% relative density

Sample C-60 : 16% fine content, 60% relative density

CPT test results

0 0.1 0.2 0.3 0.4 0.50

0.2

0.4

0.6

0.8

1

Maximum imposed hydraulic gradient, higher than is

No

rmal

ized

Bea

rin

g C

apac

ity

Nu

mb

er

SpecimenA-60

SpecimenB-60

SpecimenC-60

Flow pump

Constant-rate-flow test

Loading system with zero backlash

Pedestal

Base meshConical drainage

Top mesh

Eroded soil grain collection unit

Legend

Seepage flow

Clip gauges

Inner LVDT

Outer LVDT

LCDPT

Load cell

solenoid valves with timer

Back pressure

Load cell

Valves

Outlet

Cell pressure

Eroded soil grains

LCDPT: Low capacity difference pressure transducer

LVDT: Linear valuable displacement transducer

Constant flow rate control unit

Tap water

Triaxial Seepage Test

Seepage flow test

Shear test

Seepage test in a Triaxial cell

Flexible wall

Experiment with back

pressure

Conduct seepage test

under various stress

condition

Conclusions

The hydraulic conductivity of soils drastically increases with progress of the internal erosion. The higher the maximum assigned hydraulic gradient, the higher the fine particle loss. Suffusion would cause the soil strength reduction, the extent of which relates with the imposed hydraulic gradient.

Thank you for your attention