Characterizing Cracking Process in Clayey Soil Using ...

Characterizing Cracking Process in Clayey

Soil Using Electrical Resistivity Method

Cheng Zhu1, Chao-Sheng Tang2, De-Yin Wang2

1Department of Civil and Environmental Engineering,

Henry M. Rowan College of Engineering, Rowan University

2School of Earth Sciences and Engineering, Nanjing University

19th Annual NJDOT Research Showcase

October 13, 2017

Introduction

Desiccation cracking

Sedimentary structures formed as clay-

bearing soils dry and contract.

2

Introduction

Mechanical property change

• impaired strength

• excessive deformation

• increased compressibility

Hydraulic property change

• Increase hydraulic conductivity

• Create preferential pathways for fluids

3

Inspection of desiccation cracking

Visual observation

Excavation of trenches

4

Electrical resistivity of soil

A sensitive reflection of

• The nature of solid

(mineralogy, shape, fabric, and size distribution)

• Arrangement of voids

(porosity, tortuosity, connectivity, pore structure)

• Properties of fluid

(water content, electrical resistivity, solute concentration)

5

Electrical resistivity measurement

Schematic view of electrical resistivity method

𝜌 =∆𝑈𝑀𝑁𝐼

2𝜋

1𝑀𝐴 −

1𝑀𝐵 +

1𝑁𝐵 −

1𝑁𝐴

= 𝐾∆𝑈𝑀𝑁𝐼

• I is the injected current (A)

• ΔUMN is the measured electrical potential (V) between M and N

• K is a geometrical coefficient

• MA, MB, NA and NB represent the relative spacing (m) between electrodes6

Applications of electrical resistivity

tomography (ERT)

7

Monitoring CO2 migration in

shallow sand aquifer

Monitoring moisture change in

clay embankments

[Gunn et al., 2015][Yang et al., 2015]

Detecting sand-bedrock interface

[Ward et al., 2014]

Experimental set up

Electrical Resistivity Measurement (ERM) system

Crack Monitoring (CM) system

Temperature and Relative Humidity (RH) Monitoring (T/RHM) system

29 cm

3 c

m (a)

(b)

(c)

8

Soil specimen

Low plasticity clay (CL)

9

Installation of electrode

Twenty-one electrodes were fabricated using stainless

steel bolts with a length of 19.0 mm and a diameter of

2.0 mm.

10

• 3 mm penetration depth: to ensure sufficient soil-electrode interfacial contact.

• The electrode length to electrode spacing ratio is 0.3, a reasonably small value.

Final crack pattern

Specimen 1: electrical resistivity

Specimen 2: water content measurement

Specimen 2: for water content measurement

Specimen 1: for resistivity measurement

Same crack pattern with similar morphological characteristics

11

Image processing procedures

1) Convert original image to grey level

2) Noise removal through filter and binarization

3) Clod and crack recognition and analysis

2 cm

Clod 1 Clod 2 Clod 3 Clod 4gap Crack 3 Crack 1Crack 2

A1 A2 A3 A4

L1L2 L3 L4

W1 W2 W3 W4

(a)

(b)

(c)

12

Crack pattern characterization

• Longitudinal shrinkage strain eLon

• Lateral shrinkage strain eLat

• Surface shrinkage strain eSur

• Number of crack segments Nseg

• Number of clods Nc

• Average width of cracks Wav

• Average area of clods Aav

Crack intersection

o BA

N1

N2Crack medial axis

Crack segment

13

Laboratory T & RH conditions

The time histories of temperature and relative

humidity exhibit minor fluctuations.

0 50 100 150 200 250 3005

10

15

20

25

Tem

per

ature

(°C

)

Time (h)

Air temperature

Average line

12.29T ℃

0

20

40

60

80

100

86.21%RH

Relative humidity

Average line

Rel

ativ

e hum

idit

y (

%)

temperature varying

within 12.29 ± 1.85 oC

relative humidity varying

within 86.21 ± 13.8%

14

Water evaporation

Water content decreases with drying time.

Evaporation rate decreases with drying time.

0 50 100 150 200 250 3000

10

20

30

40

50

wr = 7.6%

w0 = 50.5%

w = 30.7 %

w = 31.1 %

Crack 3

Crack 2

Wat

er c

on

ten

t (%

)

Time (h)

Water content

Rapid evaporation stage Slow evaporation stage

waev

= 17.0%

w = 33.7 %

Crack 1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Evaportion rate

Ev

apo

rtio

n r

ate

(g/h

)

Temporal variations of water content and evaporation rate 15

Desiccation cracking process2 cm t = 0 h

t = 86.5 h

t = 88.5 h

t = 95.2 h

t = 102.7 h

t = 102.8 h

t = 103.0 h

t = 142.2 h

t = 193.0 h

t = 105.3 h

t = 113.0 h

t = 313.8 h

Crack 1

Crack 2

Crack 3

A total of three cracks were formed

nearly parallel to each other and

perpendicular to the longitudinal

direction of the specimen.

Crack 1: formed at 86.5 h



16

Geometrical parameters of crack patterns

0 50 100 150 200 250 300

0

1

2

3

4

Nse

g/N

c

Time (h)

Nseg

Nc

Crack 1

Crack 2

Crack 3

(a)

0 50 100 150 200 250 300

0.0

0.2

0.4

0.6

Shrinkage limit

(b)

Av

erag

e cr

ack

wid

th W

av(c

m)

Time (h)

Crack 1

Crack 2

Crack 3

0 50 100 150 200 250 3000

10

20

30

40

50

60

70Lateral shrinkage

(c)

Crack 3

Crack 2

Aver

age

clod a

rea

Aav

(cm

2)

Time (h)

Crack 1

Lateral and longitudinal shrinkage

Shrinake limit

0 50 100 150 200 250 300

0

5

10

15

20

25

30

Shrinkage limit

Lateral and longitudinal shrinkage

Lateral shrinkage

(d)

CrackingShri

nkag

e st

rain

(%

)

Time (h)

Lon

Lat

Sur

17

Temporal variation of 𝜌#1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13 # 14 # 15 # 16 # 17 # 18 # 19 # 20 # 21

# 1 # 2 # 3 # 4 # 5 # 6 # 7 # 8 # 9 # 10 # 11 # 12 # 13 # 14 # 15 # 16 # 17 # 18 # 19 # 20 # 21

Electrodes #1-#21

Crack 1Crack 2Crack 3

Late

ral

Longitudinal

(a)

(b)

Group 3 Group 2 Group 1

0 50 100 150 200 250 300

100

1000

10000

Phase IIPhase I

Group 2(b)

t = 101 h

Ap

pa

ren

t e

lectr

ica

l re

sis

tivity (

oh

m-m

)

Time (h)

#8-#9

#9-#10

#10-#11

#11-#12

#12-#13

#13-#14

#10-#11

#11-#12

90 92 94 96 98 100 102 104

180

200

220

240

260

280

300

0 50 100 150 200 250 300

100

1000

10000

Phase II

Group 1

#18-#19

#17-#18

t = 84.5 h

Ap

pa

ren

t e

lectr

ica

l re

sis

tivity (

oh

m-m

)

Time (h)

#14-#15

#15-#16

#16-#17

#17-#18

#18-#19

#19-#20

#20-#21

(a)

Phase I

80 81 82 83 84 85 86 87180

200

220

240

260

280

300

18

Electrical conduction behavior of

different soils

+

-

+

+- -

-- +

+

Fluid current flow

Non-conducting sand particle

Cationic migration Anionic migration

Pore fluidAir bubble

-

-

+

--

-

+

+

+

+ Fluid current flow

Matrix current flow

Clay surface conduction

Diffuse double layerClay particle

-+

+ +

+ -

-

-

-+

+ +

+ -

-

-

Fluid current flow

Matrix current flow

Fluid current flow

Matrix current flow

(a)

(b)

(c)

(d)

Saturated state with relative high water content

Saturated state with relative low water content

Unsaturated state

-

+

+

-

Exchangeable cation

Negative surface charge

19

+

-

+

+- -

-- +

+

Fluid current flow




-

-

+

--

-

+

+

+


Matrix current flow



-+

+ +

+ -

-

-

-+

+ +

+ -

-

-

Fluid current flow

Matrix current flow

Fluid current flow

Matrix current flow

(a)

(b)

(c)

(d)



Unsaturated state

-

+

+

-

Exchangeable cation


+

-

+

+- -

-- +

+

Fluid current flow




-

-

+

--

-

+

+

+


Matrix current flow



-+

+ +

+ -

-

-

-+

+ +

+ -

-

-

Fluid current flow

Matrix current flow

Fluid current flow

Matrix current flow

(a)

(b)

(c)

(d)



Unsaturated state

-

+

+

-

Exchangeable cation


+

-

+

+- -

-- +

+

Fluid current flow




-

-

+

--

-

+

+

+


Matrix current flow



-+

+ +

+ -

-

-

-+

+ +

+ -

-

-

Fluid current flow

Matrix current flow

Fluid current flow

Matrix current flow

(a)

(b)

(c)

(d)



Unsaturated state

-

+

+

-

Exchangeable cation


sand Saturated clay with relatively

low water content

Saturated clay with relatively

high water content

Unsaturated clayey soil

Electrical resistivity tomography

Crack initiation and growth

Crack pattern (location, width, geometry)

t = 22.0 h

Crack patterns Apparent resistivity images

t = 66.8 h

t = 87.0 h

t = 93.9 h

t = 117.4 h

t = 124.5 h

t = 185.4 h

t = 255.3 h

Log r(Ohm-m)

2 cm

20

Conclusions

Water evaporation is governed by the temperature and

relative humidity at the soil-air interface.

The volumetric shrinkage of the specimen shows

evident anisotropic characteristics.

The evolution of electrical resistivity in clay is

dominated by two competing effects: (1) closer packing

of soil fabric and higher concentration of ions in pore

fluids, and (2) the evaporation-induced water loss

associated with hydration film contraction and

desiccation crack insulation.

ERT is reliable to map the potential cracks’ positions.

21

Future study

Future works will be carried out to improve our

fundamental understanding of the correlations between

physical and electrical properties of soil in a higher

dimensional space at larger scale during the desiccation

cracking process.

22

Thank you!

Contact: [email protected]

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Characterizing Cracking Process in Clayey Soil Using ...

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