The relationship between clay geotechnical data and clay ... · 23 The relationship between clay...

23

The relationship between clay geotechnical data and clay electrical resistivity

M. FALLAH-SAFARI1, M.K. HAFIZI1 AND A. GHALANDARZADEH2

1 Institute of Geophysics, The University of Tehran, Iran2 Faculty of Engineering, The University of Tehran, Iran

(Received: August 11, 2011; accepted: May 21, 2012)

ABSTRACT Clays are at the centre of farming activities and civil engineering works.Characterization of subsurface clay, determination of clay moisture, and wet unitweight are prerequisites since the soft clays constitute an important part of soilfoundations. Electric resistivity can be used to characterize soil properties, e.g., watercontent, saturation, air void ratio, density. Geotechnical tests are time-consuming,destructive, and expensive. Geoelectrical methods are non-destructive, faster andcomparatively cheaper than geotechnical methods. They can thus offer a veryattractive tool for defining some of soil characteristics. In this study, the quantitativerelationship of electrical resistivity with water content, saturation, wet unit weight, airvoid ratio, and compressive strength are reported for five different clays. Research onclay has been carried out with laboratory samples. Some satisfactory quantitativecorrelations between resistivity data and geotechnical data have been obtained. Ingeneral, clays with higher water content have lower electrical resistivity and clays withhigher air void ratio have higher electrical resistivity.

Key words: air void ratio, compressive strength, geotechnical method, resistivity, saturation, water content,wet unit weight.

1. Introduction

Clays are at the centre of agricultural and civil engineering works; they are formed indiagenetic series; they crystallize in geothermal fields whose energy and mineral deposits provevaluable. Clays play an important role in everyday life, from the white-coated paper on which wewrite to the confinement of hazardous waste storage, from cosmetics to pneumatics, from paintsto building materials (Meunier, 2005).

Physical properties of clays are frequently measured in geotechnical engineering because thesoft clays constitute an important part of soil foundations for infrastructure development. Soil andclay can be easily sampled from a borehole, brought to the laboratory and then evaluated byvarious geotechnical tests. The natural chemical stability of clays, their physical properties, andchemical influences must be known in order to deal efficiently (Meunier, 2005).

Geophysical surveys conducted for petroleum, mining, hydrological, environmental,geotechnical engineering, archaeological, and agricultural applications vary dramatically in scalewith respect to the purpose of investigation (e.g., Allred et al., 2008). Many of the geophysicalmethods are widely used in non-destructive testing of civil engineering structures and materials(McDowell et al., 2002). Soft clays are absorbent materials in geophysical signals such as seismic

Bollettino di Geofisica Teorica ed Applicata Vol. 54, n. 1, pp. 23-38; March 2013

DOI 10.4430/bgta0070

© 2013 – OGS

24

Boll. Geof. Teor. Appl., 54, 23-38 Fallah-Safari et al.

and radar signals. This fact partly explains why they have not been thus far a focus in geophysicalinvestigation (Giaoa et al., 2003).

A DC resistivity survey is an inexpensive and widely used technique for investigating nearsurface resistivity anomalies. This technique has recently become popular also in investigatingsubsurface pollution problems. In principle, this approach measures the electric potential fieldgenerated by a transmission of DC electric current between electrodes implanted on the groundsurface (Vereecken et al., 2006). Brovelli and Cassiani (2011) evaluated whether the electricalresistivity and permittivity carried by one measurement type can be used to identify soilparameters that are then used to predict the other geophysical quantity.

In most Earth materials, porosity and chemical content of water filling the pore spaces aremore important in governing resistivity than the conductivity of mineral grains of which thematerial itself is composed (Dobrin and Savit, 1988; Brovelli and Cassiani, 2010, 2011).Electrical conduction in clean sands and gravels occurs primarily in the fluids filling the pores.In clayey soils and clay-bearing rocks, however, electrical conduction occurs in the pores and onthe surfaces of electrically charged clay particles. Some of pioneering research on the effects ofsurface conductivity on porous soils was carried out by Revil and Glover (1997, 1998) and Revilet al. (1998). For clays, surface conductance can be a significant factor affecting the bulkelectrical resistivity of the soil (Whitlow, 1983; Abu-Hassanein et al., 1996). Thus, for clays, clay-rich soils, and clay-rich rocks, parallel resistor models were developed to account for conductionthrough the pore fluid and along the particle surfaces (Abu-Hassanein et al., 1996).

Electrical resistivity can be used to characterize the soil properties. However, it is not a verysimple and straightforward parameter such as other geotechnical parameter (e.g., water content,liquid and plastic limits and unit weight). Experimental measurement have shown that the effectof saturation, water content and porosity on electrical resistivity can be significant (e.g., Archie,1942; Bussian, 1983; Dannowski and Yaramanci, 1999; Brovelli et al., 2005; Brovelli andCassiani, 2010). Friedman (2005), Lesmes and Friedman (2005) and Samouëlian et al. (2005)reviewed the application of electrical resistivity for characterizing soil.

In geotechnical engineering, water content determination is a routine laboratory test fordetermining the amount of water present in a quantity of soil in terms of its dry mass (Bowles,1992). Moreover, a satisfactory quantitative correlation between resistivity data and measuredwater content values has been obtained (Cosenza et al., 2006). Waxman and Smits (1968) havedeveloped relations, which relate the electrical resistivity parameters to water saturation, waterresistivity and cation-exchange capacity per unit pore volume of rock. An increase in the soilwater content results in a decrease in the electrical resistivity of soil (Cosenza et al., 2006). Thiscorrelation, once more, demonstrates that resistivity is a good indirect predictor of water content.However generally, soils with higher fines content also have higher specific surface, whichimproves surface conductance. Moreover, electrical resistivity shows definitive relationship withwet unit weight, saturation, air void ratio (ratio of the volume of air-void to the total volume ofair, water, and solids), and compressive strength (Fallah-Safari et al., 2011).

It is desirable to extrapolate and/or interpolate consistent 1D geotechnical data fromgeoelectrical measurements that are non-destructive and comparatively faster and cheaper thanthe geotechnical methods.

25

The relationship between clay geotechnical data and clay electrical resistivity Boll. Geof. Teor. Appl., 54, 23-38

2. Theory

Consider a cylinder composed of uniform material, having a length L and a cross-sectionalarea A. An electric current I is applied. The cylinder, to a lesser or greater extent, opposes thisthrough flow of electric current, thereby causing a drop in electric potential, ΔV, which occursalong the column from the end where I enters from the end and where I leaves. Electric potentialcan be characterized as the potential energy for a unit charge resulting from its position within anelectric field. As indicated by Eq. (1), ΔV is proportional to I and the proportionality constant isthe resistance R that is a characteristic of the cylinder’s overall ability to oppose the current flow:

(1)

The minus in Eq. (1) simply indicates that current flow is in a direction opposite to that ofincreasing electric potential. Eq. (1) is referred to as Ohm’s law and the resistance R of thecylinder can itself be expressed as:

(2)

where ρ is the resistivity.As Pozdnyakov et al. (2006) point out, the relationship between water content w and electrical

resistivity ρ can be expressed as the following:

(3)

The parameter a and b accounts for the effect of the soil water retention on the electricalproperties of the soil solution and the chemical properties of the soil and the soil solutionrespectively. The parameter a can depend on the physical properties of the soil that affect thewater retention in the soil such as porosity, etc. Since the nature and magnitude of the forcesinfluencing the soil water retention vary considerably in different water content conditions, theparameters a and b change with the soil water content (Pozdnyakov et al., 2006). The results ofOzcep et al. (2009) and Fallah-Safari et al. (2011) are consistent with those of Pozdnyakov et al.(2006). The relationships are site-specific and require an extensive study to determine theirvalidity and limitations, in different geological environments (Sudha et al., 2009).

There is a variety of parameters used to describe the relative proportions of air, water, andsolid in soil. This section defines these parameters and the relationships among them. The basicnotation is as follows:

Va, Vw, Vv and Vs represent the volumes of air (m3), water, voids and solids in a soil mixture;Wa, Ww, and Ws represent the weights of air (N), water and solids in a soil mixture;

w a b= −( )exp .ρ

RL

A=

ρ

ΔV RI= − .

26


Ma, Mw, and Ms represent the masses of air (kg), water and solids in a soil mixture.

Water content w is the ratio of the mass of water contained in the pore spaces of soil or rockmaterial, to the solid mass of particles in that material expressed as percentage. It is easilymeasured by weighing a sample of the soil, drying it out in an oven, and re-weighing it. Standardprocedures are described by the American Society for Testing and Materials [ASTM hereafter,see ASTM (1998a)]

(4)

Wet unit weight (mass unit weight), γm, is the weight (solids plus water) per unit of totalvolume of soil or rock mass, irrespective of the degree of saturation (ASTM, 2003). Thedensity/unit weight of a cohesionless soil may be determined by various in-place methods inthe field or by the measurement of physical dimensions and masses by laboratory soilspecimens. In some cases (for instance, in the United States oil and gas industry), density isalso defined as its weight per unit volume; although, this quantity is more properlycalled specific weight:

(5)

Degree of saturation Sr is the ratio of the volume of water to the volume of voids (ASTM,2003):

(6)

Air void ratio Av is the ratio of the volume of air-void to the total volume of air, water, andsolids (Whitlow, 1983):

(7)

Compressive strength (unconfined or uniaxial compressive strength), qu, is the load per unitarea at which an unconfined cylindrical specimen of soil or rock will fail in a simple compressiontest. The failure load is commonly the maximum that the specimen can withstand in the test(ASTM, 2000a).The primary purpose of the unconfined compression test is to quickly obtain theapproximate compressive strength of soils that possess sufficient cohesion to permit testing in the

A

V

Vva

t

= .

S

V

Vrw

v

= .

γ m

s w

s w a

t

t

W W

V V V

W

V=

++ +

= .

w

M

M

W

Ww

s

w

s

= = .

27


unconfined state.

3. Methodology

Soils consist of a mixture of particles with different size, shape, and mineralogy. Since the sizeof the particles obviously has a significant effect on the soil behaviour, the grain size and grainsize distribution are used as criteria for classifying soils. The grain size distribution concerns therelative proportions of particles of various sizes. The two traditional methods used ingeotechnical engineering are sieve analysis and hydrometer analysis.

Clays and silts, often called ‘fine-grained soils’, are classified according to their Atterberglimits, the most commonly used Atterberg limits are the Liquid limit (denoted by LL or WL) andPlastic Limit (denoted by PL or WP) (ASTM, 2000b). Atterberg limit and the grain sizedistribution of five different clays used in this study are summarized in Table 1. Also soilsclassification based on Unified Classification System (USBR Earth Manual) and ASTM D2487-98 (ASTM, 1998b) are shown. CL symbol stands for inorganic clays of low to medium plasticity,lean clays and OL symbol stands for organic silts and organic silty clays of low plasticity,respectively (Lee et al., 1983).

In this paper, a relationship between resistivity and geotechnical data (water content,saturation, wet unit weight, air void ratio and compressive strength) was researched. This studyis divided into 3 steps: (I) preparing samples, (II) measuring electrical resistivity and geotechnicalvalues, and (III) discussing the relationship between electrical resistivity and geotechnical data.

The samples were initially made from five types of clay [bentonite, kaolinite and 3 samplesof the local clay that were collected in place around Karaj, Iran (longitude 50.57° E and latitude35.48° N), C, D and E] approximately in five water content values (range of water content was12 to 40). The relative wet unit weight of each samples were then changed. The soil at a selectedwater content was placed in three layers into a mould of given dimensions (Fig. 1), with eachlayer compacted by 2 to 6 blows of a 24.4-N hammer dropped from a distance of 305-mm. Soilsamples were sampled by a smaller mould (348-mm diameter) that was pushed into prepared soil.A jack, frame or other device was adapted for the purpose of extruding compacted specimensfrom the mould. The resulting wet unit weight was determined. The procedure was repeated fora sufficient number of water contents (chemical properties of water that were used in this study

Clay type Atterberg limits grain size distribution Classificati

WP WL PI D10(µm) D30(µm) D60(µm) CU CZ

bentonite 29.81 37.99 8.18 1.21 1.7 15 12.40 0.16 CL

kaolinite 26.2 36.3 9.94 1.156 2.5 21 18.17 0.26 CL

C 22.49 36.95 14.46 1.164 2.8 30 25.77 0.22 CL

D 25.39 36.25 10.86 1.246 8 60 48.15 0.86 OL

E 24.44 30.83 6.39 1.3 60 70 53.85 39.56 OL

Table 1 - Atterberg limits and the grain size distribution of the clays; C, D and E are three samples that were collectedin different places around Karaj, Iran.

28


are summarized in Table 2). The clays did not reuse that has been previously compacted in thelaboratory. The measurements were made on 128 samples taken from five clay types.

Electrical potential of samples was measured using an ABEM RIP3 instrument. Commutatorterminals A and B are located outermost on top of the RIPS3 (Fig. 2). Terminals A and B areconnected to two sponge-filled plexiglas tubes, each of which is open at one end. These serve asthe pulse train electrodes via which the pulse train send out from the commutator is passedthrough the sample which is mounted between them (ABEM, 1953).

The platinum pick-up electrodes are connected via their leads to receiver terminals M and N(innermost on the top of the RIPS3). These electrodes are at the ends of the tubular sponges thatare in contact with the sample.

The pulse train passing through the sample gives rise to a voltage that is measured at thereceiving terminals M and N and is read from the analogue meter. The commutator controls thecapacitive analogue memories in the receiver by means of the time reference signal. Then, theresistivity can be determined by reading the memories.

Vp is the DC voltage drop determined by the current through the sample, the sample’sresistivity and its geometric factor. The sample’s resistivity can be obtained using the followingformula:

(8)

where Vp is voltage indicated on receiver analogue meter, I is current flowing through the sample,A is mean cross-sectional area of the sample and L is the overall length of the sample. Thegeometric factor for cores and sawn samples can be determined very precisely.

The values of electrical resistivity were plotted versus geotechnical data. The relationship between

ρ = ×

V

I

A

LP

Fig. 1 - Cylindrical mould.

29


Fig. 2 - ABEM RIPS3 instrument.

Table 2 - Chemical properties of water that were used in this study.

CompositionMaximum desirable

(mg/l)Maximum allowable

(mg/l)

1T.D.S

(Total Dlssolved solids)1000 1500

2 CaCo3 200 500

3 Cl 250 400

4 SO4 250 400

5 NO3 - 50

6 NO2 - 3

7 NH3 1.5 -

8 F 0.5 1.5

9 Ca 300 -

10 Mg 30 -

11 Na 200 200

12 H2S 0.05 -

13 Fe 0.3

14 Mn 0.1 0.4

15 Al 0.1 0.1-0.2

16 Zn 3 -

17 Cu 1 2

A M N B

30


electrical resistivity and water content is shown in Fig. 3. Moreover, the relationships betweenelectrical resistivity, saturation, wet unit weight, air void ratio, and compressive strength aresummarized in Figs. 4 to 7, respectively. In general, clays with higher water content or saturation havelower electrical resistivity and clays with higher air void ratio have higher electrical resistivity.

4. Results and discussion

In clays, porosity, chemical content of water filling the pore spaces and surface conductanceare the most important factors affecting resistivity. Once the chemical content of water filling thepore spaces is constant, the resistivity of clays is then mostly influenced by water content,saturation, wet unit weight, and air void ratio.

Voronin (Pozdnyakov et al., 2006) developed a concept of soil water categories based on thetheory of molecular attraction and capillary forces. The contribution of molecular attraction to the

Fig. 3 - Relationship between clay electrical resistivity andwater content: a) bentonite; b) kaolinite; c), d) and e) arethree samples that were collected in different places aroundKaraj, Iran.

a b

c d

e

w

r

= + −⎛⎝⎜

⎞⎠⎟

0 22 0 2510 44

2

. . exp.

ρ

== 0 840.

w

r

= + −⎛⎝⎜

⎞⎠⎟

0 18 40 344 33

2

. . exp.

ρ

== 0 770.

w

r

= + −⎛⎝⎜

⎞⎠⎟

=

0 19 1 234 02

2

. . exp.

ρ

00 679.

w

r

= + −⎛⎝⎜

⎞⎠⎟

=

0 18 0 236 65

2

. . exp.

ρ

00 875.

w

r

= + −⎛⎝⎜

⎞⎠⎟

0 19 50 652 51

2

. . exp.

ρ

== 0 786.

31


soil matrix potential increases with increasing water content from air-dry to water-saturated soil,because the contribution of capillary forces increases.

The electrical resistivity decreases rapidly with increasing soil water content in the adsorptionwater range, and the electrical resistivity decreases less dramatically in the range of film-capillarywater. In the gravitational water range, the mobility of water molecules does not affect the mobilityof electrical charges; therefore, the electrical resistivity is almost independent of the water content inthis range (Fig. 3). Indeed when water content is high, water is in gravitational range and saturationis high (air void ratio is low); so the electrical resistivity is independent of the saturation in highdegree of saturation or in low air void ratio (Figs. 4 and 6). Nevertheless, a small decrease in theelectrical resistivity can still occur in the gravitational water range due to the continuous dissolutionof the adsorbed and precipitated ions from the soil solid phase (Pozdnyakov et al., 2006).

Determining a point in which adsorbed water changes to film water is difficult to identify,because the measurements of electrical resistivity by the direct-current technique could notprovide a sufficient number of experimental points in the range of the adsorbed water

Fig. 4 - Relationship between clay electrical resistivity andsaturation: a) bentonite; b) kaolinite; c), d) and e) are threesamples collected from different places around Karaj, Iran.

a b

c d

e

Sr = − + −⎛⎝⎜

⎞⎠⎟

0 216 1 43545 4

. . exp.

ρ

.r 2 0 926=

Sr = − + −⎛⎝⎜

⎞⎠⎟

0 914 2 56593 8

. . exp.

ρ

.r 2 0 838=

Sr = − + −⎛⎝⎜

⎞⎠⎟

0 354 1 34312 1

. . exp.

ρ

.r 2 0 879=

Sr = − + −⎛⎝⎜

⎞⎠⎟

0 117 1 46038 7

. . exp.

ρ

.r 2 0 843=

Sr = − + −⎛⎝⎜

⎞⎠⎟

0 443 1 64614 9

. . exp.

ρ

.r 2 0 850=

32


(Pozdnyakov et al., 2006). The break point between the ranges of the adsorbed and film water canbe determined at a high measuring frequency by electrometric methods.

Archie’s law is inapplicable in instances where the matrix has significant conductivity such asfor clay because it is only valid for one conducting phase distributed within a nonconductingphase (Glover et al., 2000). Generally, soils with higher fines content also have higher specificsurface, which improves surface conductance (Whitlow, 1983; Abu-Hassanein et al., 1996).

In Fig. 3, the quantitative correlation between resistivity and water content is observed. Thereare also qualitative relations between resistivity and saturation (Fig. 4).

In this study, the relationship among resistivity, water content, and saturation in clays isexpressed as follows (Figs. 3 and 4):

(8) w a b

c= + −

⎛⎝⎜

⎞⎠⎟

expρ

Fig. 5 - Relationship between clay electrical resistivity andwet unit weight: a) bentonite; b) kaolinite; c), d) and e) arethree samples collected from different places around Karaj,Iran.

a b

c d

e

γ ρm

r

= −

=

13735 0 046

0 613

3 09

2

.

.

.

γ ρm

r

= −

=

17021 247

0 663

0 7859

2

.

.

γ ρm

r

= −

=

16435 0 0009

0 697

4 166

2

.

.

.

γ ρm

r

= −

=

16263 20 17

0 722

1 46

2

.

.

.

γ ρm

r

= +

=

4823 1144

0 699

0 66

2

.

.

33


(9)

where w is water content (%), a, a’, b, b’, c and c’ are constants within a certain clay but mayvary for different types of clay, ρ is electrical resistivity (ohm⋅m) and Sr is saturation (%).

Many of the geotechnical properties of rocks and engineering soils can be indirectlydetermined from geophysical measurements. Bulk density, porosity and permeability areexamples of physical properties that can be obtained in this way. The quantitative correlationbetween resistivity, wet unit weight and air void ratio were observed (Figs. 5 and 6). Withengineering soils, compaction will also influence the results (McDowell et al., 2002).

The state of compaction of a soil is conveniently measured using the dry density, the attainablevalues of which are related to the moisture content. As water is added to a dry soil, each particleis covered with a film of adsorbed water. As the adsorbed water films increase in thickness theparticles become lubricated and tend to pack more closely together, thus the density increases. At

w a b

c= + −

⎛⎝⎜

⎞⎠⎟

' ' exp'

ρ

Fig. 6 - Relationship between clay electrical resistivity andair void ratio: a) bentonite; b) kaolinite; c), d) and e) arethree samples that from different places around Karaj, Iran.

a b

c d

e

A

r

v = − −⎛⎝⎜

⎞⎠⎟

=

0 92 0 97109

02

. . expρ

..861

A

r

v = − −⎛⎝⎜

⎞⎠⎟

41 48 41 927536

. . expρ

22 0 672= .

A

r

v = − −⎛⎝⎜

⎞⎠⎟

=

0 27 0 44517

02

. . expρ

..788

A

r

v = − −⎛⎝⎜

⎞⎠⎟

=

0 19 0 5614

02

. . exp

.

ρ

8837

A

r

v = − −⎛⎝⎜

⎞⎠⎟

=

0 59 0 7252

02

. . exp

.

ρ

7772

34


Fig. 7 - Relationship between clay electrical resistivity, wetunit weight and compressive strength; a) bentonite; b)kaolinite; c), d) and e) are three samples collected fromdifferent places around Karaj, Iran.

a b

c d

e

q

r

u m= − − +

=

145 5518 0 0668

0 8

2 3

2

ρ γ.

. 229

q

r

u m= − − +

=

278 84893 0 0653

0

2 3

2

ρ γ.

.6698

q

r

u m= − − +

=

142 8178 0 049

0 62

2 3

2

ρ γ.

. 99

q

r

u m= − − +

=

135 21233 0 0492

0

2 3

2

ρ γ.

.6632

q

r

u m= − − +

=

38 25 22672 0 011

0

2 3

2

. .ρ γ..649

γm=17

γm=16

γm=15

γm=14

γm=19

γm=18

γm=17

γm=16

γm=20

γm=19

γm=18

γm=17

γm=20

γm=19,5

γm=19

γm=18,5

γm=19

γm=18

γm=17

γm=16

35


a certain point, however, the adsorbed films begin to push the particles apart and so with furtherincreases in moisture content the density decreases. The maximum dry density therefore occursat an optimum content (Whitlow, 1983).

From the values of dry density and moisture content obtained from samples, graphs of drydensity-moisture content are plotted. Unlike four types of clay (bentonite, C, D and E) moisturecontent of kaolinite samples is more than optimum moisture content. In other words, as thedensity increases in kaolinite the moisture content decreases. Since the moisture contentinfluences on resistivity, presumably the wet unit weight increasing in kaolinite causes resistivityto increase. Furthermore, difference between kaolinite and other clay samples in dry density-moisture content graphs causes values of the coefficient of the kaolinite sample to differ fromthose of the other samples in Fig. 5. Moreover, the surface conductivity of kaolinite is anexception (Leroy and Revil, 2004).

Compressive strength depends on the stress history, in particular on the over consolidationratio; so the used soil samples that had been previously compacted in the laboratory were notreused.

The relationship between clay electrical resistivity and compressive strength is checked. Wecannot find any relationship, so we add a third data set. Other data sets were evaluated. Wet unitweight is one of the data set that has the best equations (Fig. 7), also properties of wet unit weightand compressive strength is same.

The relationship between electrical resistivity and clay wet unit weight, air void ratio andcompressive strength are found to be given as follows:

(10)

(11)

(12)

where γm is wet unit weight (kN/m3), a, a’, a”, b, b’, b”, c, c’ and c” are constants within a certainclay but may vary for different types of clay, ρ is the electrical resistivity (ohm⋅m), Av is air voidratio (%) and qu is compressive strength (kN/m2).

5. Conclusions

Geotechnical tests are time-consuming, destructive, and expensive. Geoelectrical methodsappear to be non-destructive, faster and comparatively cheaper than geotechnical methods andcan offer a very attractive tool for soil characteristics. The main purpose of this research is to findthe relationship between electrical resistivity data and geotechnical data in clays. The selectedgeotechnical parameters are water content, saturation, wet unit weight, air void ratio andcompressive strength.

q a

bcu m= + +''

''''

ργ

2

3

A a b

cv = + −⎛⎝⎜

⎞⎠⎟

' ' exp'

ρ

γ ρm

ca b= +

36


Samples were selected from five types of clay (bentonite, kaolinite and 3 samples of the localclay collected from places around Karaj, Iran, C, D and E). Electrical potential of samples weremeasured by ABEM RIP3 instrument. The standards of the American Society for Testing andMaterials describe the procedures of geotechnical measurement.

When chemical content of water filling the pore spaces is the same, then resistivity of claysare mostly influenced by water content, saturation, wet unit weight, air void ratio and compressivestrength. The values of electric resistivity were plotted versus geotechnical data. The satisfactoryquantitative correlations between electrical resistivity, water content, saturation, wet unit weight,air void ratio and compressive strength are as follows:

where w is water content (%), a, a’, a”, a’”, aIV, b, b’, b”, b’”, bIV, c, c’, c”, c”’ and cIV areconstants within a certain clay but may vary for different types of clay, ρ is electrical resistivity(ohm·m), Sr is saturation (%), γm is wet unit weight (kN/m3), Av is air void ratio (%) and qu iscompressive strength (kN/m2).

In general, clays with higher water content or saturation have lower electrical resistivity andclays with higher air void ratio have higher electrical resistivity. In this study, kaolinite showedthe lowest correlation coefficient in root mean square. The reason for this discrepancy appears tobe the range of water content in kaolinite (16% to 30%); while the range of saturation in kaolinitewas 0.80 to 1; kaolinite was saturated by little water. Unlike four types of clay (bentonite, C, Dand E) moisture content of kaolinite samples is more than optimum moisture content. In otherwords, as the density increases in kaolinite the moisture content decreases. Since the moisturecontent influences on resistivity, presumably the wet unit weight increasing in kaolinite causesresistivity to increase. Moreover, surface conductivity of kaolinite is an exception.

Once more, these correlations demonstrate that resistivity is a good indirect predictor ofgeotechnical data in clays. Of course these correlations may vary for different types of clay andare site specific.

q a

bcu

IVIV

IVm= +

ργ

2

3

A a b

cv = + −⎛⎝⎜

⎞⎠⎟

''' ''' exp'''

ρ

γ ρm

ca b= +'' '' ''

S a b

cr = + −⎛⎝⎜

⎞⎠⎟

' ' exp'

ρ

w a b

c= + −

⎛⎝⎜

⎞⎠⎟

expρ

37


Acknowledgements. This work was supported in part by the research project developed at the Institute ofGeophysics, the University of Tehran, which is appreciated.

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Abu-Hassanein Z.S., Benson C.H. and Blotz L.R.; 1996: Electrical resistivity of compacted clays. J. Geotech. Eng.,122, 397-406.

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Corresponding author: Mehdi Fallah-SafariInstitute of Geophysics, The University of Tehran, Tehran, IranPhone: +989 102063914; e-mail: [email protected]

The relationship between clay geotechnical data and clay ... · 23 The relationship between clay...

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