Stabilized Soil Evaluation Using Laboratory Electrical Resistivity Cell

Bhangale, L.A. Bhosale, S.S.Professor

e-mail: loktalk@gmail.com e-mail: ssb.civil@coep.ac.in

Department of Civil Engineering, College of Engineering Pune, Pune

ABSTRACT

Electrical resistivity is one of the most widely used geophysical methods of investigation. In this paper results of

lime stabilized expansive soil properties determine by specially developed laboratory resistivity cell are presented.

The developed resistivity cell works on two electrodes as against field method of four electrodes. Efforts are made

to correlate the strength properties such as Unconfined compression strength and CBR of lime stabilized and

untreated expansive clay with electrical resistivity.

Indian Geotechnical Conference – 2010, GEOtrendz

December 16–18, 2010

IGS Mumbai Chapter & IIT Bombay

1. INTRODUCTION

Expansive clay causes foundation damage by excessive

deflections or differential movement. In pavement

construction , the good quality soil is needed for the sub

grade which being costlier matter in present days an

alternative method of strengthening local soil by various

techniques have gain immense importance. In India the

majority area of the states namely, Maharashtra, Karnataka,

Andhra Pradesh, Madhya Pradesh are covered by the

expansive soil like black cotton soil. Lime stabilization is

the well known method for such type of soil. The strength

of lime stabilized soil is measured by laboratory UCS and

CBR tests which are laborious and time consuming. So to

save time, indirect non destructive testing methods like

Electrical resistivity may be used.

Abu-Hassanein (1996) and Mc Carter (1984) have

correlated LL, PL and compaction characteristics of clay

with electrical resistivity. Electrical Resistivity probe and

box are developed by Sreedeep et al. (2004) to correlate

the soil degree of saturation with the four pin electrical

resistivity method .This paper presents two electrode

laboratory electrical resistivity cell which is used for

correlating UCS and lab CBR of lime stabilized expansive

clay with the resistivity.

2. TWO PIN ELECTRICAL RESISTIVITY CELL

The simple principle of resistance measurement and

resistivity calculation is shown in Figure 1. Ratio of cross

sectional area divided by the length is called as geometric/

box factor. This principle has been used to develop the

laboratory electrical resistivity cells. The Resistivity cell is

Fig. 1: Resistivity Measurement (Source – Google Images)

rectangular shaped acrylic box such that cylindrical soil

sample would fit into it and has two circular copper

electrodes at two ends of the box with acrylic end caps to

hold the end electrodes tightly against the soil sample. (Fig.

2) The resistance is measured between the two electrodes

by standard resistivity meter used for conventional four pin

electrical resistivity.

Fig. 2 Electrical Resis

Copper

Electrode

Bottom

side Top side

End Cap

Soil Sample

Fig. 2: Electrical Resistivity Cell of Box Factor 6.16

Bottom

SideTop Side

344 L.A. Bhangale and S.S. Bhosale

Two laboratory electrical resistivity cells having box

factors 6.16 and 13.88 are developed details of which are

as shown in Table 1.

Table 1: Dimensions of Electrical Resistivity Cells

Box Factor Diameter in cm Length in cm

6.16 10 12.7

13.88 15 12.7

Proctor resistivity cell (Box factor 6.16) is used to

measure resistivity of the soil which is later on correlated

to the unconfined compression strength. The UCS tests are

conducted on the soil samples extruded from the proctor

resistivity cell. While CBR resistivity cell (Box factor 13.88)

is used to measure resistivity of soil which is correlated to

the CBR value. CBR resistivity cell has such dimensions

which makes it feasible to receive soil from the CBR mould

directly.

3. MATERIALS CHARACTERIZATION

The locally available black cotton soil used for experimental

work has properties as shown in Table 2.

Table 2: Properties of Black Cotton Soil

Soil Properties Value

Maximum dry Density (gm/cc) 1.345

Optimum Moisture content (%) 30.00

Liquid Limit (%) 67.07

Plastic limit (%) 37.26

Plasticity index 29.81

Soil Classification MH

Specific Gravity

Free Swell Index (%)

Swelling Pressure (kg/cm2)

2.58

70

0.4

The determined properties indicate that the black

cotton soil has medium swelling potential. The grain size

distribution curve of the soil is shown in Figure 3.

Fig. 3: Grain Size Distribution Curve

Commercially available hydrated lime is used for the

soil stabilization. The physical and chemical properties as

provided by the supplier are shown in Table 3.

Table 3: Properties of Hydrated Lime

Chemical Content Percentage Content

Ca(OH)2 85-90% Acid Insoluble’s (as SiO2) 0.5-1.0% Moisture 0.5-1.0%

Iron (as Fe2O3) 0.08%

Alumina (as Al2O3) 0.07% Magnesia(as MgO) 1.0% Carbon dioxide (as CO2) 2.5%

Physical properties

Colour White Particle Size 250 mesh

Physical State Powder

4. EXPERIMENTAL STUDY

The clods of oven dried BC soil are pulverized to 4.75mm

particle size and stabilized by 4% of lime ( SSP,1994). The

lime percentage is based on oven dry weight of soil .UCS

and unsoak CBR are conducted on statically compacted

soil at constant MDD with varying water content for

untreated (25-32 % ) and lime treated soil (25-40%) for

achieving variation in degree of saturation. For every trail

mould resistivity is measured. The compacted soil is

extracted from the moulds and placed in resistivity cell

and then its resistance is measured by applying 100V AC

voltage across the two copper electrodes with the standard

resistivity meter calibrated for two pin method of resistance

measurement as shown in Figure 4. The resistivity is

computed by multiplying measured resistance with

respective box factors. The soil moulds are then inserted

again in the compaction/CBR moulds for UCS and CBR

testing. Samples for UCS test are extracted from the

compaction mould. In case of lime treated soil such tests

are performed on samples of 0, 7,14 and 28 days curing

periods.

Battery 24V

Resistivity cell

Resistivity meter with inbuilt DC to AC

converter

Fig. 4: Setup for Resistivity Measurement

5. RESULT AND DISCUSSION

The Untreated soil sample is designated as “UTS sample

no_water content” for example, UTS3_30 indicates

Stabilized Soil Evaluation Using Laboratory Electrical Resistivity Cell 345

untreated soil sample number 3 having water content 30%.

The letter (o) after water content denotes that this is OMC.

The Laboratory test results for untreated soil are as shown

in Table 4.

Table 4: Test Results for Untreated Soil

Soil Sample UCS

(kg/cm2)

Resistivity

(Box

Factor

6.16)

(Ohm-cm)

CBR

(%)

Resistivity

(Box

factor13.88)

(Ohm-cm)

UTS1_25 6.62 4.937717 6.237 4.37 UTS2_28 5.10 2.715745 5.21 2.64

UTS3_30 (o) 4.56 2.437998 4.912 2.53 UTS4_32 3.46 1.975087 3.23 2.39

As observed from Table 4 resistivity decrease with

increase in water content which validates working of

developed resistivity cells.

The lime treated soil found to have MDD of 1.30 g/cc

and OMC of 33%. The UCS strength and lab CBR value of

treated soil increases with curing period for each water

content. 14 and 28 days cured soils found to have highest

UCS and CBR respectively. Figures 5-6 shows variation of

UCS and CBR with water content as well as curing period.

Fig. 5: Effect of Curing on UCS for Lime Treated Soil

The resistivity value is high when soil has water content

less than optimum and then decreases and has constant

value after optimum and is independent of curing period

for both box factors. Figures 7-8 shows effect of curing on

the measured resistivity for proctor resistivity cell and CBR

resistivity cell respectively.

Fig. 6: Effect of Curing on CBR for Lime Treated Soil

Fig. 7: Effect of Curing on Resistivity (Box factor 6.16) for

Lime Treated Soil

Fig. 8: Effect of Curing on Resistivity (Box factor 13.88)for Lime Treated Soil

Figure 9 shows variation of UCS with resistivity (Box

factor 6.16 ). It is observed from the Figure 9 that UCS

values increase rapidly to a certain resistivity value and

then become almost a constant. It is further noted that the

peak value of UCS increase with resistivity and curing

period. The curves follow two term power equation with

fit coefficient 0.76 ,0.97 and 0.99 for curing periods of 0,14

and 28 days respectively. Table 5 shows the regression

equations for UCS of treated soil. These equations can be

utilized to find the UCS strength once Resistivity of soil is

known. Seven day curing results do not show this trend.

This may be attributed to insufficient chemical reaction.

Though resistivity continuously increases the UCS value

remains constant for each cured sample. This is due to

brittleness of treated soil matrix In case of untreated soil

UCS continuously increases with resistivity which may be

due decrease in water content.

Fig. 9: Plot of Resistivity (Box Factor 6.16 ) Versus

UCS(kg/cm2)

346 L.A. Bhangale and S.S. Bhosale

Table 5: Equations of Resistivity (Box Factor) vs. UCS

Curing

Period

Equation R2

0 days y=–4.931*x-15.94+3.031

0.76

7 days NA NA

14 days y=–26.79*x-6.245+6.071

0.97

28 days y=–297.2*x-14.03+6.071

0.99

The variation of CBR with the Resistivity (Box factor

13.88) for treated soil shown in the Figure 10 .The curves

also follow the two term power equation with fitness

coefficient ranging from 0.7 to 0.9. Table 6 shows the

regression equations for unsoak CBR of treated soil.These

equations are used to evaluate the CBR value directly from

Resistivity.

Table 6: Equation of Resistivity (Box Factor) vs. CBR

Curing

Period

Equation

R2

0 days y= 0.1287*x2.146+11.66

0.99

7 days y= – 4.623*10-15x18.17+28.88

0.80

14 days y= – 1.04*10-14x15.57+ 26.97

0.70

28 days y= –0.007*10-14x4.034+ 32.88

0.93

2 3 4 5 6 7 8 90

5

10

15

20

25

30

35

Resistivity (Ohm-cm)

CB

R

Box Factor 13.88

Untreated Soil

Soil + 4% Lime

0 days

7 days

14 days

28 days

Fig. 10: Plot of Resistivity (Box Factor 13.88) Versus CBR

6. CONCLUSIONS

In case of lime treated soil the resistivity and UCS increases

with curing period which indicate decrease in water content

due to chemical reaction. The treated soil matrix become

brittle if prepared dry of optimum.

The deduced regression equations deduced are soil

specific and hence need to be generalized with further

studies. The UCS Strength of Treated soil increases with

the curing period and it has increased by 198% as compare

to 0 days curing at OMC. The maximum value is observed

at 14 days curing period at all moisture contents. The CBR

value of Treated soil increases with the curing period and

it has increased by 258 % as compare to 0 days curing at

OMC. The maximum value is observed at 28 days curing

period at all moisture content values.

ACKNOWLEDGMENTS

The Director, College of Engineering, Pune, is gratefully

acknowledged.

REFERENCES

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