Stabilized Soil Evaluation Using Laboratory Electrical Resistivity Cell
Bhangale, L.A. Bhosale, S.S.Professor
e-mail: [email protected] e-mail: [email protected]
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.
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