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Effect of Soil Composition on Electrokinetic Grouting
IGC 2009, Guntur, INDIA
EFFECT OF SOIL COMPOSITION ON ELECTROKINETIC GROUTING
L.S. Thakur
Lecturer, Applied Mechanics Department, Faculty of Tech. & Engg., M.S.U., Vadodara–390001, India.
E-mail: [email protected]
V.H. Padhiar
P.G. Student, Applied Mechanics Department, Faculty of Tech. & Engg., M.S.U., Vadodara–390001, India.
D.L. Shah
Professor, Applied Mechanics Department, Faculty of Tech. & Engg., M.S.U., Vadodara–390001, India.
E-mail: [email protected]
ABSTRACT: Sometimes it is difficult to find an ideal place for construction purpose, but if it cannot be avoided, than, such
cases necessitates use of various ground improvement techniques. Electrokinetic grouting is one such process that has shown
a great potential for remediating soil with low shear strengths as well as low permeability and strengthening of deep seated
soil layers under the existing structure, using combined effect of electric, chemical and hydraulic potential for treatment. The
work presented in this paper is aimed for understanding various factors that play a role in grouting various types of soils using
electrokinetics such as soil composition, grout concentration and voltage gradient using laboratory scale models. Maximum
increase in strength and decrease in permeability was observed for grout with 25% sodium silicate. The study of effect of soil
composition showed maximum percentage improvement in shear strength for black cotton soil samples.
1. INTRODUCTION
If one goes to see, a very small part of the total available land
is available for erection of structures and for a country with a
population of over 100 million, land area is becoming scarce
day by day. So a need arises that one should utilize the lands
for construction; residential and industrial mainly; which
were considered unfit for structural erections and were rejected
earlier. Moreover, the numerous existing structures are also
needed to be treated for their foundation problems such as low
bearing capacity of soil, etc. as they were built on soil with
poor geotechnical properties. For this purpose, stabilization of
soil needs to be carried out. Although many different in-situ
soil stabilization techniques are available, electrokinetics offers
a number of advantages over other methods. Electrokinetics
is a process that has shown a great potential for remediating
soil with low shear strengths and strengthening of deep
seated soil layers under the existing structure.
the chemical grout chambers and sand filter chambers. Two
10 mm diameter and 30 cm long carbon electrodes were
placed in each anode and cathode compartments, connected
in series by copper wire, and finally connected to an AC-DC
converter unit (Fig. 1).
Table 1: Property of Various Soils
Black
Properties
Silty soil
Sandy silt
cotton soil
Specific gravity
2.59
2.58
2.72
Liquid limit (%)
28.00
59.70
Plastic limit (%)
24.67
38.24
NP
Shrinkage limit (%) 23.00
11.64
Free swell (%)
10
75
NS
OMC (%)
13.80
20.50
14.00
MDD (gm/cc)
1.76
1.59
2.00
Coefficient of
1.38 ×10–6 1.283 × 10–7 1.979 × 10–2
Permeability (cm/s)
Sodium Content
40.70
45.80
54.00
(mg/kg)
2. LABORATORY EXPERIMENTS
The present investigation was performed on three different
soils; the first was procured from the Institute (Silty soil), the
second soil procured from Karjan (Black cotton soil) whereas
the third sample was procured from Hazira, Surat (Sandy
Silt) (Table 1). Three laboratory scale models were prepared
using 10 mm waterproof plywood sheet having dimensions
as 60 cm length, 30 cm width and 20 cm height. PVC
flooring material was used to prevent the wood from being
corroded and soiled due to various chemicals to be used.
Acrylic sheet 1.5 mm thick was used to create partitions for
The soil mass was compacted in the models using hand
compaction at 85% of their respective maximum dry
densities keeping the moisture content on wet side of OMC.
Duration of all tests was kept as 35 days. The voltage was
kept 25 volts DC and concentration of sodium silicate
(Na2SiO3) at cathode was kept 25% and concentration of
calcium chloride (CaCl2) at anode was kept 2%. Readings for
voltage, current, pH, ambient temperature, temperature at
392
Effect of Soil Composition on Electrokinetic Grouting
surface and at depth etc were being noted at regular interval
of time.
A.C.-D.C. converter
Anode compartment
filled with calcium
chloride
10.0
Graphite electrode
Sand filter
Graphite electrode
Sand filter
Cathode
compartment filled
with sodium silicate
5.0 5.0
Figure 4 denotes the variation of voltage at different sections
along the length of the sample. During the first two days,
high voltage was noted. But then it gradually started to
decrease and thus stabilized after two weeks which suggest
that the resistance of the mass has almost achieved a constant
value suggesting homogenous flow of grout in the soil.
Figure 5 depicts variation of current and current density with
respect to time. The current density was unstable during the
first week, but then stabilized and remained constant again
suggesting uniform flow of grout.
Fig. 1: Schematic Diagram of Laboratory Model
14
12
Voltage in Volts
3. RESULTS
3.1 Silty Soil
Figure 2 shows variation of pH along the length of the soil
sample noted near anode, at mid-section and near cathode for
silty soil sample. During the initial days, pH shows values on
the alkaline side reducing as time progresses to a minimum
value of 8 but increases again when the consumption of the
solution increased after the second week. Then it stabilizes
and remains more or less around 11 throughout the
experiment. Reading for temperature along the length of the
sample as well as ambient temperature was noted at a fixed
time every day (Fig. 3).
10
8
6
4
2
0
0
3
6
9 12 15 18 21 24 27 30 33 36 39
Number of days
Fig. 4: Variation of Voltage at Different Sections along
Length of Sample
Number of days
Fig. 2: Variation of pH along the Length of Sample
0.4
Current Density
0.35
0.3
Current
0.25
0.2
0.15
0.1
0.05
Current (in Amp)
pH
Current Density (in Amp/sq.cm)
0.45
0
1
4
7 10 13 16 19 22 25 28 31 34
Temperature in Degree C
Number of days
Fig. 5: Variation of Current and Current Density
Time in days
Fig. 3: Variation of Temperature at Surface and Ambient
Temperature with Time
The shear strength of the virgin soil with same moisture
content was 0.088 kg/cm2 before the start of the
electrokinetic grouting experiment. It started increasing
because of gel formation of chemical grout into the soil,
flowing due to electro-osmosis effect under the influence of
applied electric field. The average shear strength was noted 3
times more than the original strength after 35 days. Figure 6
shows the variation in shear strength of soil along the cross
section of the model with increase in duration of experiment.
The maximum shear strength obtained was at the cathode
section, measured as 0.335 kg/cm2.
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Effect of Soil Composition on Electrokinetic Grouting
Virgin soil
7 days
14 days
21 days
28 days
35 days
comparison for permeability which shows noticeable
reduction in permeability of soil due to grouting.
Permeability reduction is more at mid section than at anode
and cathode similar to sandy soil as mentioned above, with
average reduction of 81%.
0.3
0.2
0.20
Shear strength in kg/sq cm2
Shear strength in kg/sq cm
0.4
0.1
0.0
near anode
at centre
near cathode
Fig. 6: UCS of Silty Soil at Various Time Duration
(Silty Soil)
Three undisturbed samples collected from near anode, at mid
section and near cathode were tested for permeability. Figure
7 compares permeability of soil after electrokinetic grouting
experiment with that of virgin soil. It showed noticeable
reduction in permeability of soil due to grouting.
Permeability reduction is more at mid section than near
anode and near cathode. On an average, there was 76%
reduction in permeability. This result supports flow of
calcium chloride and sodium silicate into the soil and
formation of gel due to applied electric potential.
7 days
28 days
14 days
35 days
0.15
0.10
0.05
0.00
near anode
at centre
near cathode
Fig. 8: UCS of Silty Soil at Various Time Duration
(Sandy Silt)
2.5 E-2
Coefficient of Permeability, cm/sec
Coefficient of Permeability, cm/sec
1.8E-6
Virgin soil
21 days
Virgin soil
1.6E-6
1.4E-6
1.2E-6
1.0E-6
8.0E-7
6.0E-7
Virgin soil
2.0E-2
1.5E-2
1.0E-2
5.0E-3
0.0E+0
4.0E-7
After Electrokinetic Grouting
near anode
at centre
near cathode
Fig. 9: Permeability of Soil Before and After Electrokinetic
Grouting (Sandy Silt)
2.0E-7
0.0E+0
near anode
at centre
near cathode
3.3 Black Cotton Soil
Fig. 7: Permeability of Soil Before and After Electrokinetic
Grouting (Silty Soil)
3.2 Sandy Silt
Figure 8 shows comparison of shear strength of sandy silt at
various intervals of time. It shows nominal increase in
strength at the end of the first week, but increased gradually,
with the maximum shear strength showing about 72%
increase with an average value of 63%. Figure 9 shows
The shear strength of black cotton soil shows a considerable
increase after electrokinetic grouting with a major part of it
being gained in the last two weeks, with an average increase
of about 11 times (Fig. 10). The comparison of permeability
of soil before and after electrokinetic grouting experiment is
shown in Figure 11. Average decrease in the initial
permeability of soil was found to be 24.41%. Here the
overall decrease in permeability obtained was less compared
to the other two experiments. (Fig. 11)
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Effect of Soil Composition on Electrokinetic Grouting
7 days
28 days
14 days
35 days
Shear strength in kg/sq cm2
Virgin soil
21 days
Fig. 12: Comparison of UCS for Different Soils
near anode
at centre
near cathode
Fig. 10: UCS of Silty Soil at Various Time Duration
(Black Cotton Soil)
After Electrokinetic
Electrokinetic Grouting
Grouting
After
Fig. 13: Comparison of Permeability for Different Soils
Coefficient of Permeability, cm/sec
Virgin soil
Fig. 14: Comparison of Sodium Content for Different Soil
after Test
4. CONCLUSIONS
near anode
at centre
near cathode
Fig. 11: Permeability of Soil Before and After Electrokinetic
Grouting (Black Cotton Soil)
Figure 12 shows the comparison of UCS for all three soils
selected for the study, the maximum increase in the shear
strength was observed when black cotton soil was treated
with electrokinetic grouting technique. By comparison, 75%
of the total increase in shear strength was in black cotton soil
compared to other two soils i.e. silty soil and sandy silt. The
minimum increase in shear strength (0.6 times) which was
4% by comparison, was noted for sandy silt.
The comparison of permeability for the same is shown in
Figure 13. The maximum decrease in the permeability was
observed for sandy silt soil which is 81.14% average and
45% compared to the other two soils. The minimum decrease
in permeability comparatively was noted when black cotton
soil was treated with the electrokinetic grouting technique
which was 24.41% in average and 13% comparatively. This
may be due to the highly expansive nature of the black cotton
soil. Figure 14 shows the comparison of the sodium content
which has increased many folds indicating proper grouting of
all soils used.
It can be concluded from the work that electrokinetic
grouting is very effective method for grouting soils with very
low permeability. It is also seen that it work more efficiently
when used for clayey types of soils. The strength increase is
maximum when the selected grout mix used with black
cotton soil. The decrease in permeability for black cotton soil
is relatively less since the permeability before test itself is
very low. Fine silty sands can be also easily grouted with a
considerable decrease in their permeability as well as
appreciable increase in the strength.
REFERENCES
Alshawabkeh A.N. and Açar Y.B. (1996). “Electrokinetic
Remediation II: Theoretical Model”, ASCE Journal of
Geotechnical Engineering, 122(3): 186–196.
Esrig M.I. and Gemienhardt J.P. (1967). “Electrokinetic
Stabilization of an Illite Clay”, Journal of the Soil
Mechanics and Foundations Division, Proceedings of the
American Society of Civil Engineers, 93(SM3): 109–128.
Lo K.Y., Inculet I.I. and Ho K.S. (1991). “Electroosmotic
Strengthening of Soft Sensitive Clays”, Canadian
Geotechnical Journal, 28: 62–73.
Micic S., Shang J.Q. and Lo K.Y. (2001). “Electrokinetic
strengthening of a Marine Sediment Using Intermittent
Current”, Canadian Geotechnical Journal, 38: 287–302.
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