IGC 2009,
Guntur,
INDIA of Expansive Clay-Sand Blends
Volume
Change
Behaviour
VOLUME CHANGE BEHAVIOUR OF EXPANSIVE CLAY-SAND BLENDS
B.R. Phanikumar
Professor, Department of Civil Engineering, VIT University, Vellore–632 014, India.
E-mail: [email protected]
C. Amshumalini
Research Student, Department of Civil Engineering, VIT University, Vellore–632 014, India.
E-mail: [email protected]
R. Karthika
Research Student, Department of Civil Engineering, VIT University, Vellore–632 014, India.
E-mail: [email protected]
ABSTRACT: Expansive soils are highly problematic because they undergo detrimental volumetric changes corresponding
changes in moisture regime. They swell when they absorb water and shrink when water evaporates from them. As a result,
civil engineering infrastructure is severely cracked. Of various innovative foundation techniques, chemical stabilization and
blending expansive clay with non-expansive material have also been found to have met with success. This paper presents
swell-consolidation behaviour of artificially prepared sand-expansive clay mixtures, wherein fine sand was used in the blends
at different contents. The fines content in the expansive clay was also varied. Swell potential, swelling pressure, compression
index and co-efficient of consolidation were studied.
compression index, compressibility and volume compressibility
were determined.
1. INTRODUCTION
1.1 Problems with Expansive Soils
Expansive soils pose the problem of swelling on absorption
of water during monsoon and shrinkage on evaporation of water
in summer (Chen 1988, McKeen 1988, Nelson & Miller
1992, Kenneth 1993). As a result of the swell-shrink behaviour
of expansive soils, lightly loaded structures such as foundations,
pavements, canal beds, and linings and residential buildings
founded in them are severely damaged (Chen 1988). The
annual cost of damage to civil engineering structures is
estimated at $1000 million in the USA, £150 million in the UK,
and many billions of pounds worldwide (Gourley et al. 1993).
2. EXPERIMENTAL INVESTIGATION
2.1 Test Materials
Remoulded expansive clay and fine sand was the test materials
used in the investigation. The soil was collected from
Amalapuram, Andhra Pradesh. It is a highly swelling soil
with a Free Swell Index (FSI) of 180%. It was a CH soil as
per USCS classification. Table 1 shows the index properties
of the expansive soil.
Table 1: Index Properties of Expansive Soil
1.2 Innovative Foundation Techniques
Several innovative foundation techniques have been suggested
for counteracting the problem of heave and shrinkage of
expansive soils, which include sand cushion technique
(Satyanarayana 1966), under-reamed piles (Sharma et al., nd
granular pile-anchors (Phanikumar 1997, 1978) a Phanikumar
et al. 2004). Belled piers, lime-slurry pressure injection and
geomembranes as moisture barriers are also among the
innovative techniques in practice (Chen 1988). Apart from
these techniques, stabilization of expansive soils with various
additives including fly ash, lime, cement and calcium chloride
(Sankar 1989, Desai and Oza 1997, Hunter 1988, Phanikumar
& Sharma 2004) has also met with considerable success.
Specific gravity
Liquid limit (%)
Plastic limit
Plasticity index
This paper presents results of laboratory tests performed on
artificially prepared expansive soil-sand mixtures. Swell
potential, swelling pressure and compression characteristics like
2.72
100
27
73
Gravel (%) (>6.20–4.75 mm)
0
Sand (%) (4.75–0.075 mm)
7
Silt (%) (0.075–0.002 mm)
24
Clay (%) (<0.002 mm)
69
Free Swell Index (FSI)
250
USCS classification
CH
Fine sand, used as the blend material, had the particle size
between 0.075 mm and 0.425 mm.
84
Volume Change Behaviour of Expansive Clay-Sand Blends
void ratio at the end of the process of swelling was different
for different specimens having varying sand content. The
equilibrium void ratio was the highest for the sample having
0% sand content or the unblended sample. However, the
equilibrium void ratio decreased with increase in sand content,
indicating that swelling or volume increase decreased with
increasing sand content. The amount of fine sand used in
different samples replaced the particles of swelling clay and
thus reduced expansion or volume increase of clay-sand blends.
Hence, the equilibrium void ratio of sample having 20% sand
content starting with the same initial void ratio was the lowest.
Thus the e-log p curves got shifted to lower positions as the
fine sand content in the blend increased. This means that the
amount of heave ∆H or swell potential ((∆H/H)  100)
undergone by the specimen reduced with increase in sand
content. Swell potential written in the form of the following
equation was determined according to the definition given by
Jennings in 1963.
2.2 Tests Conducted and Variables Used
1-D swell-consolidation tests were conducted on the sandclay blends by free swell method. Swell potential, swelling
pressure, compression index and coefficient of consolidation
were determined.
The dry unit weight of the expansive soil was kept constant
at 12 kN/m3. Oven-dry soil passing 425 µm and 75 µm sieve
was used for the preparation of the test specimens, which
meant that the initial water content of the specimens was 0%.
Oven-dry samples were used in the testing in order to obtain
measurable swell potential and swelling pressure. Fine sand
content was varied as 0%, 10%, 20% and 30% by dry weight
of the expansive soil.
2.3 Preparation of Test Specimens and Test Procedure
Oven-dry soil passing 425 µm sieve and 75 µm sieve was
weighed corresponding to the prefixed dry unit weight of 12
kN/m3 based on the volume of the consolidometer ring (dia =
60 mm and height = 20 mm). Fine sand was also weighed
according to its content in the blend and mixed with the
expansive soil thoroughly. The blend was statically compacted
into the consolidometer ring in four layers, each layer having
a thickness of 5 mm. A thin layer of silicone grease was applied
to the inner wall of the consolidometer ring to minimize friction
between the swelling soil and the consolidometer ring. The
compacted blend specimen was sandwiched between two filter
papers and porous discs.
It is written as,
S = ((∆H/H)  100)
(1)
Figure 2 shows the variation of swell potential with sand
content. Swell potential decreased significantly with increase
in sand content.
1.6
1.5
The consolidometer was arranged in position and loading
hanger was placed upon the specimen. A seating load of 5 kPa
was applied on the specimen. Swelling was allowed in the blend
by free swell method, wherein the sample was inundated with
water. Heave was monitored with a dial gauge corresponding
to time intervals of 0, ½, 1, 2, 5, 10, 20, 30, 60 and 120 minutes
and thereafter every 24 hours from the time of inundation. When
equilibrium heave was reached by the specimen, compressive
load increments were applied on the specimen to bring the
swollen sample to its initial void ratio. Equilibrium heave was
understood to have reached when there was no further
movement in the dial gauge. The sample was consolidated under
load increments of 10 kPa, 20 kPa, 40 kPa, 80 kPa, 160 kPa and
320 kPa. At the end of consolidation, the sample was removed
and water content determined.
e
1.4
1.3
1.2
1.1
1
1
10
Fig.
log p
100
1000
1. e - log
curves
1:Fige-log
ppCurves
20
Fines passing 425
micron
Swell potential (%)
16
3. DISCUSSION OF TEST RESULTS
Figure 1 shows the e-log p curves of the expansive clay (425
µm)-sand blends for varying sand content (0%, 10%, 20%
and 30%). Similar e-log p curves were obtained in the case of
75 µm fraction also, but they are not being presented here.
Fines passing 75
micron
12
8
4
0
As the initial dry unit weight of the specimens was kept
constant at 12 kN/m3, the initial void ratio (eo) of all the
specimens was the same at 1.21. Though all the specimens had
an initial surcharge pressure of 5 kPa, the final or equilibrium
0
5
10
15
20
25
30
35
Sand content (%)
Fig 2. Variation
of swell
potential with
sand
content
Fig. 2: Variation
of Swell
Potential
with
Sand
Content
85
Volume Change Behaviour of Expansive Clay-Sand Blends
8
Swelling pressure (ps) is defined as the pressure required to
bring back a completely swollen soil specimen to its initial
void ratio (eo). This can be obtained from the e-log p curves
as the pressure corresponding to the initial void ratio.
Swelling pressure of clay sand blends was determined from
the e-log p curves shown in Figure 1 by drawing a horizontal
line from the point corresponding to the initial void ratio.
Swelling pressure clay-sand blends having a sand content of
0%, 10%, 20% and 30% were found to be respectively equal
to 230 kPa, 150 kPa, 110 kPa and 75 kPa when the fines
content in the blend was passing through 425 µm sieve.
7.5
-4
7
mv x 10
Coefficient of volume compressibility,
As the sand content in the blends was decreased from 0% to
30%, the swell potential decreased by 71% and 50%
respectively in blends containing 425 µm and 75 µm fractions.
This suggests that physical alteration of expansive clay soils
blending them with non-swelling sandy materials significantly
reduces their swelling nature.
6.5
6
5.5
5
0
5
10
15
20
25
30
Sand content (%)
Fig 5. Influence of sand content (%) on coefficient of volume
compressibility, mv
35
Fig. 4: Influence of Sand Content (%) on Coefficient of
Volume Compressibility, mv
0.52
Figure 3 shows the variation of swelling pressure with sand
content for fines passing both 425 µm and 75 µm sieves.
Significant reduction in swelling pressure with increase in
sand content was observed in the specimens of both clay
fractions. Swell potential decreased significantly with increase
in sand content. As the sand content in the blends decreased
from 0% to 30%, swell potential decreased by 71% and 50%
respectively in blends containing 425 µm fraction and 75 µm
fractions.
0.48
0.44
Cc
0.4
0.36
0.32
0.28
0.24
0.2
300
0
5
10
15
20
25
30
35
Sand content (%)
Fig 5. Variation of C with sand content (%)
Fig. 5: Variation
of Cc with Sand Content (%)
Swelling pressure (kPa)
250
c
Fines passing 425 micron
200
Fines passing 75 micron
Cc decreased significantly with increase in sand content,
indicating that the compressibility of the blends also decreased
significantly with increase in sand content. As the sand
content increased from 0% to 30%, Cc decreased by 55%. This
shows that blending expansive clays with fine sand improves
their compressibility characteristics apart from reducing their
swelling nature.
150
100
50
0
0
5
10
15
20
25
30
35
4. CONCLUSIONS
Sand content (%)
Swell potential (S%) decreased significantly with increase in
sand content. As the sand content in the blends was decreased
from 0% to 30%, swell potential decreased by 71% for 425
µm fraction. Swelling pressure also decreased as sand content
in the blends increased. Similar observations were made in the
case of 75 µm fraction also. Coefficient of volume
compressibility (mv) also decreased with increasing sand
content. Coefficient of volume compressibility reduced by
30% as the sand content was increased from 0% to 30% in
the case of 425 µm fraction. Compression index (Cc) too
decreased significantly with increase in sand content, indicating
that the compressibility of the blends decreased significantly
with increase in sand content. As the sand content increased
from 0% to 30%, Cc decreased by 55%.
Fig 3.
of sand
contenton
on swelling
pressure
Fig. 3: Effect
ofEffect
Sand
Content
Swelling
Pressure
Figure 4 shows the influence of sand content on coefficient
of volume compressibility of the blends decreased. Coefficient
of volume compressibility (mv) also decreased moderately up
to a sand content of 20%, but decreased steeply as the sand
content was increased from 20% to 30%. Data for 425 µm
fraction alone are available.
The slope of the linear portion of the e-log p curve is called
the compression index (Cc). This indicates the amount of
compression or reduction in void ratio under a given stress
range. Figure 5 shows the variation of Cc with sand content in
the blends. Data for 425 µm fraction alone are available.
86
Volume Change Behaviour of Expansive Clay-Sand Blends
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