Unstable were carried out on specimens of 50 mm

Unstable zone of sand-silt mixture using static
triaxial tests

N. J. Sahare

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Post Graduate Student, (Geotechnical Engineering Department), College of
engineering Pune-411005, Maharashtra, India.

 Email: [email protected],  Contact
Details- 9404824292

N. T. Chaudhari

Post Graduate Student, (Geotechnical Engineering Department), College of
engineering Pune-411005, Maharashtra, India.

Abstract

Present
work deals with the identification of unstable zone of sand with fines content
using static triaxial tests. A series of undrained monotonic triaxial
compression tests were conducted on saturated samples of clean sand with
variation in silt content as 0 %, 15 %, 25 % and 35 %. Total 24 tests were carried
out on specimens of 50 mm in diameter and 100 mm in height with three different
confining pressures of 60 kPa, 120 kPa and 240 kPa. The specimens were prepared
at 30 % and 50 % relative densities using moist tamping method of sample
preparation. It was observed that the limiting fines content and relative
density played an important role in deciding the undrained behavior of a
mixture of sand and silt. Furthermore, it was observed that liquefaction
resistance of soil decreased with an increase in silt content until the
limiting fines content was reached. However, a further increase in the silt
content beyond the limiting silt content increased the liquefaction resistance.

Keywords:- sand-silt mixture, liquefaction,
triaxial test, limiting fines content, unstable zone.

1  Introduction

Instability
has been observed to occur for saturated loose sand under undrained conditions
(Lade andPradel, 1990; Leong et al., 2000) and for saturated
medium to dense sand under strain-controlled conditions (Chu, 1991; Chu et al.,
1993; Chu & Leong, 2001). The term instability refers to a behaviour in
which large plastic strains are generated rapidly owing to the inability of a
soil element to sustain a given load or stress. In recent years, instability
has been considered as one of the failure mechanisms that lead to flow slides
or collapse of granular soil slopes in a number of case studies (e.g. Kraft et
al., 1992; Lade, 1993; Hight et al., 1999; Olson et al., 2000). It has been
established by Lade & Pradel (1990) that instability occurs when the stress
ratio at the onset of instability is above the instability line.

The
behavior of silty sandy soils such as hydraulic fills, landfills or alluvial
deposits not clearly known during earthquake. Therefore, a thorough understanding
of unstable behavior of silty sand is needed. The main focus of the present
work is to conduct static triaxial tests to study undrained behavior of sand
silt mixture. Isotropically consolidated undrained triaxial tests are performed
on 50 mm × 100 mm sample size with varying fines content. Tests are conducted
at two different relative densities at three confining pressures.

2  Experimental Investigation

The clean sand used in these
experiments was silica
sand obtained locally and has been classified as SP according
to the unified soil classification system (USCS). Silt used in this study is non plastic and obtained from quarry dust.
Figure 1 indicates grain size distribution curves of clean sand-silt and
various sand-silt mixture. The index properties of clean sand and silt are
shown in Table 1.

Table 1: Index properties of sand and silt

 

G

D50

?max

?min

emax

emin

Cu

Cc

Sand

2.416

0.28

15.16

13.98

0.745

0.609

2.24

0.85

Silt

2.751

0.06151

15.70

12.65

1.175

0.753

3.6714

1.1633

 

The tests are conducted on clean sand and mixture of sand-silt at 30% and 50% relative densities for three confining pressures of 60 kPa, 120 kPa and 240 kPa with varying silt content as 15%, 25% and 35%. Total twenty four tests were
conducted on 50 mm ×100 mm sample size where 50 mm is diameter and 100 mm is
the height of the sample (H/D = 2). Detail test program is as shown in Table 2.

Fig. 1: Soil gradation curve

Table 2: Test program

Soil

Fines
Content (%)

Relative
Density (%)

Confining
Pressure (kPa)

No. of tests

Sand

0

30

60,
120, 240

3

50

60,
120, 240

3

15

30

60,
120, 240

3

50

60,
120, 240

3

25

30

60,
120, 240

3

50

60,
120, 240

3

35

30

60,
120, 240

3

50

60,
120, 240

3

 

 

 

Total

24

 

Moist tamping method was
adopted to prepare a sample of clean sand and mixture of sand-silt to perform
triaxial tests on the cylindrical specimen of size 50 mm × 100 mm. In this
method, a known quantity of soil for achieving particular density was mixed
with 5 % of water by weight added in soil and placed in a split mould in
layers. In order to achieve particular density tamping was done for each layer
with the help of a hammer. Fig.2 shows details of triaxial test instrument.

3  Results and Discussion

3.1 Clean sand specimen

Fig. 3 shows the
deviator stress-strain behaviour of clean sand for 60 kPa, 120 kPa and 240 kPa
confining pressures for 30% relative density. It was observed that for clean
sand specimen, stress-strain relation becomes curved at very small strains and
achieve a peak at a strain of about 3% for effective confining pressure 120 and
240kPa. The resistance of soil then gradually decreases until this test was
arbitrarily stopped at a strain of 20%. It is also seen that as the confining
pressure increases from 60 kPa to 240 kPa peak value of deviator stress
increases from 66.9993 to 262.1799 kPa and steady state has been achieved at
large percentage of strain (20% strain).

Fig. 3: Deviator stress vs axial strain response of clean sand
specimen (Relative density = 30%)

Fig. 4: Excess pore pressure vs axial strain response of clean sand
specimen (Relative density = 30%)

Typical graph of
increase in excess pore pressure with axial strain during shearing is shown in
Fig. 4. For clean sand it is observed that peak value of pore pressure was
reached within strain of about 2% to 3% and remain constant at large percentage
of strain (about 15% axial strain) for all confining pressures. Further it is
also observed that as confining pressure increases, peak pore pressure also increases.

P’-q plot for
clean sand specimen for all confining pressures are shown in the Fig. 5. For
all confining pressures, contractive behavior has been observed.

Fig. 5: p’ vs q response of clean sand specimen
(Relative density = 30%)

Similar observation have been observed for specimens
of clean sand with relative density 50%.

3.2 Specimen of
mixture of sand-silt

Fig. 6 shows
deviator stress vs axial strain of specimen of sand with 15% silt content
prepared at relative density of 30% and for all confining pressures. It is
observed that deviator stress increases as confining pressure increases from 60
kPa to 240 kPa. However peak value of deviator stress was varied from 62.0795
kPa to 116.6601 kPa. All the specimens achieve steady state at the large strain.

Typical graph of
increase in excess pore pressure with axial strain during shearing for sand
with 15 5 silt content is shown in Fig.7. It is observed that peak value of
pore pressure was reached at a strain of about 5% and remain constant at large
percentage of strain for all confining pressure.

Fig. 6: Deviator stress vs axial strain response of specimen of sand
with 15% silt (Relative density = 30%)

Fig. 7: Excess pore pressure vs axial strain response of specimen of
sand with 15% silt (Relative density = 30%)

P’-q for sand
with 15% silt content prepared at relative density of 30%for all confining
pressure is as shown in Fig. 8. It is observed that all samples show
contractive behavior. Excess pore pressure developed in sand with 15% silt is
higher than that of clean sand, whereas peak deviator stress developed in sand
with 15% silt is less than that of clean sand.

Fig. 8: p’ vs q response of specimen of sand with
15% silt (Relative density = 30%)

Similar observations were made
for 50 % relative density of sand with 15% siltas well as for loose and medium
dense specimens of sand with varying silt content of 25% and 35%.

3.3 Effect of
confining pressure

The variations
in peak deviator stress and excess pore pressure generation with increase in confining
pressure for clean sand as well as sand with silt content of 15%, 25% and 35%.
It is observed that as the confining pressure increases deviator stress as well
as peak pore pressure increases for all the cases studied.

Similar behaviour is observed for specimens prepared with 50 % relative
density. However
peak value of deviator stress for 50 5 relative density is higher than that of
30 % relative density.

3.4 Effect of
silt content

Small fines content can affect undrained behaviour of clean sand
considerably, therefore study is needed to check the effect of fines content on
undrained behaviour. The effect of fines content on peak deviator stress of
specimens prepared at loose and medium dense state (i.e. relative density 30 and 50%) with increase in silt content from 15%, 25% and 35%
at various effective confining pressures is as shown in Figs. 9 and 10.

As the fines content increase, peak deviator stress
decreases up to limiting fines content after that peak deviator stress
increases. Addition of fines reduces value of peak deviator stress
before limiting fines content because fines do not participate in capacity of
carrying load. Addition of fines beyond the point of limiting fines content
changes behaviour of soil from sand dominated to silt dominated. For
maintaining same relative density, soil needs to be compacted densely in
specimen with fines content more than limiting fines content. So, load carrying
capacity increases for sand with 35% silt content. Similar behaviour is
observed for specimens prepared at medium dense state (50 %). (Fig. 10).

Fig. 9: Peak deviator stress vs silt content (Relative density = 30%)

Fig. 10: Peak deviator stress vs silt content (Relative density = 50%)

3.5 Residual
strength

In order to
check slope stability of liquefied soil masses knowledge of residual strength
is important. When loose and medium dense sandy soils are subjected to
undrained loading beyond the point of peak strength, the undrained shear
strength declines to near constant value over large deformation. Conventionally
this  strength is called the undrained
residual strength. Castro and Polous (1985) determined the
residual strength of silty sand using following equation:

                 …..eq 1               Where,     Su :
residual strength

                  …..eq 2               
               ?s : inter granular 
friction angle

              …..eq 3                                qs  : deviator stress (at 20% strain)

3.6 Unstable
zone

In order to classify the liquefaction behavior of soil,
Pathak and Dalvi (2011) have established unstable zone plotted between Kf
line and peak pore pressure line on effective stress path plot. Unstable zone
has been obtained by plotting effective stress path for all three confining
pressures for each relative density for each percentage of fines content. Kf
line is plotted using p’ and q values corresponding to peak deviator stress
valueand line is joined with origin. p’ and q values are
calculated by peak effective major principal stress (?1′) and corresponding
effective minor principal stress (?3′) using following equations:

q  =  ?1′
– ?3′                                 …..eq 4                                           Where,   ?1′ :
effective major principal stress

p’ =                                  …..eq 5                                                            ?3′ : effective minor principal stress

Peak pore
pressure line is plotted using p’ and q values corresponding to  peak pore pressure point.

Unstable zone for clean sand and clean sand with 15%, 25% and 35% silt is shown in Fig. 11(a),(b), (c) and (d)
respectively. It is observed that as fines content increase upto limiting fines
content,
unstable zone becomes wider. Further increase in fines content
beyond limiting fines content, narrows down the unstable zone. This may be an
effect of excess pore water pressure, as fines content increased to limiting
fines content (25
%) excess pore water pressure increases.

(a)

(b)

(c)

(d)

Fig. 11: Variation of unstable zone with increase in fines content at
relative density 30%

Similar trend of unstable zone
obtained for specimen prepared at relative density 50% as that at relative
density 30%.

4  Conclusion

From the present study following conclusions are made:

As the confining pressure increases
peak deviator stress and excess pore water pressure increases for all the
specimens.Peak deviator stress increases as
the relative density increases at all the confining pressures.For both loose and medium dense
specimens, peak deviator stress decreases as fines content increases up to
limiting fines content. Further increase in fines content increases the
peak deviator stress value.As the confining pressure
increases, residual strength also increases.Residual strength decreases as
fines content increases up to limiting fines content and after that it
increases as fines content increase.Unstable zone of sand-silt mixture
widens as fines content increases up to limiting fines content. Further
increase in fines content narrows down the unstable zone for both loose
and medium dense specimens.

5  References

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