improving strength of soil

7/28/2019 improving strength of soil

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PRESENTATION OUTLINE


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ABSTRACT OF RESEARCH PAPER

In order to improve the strength of soil many methods are incorporated

Including fiber reinforcement, cementious material or mixing other soil.

All of the above mentioned method need research related to the test of

Soil strength and its behavior under high stress.

Unfortunately there are limited number of test conducted on

mechanically improved soil under low confining pressure and the soil

behavior is not fully understood.

The aim of this research paper is to test fiber reinforced cementious soil

under high confining pressure of 20 MPa using GDS triaxial test system

and study its mechanical behavior .


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Material and Equipment

Portaway sand was fiber reinforced and cemented and test using GDS

High pressure triaxial testing system under confining pressure of 1,4,10,20MPa both un drained and drained test have been conducted.

Discrete monofilament polypropylene fibres were used to strengthen

soil along with ordinary Portland cement. Their characteristics were

22 mm in length, 0.023 mm in diameter, specific gravity of 0.91, tensile

strength and elastic modulus of 120 and 3,000 MN/m2 respectively,

and linear strain of 80% at failure.

Standard methods were used to prepare sample of size 50mm diameter

And length of 100mm.


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Microphotographs of Portaway sand


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Drained behaviour at high pressures:-

a) Effect of fibre and cement on sand

b) Effect of confining pressure

c) Effect of fibre and cement on shear strength parameters


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A) Effect of fibre and cement on sand

Effect of addition of 0.5% fibre

and 5% cement at 4MPa

The Graph is between deviator

stress and axial strain.

%

1. Addition of fiber has

increased peak deviator

stress for both clean sandand cemented sand

2. Cement also plays important

roll since it also increases

deviator stress more than

fiber effect.

0

4

8

12

q(MPa)

s '3 = 4 MPa

Sand, Cement & FibreSand & CementSand & FibreSand only

(a)

0 10 20 30


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0

0.4

0.8

1.2

1.6

q/p'

(b)

s

Sand, Cement & Fibre

Sand & CementSand & FibreSand only

%

The Graph is between deviator

stress ratio and axial strain.

s '3 = 4 MPa

1. At 4MPa confining pressure

graph lines tends to

converge on unique stressratio of 1.3.

2. Cement also plays important

roll since it also increases

deviator stress more than

fiber effect.

Effect of addition of 0.5% fibreand 5% cement at 4MPa

0 10 20 30


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The Graph is between volumetric

strain and axial strain.

Effect of addition of 0.5% fibre

and 5% cement at 4MPa

0

5

10

15

0 10 20 30

v

(%)

(c)

%

1. Fiber also reduces

volumetric strain and graph

lines gets more flatter.2. Cement also plays important

roll since it also reduces

volumetric strain more than

fiber effect.

s '3 = 4 MPa

s

Sand, Cement & FibreSand & CementSand & FibreSand only


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0

50

100

150

200

250

0 5 10 15 20

increa

seqmax

(%)

p' (MPa)

% increase in sand with fibre & cement

% increase in sand with f ibre


The Graph is between increase

in qmax(%) and confiningpressure.

1. The effect of fiber decreaseswith increase in the confining

pressure. At low confining

press the effect of fiber is

more prominent.


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B) Effect of confining pressure

0 10 20 30 40a (%)

0

10

20

30

q(Mpa)

Fibre reinforced cemented sand

10MPa

4MPa

1MPa

(a)

0 10 20 30 40a (%)

0

10

20

30

q(Mpa)

Fibre reinforced sand

10MPa

4MPa

1MPa

(a)

1. Both graphs show that addition of fiber in cemented soil sampleincrease the peak deviator stress and delays axial strain. Similar

effect of cement addition in soil samples can also be observed.


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0

0.4

0.8

1.2

1.6

2

q/p'

(b)

0 10 20 30 40a (%)

10MPa

4MPa

1MPa


0

0.4

0.8

1.2

1.6

2

q/p'

(b)

0 10 20 30 40a (%)


10MPa

4MPa

1MPa

1. Both graphs show that addition of fiber in cemented soil sampleincrease the peak deviator stress and delays axial strain. Similar

effect of cement addition in soil samples can also be observed. For

fiber reinforced soil the graph lines tend to intersect at unique

stress ratio of 1.3.


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C) Effect of fibre and cement on shear strength parameters

25

35

45

55

65

75

0 5 10 15 20

max

p' (MPa)

Sand with cement & fibre

Sand with cement

Sand with fibreSand only

25

35

45

55

65

75

0 200 400 600

max

p' (kPa)

Sand with cement & f ibre

sand with cement

Sand with fibre

Sand only

1. Graph on left shows that increase in confining pressure diminishesthe effect of fiber on friction angle .It can be observed on right

graph that cement in clean soil show more shift in graph then fiber

addition.


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C) Effect of fibre and cement on shear strength parameters

0

10

20

30

40

50

0 10 20 30 40

q(MPa)

p' (MPa)

sand with fibre & cement

Sand with fibre

Sand only

0

500

1000

1500

2000

0 200 400 600 800 1000

q(kPa)

p' (kPa)

Poly. (Sand with fibre & cement)Poly. (Sand with fibre)Poly. (Sand only)

1. Graph on left shows that failure envelops of all sample followssame path .It can be observed on right graph that at low confining

pressure fiber addition shifts the graph line upwards to form

cohesion intercept. This cohesion in soil is due to cement addition.


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Un drained behaviour at high pressures:-

a) Effect of fibre and cement on sand

b) Effect of confining pressure


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0

4

8

12

16

20

0 10 20 30 40

q(MPa)

a %


20MPa 10MPa

4MPa 1MPa

(a)

1. At low confining pressure the soil sample shows gradual

behavior. This behavior is effect by addition of fiber and

cement.

2. At high confining pressure the soil sample shows brittle

behavior by delaying axial strain. This behavior is effect by

addition of fiber and cement

0

2

4

6

8

0 10 20 30 40

q(MPa)

a %


1MPa

4MPa

10MPa

(a)


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0

4

8

12

16

20

0 5 10 15 20 25

q(MPa)

p' (MPa)

(c)


0

2

4

6

8

0 2 4 6 8 10 12

q(MPa)

p' (MPa)

FL

(c)

20MPa 10MPa

4MPa 1MPa



1. Fiber addition to sample increases the strength of soil and

shifts the failure envelop graph upwards.

1MPa

4MPa

10MPa


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0

10

20

30

40

50

0 10 20 30 40

q(MPa)

p' (MPa)

sand with fibre & cement

Sand with fibreSand only

0

4

8

12

16

20

0 5 10 15 20 25

q(MPa)

p' (MPa)

(c)


drained tests

ndrained tests

1. Both drained and un drained test shows that stress path ofdrained test is crossed by soil test in un drained conditions

proving that soil can attain more strength in un drained

conditions.


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Conclusion

1. Both drained and un drained test shows that addition of fiber

increases the strength and increases the friction angle.2. Both test shows that cement plays an important role in

producing cohesion in soil and its strength.

3. More research is recommended for this field as complete

behavior of soil fiber interaction is not understood well.

4. Arrangement of fibers in soil structure also plays an importantroll.

5. Also other soil of good strength can be mix in proportion to

improve the target soil.

6. It was observed that at confining pressure of 10MPa the

effectiveness of fiber was lost where as the cement plays its roll

with minor effects.

improving strength of soil

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