an experimental study of axially loaded … abstract: the aim of this experimental study is to...

1

ABSTRACT:

The aim of this experimental study is to improve the bearing capacity and reduce the settlement of strip footing, resting upon sand deposit, using Geogrid.

For this purpose, model laboratory plate load tests are performed on sand with and without multi-layers of geogrid at different depths below the footing. The load settlement characteristics for each soil-geogrid system are observed.

This paper reports the results of model plate load tests performed on axially loaded strip footing supported by multi-layered geogrid-reinforced sand. Only one type of geogrid and sand at one relative density of compaction are considered. This paper also discusses the reinforcement mechanisms using a small scale laboratory plate load test.

Key words: Shallow foundation, Strip footing, Bearing capacity, Sand bed, Settlement, Geogrid.

INTRODUCTION:

In the past half century a significant progress has been achieved in the research and application of reinforced soil earth structures. The concept of reinforced soil is based onthe existence of tensile strength of reinforcement and soil-reinforcement interaction due to frictional, interlocking and adhesion properties. It was first commercially introduced in the construction industry by French architect Henri Vidal in 1965. Since then, this technique has been widely used in geotechnical engineering practice. The reinforcing materials range from stiff metal to flexible geosynthetic materials and can be classified as either extensible reinforcements or inextensible reinforcements (McGown et al., 1978).

Since 1985, the results of a number of studies relating to theEvaluation of the ultimate bearing capacity of foundations onSand reinforced with geogrid layers have been published

(e.g. Omar et al., 1993; Adam and Collin, 1997; Yetimoglu et al.,

1994). practically all of these studies are based on small-scale Model tests conducted in the laboratory. However, all of the investigations reported so far are for surfacefoundation condition (that is, depth of foundation, Df = 0).

More recently Huang and Menq (1997) have provided a tentative guideline for estimating the ultimate bearing capacity for surface strip foundations supported by geogrid reinforced sand. This theory is primarily based on the so called “wide slab” failure mechanism in soil proposed by Schlosser et al. (1983).

The purpose of this paper is to present some recent laboratory Model test results on a strip foundation on geogridreinforced Sand with the depth of embedment Df equal to and greater than zero. The experimental results will be compared with the bearing capacity theory of Huang and Menq (1997).

REINFORCEMENT MECHANISMS:

Compared to the number of experimental studies, theoretical analysis of bearing capacity of footings on reinforced soil is relatively scarce. The proposed reinforcement mechanisms in the literature can be categorized as follows:

(1) RIGID BOUNDARY (Figure.1 (a)): If the depth to the first layer of reinforcement (u) is greater than a specific value, the reinforcement would act as a rigid boundary and the failure would occur above the reinforcement. Binquet and Lee (1975b) were the first who reported this finding. Experimental study conducted by several researches (Akinmusuru and Akinbolade, 1981; Mandal and Sah, 1992;

AN EXPERIMENTAL STUDY OF AXIALLY LOADED

STRIP FOOTING RESTING ON GEOGRID REINFORCED

SAND BED

PRATIK B. SOMAIYA,

Student (M.Tech.-Geotechnical Engineering), Dharmsinh Desai University, Nadiad.

E-mail ID: [email protected]

Dr. A.K.VERMA

Professor and head, BVM Engineering College, V.V.Nagar-388120.

E-mail ID: [email protected]

13-14 May 2011 B.V.M. Engineering College, V.V.Nagar,Gujarat,India

National Conference on Recent Trends in Engineering & Technology

2

Khing et al., 1993; Omar et al., 1993b; Ghosh et al., 2005) confirmed this finding subsequently.

(2) MEMBRANE EFFECT (Figure.1 (b)): With the applied load, the footing and soil beneath the footing move downward; the reinforcement is deformed and tensioned. Due to its stiffness, the curved reinforcement develops an upward force to support the applied load. A certain amount of settlement is needed to mobilize tensioned membrane effect and the reinforcement should have enough length and stiffness to prevent it from failing by pull-out and tension. Binquet and Lee (1975b) were perhaps the first who applied this reinforcement mechanism to develop a design method for a strip footing on reinforced sand with the simple assumption made for the shape of reinforcement after deformation. Kumar and Saran (2003) extended this method to a rectangular footing on reinforced sand.

(3) CONFINEMENT EFFECT (lateral restraint effect) (Figure.1(C)): Due to relative Displacement between soil and reinforcement, the friction force is induced at the soilreinforcement interface. Furthermore, the interlocking can be developed by the interaction of soil and geogrid. Consequently, lateral deformation or potential tensile strain of the reinforced soil is restrained. As a result, vertical deformation of soil is reduced. Since most soils are stress-dependent materials, improved lateral confinement can increase the modulus/compressive strength of soil, and thus improve the bearing capacity.

Huang and Tatsuoka (1990) substantiated this mechanism by successfully using short reinforcement having a length (L) equal to the footing width (B) to reinforce sand in their experimental study. Michalowski (2004) applied this reinforcing mechanism in the limit analysis of reinforced soil foundation and derived the formula for calculating theultimate bearing capacity of strip footings on reinforced soils.

Figure.1 Reinforcement mechanisms

LABORATORY MODEL TESTS:

The model foundation used for this study having a width of 75 mm and a length of 560 mm. It was made out of a mild steel plate with a thickness of 20 mm. The bottom of the model foundation was made rough by coating it with glue and then rolling it over sand. Bearing capacity tests wereconducted in a box measuring 0.6 m (length) × 0.6 m (width) × 0.75 m (depth) and having transparent sides.

In this tank, the sand was filled with average medium densities varying from 15.8kN/m3 to 18.5kN/m3 and relative density of compaction is 45%. The sand used for the tests had 100% passing 1.18-mm size sieve and 0% passing 0.075-mm size sieve. The average peak friction angle φ of the sand at the test conditions as determined from direct shear tests was 34°. Geogrid are used for the present tests and the physical properties of the geogrid are given in Table 1. The load was applied by manually applied loading jack having capacity 5 tones.

In conducting a model test, sand are placed in layers of 150 mm in the test box. For each layer, the amount of soil required to produce the desired unit weight are weighed and compacted using a flat bottomed wooden block. Geogrid layers are placed in the sand at desired values of u/B and h/B. The model foundation is placed on the surface of sand bed. Load to the model foundation are applied through a manually operated loading jack. The settlement of the foundation arerecorded by two dial gauges having 0.01-mm accuracy and placed on either side of the model foundation. Loads areapplied in small increments and the resulting deformations recorded so that the entire load settlement curve could be obtained. Since the length of the model foundation isapproximately the same as the width of the test box, it can be assumed that an approximate plane strain condition is exist during the tests.

Table 1. Physical Properties of the Geogrid.

NETLON GEOGRID CE121

Aperture size 8mmx8mmPolymer typeTensile strength

Polypropylene7.68kN/m

GEOMETRIC PARAMETERS:

Figure 2. Shows a strip foundation of width B on geogrid-reinforced sand. The depth of the foundation is Df. The firstLayer of geogrid is located at a depth u below the bottom of the foundation, and the distance between the consecutive layers of geogrid is h. The width of each geogrid layer is b. The depth of reinforcement is d, or

d = u + (N − 1) hWhere, N = number of geogrid layers.

… (1)



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Figure.2 Geometric parameters studied in the Laboratory Model tests.

For the present test program, the following parameters wereadopted for the geogrid reinforcement layers: u/B = 0.2, 0, 5, 0.7, h/B = 0.3, b/B =1 to 5, N =1, 2, and 3. the sequence of the model tests is given in Table 2.

Table 2. Details of Model Tests.

Test

No

u/B N b/B

1-3 0.2, 0.5, 0.7 1 1

4-6 0.2, 0.5, 0.7 1 2

7-9 0.2, 0.5, 0.7 1 3

10-12 0.2, 0.5, 0.7 1 4

13-15 0.2, 0.5, 0.7 1 5

16-18 0.2, 0.5, 0.7 2 1

19-21 0.2, 0.5, 0.7 2 2

22-24 0.2, 0.5, 0.7 2 3

25-27 0.2, 0.5, 0.7 2 4

28-30 0.2, 0.5, 0.7 2 5

31-33 0.2, 0.5, 0.7 3 1

34-36 0.2, 0.5, 0.7 3 2

37-39 0.2, 0.5, 0.7 3 3

40-42 0.2, 0.5, 0.7 3 4

42-45 0.2, 0.5, 0.7 3 5

RESULTS AND DISCUSSION:

A total of 46 tests were carried out on a model strip footing supported on geogrid-reinforced sand. The effects of geogrid parameters with unsymmetrical anchorage extension on the footing bearing load and displacement were obtained and discussed. The bearing capacity improvement of the footing on the reinforced sand is represented using a non dimensional factor, called BCR. This factor is defined as the

ratio of the footing ultimate pressure on reinforced soil qu (reinforced) to the footing ultimate pressure in tests without reinforcement (qu). The footing settlement (S) is also expressed in non dimensional form in terms of the footing width (B) as the ratio (S B, %). Footing tilt is calculated as the ratio of the difference of the two dial gauge readings to the distance between them. The ultimate bearing capacities for the footing-soil systems were determined from the load-displacement curves as the pronounced peaks, after which the footing collapses and the load decreases. In curves, which did not exhibit a definite failure point, the ultimate load was taken as the point at which the slope of the load-settlement curve first reach 0 or steady minimum value (Vesic 1973).

Settlements were measured from the tests results for footing supported on both reinforced and non reinforced sands given in Table3. Also, the measured and calculated ultimate loads for footing supported on both reinforced and non reinforced sands for the different studied parameters are given in Tables 4.

0.2

34.3

013

.86

20.9

347

.44

18.4

953

.57

15.5

660

.92

13.9

065

.09

0.5

36.0

49.

4923

.04

42.1

419

.87

50.1

016

.26

59.1

715

.03

62.2

6

0.7

39.5

80.

6027

.13

31.8

723

.34

41.3

918

.52

53.4

917

.63

55.7

3

0.2

20.0

849

.57

16.0

559

.69

11.9

470

.02

11.8

570

.24

9.73

75.5

7

0.5

25.5

835

.76

18.4

553

.67

13.2

666

.70

12.3

069

.11

10.5

673

.48

0.7

27.0

831

.99

22.4

843

.55

16.8

057

.81

13.7

365

.52

11.1

572

.00

0.2

17.1

257

.01

11.0

572

.25

9.00

77.4

08.

9377

.57

7.45

81.2

9

0.5

19.5

051

.03

14.0

364

.77

10.9

972

.40

9.46

76.2

47.

9380

.09

0.7

22.5

243

.45

16.8

557

.68

13.3

366

.52

10.3

374

.06

10.0

74.8

9

SETTLEM

EN

T in

mm

For

N=1

to 2

, u/B

= 0

.2, 0

.5, 0

.7 a

t LO

AD

,Q= 4

20kg

.

Red

uction

in

Set

tlem

ent(%

)

u/B

No

of

Geo

grid

(N)

N=3

N=3

N=3

Set

tlem

ent in

mm

N=1

N=2

N=2

N=2

N=1

N=1

B'=

BB'=

2BB'=

3BB'=

4BB'=

5B

Red

uction

in

Set

tlem

ent(%

)

Red

uction

in

Set

tlem

ent(%

)

Red

uction

in

Set

tlem

ent(%

)

Red

uction

in

Set

tlem

ent(%

)

Tab

le.3

SE

TT

LE

ME

NT

in m

m F

or N

=1

to 2

, u/B

= 0

.2, 0

.5, 0

.7 a

t L

OA

D,Q

= 4

20k

g.



4

The Ultimate pressure-settlement/Width curves of the model tests for one, two and three Layer of reinforcement placed at different top layer spacing and different size of Width of the Geogrid are shown in Figures 3 to 7. Figure 3. Shows that the ultimate bearing pressure increases from 22 kPa to 26 kPa as Number of layer of reinforcement (N) increasing from 1 to3 and also increases with increases in width of reinforcement (B’).Similar Phenomenon was shown in figure 4, 5, 6 and 7 and model tests results are shown in Table 4.

Figure 3. Ultimate Pressure Vs. Setllement/Width For B’=B.

Figure 4. Ultimate Pressure Vs. Setllement/Width For B’=2B.

0.2

21.0

01.

3131

.25

22.5

01.

4140

.63

27.0

01.

6968

.75

32.0

02.

0010

0.00

29.0

01.

8181

.25

0.5

18.0

01.

1312

.50

21.5

01.

3434

.38

24.0

01.

5050

.00

33.5

02.

0910

9.38

24.0

01.

5050

.00

0.7

17.0

01.

066.

2517

.00

1.06

6.25

18.0

01.

1312

.50

26.0

01.

6362

.50

20.0

01.

2525

.00

0.2

24.0

01.

5050

.00

31.0

01.

9493

.75

33.0

02.

0610

6.25

40.0

02.

5093

.72

38.5

02.

4114

0.63

0.5

21.0

01.

3131

.25

26.5

01.

6665

.63

33.0

02.

0610

6.25

39.5

02.

4793

.80

34.0

02.

1311

2.50

0.7

20.0

01.

2525

.00

20.0

01.

2525

.00

29.0

01.

8181

.25

32.0

02.

0094

.98

31.5

01.

9796

.88

0.2

29.7

01.

8685

.63

39.0

02.

4414

3.75

46.0

02.

8818

7.50

47.5

02.

9792

.54

52.0

03.

2522

5.00

0.5

26.5

01.

6665

.63

31.5

01.

9796

.88

33.5

02.

0910

9.38

40.5

02.

5393

.64

48.0

03.

0020

0.00

0.7

26.0

01.

6362

.50

28.0

01.

7575

.00

32.5

02.

0310

3.13

35.0

02.

1994

.51

39.5

2.47

146.

88

No

of

Geo

grid

(N)

u/B

Bea

ring

Cap

acit

y R

atio

(BC

R)

= q

r / q

o , (

qo =

16kN

/m2 )

B'=

BB

'=2B

B'=

3BB

'=4B

B'=

5B

qr

BC

R=

qr

/ qo

Incr

ease

s

in

B.C

(%

)q

rB

CR

=

qr

/ qo

Incr

ease

s

in

B.C

(%

)q

rB

CR

=

qr

/ qo

Incr

ease

s

in

B.C

(%

)q

rB

CR

=

qr

/ qo

Incr

ease

s

in

B.C

(%

)q

rB

CR

=

qr

/ qo

Incr

ease

s

in

B.C

(%

)

N=1

N=1

N=1

N=3

N=2

N=2

N=2

N=3

N=3

Tab

le 4

. q

ult

is @

(S

ettl

emen

t/w

idth

) of

0.0

9% in

UR

bed

For

N=

1 to

2, u

/B=

0.2

, 0.5

, 0.7

at

LO

AD

,Q=

420

kg.



5




CONCLUSIONS:

Laboratory model tests for strip foundation supported by geogrid-reinforced sand have been presented. The ultimate bearing capacities and settlement obtained from these tests have been presented. Based on the present tests, the following conclusions can be drawn:

1. For the same soil, geogrid, and configuration, the ultimate bearing capacity increases with the increase in the width of geogrid (B’) and no of layer of geogrids. The ultimate bearing capacity of sand without geogrids which was 16kN/m2, increased to 52 kN/m2 at settlement/width (S/B) 9% in unreinforced bed, u/B=0.2, N=3 and B’=5B showing 225% improvement.

2. For the same soil, geogrid, and configuration, the settlement decreases with the increase in the width of geogrid (B’) and no of layer of geogrids. The settlement of footing was decreases with increases No of layer of Geogrids (N) and increases in width of geogrid (B’). Settlement of footing without geogrids which was 39.82 mm, decreased to 7.45mm at u/B=0.2, B’=5B and N=3 showing 81.29% decreased.

3. The optimum width of Reinforcement for getting maximum in bearing capacity of sand was found to be about 4.0B. Any additional width of geogrid beyond optimum value will be ineffective.

4. The bearing capacity ratio (BCR) decreases with increasing top layer spacing of geogrids layers and increases with increases width of geogrid (B’) and also depends on Number of layer of geogrids. For design purposes, engineers need to balance between reducing spacing and increasing geogrid tensile modulus. The author believes that a value of h/B = 0.3 can be a reasonable value for use in the design of reinforced soil.

REFERENCES

[1] Huang, C.C., and Menq, F.Y, 1997. “Deep-footing and wide-slab effects in reinforced sandy ground.” Journal of Geotechnical and Geoenviromental Engineering, ASCE, Vol. 123, No.1, pp. 30-36.

[2] Binquet, J., and Lee, K.L., 1975a. “Bearing capacity tests on reinforced earth slabs.” Journal of Geotechnical Engineering Division, ASCE, Vol. 101, No.GT12, pp. 1241-1255.

[3] Michalowski, R.L., April 2004. “Limit loads on reinforced foundation soils”, Journal of Geotechnical and Geoenviromental Engineering, ASCE, Vol. 130, No.4, pp. 381-390.

[4] Huang, C.C., and Tatsuoka, F., 1990. “Bearing capacity reinforced horizontal sandy ground.” Geotextiles and Geomembranes, Vol. 9, pp. 51-82.



an experimental study of axially loaded … abstract: the aim of this experimental study is to...

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