mtech project_2013_ppt

Presented by

(1RV11CSE02)

IV Semester, M Tech- Structural Engg. RVCE

Under the Guidance of

Associate Professor, Dept. of Civil Engg., RVCE

2

OUTLINE

• INTRODUCTION

• LITERATURE SURVEY

• PROJECT OBJECTIVE & METHODOLOGY

• VALIDATION OF ANSYS

• PARAMETRIC STUDIES

• CONCLUSIONS

• SCOPE FOR FURTHER STUDIES

• REFERENCES

3

INTRODUCTION

FOUNDATION

SHALLOW FOUNDATION

ISOLATED FOOTING

COMBINED FOOTING

STRAP FOOTING

RAFT FOUNDATION

DEEP FOUNDATION

PILE FOUNDATION

PIER FOUNDATION

WELL FOUNDATION

4

(Df/B)<=1

(L/d) > 1

INTRODUCTION

RAFT FOUNDATION

• A flat slab transmits the entire structural load or

load from several columns to the underlying

rock or soil

• where the area is covered by conventional

spread footings which is more than 50% of total

plan area

• Can spread over a large area where the soil

below has low bearing capacity

Fig.1 Raft Foundation

5

INTRODUCTION

PILE FOUNDATION

• Long slender members

• Transfer load to the hard stratum lying

below the soft stratum

• Transfer load by skin friction

• It can withstand uplift forces in foundations

in expansive soil.

• to assist structures in resisting lateral and

overturning forces.

Fig.2(a) End Bearing Piles

Fig.2(b) Friction Piles 6

PILED RAFT FOUNDATION

NEED FOR PILED RAFT FOUNDATION

• The major disadvantage of Rafts are they will undergo

excessive settlement in case of weak soil & with higher

loads

• In case of free standing pile foundation, they are prone to

differential settlement since the loads from the

superstructure are not equally distributed on all the piles

Piled Raft foundation system which is a combination of

both Rafts and piles are used to overcome the above

disadvantages.

7


8

RAFT FOUNDATION

PILE FOUNDATION


reduction of settlement &

load transfer mechanism

Fig.3

Fig.4 Fig.5

WORKING MECHANISM OF PILED RAFT

FOUNDATION

• reduction of settlement & load transfer mechanism

• A geotechnical assessment for design of such a

foundation system therefore needs to consider not only

the capacity of the pile elements and the raft elements, but

their combined capacity and interaction under

serviceability loading.

9

ANALYSIS OF PILED RAFT FOUNDATION

ANALYSIS OF PILE RAFT

FOUNDATION

ANALYTICAL METHODS

EXPERIMENTAL METHODS

NUMERICAL METHODS

FUNCTIONAL APPROXIMATION

METHOD

FINITE DIFFERENCE

METHOD

FINITE ELEMENT METHOD

ANSYS

10

To assess the behavior of

foundation under different

parameters & conditions

FINITE ELEMENT METHOD OF ANALYSIS

• BASIS : representation of a body or a structure by an assemblage of

subdivisions called finite elements.

• all the complexities like material properties, boundary conditions, varying

loads can be taken care of.

• Approximate but acceptable solutions are obtained

Use of computer is the basic and essential part of the finite element analysis

with variety of computer software packages have been used of which one we

are using is ANSYS.

With the advent of computer, this technique can be used for getting better

engineering analysis results.

11

• A sophisticated and leading FEM integrated software package

consisting of whole module required for modeling, meshing, analysis

and post processing of results.

• tailor made for analysis of foundation as it is more versatile and

flexible as the field conditions can be incorporated easily

• capable of analyzing unusual and complex raft shapes, rafts with

different thicknesses, piles with different shapes, piles with different

lengths and diameters of pile, interaction between piles, raft and soil.

ANSYS

12

STEPS IN ANSYS

• Pre Processing mode

Defining element type (for Pile, raft and soil) and element behavior

(Eg: Plane strain)

Assigning material properties

Modeling by joining key points through lines and create areas for

entire dimension of the model.

Meshing the entire model by assigning material properties

Also, model contact elements at the interfaces

Apply Boundary conditions to incorporate field conditions

• Solution and Post Processing mode

A Non-linear analysis is carried out

Results are observed ( Ultimate load carrying capacity and

settlements are observed) from the analysis

13

LITERATURE STUDY [3] Oh. et.al.(2008) FINITE ELEMENT MODELING OF PILED RAFT IN SAND

using PLAXIS.

Raft thickness doesn’t affect the load carrying capacity of foundation

[17] Ningobham Thoiba Singh & Baleshwar Singh studied INTERACTION ANALYSIS FOR PILED RAFTS IN COHESIVE SOILS (2008) using Ansys

Increase in raft thickness hasn’t improved the foundation behavior

greater raft thickness is not generally advantageous in reducing overall or maximum settlement, provide optimum raft thickness

Addition of piles increases the load carrying capacity of a raft with reduction in settlement.

Axial load is maximum at top portion of pile and is minimum at the tip.

[5] Srilakshmi & Chetan Gowda suggested that raft thickness does not have

influence on unifrom settlement & ultimate load capacity of foundatiom 14

LITERATURE STUDY Poulos et al.[7] Piled Raft Foundations for Tall

Buildings (2011) have used PLAXIS-3D to study

proposed piled raft foundation for INCHEON Tower at

S.Korea using the concept of Base embedment of Raft

with & without Soil for cases under horizontal &

vertical loading

• the bending down of rafts at corners and centre is

reduced in the case of basement wall embedment .

They have found out that Piled raft behaves

safely in high raised buildings when subjected to

horizontal & vertical loads.

15

Fig.6 Foundation layout

172 piles of 2.5m dia

lengths varying from

36m to 66m

Raft thickness of 5.5m

vertical basement of

the raft 14.6m(9.1+5.5)

below ground surface

level and 1.2m in

thickness

Fig.6

LITERATURE STUDY FINDINGS

• It is found out that piled raft foundation is the most effective

system for high rise structures

• Most of the studies were comprised of 2D analysis on piled

raft foundation .

• Most of the analysis were done using softwares other than

ANSYS.

• Influence of raft thickness & pile spacing is stressed often.

17

• Piled raft has major role to play in case of high rise structures

• Analyses carried out on piled rafts mainly dealt with raft

thickness and dimensions of pile

• The work was concentrated on uniform diameter & uniform

length of piles

In this study, the influence of different diameters and different

lengths of pile on the performance of foundation system is

considered.

18

MOTIVATION FOR THE PROJECT

OBJECTIVES OF THE PROJECT

• To study the variables which influence the load carrying

capacity of piled raft foundation & finally to know the

performance of the piled raft foundation system on uniform

sand & clay under vertical load- Parametric study using

FEM Analysis.

• To study the effect of parameters like

Pile diameters in different combinations

Pile length in different combinations

Raft thickness

19

METHODOLOGY

• Preprocessing:

Elements used- PLANE 82 for pile, raft & soil and Element behavior – plane strain.

TARGET 169 & CONTACT 172 for creating contact b/n pile & soil.

Assign material properties

H=5D, V=3D

Meshing entire model

Contact is created at the interface of piled raft & soil.

Apply Boundary conditions

• Solution & Post processing:

Non linear static analysis is carried out

Ultimate load carrying capacity & corresponding settlement is found out.

20

Fig.7 Plane strain consideration of geometric

modeling of piled raft foundation

Fig.8 Discretized geometric model with

boundary condtions

SCOPE OF THE WORK

Influence of different parameters on Ultimate load carrying

capacity of piles will be clearly observed.

The combination of suitable length, diameter of pile, thickness

of raft and type of soil from this study can be optimized for

getting good results in terms of Ultimate load carrying

capacity

21

VALIDATION OF ANSYS

VALIDATION 1 using formula

• Unpiled Raft foundation of size 5m×5m ×0.5m

[3,5,6,11,16,29,30,31]on a single layer soil.

The calculated settlement was 94.3mm.

22

Fig.9: Settlement contours for a

raft foundation

Here, from 2D

analysis, the

Settlement is found

out to be 95.4mm.

VALIDATION OF ANSYS

23

VALIDATION 2 using literature

• Piled raft foundation as shown

in table 1.

Fig.10. modelled

piled raft foundation

Properties Piled raft Soil layer ( 0 – 35m)

Size of mat 8m*8m*1.5m -

Pile length 16m -

Pile dia 0.7m -

No. of piles 16 -

Modulus of Elasticity,

E

2.5*107 kPa 9*104 kPa

Poisson’s ratio,µ 0.15 0.3

Density, ρ 2500 kg/m3 1700 kg/m3

Cohesion, c - 45 kPa

Friction angle, ϕ - 22.5o

Flow angle, ψ - 6.75o

Pressure applied on

raft

600 kN/m2

Table1

24

Fig.11: Displacement contours along y-

direction

VALIDATION 2 using literature

From the literature[3], the

settlement was found out to be

64.3mm.

From ANSYS, the settlement

obtained was 69.414mm.

Hence, the results obtained from ANSYS are compatible with the

calculated theoretical results.

PARAMETRIC STUDIES STUDY ON SAND Effect of pile diameter:

• No. of piles(16), Pile

length(3m), raft

thickness(0.6m),pile spacing

(3d) are kept constant

•The pile diameter in

different combinations of

0.3m, 0.4m & 0.5m are

shown in table 3

25

Table 2 : Material properties

Properties Piled raft Soil ( medium sand)

Modulus of Elasticity, E 2.5*107 kPa 4*104 kPa



Cohesion, c - 0 kPa (minimum of 5 kpa in ANSYS)

Friction angle, ϕ - 35o

Flow angle, ψ - 10.5o

Pile diameter(m) Raft

size

(m*m)

Pile

length(m

)

Pile

spacing(

m)

No. of

piles Centre

portion of

the raft

Edges of

the raft

0.4 0.4 4.4*4.4 3 3*Pile dia 16

0.3 0.4 3.8*3.8 3 3*Pile dia 16

0.3 0.5 4.2*4.2 3 3*Pile dia 16

0.4 0.3 4.0*4.0 3 3*Pile dia 16

0.5 0.3 4.6*4.6 3 3*Pile dia 16

0.3 0.3 3.4*3.4 3 3*Pile dia 16

0.5 0.5 5.4*5.4 3 3*Pile dia 16

0.5 0.4 5.0*5.0 3 3*Pile dia 16

0.4 0.5 4.8*4.8 3 3*Pile dia 16

Table 3 : parameters

considered for analysis

Results & Discussion • From table 4, the ultimate

load carrying capacity

increases with increase in pile

diameter.

26

Pile diameter(m) Ultimate load

P (MN)

Settlement δ

(mm) Centre

portion of

the raft

Edges of the

raft

0.3 0.3 2.12 19.57

0.4 0.4 3.388 24.59

0.5 0.5 4.808 26.903

Table 4


P (MN)

Settlement δ

(mm) Centre

portion of

the raft

Edges of the

raft

0.4 0.3 3.616 28.16

0.5 0.3 3.926 25.97

0.3 0.4 3.850 30.85

0.3 0.5 3.486 24.55

0.5 0.4 4.45 26.76

0.4 0.5 3.581 22.15

Table 5

• From table 5,

For the equal pile diameter combination of 0.3m as either of inner & outer pile diameter is increased from 0.3m to 0.5m , the ultimate load increases around 80% whereas settlement decreases around 30%. The pile diameter combination of 0.5m along the centre portion of the raft with 0.3m along the edges also shows credible results.

0

2

4

6

0.2 0.4 0.6U

ltim

ate

load

(MN

)

diameter of pile(m)

Fig.12: Variation of ultimate load

with diameter of pile

• For a uniform pile diameter

combination of 0.4m, , as the inner pile

diameter increases from 0.4m to 0.5m,

the ultimate load increases significantly

by 30% whereas the settlement

increases slightly by by 10%. Hence,

the pile diameter combination of 0.5m

along the centre portion of the raft with

0.4m along the edges also shows

incredible results from ultimate load

point of view.

• However, The combination of 0.3m with

0.4m shows good results with economic

point of view.

• Hence, the combination of 0.5m with

0.3m & 0.5m with 0.4m is preferred.

27

0

1

2

3

4

5

6

0.3 0.4 0.5

Ult

ima

te l

oa

d(M

N)

Inner pile diameter(m)

outer pile dia of 0.3m



Fig.13: Variation of ultimate load with inner &

outer pile diameter in combination

0

0.5

1

1.5

2

2.5

3

3.5

4

0 5 10 15 20 25 30

Ult

ima

te l

oa

d(M

N)

Settlement(mm)

Fig.14: Ultimate load vs settlement plot

for the case of uniform pile diameter of 0.4m

Fig15. Displacement contours in Y-direction at ultimate load for

the pile diameter combination of 0.5m along centre with 0.3m

along edges of the raft

28

Fig 16. Displacement contours in Y-direction at ultimate load for the

pile diameter combination of 0.5m along centre with 0.4m along

edges of the raft

Number of nodes present : 7460 Number of nodes present : 8678

Even though relatively it is difficult for casting different diameters, but it is giving relatively higher values of ultimate load which is cost effective also.

Since, the load carried in case of piled raft foundation is more at the centre of the raft, there is a need to provide larger pile diameter in the central portion of the piled raft.

Greater the pile diameter at the centre of raft, greater is the ultimate load carrying capacity of the foundation system.

Hence, t is ideal to provide a combination of different diameter piles rather than equal diameter piles throughout where different pile diameters lead for better function of foundation.

29

Findings

STUDY ON SAND Effect of pile length:

•Material properties

same as table 2.

•No. of piles(16), Pile

diameter(0.4m), raft

thickness(0.6m),pile

spacing (3d) are kept

constant

•The pile length in different

combinations of 3m, 4m &

5m are shown in table 6

30

Case Pile length(m) Raft size

(m*m)

Pile

diameter(m)

Pile

spacing(m)

No. of

piles Centre

portion of

the raft

Edges of

the raft

1 4 4 4.4*4.4 0.4 3*Pile dia 16

2 3 3 4.4*4.4 0.4 3*Pile dia 16

3 5 5 4.4*4.4 0.4 3*Pile dia 16

4 3 4 4.4*4.4 0.4 3*Pile dia 16

5 3 5 4.4*4.4 0.4 3*Pile dia 16

6 4 3 4.4*4.4 0.4 3*Pile dia 16

7 4 5 4.4*4.4 0.4 3*Pile dia 16

8 5 4 4.4*4.4 0.4 3*Pile dia 16

9 5 3 4.4*4.4 0.4 3*Pile dia 16

Table 6 : parameters considered for analysis

Results & Discussion

Pile length(m) Ultimate load P

(MN)

Settlement δ

(mm) Centre

portion of

the raft

Edges of the raft

3 3 3.388 24.59

4 4 3.037 20.99

5 5 2.251 15.12

31

Table 7

•As the pile length increases from 3m to

4m & 5m, the ultimate load carrying

capacity decreases very slightly around 10

% & 30%.

•For an equal pile length combination of

4m,as the outer pile length decreases from

4m to 3m, the ultimate load increases

highly by 8%, thereby the settlement

increases by 8%. Here the pile length

combination of 4m along the centre

portion of the raft with 3m along edges

of the raft gives satisfactory results.


(MN)

Settlement δ (mm)

Centre

portion of the

raft

Edges of the raft

4 3 3.29 22.84

5 3 2.00 12.86

3 4 3.099 22.306

3 5 3.78 27.65

5 4 1.22 7.34

4 5 2.59 17.93

Table 8

Findings • The ultimate load carrying capacity does not

get affected much with the increase in pile

diameter

• From economic point of view, it is ideal to

provide a combination of different length

piles rather than equal length piles

throughout

32

0

0.5

1

1.5

2

2.5

3

3.5

0 5 10 15 20 25

Ultimate

load(MN)

Settlement(mm)

Fig.17 Ultimate load vs settlement

plot for the case of uniform pile

length of 4m

Fig 18. Displacement contours in Y-

direction at ultimate load for the pile

length combination of 4m along centre

with 3m along edges of the raft

PARAMETRIC STUDIES STUDY ON CLAY Effect of pile diameter:

• No. of piles(16), Pile

length(3m), raft

thickness(0.6m),pile spacing

(3d) are kept constant

•The pile diameter in

different combinations of

0.3m, 0.4m & 0.5m are

shown in table 3

33

Table 9 : Material properties


size

(m*m)

Pile

length(m

)

Pile

spacing(

m)

No. of

piles Centre

portion of

the raft

Edges of

the raft

0.4 0.4 4.4*4.4 3 3*Pile dia 16

0.3 0.4 3.8*3.8 3 3*Pile dia 16

0.3 0.5 4.2*4.2 3 3*Pile dia 16

0.4 0.3 4.0*4.0 3 3*Pile dia 16

0.5 0.3 4.6*4.6 3 3*Pile dia 16

0.3 0.3 3.4*3.4 3 3*Pile dia 16

0.5 0.5 5.4*5.4 3 3*Pile dia 16

0.5 0.4 5.0*5.0 3 3*Pile dia 16

0.4 0.5 4.8*4.8 3 3*Pile dia 16

Table 10 : parameters

considered for analysis

Properties Piled raft Cohesive soil ( Clay)

Modulus of Elasticity, E 2.5*107 kPa 4*103 kPa



Cohesion, c - 25kPa

Friction angle, ϕ - 10o

Flow angle, ψ - 3o

Results & Discussion • From table 4, the ultimate

load carrying capacity

increases with increase in pile

diameter.

34

Table 11

Table 12

• From table 5,

For the equal pile diameter combination of 0.3m as either of inner & outer pile diameter is increased from 0.3m to 0.5m , the ultimate load increases around 20% whereas settlement decreases around 30%. The pile diameter combination of 0.5m along the centre portion of the raft with 0.3m along the edges also shows credible results.

Fig.19: Variation of ultimate load

with diameter of pile


P (MN)

Settlement δ

(mm) Centre

portion of

the raft

Edges of the

raft

0.3 0.3 2.01 201.34

0.4 0.4 2.323 144.56

0.5 0.5 3.207 156.92

2.01 2.323

3.207

0

0.5

1

1.5

2

2.5

3

3.5

0 0.2 0.4 0.6

Ult

ima

te l

oa

d(M

N)

diameter of pile(m) Pile diameter(m) Ultimate load

P (MN)

Settlement δ

(mm) Centre

portion of

the raft

Edges of the

raft

0.4 0.3 2.112 140.97

0.5 0.3 2.412 136.96

0.3 0.4 2.357 192.37

0.3 0.5 2.593 176.57

0.5 0.4 3.225 176.89

0.4 0.5 3.064 176.94

• For a uniform pile diameter

combination of 0.4m, , as the inner

pile diameter increases from 0.4m to

0.5m, the ultimate load increases

significantly by 38% whereas the

settlement increases slightly by 20%.

Hence, the pile diameter combination

of 0.5m along the centre portion of the

raft with 0.4m along the edges also

shows incredible results from ultimate

load point of view.

• However, The combination of 0.3m

with 0.4m shows good results with

economic point of view.

• Hence, the combination of 0.5m with

0.3m & 0.5m with 0.4m is preferred . 35

0

0.5

1

1.5

2

2.5

3

3.5

4

0 5 10 15 20 25 30

Ult

ima

te l

oa

d(M

N)

Settlement(mm)

Fig.21: Ultimate load vs settlement plot

for the case of uniform pile diameter of 0.4m

0

0.5

1

1.5

2

2.5

3

3.5

0.3 0.4 0.5

Ult

ima

te l

oa

d(M

N)

Inner pile diameter(m)




Fig.20 Variation of ultimate load with inner

& outer pile diameters in combinations

Findings on Effect of pile diameter

36

Table 13. Ultimate load & settlement values for different

pile diameter combinations

• The Ultimate load capacity is around 30% more in case of sand when compared with clay.

• Similarly, the corresponding settlement for clay is around 4 to 5 times that of sand.

• The reason could be that piles in sand have both skin friction & end bearing effect, whereas piles in clay may take only end bearing effect into consideration.

Pile diameter combination Uniform

soil

condition

Ultimate

load

P(MN)

∆P ( %) Settlemen

t δ(mm)

∆δ

(%)

Centre of the raft Edges of the raft

0.5 0.3

Sand 3.926 25.97

Clay 2.412 39% ↓ 136.96 427%↑

0.3 0.4

Sand 3.85 30.85

Clay 2.357 39% ↓ 192.37 524%↑

0.5 0.4

Sand 4.45 26.76

Clay 3.225 27.5% ↓ 176.89 561%↑

STUDY ON CLAY Effect of pile length:

•Material properties

same as table 9.

•No. of piles(16), Pile

diameter(0.4m), raft

thickness(0.6m),pile

spacing (3d) are kept

constant

•The pile length in different

combinations of 3m, 4m &

5m are shown in table 6

37

Pile length(m) Raft size

(m*m)

Pile

diameter(m)

Pile

spacing(m)

No. of

piles Centre

portion of

the raft

Edges of

the raft

4 4 4.4*4.4 0.4 3*Pile dia 16

3 3 4.4*4.4 0.4 3*Pile dia 16

5 5 4.4*4.4 0.4 3*Pile dia 16

3 4 4.4*4.4 0.4 3*Pile dia 16

3 5 4.4*4.4 0.4 3*Pile dia 16

4 3 4.4*4.4 0.4 3*Pile dia 16

4 5 4.4*4.4 0.4 3*Pile dia 16

5 4 4.4*4.4 0.4 3*Pile dia 16

5 3 4.4*4.4 0.4 3*Pile dia 16

Table 14 : parameters considered for analysis

Results & Discussion

38

Table 15 Ultimate load & corresponding

settlement values •As the pile length increases from 3m to

4m & 5m, the ultimate load carrying

capacity decreases very slightly around 4

% & then increases slightly by 12%

whereas settlement increases slightly by

10% & 20% respectively.

•For an equal pile length combination of

3m, as the inner pile length increases from

3m to 4m, the ultimate load slightly

increases by 34%, thereby the settlement

decreases by 66%. Here the pile length

combination of 4m along the centre

portion of the raft with 3m along edges

of the raft gives satisfactory results.

Table 16


(MN)

Settlement δ

(mm) Centre

portion of

the raft

Edges of the

raft

3 3 2.323 144.56

4 4 2.304 137.78

5 5 2.594 159.21


(MN)

Settlement δ (mm)

Centre

portion of the

raft

Edges of the raft

4 3 3.12 241.08

5 3 2.304 135.5

3 4 2.168 137.2

3 5 2.401 163.29

5 4 2.502 154.53

4 5 2.323 146.74

Discussion • The ultimate load carrying capacity does not

get affected much with the increase in pile

diameter

• From economic point of view, it is ideal to

provide a combination of different length

piles rather than equal length piles

throughout

39

Fig.22 Ultimate load vs settlement

plot for the case of uniform pile

length of 4m

Fig 18. Displacement contours in Y-

direction at ultimate load for the pile

length combination of 4m along centre

with 3m along edges of the raft

0

0.5

1

1.5

2

2.5

0 50 100 150

Ult

ima

te l

oa

d(M

N)

Settlement(mm)

Findings on effect of pile length

Pile length combination Uniform soil

condition

Ultimate

load

P(MN)

∆P ( %) Settlement

δ(mm)

∆δ (%)

Centre of the raft Edges of the raft

4 3

Sand 3.29 22.84

Clay 3.12 5%↓ 241.08 950%↑

40

Table 17

The Ultimate load capacity is around just 5% more in case of

sand when compared with clay.

Similarly, the corresponding settlement for clay is around 9 times

that of sand.

STUDY ON EFFECT OF RAFT

THICKNESS on Sand

• Study is carried out on the combinations of pile diameters

combinations and pile lengths separately.

• Study on pile diameter combination of 0.5m along centre with

0.3m along edges of raft

41

Table 18 Parameters considered for the analysis (sand)

Pile diameter(m) Raft size

(m*m)

Pile

length(m)

Raft thickness(m) Pile spacing(m) No. of piles

Centre portion

of the raft

Edges of the

raft

0.5 0.3 4.6*4.6 3 0.6 3*Pile dia 16

0.5 0.3 4.6*4.6 3 0.4 3*Pile dia 16

0.5 0.3 4.6*4.6 3 0.2 3*Pile dia 16

0.5 0.3 4.6*4.6 3 0.8 3*Pile dia 16

Pile diameter(m) Raft thickness(m) Ultimate load

P (MN)

∆P (%) Settlement δ

(mm)

∆ δ (%)

Centre

portion of

the raft

Edges of

the raft

0.5 0.3 0.6 3.926 25.97

0.5 0.3 0.4 4.168 6% ↑ 27.84 7% ↑

0.5 0.3 0.2 4.38 11% ↑ 29.69 14% ↑

0.5 0.3 0.8 4.13 5% ↑ 27.38 5% ↑

42

Results & Discussion Table19. showing Ultimate load & settlement values

4.38 4.168 3.926 4.13

0

1

2

3

4

5

0 0.2 0.4 0.6 0.8 1

Ult

ima

te l

oa

d(M

N)

Raft thickness(m)

29.69 27.811

25.97 27.38

0

5

10

15

20

25

30

35

0 0.2 0.4 0.6 0.8 1

Set

tlem

ent

(mm

) Raft thickness(m)

Fig.19 plot of ultimate load for different raft

thickness for the pile diameter combination of

0.5m along the centre portion of the raft with

0.3m along the edges of the raft

Fig.20 plot of settlement values for different

raft thickness for the pile diameter combination

of 0.5m along the centre portion of the raft with

0.3m along the edges of the raft

Varying raft thickness has no significant effect on the piled raft

foundation with varying pile diameter combination.

STUDY ON EFFECT OF RAFT

THICKNESS on Sand • Study on pile length combination of 4m along centre with 3m

along edges of raft

43

Table 20. Parameters considered for the analysis (sand)

Pile length(m) Raft size

(m*m)


thickness(m)

Pile

spacing(m)

No. of piles

Centre portion

of the raft

Edges of the

raft

4 3 4.4*4.4 0.4 0.6 3*Pile dia 16

4 3 4.4*4.4 0.4 0.4 3*Pile dia 16

4 3 4.4*4.4 0.4 0.2 3*Pile dia 16

4 3 4.4*4.4 0.4 0.8 3*Pile dia 16

Results & Discussion Table21. showing Ultimate load & settlement values

Pile length(m) Raft thickness(m) Ultimate load P

(MN)

∆P (%) Settlement δ

(mm)

∆ δ (%)

Centre

portion of

the raft

Edges of

the raft

4 3 0.6 3.2912 22.84

4 3 0.4 3.454 5% ↑ 24.044 5% ↑

4 3 0.2 3.2912 0 % ↑ 22.902 0.3% ↑

4 3 0.8 3.543 8% ↑ 24.319 6% ↑

44

Fig.21 plot of ultimate load for different raft

thickness for the pile length combination of 4m

along the centre portion of the raft with 3m

along the edges of the raft

Fig.22 plot of settlement values for different

raft thickness for the pile length combination of

4m along the centre portion of the raft with 3m

along the edges of the raft

• Varying raft thickness has no significant effect on the piled raft

foundation with varying pile length combination.

3.2912 3.454

3.2912 3.543

0

0.5

1

1.5

2

2.5

3

3.5

4

0 0.2 0.4 0.6 0.8 1

Ult

ima

te l

oa

d(M

N)

Raft thickness(m)

29.69 27.811

25.97 27.38

0

5

10

15

20

25

30

35

0 0.2 0.4 0.6 0.8 1

Set

tlem

ent

(mm

)

Raft thickness(m)

COMBINED MODEL OF BOTH PILE DIAMETER &

PILE LENGTH IN DIFFERENT COMBINATION

Type of soil Pile length(m) Pile diameter(m) Raft size

(m*m)

Raft

thickness

(m)

Ultimate

load

P (MN)

Settlement

δ (mm) Centre

portion

of the

raft

Edges

of the

raft

Centre

portion

of the

raft

Edges

of the

raft

Medium

sand

4 3 0.5 0.3 4.6*4.6 0.6 3.6 22.813

Clay 4 3 0.5 0.3 4.6*4.6 0.6 3.1 203.08

45

Fig.19 modeled piled raft foundation Fig.20 displacement contours in y-direction at ultimate load

Table22 showing Proposed model & its Ultimate load & settlement values

• Hence, a pile diameter combination of 0.5m with 0.3m for a uniform pile length

of 3m is preferred instead of varying pile length.

STUDY ON LAYERED SOIL • The combination used here is

shown in table 21.

46

Raft size

(m*m)

Raft

thickness(m)

Pile diameter(m) Length

of

pile(m)

Centre

of the

raft

Edges of

the raft

4.6*4.6 0.6 0.5 0.3 3

Table 23 piled raft dimensions

• Material properties for pile and raft, Clay & sand are

same as previous studies.

sand

clay

clay

sand

Fig 21. soil profile

Fig.22 model showing soil to be double

layered at different depths

Results & Findings

47

• It is observed that the layered soil

with clay overlying sand is

giving relatively higher load

which is an interesting

observation that the compressible

layers below the depth of

foundation also should be taken

care of.

Table 24.Values of ultimate load and settlement values

Soil profile Ultimate load

(MN)

Settlement(m

m)

Sand 3.926 25.97

Clay 2.412 136.96

Sand/Clay

Sand up to 1.5m depth 2.666 118.39

Sand up to 3m depth 1.524 56.035

Sand up to 4.5m depth 0.34 8.572

Clay/Sand

Clay up to 1.5m depth 2.2112 21.4

Clay up to 3m depth 3.413 83.65

Clay up to 4.5m depth 4.5071 152.52

Table 25.Displacement contours in y-direction

at ultimate load

CONCLUSION

• Pile diameter has significant influence on the ultimate capacity

of piled raft foundation.

• Varying Pile diameters that too in combination has significant

effect on the ultimate capacity of the foundation system. Instead

of equal pile diameters throughout in a piled raft system, a

combination of different inner & outer pile diameters in

combination is suggested.

• In case of foundation in sand, ultimate load carrying capacity of

foundation increases by 126% with increase in pile diameter

from 0.3 to 0.5 m uniformly.

48

CONCLUSION

• In case of foundation in sand, the pile diameter combination of 0.3m along

the centre portion of the raft with 0.4m along the edges of the raft shows

best results with respect to ultimate load capacity point of view.

Similarly, pile diameter combination of 0.5m along the centre portion of the

raft with 0.3m along the edges also shows acceptable results.

From economic point of view, the pile diameter combination of 0.3m along

the centre with 0.4m along the edges of raft is suggested

• Same combinations for foundation in clays showed good results.

• In case of foundation in clayey soil, the ultimate load capacity increases by

60% with increase in pile diameter from 0.3 to 0.5 m uniformly. This

implies that it has a considerable effect on the behavior of the foundation.

• It is concluded that piled raft foundations having higher pile diameter at the

centre and smaller diameter at the edges is preferred for higher ultimate

loads.

49

CONCLUSION • Varying pile lengths throughout uniformly does not have significant

effect on the ultimate load carrying capacity of the foundation.

• Hence, pile length has no significant effect on the behavior of the foundation on both sand & clay.

• However, in case of foundation in both sand & clay, a pile length combination of 4m along the centre with 3m along the edges of raft shows credible results.

• Further parametric studies shows that the raft thickness does not affect much on the ultimate load carrying capacity of the foundation.

• Ultimate capacity of foundation in sand is around 50% more than that of clay.

• The layered soil with clay overlying sand is giving relatively higher loads when compared with that of sand overlying clay.

50

SCOPE FOR FURTHER STUDY

• Foundation system can be analyzed by different soil in multiple

layers for the combination of different pile lengths & pile diameters.

• Research can be carried on 3-D modeling for analysis of piled raft

under load & bending moment.

51

JOURNALS SUBMISSION STATUS

ANALYSIS OF PILED

RAFT FOUNDATION

USING FINITE

ELEMENT METHOD

Effect of pile diameter &

pile length in sand

Submitted & accepted by

IJERST –

ISSN : 2319-5991

Vol. no., No.3, Aug.2013

Pg. 89-96

Effect of pile diameter &

pile length in clay

To be submitted

52

REFERENCES

1. Dang Dinh Chung Nguyen, Seong-Bae Jo and Dong-Soo Kim(2013), “Design Method of

Piled-Raft Foundations under Vertical Load Considering Interaction Effects”, Science direct-

Computers and Geotechnics, Vol. 47, pp. 16–27

2. Der-Guey Lin and Zheng-Yi Feng(2006), “A Numerical Study Of Piled Raft Foundations”,

Journal of the Chinese Institute of Engineers, Vol. 29, , pp. 1091-1097

3. E.Y. N Oh, M. Huang, C. Surarak and A. S. Balasubramaniam (2008) , “Finite Element

Modeling For Piled Raft Foundation in Sand”, Eleventh East Asia-Pacific Conference on

Structural Engineering & Construction (EASEC-11) ; Building a Sustainable Environment;

Taipei; Taiwan, pp. 1-8

4. Emilios M. Comodromos , Mello C. Papadopoulou and Ioannis K. Rentzeperis(2009), “Pile

Foundation Analysis and Design Using Experimental Data And 3-D Numerical Analysis”,

Science direct - Computers and Geotechnics, Vol. 36 , pp. 819–836

5. G.Srilakshmi and Chethan Gowda R.K (2012), “Analysis of Piled raft foundation by Finite

Element method”, Dissertation report to VTU Belgaum.

6. G. Srilakshmi and Rekha (2011), “Analysis of Mat Foundation Using Finite Element

Method”, International Journal of Earth Sciences and Engineering, Vol. 4, pp. 113-115

53

REFERENCES 7. H.G. Poulos, J.C. Small and H. Chow (2011), “Pile Raft Foundation For Tall Buildings”,

Geotechnical Engineering Journal of the SEAGS & AGSSEA, Vol.42, pp. 78-84

8. Helen Sze Wai Chow(2007), “Analysis of piled raft foundations with piles of different and diameters”, Doctoral Thesis submitted to University of Sydney School of Engineering, August 2007.

9. J.C. Small and H.H. Zhang (2002), “Behavior of Piled Raft Foundations under Lateral and Vertical Loading”, The International Journal of Geo mechanics, Vol.2, pp. 29–45

10. Jaeyeon Cho, Jin-HyungLee, SangseomJeong and JaehwanLee (2012), “The Settlement Behavior Of Piled Raft In Clay Soils”, Science Verse – Science direct - Ocean Engineering, Vol.53,pp. 153–163

11. Joseph E Bowles, Foundation Analysis and Design, McGraw- Hill Companies, 5th edition, New York, 1997.

12. JinHyung Lee, Youngho Kim and Sangseom Jeong(2010), “Three-Dimensional Analysis of Bearing Behavior Of Piled Raft On Soft Clay”, Science direct - Computers and Geotechnics, Vol. 37, pp. 103–114

54

REFERENCES

13. Luca de Sanctis and Alessandro Mandolini(2006), “Bearing Capacity Of Pr On Soft Clays”,

Journal of Geotechnical And Geoenvironmental Engineering; vol. 132, pp.1600-1610.

14. Luca de Sanctis and Gianpiero Russo (2008), “Analysis and Performance of Piled Rafts

Designed Using Innovative Criteria”, Journal of Geotechnical And Geoenvironmental

Engineering, vol.134, pp. 1118-1128

15. M. H. Baziar, A. Ghorbani and R. Katzenbach (2009), Three-Dimensional Analysis of Pile-

Raft Foundation On Medium-Dense Sand, International Journal of Civil Engineering, Vol. 7,

pp. 170 -175.

16. Nainan P Kurian and G.Srilakshmi(2003), “Studies on behavior of under reamed piles in

normal and expansive soils by finite element method”, Doctoral Thesis, Indian Institute of

Technology, Madras, Chennai, India

17. Ningombam Thoiba Singh and Baleshwar Singh(2008), “Interaction Analysis For Piled

Rafts In Cohesive Soils”, The 12th International Conference of International Association for

Computer Methods and Advances in Geomechanics (IACMAG); 1-6 October 2008; Goa;

India, pp. 3289-3296

55

REFERENCES 18. Oliver Reul (2004), “Numerical study of the bearing behavior of piled rafts”, International

Journal Of Geomechanical Engineering, Vol.4, pp. 59-68

19. Oliver Reul and Mark F. Randolph (2004), “Design Strategies For Piled Rafts Subjected to

Nonuniform Vertical Loading”, Journal of Geotechnical & Geoenvironmental Engineering,

vol 130, pp. 1-13.

20. Phung Duc Long (2010), “Piled Raft – A Cost-Effective Foundation Method For High-

Rises”, Geotechnical Engineering Journal of the SEAGS & AGSSEA, Vol. 41, pp. 1-12

21. R. P. Cunha, H. G. Poulos and J. C. Small(2001), “Investigation of Design Alternatives for a

Piled Raft Case History”, Journal Of Geotechnical And Geoenvironmental Engineering, vol.

127, pp. 635-641

22. R. Ziaie-Moayed, M.Kamalzare and M. Safavian(2010), “Evaluation Of Piled Raft

Foundations Behavior With Different Dimensions of Piles”, Journal of Applied Sciences

,Vol.10, pp.1320-1325

23. Rajendra Singh Bisht and Baleshwar Singh(2012) , “Study On Behaviour Of Piled Raft

Foundation By Numerical Modelling” , SAITM Research Symposium on Engineering

Advancements (SAITM – RSEA 2012),Indian Institute of Technology, Guwahati.

56

REFERENCES 24. Widjojo A. Prakoso and Fred H. Kulhawy(2001), “Contribution To Piled Raft Foundation Design”,

Journal of Geotechnical & Geoenvironmental Engineering , vol. 127, pp. 17-24

25. Y. F. Leung , A. Klar, and K. Soga(2010), “Theoretical Study On Pile Length Optimization Of Pile

Groups And Piled Rafts”, Journal Of Geotechnical And Geoenvironmental Engineering, vol. 136, pp.

319-330

26. Zehai Cheng Zhejiang (2011) , “Prediction And Measurement Of Settlement Of A Piled Raft

Foundation Over Thick Soft Ground”, EJGE ,Vol. 16 ; Bund. A, pp. 125-136

27. CODE OF PRACTICE FOR DESIGN AND CONSTRUCTION OF PILE FOUNDATIONS.

IS:2911(PART I/SECI)-1979

28. CODE OF PRACTICE FOR CALCULATION OF SETTLEMENTS OF FOUNDATIONS.

IS:8009(PART I)-1976

29. CODE OF PRACTICE FOR DETERMINATION OF BREAKING CAPACITYOF SHALLOW

FOUNDATIONS. IS 6403:1981

30. CODE OF PRACTICE FOR DESIGN AND CONSTRUCTION OF RAFT FOUNDATIONS.

IS:2950(PART I)-1981

31. V.N.S Murthy, Soil mechanics and Foundation Engineering, CBS Publishers and Distributors

Bangalore, 2007

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