Experimental Observations and Parametric Study of Piled Raft Foundation Using Plaxis

8/10/2019 Experimental Observations and Parametric Study of Piled Raft Foundation Using Plaxis

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Experimental Observations and Parametric Study

of Piled Raft Foundation Using Plaxis

N. VenkateshwaranFaculty,

National Institute of Technology,

Tiruchirapalli, 620015, India

Email: [email protected]

Dr.S.P. Jeyapriya

Assistant Professor,

Government College of Technology,Coimbatore 641013, India


AbstractThe foundations for tall buildings are usually

subjected to a combination of vertical, lateral and

overturning forces. Piled raft foundation is an

economical and efficient foundation for tall buildings in

which the load from the superstructure is transferred to

the soil by load sharing mechanism between raft and pile

and thereby reducing the settlement. In the present

study, laboratory model tests and numerical analysis

were carried out to study the behaviour of piled raft

foundation subjected to vertical load. The test was

conducted on plain raft and piled raft model with

different configurations viz. 1x1, 2x2 and 3x3 in

homogeneous loose sand. The experimental results were

analysed and calibrated using PLAXIS 2D. The

percentage settlement reduction observed experimentally

and numerically were compared and reported. It was

observed that the settlement has reduced considerably

for piled raft foundation and it was about 71% reductionfor 3x3 configuration. Also, the load carrying capacity

has increased significantly from 234N to 578N when 3x3

pile configuration was used which accounts about 146%

increase .

Keywords-component; piled raft modellng;

experimental program; PLAXIS.

I. INTRODUCTION

One of the most important aspect of Civil

Engineering projects is the foundation system.

Designing a foundation system carefully and properly,

will not only lead to a safe and efficient structure, butalso an overall economy of the project. Until recently,

there were some individual systems such as shallow

foundations like rafts and deep foundations like piles.

By combining these two systems, foundation

engineers will provide necessary values for the design,

obtain the required safety and also come out with more

economical solutions. A piled raft is a composite

foundation in which the piles are used as settlement

reducers and they share with the raft, the load from the

superstructure [5],[8]. In the design of piled rafts, the

load is shared between the piles and raft. The piles in

piled raft are used up to a load level that can be of thesame or even greater magnitude as that of a

comparable single pile.

Th f th il d ft f d ti ll

rectangular tank of size 450mm x 300mm x 450mm

and modelled plain raft of size 70mm x 70mm and

thickness 6mm [2]. In piled raft, the raft is of samesize and piles of 8mm diameter and length 180mm

with a spacing of 3.5 times the diameter of pile was

used. The configuration of piles used in this study was1x1, 2x2 and 3x3. Results showed that the piled raftfoundation in sand considerably reduces the

settlement compared to that of plain raft foundation.

Methods of Analysis

Numerous methods were available to

establish the interaction of piled raft combination. Thisinclude simplified calculation methods, approximate

computer-based methods and more rigorous computer

based methods. The simplified calculation methods are

empirical methods with numerous simplifications,mostly concerning the soil profile and the load

condition. In approximate computer-based methods,

the raft is modelled as a strip and the soil and the pilesare modelled with springs of varying stiffness. The

more rigorous computer-based methods consist of

different numerical methods and this research work is

mainly focused on the numerical analysis using

PLAXIS 2D software.

Plane strain FEM-model for piled rafts

The main problem when modelling a piledraft with a plane strain model is the transition from

three to two dimensions. The wall element is defined

per meter in the plane strain analysis using PLAXIS,the raft and piles were modelled as series of beam

elements with the appropriate geometrical parameters

and geometrical boundaries.

II. PARAMETRIC STUDY

Parametric study of piled raft refers to

studying the behaviour of the structure by changingtheir dimensions [1], [4]. The details of model are

Case 1: Plain raft 70mm 70mm square and of 6mm

thick.Case 2: Piled raft of 70mm 70mm with raft

thickness of 6mm and pile length of 180mm. The pileconfiguration is 1x1 and is named as PR 1x1.

Case 3: Piled raft of 70mm 70mm with raftthi k f 6 d il l th f 180 Th il


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A. Properties of Sand

All the laboratory tests were performed as per

IS specifications. The sand was tested to determine the

following preliminary properties and the results aretabulated in Table 1.

1. Gradation of sand

2.

Specific Gravity3. Unit weight

4. Maximum and minimum dry densities

5. Angle of internal friction6. Maximum and minimum void ratio.

Unit Weight

Dry unit weight of sand was determined by

vibrating table method conforming to IS 2720-Part 14.To determine the unit weight, several trials have been

carried out for varying heights of fall. It was found

that as the height of fall of sand increases, the density

of sand also increases. To verify this, a cylindricalcalibrating container conforming to IS 2720-Part 28was used to pour the sand with the help of a hopper.

The cylinder was filled with sand for different heights

of fall of 0cm, 10cm, 20cm, 25cm, 30cm, 35cm,

40cm, 50cm, 60cm, 70cm, 80cm, 90cm and 100cm.The height of fall of 25cm, 30cm and 35cm was fixed

for filling tank to achieve loose sand condition

corresponding to 20.55%, 26.63% and 32.97% relativedensity respectively.

Void ratio and Relative density

To determine the void ratio and relative

density of sand for varying heights of fall,

corresponding value of unit weights obtained from

previous study was used. The value of relative densityvaried from 0% to 100% for a void ratio of 0.72 to

0.47 respectively. The results of unit weight, void ratio

and relative density corresponding to various heightsof fall are shown in Table 2. The relationship of void

ratio vs height of fall,, relative density vs height of

fall, unit weight vs height of fall are illustrated in

figures 1 to 3 respectively.

Modulus of Elasticity (E)

Shear strength parameters of soil wasdetermined from Direct Shear test. The Modulus of

Elasticity was indirectly determined from shear

modulus and the value was found to be 10.24 MN/m2

for the relative density of 26.63%.

TABLE 1 PROPERTIES OF SAND

Sl.

No.Description Values

1 Specific gravity 2.65

2 Coarse sand 4.6

3 Medium sand 71.7

4 Fine sand 23.7

5 Uniformity coefficient 2.55

6 Coefficient of curvature 0.76

7 Classification of sandPoorly graded

sand (SP)

8 Maximum dry unit weight 17.63 kN/m3

9 Minimum dry unit weight 15.10 kN/m3

10 Maximum void ratio 0.72

11 Minimum void ratio 0.47

12 Unit weight 15.71 kN/m3

13 Relative density 26.63%

14 Angle of internal friction 30

15 Modulus of Elasticity 10.24 MN/m2

TABLE 2 HEIGHT OF FALL METHOD

Sl.

No.

Height

of fall

(cm)

Unit

weight

(kN/m3)

Relative

density

(%)

Void

ratio

Angle

of

intern

al

frictio

n ()

1 0 15.10 0 0.72 24

2 10 15.18 3.67 0.71 24

3 20 15.42 14.46 0.69 27

4 25 15.57 20.55 0.67 285 30 15.70 26.63 0.66 30

6 35 15.85 32.97 0.66 31

7 40 16.00 39.20 0.62 32

8 50 16.40 55.23 0.59 34

9 60 16.78 69.77 0.55 37

10 70 17.10 81.52 0.52 38

11 80 17.28 87.91 0.50 39

12 90 17.34 90.00 0.49 40

13 100 17.38 91.41 0.49 40

Fig 1. Relationship of void ratio and height of fall


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Fig 3. Relationship of Unit weight and height of fall

IV. EXPERIMENTAL PROGRAM AND

NUMERICAL ANALYSIS

Mild steel rods were used as model pileshaving diameter 8 mm and length 180 mm, forming

configurations 1x1, 2x2, 3x3 and the piled raft model

used in the study is shown in figures 4 to 6. The pilesare rigidly fixed to the raft by welding. The property

of mild steel is shown in Table 3.

TABLE 3 PROPERTIES OF MILD STEEL SPECIMEN

Sl.No. Properties Values

1 Yield stress (N/mm2) 416.74

2 Ultimate stress (N/mm2) 436.59

3 Modulus of Elasticity (N/mm2) 2.34 x105

Fig.4 Piled raft model of configuration 1x1

Fig.6 Piled raft model of configuration 3x3

A. Preparation of foundation medium

The sand was placed in the tank with

predetermined relative density of 26.63%, by

maintaining a dropping height of 30cm upto top levelor by taking the weight of sand equal to 21.60 kg that

is poured to the tank at every 100cm height until the

tank is filled upto the surface that corresponds to arelative density of 26.63%. After each test, the sand

box should be emptied to a depth below the zone of

influence (which was considered as L below the tip of

pile, where L is the pile length). During the process of

sand raining, the piled raft model was placed at thecenter of the tank and under the loading ring, a bubble

balance was used to ensure the level of the raft, then

the sand raining was continued upto the top slightly

lower than the raft. The final layer of the sand islevelled by a sharp edge ruler.

B. Loading Setup

The load frame consists of proving ring of

2kN capacity with 0.0025kN accuracy for load

determination and two numbers of 25mm dial gauge

with 0.01mm sensitivity for settlement prediction. The

entire loading setup is shown in figure 7.


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C .Application of vertical load and calibration in

PLAXIS

The vertical load was applied at a constant

loading rate of 1mm/min throughout the entire testing

program. The settlement of the system was measuredusing dial gauges for each load increment until the

settlement is less than 0.02mm/min. The load vs

settlement graph was plotted for each configuration ofpiled raft system. The obtained curves are calibrated in

PLAXIS 2D and parametric study for these

configurations were analysed. The ultimate load isdetermined by considering 10 % width of the footing.

Figures 8 to 10 show the parametric study using

PLAXIS 2D.

Fig.8 PR 1x1 in PLAXIS 2D

Fig.9 PR 2x2 in PLAXIS 2D

V RESULTS AND DISCUSSIONS

From the experimental study and FEM

analysis, it was observed that for plain raft and piled

raft (PR 1x1, PR 2x2 and PR 3x3), the load settlementvariation between the experimental study and PLAXIS

2D is within 15% upto the ultimate load and also the

FEM analysis underestimates the settlement andoverestimates the ultimate load when compared to that

of experimental study. Figures 11 to 14 show the

comparison between experimental study and PLAXIS2D. In all these graphs, it was found that the numerical

analysis showed slightly higher value of load than the

experimental work. Figure 15 shows the comparison

between load vs settlement for plain raft and piled raft

with different configurations namely PR 1x1, PR 2x2and PR 3x3. As the number of piles is increased, the

load carrying capacity of piled raft system is increased

significantly which can be observed from Table 4 andFigure 16. Figure 17 shows the settlement reduction

with pile raft combination as reported in Table 5 forvarying area ratio (ratio between the cross sectional

area of pile and area of raft).

Fig.11 Comparison of Plain raft in experimental and PLAXIS study


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VI CONCLUSIONS

Laboratory study with plain raft and piled raft

on loose sand foundation medium was conducted

and the following observations were made.(1)The failure load has increased from 234N for

plain raft to 390N, 505N and 578N for piled raft

of 1x1, 2x2 and 3x3 configurations respectively.This account to about 67%, 116% and 146%

increase in the carrying capacity for piled raft of

1x1, 2x2 and 3x3 respectively.(2)The settlement of plain raft is 7mm and it has

reduced to 3.19mm, 2.40mm and 2.02mm when

piled raft of 1x1, 2x2 and 3x3 configuration is

used. This is about 54%, 66% and 71% for the

respective combination.(3)With the increase in area ratio, the % reduction in

settlement initially increase rapidly and and

thereafter attains a marginal value and also the %increase in load carrying capacity increases

rapidly at first and then it maintains a gradualincrease .

(4) Analysis using PLAXIS 2D compares well withthe experimental values and hence it can be used

for studies on piled raft system of other

configurations.

VII REFERENCES

1. MeisamRabiei, 2009, Parametric Study for

Piled Raft Foundations, Electronic Journal

of Geotechnical Engineering ,Vol.14.2. S.P.Bajad, R. B. Sahu, 2012, An

Experimental Investigation on Interference of

Piled Rafts, Civil and EnvironmentalResearch, Vol.2, No.2, pp 49-58.

3. Rolf Katzenbach, Christian Gutberlet, Gregor

Bachmann, 2007, Soil-Structure Interactionaspects for ultimate limit state design of

complex foundations, First International

Symposium on Geotechnical Safety & Risk,

Shanghai, Tongji University, China, pp 585-

596.4. M. H. Baziar, A. Ghorbani , R. Katzenbach,

2009, Small-Scale Model Test and Three-

Dimensional Analysis of Pile-RaftFoundation on Medium-Dense Sand,

International Journal of Civil Engineerng,Vol.7, No.3, pp 170-175.

5. Poulos.H.G., 2001,Piled Raft FoundationDesign and Applications, Geotecnique,

Vol.51, No.2, pp 95-113.

6. Young-Kyo Seo and Kyung-sik Choi, Sung-

Gyo Jeong, 2003, Design Charts of Piled

Raft Foundations on Soft Clay, Proceedingsof The Thirteenth International Offshore and

Polar Engineering Conference, Honolulu,

Hawaii, USA, May 2530, pp 753-755.7. K. Yamashita, T. Yamada, 2009, Settlement

and load sharing of a piled raft with ground

improvement on soft ground, Proceedings ofthe 17

thInternational Conference on Soil

Mechanics and Geotechnical Engineering, pp

1236-1239.

8. H.G. Poulos, J.C. Small, H. Chow, 2009,

Piled Raft Foundations for Tall Buildings,Geotechnical Engineering Journal of the

SEAGS & AGSSEA, Vol. 42, No.2, pp 78-

84.

AUTHORS PROFILE

1. N. Venkateshwaran, M.E., Geotech

Faculty,

National Institute of Technology,

Thiruchirapalli, 6201015, India


2. Dr.S.P. Jeyapriya

Assistant Professor,

Government College of Technology,

Coimbatore 641013, India



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Experimental Observations and Parametric Study of Piled Raft Foundation Using Plaxis

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