 Open Access
 Authors : Nirav Agrawal , Dr. Vinubhai Ratilal Patel
 Paper ID : IJERTV10IS090124
 Volume & Issue : Volume 10, Issue 09 (September 2021)
 Published (First Online): 22092021
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
A Study on Load Distribution Mechanism of PileRaft Foundation Systems
Nirav Agrawal 1, Dr. Vinubhai Ratilal Patel 2
1Post Graduate Student, Civil Engineering Department, MS University, Vadodara,
2Associate Professor Applied mechanics and structural engineering department, M S University, Vadodara, Gujarat , India,
Abstract: Design concepts and load sharing mechanism of piled raft foundations have been studied in this thesis. In the conventional piled foundations, the load transferred only by the piles and the piles are used for the reducing of both total and differential settlements and the contribution of the raft is generally disregarded. In the first part of the thesis, design approaches in the literature have been discussed. In the second part of the thesis, parametric analyses have been conducted for typical foundation in over consolidated Bhaili clay and finite element analyses have been done for Aura prime building. Three dimensional analyses have been made by the widely used commercial software of Plaxis 3D and SAFE2016, which solve the models by using the Finite Element Method. The foundation settlement and the load sharing between raft and pile have been investigated to identify the contribution of raft to the total capacity of piled raft foundations. The results showed that the raft can carry up to the 41.52% of the total applied load for an optimum number of piles for acceptable settlement levels.
Keywords: Pile, raft, foundation, plaxis3d, deepfoundation, soilinteraction, settlement, soilpile interaction, safe2016
INTRODUCTION
One of the most important aspects of a civil engineering project is the foundation system. Designing the foundation system carefully and properly, will surely lead to a safe, efficient and economic project overall. In other words, foundation system design is one of the most critical and important step when a civil engineering project is considered. Until quite recently, there were some separately used systems like shallow foundations such as rafts and deep foundations such as piles. However, lately the foundation engineers tend to combine these two separate systems. By combining these two systems, the foundation engineer will provide the necessary values for the design obtain the required safety and also come out with a more economical solution.
To carry the excessive loads that originate from the superstructures like elevated structures, spans, power plants or other common structures and to prevent excessive settlements, piled foundations have been created and broadly utilized in late decades. However, it is seen that the design of foundations considering just the pile or raft is not a feasible solution because of the load sharing mechanism of the pileraftsoil. In this manner, the blend of two separate systems, to be specific "Piled Raft Foundations" has been created (Clancy and Randolph (1993)).
Piled Raft foundation system is verified to be economical foundation type comparing with the traditional piled foundations , where, just the piles are utilized for the diminishing both aggregate and differential settlements and the contribution of the raft is commonly ignored.
In this examination, conduct of the piled raft foundation systems under axial loads has been explored by comparing the traditional design approaches and investigating parametric analyses. In the literature, there are a lot of explores centering of these boundaries, like; the number of piles, length of piles, diameter of piles, pile spacing ratio, location of piles, stiffness of piles, distribution of load, level of load, raft thickness, raft dimensions and type of soil. In any case, through these boundaries, the number of piles, length of piles and level of load are emphasized in this examination. Effects of these parameters are discussed with the solutions of finite element models. To this end, parametric investigations are directed through the software Plaxis 3D and SAFE2016 with the comparisons.
FINITE ELEMENT ANALYSIS OF PILED RAFT FOUNDATIONS
Geotechnical and Material Parameters for Input
Piled raft on two different softwares were analyzed in this study. A typical 23storey building in Bill village soil, in software PLAXIS3D. The second one is in software SAFE2016 with same building and soil parameters.
A typical 23storey building in BHAILI
Soil Properties: The SPT data was taken from a report for a construction site on Bhaili, where the soil is commonly overconsolidated clay. Raw SPTN values for four different boreholes are shown in Figure1.
Average of the SPTN values has been taken to derive a single corrected SPT value, N60, which is shown in Figure2. It is assumed that the stiffness of the soil is linearly increasing; therefore the profile of the corrected N values is deeper than the
profile of raw SPTN values. For the input parameters of the Plaxis 3D, it is needed to convert the SPTN values to soil stiffness parameter E. The correlation of Stroud (1975) has been used for the N60 – E relationship.
E = N60 0.8
Where E is in MPa
10
10
20
20
30
30
40
40
50
50
BH2
BH1 BH3
BH4
BH2
BH1 BH3
BH4
DEPTH(m)
DEPTH(m)
Corresponding E values vs. depth has been shown in the Figure3. E increases averagely 1.600 MPa per one meter depth, starting from the 10.24 MPa at ground level.
SPTN
0
2 0
4
6
8
10
12
14
16
18
SPTN
0
2 0
4
6
8
10
12
14
16
18
Figure 1 SPTN values vs Depth(m)
N60
17 
20 
28 
33 
38 
42 

BH2 BH1 BH3 BH4 

17 
20 
28 
33 
38 
42 

BH2 BH1 BH3 BH4 

0
2
4
DEPTH(m)
DEPTH(m)
6
8
10
12
14
16
18
Figure 2 Corrected SPTN values
0
2
4
Depth(m)
Depth(m)
6
8
10
12
14
16
18
102 
40 112 
00 137 
60 153 
60 169 
60 182 
40 
102 
40 112 
00 137 
60 153 
60 169 
60 182 
40 
E'(Kpa)
Depth(m)
Figure 3 E (kPa) vs. Depth (m)
Table 1 – Soil Properties for BHAILI CLAY
Parameter 
Symbol 
Overconsolidated Clay 
Unit 
Material Model 
– 
MohrCoulomb 
– 
Unsaturated weight 
unsat 
18 
kN/m3 
Saturated weight 
sat 
18 
kN/m3 
Stiffness 
E 
10,240 
kN/m2 
Stiffness Increment 
Eincrement 
1600 
kN/m2/m 
Cohesion 
cref 
12 
kN/m2 
Friction angle 
25 
– 

Poisson ratio 
' 
0.25 
– 
Lateral pressure coefficient (K0x=K0z) 
K0 
0.8 
– 
Interface stiffness ratio 
Rinter 
1.0 
– 
Drainage Type 
– 
Drained 
– 
Structural Properties: The building is assumed as reinforced concrete building with 23 storey and 28x22m floor area. Piled raft foundation located at 5m below ground level. Raft thickness and the length of the piles are variable. Raft and pile properties used in the Plaxis 3D model are shown in Table 2 and Table 3. Piles are designed as embedded piles. Massive circular pile was selected among the predefined pile types for embedded piles. Skin resistance and the base resistance of embedded piles must be calculated and specified for the material input phase. The correlation of Stroud (1975), shown in Figure 28, has been used for the conversion of SPTN values to shear strength of soil by taking the coefficient f1 as 4.6. In addition the unit skin friction multiplier is taken as 0.35 of Cu. With the multiplication of these coefficients by related area and circumference values of piles,
maximum skin resistance at the top (Ttop ) and the bottom (Tbottom) of the pile and the base resistance were calculated for the
max max
piles with a diameter of 1m. Material and section properties, used in SAFE2016 Analyses of raft and pile are shown in Table 4 and Table 5 for Building in BHAILI CLAY.
Table 2 Raft Properties for BHAILI CLAY
Parameter 
Symbol 
Raft 
Unit 
Material Model 
– 
LinearIsotropic 
– 
Unit weight 
25 
kN/m3 

Stiffness 
Eref 
2.7386E+07 
kN/m2 
Poisson ratio 
0.2 
– 

Thickness 
t 
1.6 
m 
Width – Breadth 
W x B 
30 x 24 
m 
Table 3 Embedded Pile Properties for BHAILI CLAY
Parameter 
Symbol 
Embedded Pile 
Unit 

Material Model 
– 
Linear Elastic 
– 

Unit weight 
30 
kN/m3 

Stiffness 
Eref 
2.7386E+07 
kN/m2 

Diameter 
d 
0.75 
m 

Length 
L 
15 
25 
m 
Ttop max 
– 
16.57 
16.57 
kN/m 
Tbottom max 
– 
55.79 
81.94 
kN/m 
Base resistance 
Fmax 
281.46 
410 
kN 
No. of piles 
– 
110 & 210 
– 
Table 4 Material and section properties of Raft in BHAILI CLAY for SAFE2016 Analyses
Parameter 
Symbol 
Raft 
Unit 
Property 
– 
Shell Thick 
– 
Unit weight 
25 
kN/m3 

Modulus of Elasticity 
E 
2.7386E+07 
kN/m2 
Poisson ratio 
0.2 
– 

Thickness 
t 
1.6 
m 
Width – Breadth 
WxB 
30 x 24 
m 
Table 5 Material and section properties of Pile in BHAILI CLAY for SAFE2016 Analyses
Parameter 
Symbol 
Pile 
Unit 
Property 
– 
Frame – Pile 
– 
Unit weight 
25 
kN/m3 

Modulus of Elasticity 
E 
2.7386E+07 
kN/m2 
Poisson ratio 
0.2 
– 

Diameter 
d 
0.75 
m 
Length 
L 
15 & 25 
m 
No. of piles 
– 
110 & 210 
– 
Building1: A typical 23storey building in BHAILI
The building is a reinforced concrete building with 23storey and 28x22m floor area. Piled raft foundation located at 5m below ground level (assuming two basements). Lengths of the piles are variable. Due to loading and the shape/geometry of the structure and also the soil beneath the foundation only a quarter of the foundation has been taken into account and the center of the model foundation has been placed in alignment with the zaxis the as shown in Figure 5. For the parametric analyses of the Building1, subcases have been used, which are listed in the Table 6.
Table 6 Subcases of Building1
SubCase 
Number of piles 
Pile length (m) 
Distributed load (kPa) 
Model1 
110 
15 
390 
Model2 
110 
25 
390 
Model3 
210 
15 
390 
Model4 
210 
25 
390 
Figure 5 Deformed shape of Finite Element method of the piled raft foundation of Model4
Raft and pile properties, including the geometrical properties, were previously tabulated in Table 5 and in Table 6. The raft thickness is taken as 1.6m. Ultimate capacity of 15m and 25m length piles are 3347kN and 7374kN respectively.
The maximum total (dead and live) load has been taken as 16.9kN/m2/floor. Therefore, the maximum total design load of the structure becomes 202034KN for the selected case with 23 storey building. Total 390 kN/m2 load applied to the foundation in the direction of gravity as distributed load.
SAFE2016 Analyses
Simplified Method (Using the outputs of Manual Calculation):
SAFE2016 software has been used for back analyses of the model. As mentioned before, the main supports are joint springs and area springs in SAFE2016. The stiffness of soil is modeled by the joint and area springs. A sample of a 3D view of the Sap2000 model is shown in Figure 30. Raft settlements are taken as average by the suggested formula of Davis & Taylor (1962) as following;
Savg =
1
(2Scentre + Scorner)
3
To find the spring constant of raft and pile separately, a soil capacity is calculated by using soil parameters of site in BHAILI SOIL. Skin Resistance at bottom of pile is calculated by theoretical equation for 15m length and 25m length piles are 55.796KN/m and 81.94KN/m respectively. Similarly, End Bearing RÃ©sistance for 15m length and 25m length are calculated as 281.46KN and 410KN respectively. By considering the different allowable displacement of pile as 5mm, 10mm, 20mm, 50mm different spring constant are calculated shown in Table7
Figure 6: Plan View of SAFE2016 model for Model1
TABLE 7 FOR PILE SPRING CONSTANT 

Allowable displacement (mm) 
5(mm) 
10(mm) 
20(mm) 
50(mm) 
15m length pile 
171546KN/m 
85773 KN/m 
42886KN/m 
17154KN/m 
25m length pile 
413560KN/m 
206780KN/m 
103390kN/m 
41356KN/m 
Disregarding the outputs of Plaxis
Separate analyses have been conducted to observe the behavior of piled raft by disregarding the outputs of Plaxis. In these analyses, allowable loading capacities of piles have been divided to the variable allowable settlements (at the top of the pile) to the usage of spring stiffness of piles. Initially, maximum allowable settlement is taken as 0.01m. This results in to a spring constant of 85773 KN/m and 206780KN/m for 15m and 25m length piles, respectively. Also, to analyze the effect of raft contribution, raft springs have been used as; 0, 1000, 2000, 5000, 10000 kN/m/m2. This process has been repeated for the allowable settlements; 0.05m, 0.02m, 0.01m and 0.005m. All results are listed in Appendix V. Critical pile loads are listed in Table 8 and the locations of piles are shown in a quarter of the piled raft in Figure 7.
Figure 7 Piles taken as reference shown in a quarter of the piled raft.
Table 8 Comparison of pile loads (Model115m pile length)
Springs 
Pile Loads (kN) 

Pile (kN/m) 
Raft (kN/m/m2) 
A 
B 
C 

SAFE 2016 
k for 0.005m 
171546 
10,000 
1458.165 
2181.863 
1782.005 
k for 0.010m 
85773 
10,000 
1230.728 
1585.339 
1413.817 

k for 0.020m 
42886 
10,000 
971.77 
1091.729 
1045.988 

k for 0.050m 
17154 
10,000 
664.295 
645.723 
662.954 
Pile load distributions
Case 1: A typical 23storey building in BHAILI
Axial load distributions are plotted in Figure 8. All the piles are 15 m length. It is observed that the center piles have lower axial loads on both 110 piles and 210 piles situations. This is because of the pile group effects. However, for 210pile case, it is observed that the axial load of the center pile is almost half of the outer piles. Center pile can approach to the load level of outer piles after the depth of 10m and moves parallel after this point. This may be the effect of small ratio of pile spacing and pile diameter, which causes the block movement of soil just beneath the center of raft. Therefore, considering the movement of the soil beneath the center of raft, shown in Figure 9 and Figure 10, the behavior of the center and corner piles is in reasonable.
Axial Load (KN)
0
0 200 400 600 800 1000
5
10
15
20
25
110pilesCenter
110pilesMid Edge
110pilesCorner
Figure 8 Comparison of axial load distributions along piles for different number of piles (Model1 (110piles, 15m, 390kPa) vs. Model3(210piles, 15m, 390kPa))
0
0
0
500
1000
1500
2000
0
500
1000
1500
2000
5
10
15
15mCenter Pile
20 15mMidedge pile
15mCorner pile
25 25mCenter pile
30 25mMid edge pile
25mCorner pile
35
5
10
15
15mCenter Pile
20 15mMidedge pile
15mCorner pile
25 25mCenter pile
30 25mMid edge pile
25mCorner pile
35
Figure 9 Comparison of axial load distributions along piles for different length of piles (Model1 (110piles, 15m, 390kPa) vs. Model2 (110piles, 25m, 390kPa))
0
0
0
200
400
600
800
1000
1200
0
200
400
600
800
1000
1200
5
10
15
20
25
30
35
15m Center piles
15mMidedge piles 15mCorner piles 25mCenter piles 25mMid edge piles
25mCorner piles
5
10
15
20
25
30
35
15m Center piles
15mMidedge piles 15mCorner piles 25mCenter piles 25mMid edge piles
25mCorner piles
Figure 10 Comparison of axial load distributions along piles for different length of piles (Model3 (210piles, 15m, 390kPa) vs. Model4 (210piles, 25m, 390kPa))
Load Sharing of Raft
After all of the analyses have been carried out, the loads on each pile are added and subtracted from the total applied load to find the total load carried by the raft. The values are tabulated in Table 9. As stated in the previous subject, the usage of longer piles increases the shared load of piles. Other points can be listed as; total load carried by the raft increases in higher load levels and it also increases in higher number of piles for longer piles. However, the load on raft is decreased by the higher number of piles for shorter piles.
Table9 Model cases for Building1 with variable number of piles, pile length
Models 
Number of piles 
Pile Length(m) 
Dist. Load(kPa) 
Total Load(KN) 
Pile Load(KN) 
Raft Load(KN) 
Load on Piles(%) 
Load on raft(%) 
1 
110 
15 
390 
280800 
86285 
194514 
30.73% 
69.27% 
2 
110 
25 
390 
280800 
172359 
108440 
61.38% 
38.62% 
3 
210 
15 
390 
280800 
164040 
116759 
58.42% 
41.58% 
4 
210 
25 
390 
280800 
23036 
50563 
81.99% 
18.01% 
Table10 Comparison of load and settlement with different spring constant
Spring constant 
Load (KN) 

Max Allow. Sett 
Pile (KN/m) 
Raft (KN/m/m2) 
Pile (KN) 
Raft (KN) 
Pile load/Total load 
5mm 
171546 
0 
231324 
0 
100% 
171546 
5000 
195776 
35548 
84.6% 

171546 
10000 
171005 
60318 
73.92% 

10mm 
85773 
0 
231324 
0 
100% 
85773 
5000 
171982 
59341 
74.34% 

85773 
10000 
139650 
91673 
60.36% 

20mm 
42886 
0 
231324 
0 
100% 
42886 
5000 
140426 
90898 
60.70% 

42886 
10000 
105837 
231324 
45.75% 

50mm 
17154 
0 
231324 
0 
100% 
17154 
5000 
96046 
135277 
41.52% 

17154 
10000 
68968 
162356 
29.81% 
Table18 Maximum and minimum Settlement of Rafts
PLAXIS 
SAFE2016 (Considering raft spring 2000KN/m2) 

Max. Sett. (mm) 
Min. Sett. (mm) 
Max. Sett. (mm) 
Min. Sett. (mm) 

Model1(110 piles 15m) 
134 
105 
80 
65 
Model2(110 piles 25m) 
73 
60 
51 
34 
Model3(210 piles 15m) 
98 
80 
54 
42 
Model4(210 piles 25m) 
72 
58 
31 
20 
0
0
Raft spring0KN/m2 Raft spring1000KN/m2 Raft spring2000KN/m2 Raft spring5000KN/m2
Raft spring10000KN/m2
PLAXIS3D
Raft spring0KN/m2 Raft spring1000KN/m2 Raft spring2000KN/m2 Raft spring5000KN/m2
Raft spring10000KN/m2
PLAXIS3D
0
0
20
20
40
40
60
60
80
80
100 120
100 120
50
50
100
100
150
150
PILE NUMBER
PILE NUMBER
SETTLEMENT(mm)
SETTLEMENT(mm)
Figure 11 Settlements at the head of piles (i.e. Outer piles)
(Model 1110 piles15m)
SUMMARY AND CONCLUSION
Piled raft systems are verified to be an economical foundation type comparing the conventional piled foundations, where, only the piles are used for the reducing of both total and differential settlements and the contribution of the raft is generally disregarded. In this study, the foundation settlement and the load sharing between raft and pile have been investigated to identify the contribution of raft to the total capacity of piled raft foundations.
In the first part of this study, a detailed literature review for the design of piled raft foundations has been presented. Advantages and disadvantages of different approaches have been discussed to model the piled raft foundation systems. Also the factors affecting the behavior of piled raft foundations have been discussed. Discussed factors are; the number of piles, length of piles, diameter of piles, pile spacing ratio, location of piles, stiffness of piles, distribution of load, level of load, raft thickness, raft dimensions and type of soil.
In the second part, a case has been created for Bhaili Clay and parametric analyses have been conducted with the help of Plaxis 3D software. Variables for the parametric analyses are pile number, pile length. In addition, a case study, SAFE2016 Analyses validate the method of the calculation. Results show that the calculation method is in line with the actual piled raft behavior.
CONCLUSIONS
A method presented to find the settlements in Safe2016 by using the outputs of the Plaxis. Using the embedded pile feature of Plaxis, raft and pile load sharing is calculated and the corresponding loadsettlement curves are plotted. Taking the constant of the slope of this curve as the total spring of elements, corresponding average spring constants are assigned to the piles and raft in Safe2016.
The proposed method of analysis becomes more realistic with the additional soil springs connected to the raft. This may be helpful for structural/foundation engineers to calculate the deformations more accurate and the structural properties of raft more effective.
Average percentage of raft load share is 41.52% for the allowable settlement of 65mm for the Plaxis Analysis. For the SAFE 2016 Analysis, at the final design load, raft can carry up to 29.81% of applied load. The main reasons for the difference in these two cases are the Raft Spring, the foundation characteristic (number of piles and length of piles) and the soil stiffness.
HYP8: The principle of consuming income and not capital is not applied in construction project management.
REFERENCES

Behavior of large piledraft foundation on clay soil by Shivanand Mali, Baleshwar Singh IIT Guwahati

Physical modeling of behaviors of castinplace concrete piled raft compared to freestanding pile group in sand by Mehdi Sharafkhah, Issa Shooshpasha (Babol Noshiravani University of Technology, Iran)

Modelling the behaviour of piled raft applying Plaxis 3D Foundation Version 2 by (Yasser ElMossallamy, Associate Prof., Ain Shams University, Cairo, Egypt c/o ARCADIS GmbH, Berliner Allee 6, D – 4295 Darmstadt, Germany).

2D and 3D Numerical Simulation of LoadSettlement Behaviour of Axially Loaded Pile Foundations by Gowthaman S(Uni. Of Jaffna), Nasvi MCM (Uni. Of Peradeniya) Sri Lanka.

3DAnalysis of SoilFoundationStructure Interaction in Layered Soil by Mohd Ahmed1*, Mahmoud H. Mohamed2, Javed Mallick3, Mohd Abul Hasan4 (Civil Engineering Department, Faculty of Engineering, King Khalid University, Abha, Saudi Arabia)

An Analytical Approach for PiledRaft Foundation Design Based on Equivalent Pier and Raft Analyses by Using 2D Finite Element Method by Fatih Celik (Saudi Society for Geosciences 2019)

Analysis of Piled Raft Foundations by Jayarajan P, Kouzer KM. Analysis of Piled Raft Foundations. Indian Journal of Science

DESIGN METHODS FOR PILE GROUPS AND PILED RAFTS METHODES by M.F. Randolph (the University of Australia)

EQUIVALENT PIER THEORY FOR PILED RAFT DESIGN by Balakumar Venkatraman(Simplex Infrastructure limited), Erwin Oh (Griffith University), Arumugam Balasubramaniam (Griffith University)

Soilstructure interaction in a combined pileraft foundation a case study by Ashutosh Kumar, Milind Patil, Deepankar Choudhury (IIT BOMBAY)