1. Foundation design for tall buildings
From Pile Groups to Piled Rafts
Benoît Latapie
Technical Manager – Ground Engineering
16 November 2016 1
Underground Infrastructure and Deep Foundations UAE

2. 1. Introduction to Combined Pile Raft Foundations (CPRF)
2. Foundation design procedure / Case study
3. Existing buildings on CPRF in Dubai
4. Why carry out a preliminary test pile early and to failure?
5. Benefits of using BS8004:2015
6. Common ME practice VS Atkins geotechnical approach
7. Conclusions
8. References
Foundation design for tall buildings
16 November 2016 2
From Pile Groups to Piled Rafts – Contents

3. 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0 1 2 3 4 5 6 7 8 9 10
Pile spacing in multiples of its diameter Ø
Negligible pile interaction
Intro. to Combined Pile Raft Foundation
16 November 2016 3
Pile group interaction
Shearstressnormalisedbyshearstressatpile/rockinterface(-)
Most CPRF have average
pile spacing larger than 5Ø
Typical range of fully
piled foundations
High pile interaction
Piles close to each other
are not efficient.
Larger pile spacing have
the following benefits:
• Decreases pile-to-pile
interactions
• Increases pile
utilisation
• Increases foundation
efficiency
After the studies of Cooke (1974), Frank (1974) and Baguelin et al. (1975).

4. • Composite foundation that combines the bearing resistance of both raft and piles
• Building loads shared between the piles and the raft
• Piles are settlement reducers
• The raft provides additional load capacity to the piles
• Stiffness governs the design over bearing capacity
Intro. to Combined Pile Raft Foundation
16 November 2016 4
Design philosophy
Piled foundation design = Capacity Limiting + Settlement Check
CPRF design = Settlement Limiting + Capacity Check

5. Intro. to Combined Pile Raft Foundation
16 November 2016 5
The different types of foundations using rafts and piles
Source:Burland,J.,Chapman,T.,Skinner,H.D.andBrown,
M.,2012.ICEManualofGeotechnicalEngineering.

6. Piles located to reduce
shear and bending in raft.
Pile capacity fully
mobilised.
B - Pile-enhanced raft
Piles at larger spacing than
conventional pile group.
Piles located to minimise
differential settlement.
Intro. to Combined Pile Raft Foundation
16 November 2016 6
The different types of foundations using rafts and piles
Source: Burland, J., Chapman, T., Skinner, H.D. and Brown, M., 2012. ICE Manual of Geotechnical Engineering.
C - raft-enhanced pile group

7. Intro. to Combined Pile Raft Foundation
16 November 2016 7
Complex soil-structure interaction mechanisms
1. Raft–soil interaction: The contact stresses between the raft and
the soil are transmitted into the soil and settlement of the raft
takes place.
2. Raft–soil–raft interaction: Interaction takes place through the soil
with other parts of the raft.
3. Raft–soil–pile interaction: The raft contact stresses are also
transmitted through the soil and interact with the piles.
4. Raft–pile interaction: Loads are transmitted into the piles directly
by the raft.
5. Pile–soil interaction: The pile loads disperse into the ground
surrounding the piles.
6. Pile–soil–pile interaction: Interaction takes place between each
pile through the soil to other piles.
7. Pile–soil–raft interaction: Interaction also takes place between
each pile through the soil to the underside of the raft.
Source: Burland, J., Chapman, T., Skinner, H.D. and Brown,
M., 2012. ICE Manual of Geotechnical Engineering.

8. 1. Introduction to Combined Pile Raft Foundations (CPRF)
2. Foundation design procedure / Case study
3. Existing buildings on CPRF in Dubai
4. Why carry out a preliminary test pile early and to failure?
5. Benefits of using BS8004:2015
6. Common ME practice VS Atkins geotechnical approach
7. Conclusions
8. References
Foundation design for tall buildings
16 November 2016 8
From Pile Groups to Piled Rafts – Contents

9. 9
Foundation Design Procedure
16 November 2016
Outputs from the geotechnical analysis
For all structural elements, deflections, axial
forces, shear forces and bending moment
diagrams are available in the output.

10. Confidential project – case study
16 November 2016 10

11. Confidential project – case study
11
Plaxis 3D settlements
Maximum settlement: 42mm
16 November 2016

12. Confidential project – case study
16 November 2016 12
SAFE settlements with Springs from Plaxis 3D
Maximum settlement: 47mm

13. Confidential project – case study
16 November 2016 13
SAFE settlements with Springs from SI Report
Maximum settlement: 13mm

14. 1. Introduction to Combined Pile Raft Foundations (CPRF)
2. Foundation design procedure / Case study
3. Existing buildings on CPRF in Dubai
4. Why carry out a preliminary test pile early and to failure?
5. Benefits of using BS8004:2015
6. Common ME practice VS Atkins geotechnical approach
7. Conclusions
8. References
Foundation design for tall buildings
16 November 2016 14
From Pile Groups to Piled Rafts – Contents

15. Existing tall buildings with CPRF in Dubai
16 November 2016 15
A few numbers from published literature
Building Height
Foundation System
(dimensions)
Predicted
Settlement
Built
Emirates Twin Towers 355m
Combined Piled Raft Foundation
(1.2mØ, 45m piles & 1.5m thick raft)
140mm
(1/378)
Burj Khalifa 828m
Combined Piled Raft Foundation
(1.5mØ, 50m piles & 3.7m thick raft)
80mm
(1/250)
OnHold
Pentominium Tower 516m
Combined Piled Raft Foundation
(1.2-1.5mØ, 32-42m piles & 5m thick raft)
92mm
Nakheel Tall Tower 1,000m
Combined Piled Raft Foundation
(2.8mx1.2m, up to 58m barrettes & up to 8m thick raft)
Up to
100mm

16. Existing CPRF arrangements in Dubai 1/2
16 November 2016 16
Emirates Twin Towers Burj Khalifa
Contours of maximum axial load [kN]
Typ. layout of one tower

17. Existing CPRF arrangements in Dubai 2/2
16 November 2016 17
Pentominium Tower (on hold) Nakheel Tall Tower (on hold)

18. 1. Introduction to Combined Pile Raft Foundations (CPRF)
2. Foundation design procedure / Case study
3. Existing buildings on CPRF in Dubai
4. Why carry out a preliminary test pile early and to failure?
5. Benefits of using BS8004:2015
6. Common ME practice VS Atkins geotechnical approach
7. Conclusions
8. References
Foundation design for tall buildings
16 November 2016 18
From Pile Groups to Piled Rafts – Contents

19. Recommendations – PTP to failure
16 November 2016 19

20. Recommendations – PTP to failure
16 November 2016 20

21. 0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
0 10 20 30 40 50 60 70 80 90 100
TotalAxialLoad[MN]–Example,typical
Deflection at Pile's Head [mm] – Example, typical
Pile Load Test To Failure
Conservative Pile Load Test
Recommendations – PTP to failure
16 November 2016 21
Optimised
working load
We propose to test the pile
to geotechnical failure
FoS=2
FoS=2
Conservative test stops
at empirical capacity
Under estimated
working load

22. 1. Introduction to Combined Pile Raft Foundations (CPRF)
2. Foundation design procedure / Case study
3. Existing buildings on CPRF in Dubai
4. Why carry out a preliminary test pile early and to failure?
5. Benefits of using BS8004:2015
6. Common ME practice VS Atkins geotechnical approach
7. Conclusions
8. References
Foundation design for tall buildings
16 November 2016 22
From Pile Groups to Piled Rafts – Contents

23. Benefits of using BS 8004:2015
16 November 2016 23
Time for an update?
https://login.dm.gov.ae/wps/wcm/connect/29d53e7c-7f22-4c42-aded-
066727d87c0f/Structural+Codes+%26+guidelines+4.pdf?MOD=AJPERES

24. • BS 8004:2015 was updated in line with the Eurocodes
• Partial factors may be reduced if pile load tests are carried out
• Clear and detailed guidance on foundation design
• Piled rafts are addressed with reference to
ICE Manual of Geotechnical Engineering (2012), Volume II, Chapter 56 is provided.
Benefits of using BS 8004:2015
16 November 2016 24
Time for an update?

25. Benefits of using BS 8004:2015
16 November 2016 25
Partial factors reduction based on in situ pile testing
• Two partial factors are used in pile design to BS 8004:2015 (or/and BS EN 1997-1)
 γRd model factor, used to obtain the characteristic ultimate pile
compressive resistance Rc,k
 γs and γb partial resistance factors for shaft and end bearing respectively.
Partial
factor
Default
Value
Testing to BS 8004:2015
(i.e. UK NA to BS EN 1997-1)
Revised
Value
Saving
γRd 1.4
Maintained load test taken to the required
unfactored ultimate resistance
1.2 14%
γs 1.6 Explicit verification of SLS:
Working test on 1% of constructed piles to loads
not less than 1.5 times the representative load.
1.4 12%
γb 2.0 1.7 15%
Total SAVING
Minimum 25%

26. 1. Introduction to Combined Pile Raft Foundations (CPRF)
2. Foundation design procedure / Case study
3. Existing buildings on CPRF in Dubai
4. Why carry out a preliminary test pile early and to failure?
5. Benefits of using BS8004:2015
6. Common ME practice VS Atkins geotechnical approach
7. Conclusions
8. References
Foundation design for tall buildings
16 November 2016 26
From Pile Groups to Piled Rafts – Contents

27. Common geotechnical approach in ME
• Early stage project involvement
• Desk study
• Design/scoping and specification of high quality GI
• Carry out GI
• Full time GI supervision
• Factual reporting by contractor
• Review of factual reporting
• Interpretation
• Ground model + parameters + design
27
STANDALONE
GROUND
INVESTIGATION
CONTRACTOR
CONSERVATIVE / UN-ECONOMIC / RISKY SOLUTION

28. ECONOMIC RISK CONTROLLED FOUNDATION SOLUTION
• Early stage project involvement
• Desk study
• Design/scoping and specification of high quality GI
• Carry out GI
• Full time GI supervision
• Factual reporting by contractor
• Review of factual reporting
• Interpretation
• Ground model + parameters + design
Atkins proposal to reduce geotechnical risk
28
SPECIALISTGEOTECHNICAL
CONSULTANT
By GI Contractor

29. 1. Foundation design for tall buildings is complex and requires sophisticated analyses
2. 3D FE analyses encompass soil-structure interactions
3. CPRF are economical and proven in Dubai
4. Preliminary test piles with O-cells help reduce cost and risk
5. BS8004:2015 is recommended for the design of foundations
6. Ignoring project specific data raises the foundation risk
7. Employing a specialist geotechnical consultant is recommended to maximise
opportunities
Conclusions
16 November 2016 29

30. • BS 8004:2015 Code of practice for foundations
• Haberfield, C.M. and Paul, D.R. 2011. Footing Design of the Nakheel Tower, Dubai,
UAE in Workshop on soil-structure interaction and retaining walls. Proceedings of the
Technical Meeting TC207 ISSMGE, Dubrovnik, pp 35 to 52.
• Poulos, H.G. and Bunce, G., 2008. Foundation design for the Burj Dubai–the world’s
tallest building. 6th International Conference on Case Histories in Geotechnical
Engineering, Arlington, VA.
• Poulos, H.G. and Davids, A.J., 2005. Foundation design for the Emirates twin towers,
Dubai. Canadian Geotechnical Journal, 42(3), pp.716-730.
• Ibrahim, K., Bunce, G. and Murrells, C., 2009, December. Foundation design for the
Pentominium tower in Dubai, UAE. In Proceedings of the Institution of Civil
Engineers-Civil Engineering (Vol. 162, No. 6, pp. 25-33). Thomas Telford Ltd.
References
16 November 2016 30

31. Foundation design for tall buildings
From Pile Groups to Piled Rafts
Benoît Latapie
Technical Manager – Ground Engineering
benoit.latapie@atkinsglobal.com
16 November 2016 31
Underground Infrastructure and Deep Foundations UAE
QUESTIONS?