Foundation design for tall buildingsFrom Pile Groups to Piled Rafts

Benoît LatapieTechnical Manager – Ground Engineering

16 November 2016 1

Underground Infrastructure and Deep Foundations UAE

1. Introduction to Combined Pile Raft Foundations (CPRF)2. Foundation design procedure / Case study3. Existing buildings on CPRF in Dubai4. Why carry out a preliminary test pile early and to failure?5. Benefits of using BS8004:20156. Common ME practice VS Atkins geotechnical approach7. Conclusions8. References

Foundation design for tall buildings

16 November 2016 2

From Pile Groups to Piled Rafts – Contents

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0 1 2 3 4 5 6 7 8 9 10Pile spacing in multiples of its diameter Ø

Negligible pile interaction

Intro. to Combined Pile Raft Foundation

16 November 2016 3

Pile group interactionS

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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).

• 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

Intro. to Combined Pile Raft Foundation

16 November 2016 5

The different types of foundations using rafts and piles

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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

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.

1. Introduction to Combined Pile Raft Foundations (CPRF)2. Foundation design procedure / Case study3. Existing buildings on CPRF in Dubai4. Why carry out a preliminary test pile early and to failure?5. Benefits of using BS8004:20156. Common ME practice VS Atkins geotechnical approach7. Conclusions8. References

Foundation design for tall buildings

16 November 2016 8

From Pile Groups to Piled Rafts – Contents

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.

Confidential project – case study

16 November 2016 10

Confidential project – case study

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Plaxis 3D settlements

Maximum settlement: 42mm16 November 2016

Confidential project – case study

16 November 2016 12

SAFE settlements with Springs from Plaxis 3D

Maximum settlement: 47mm

Confidential project – case study

16 November 2016 13

SAFE settlements with Springs from SI Report

Maximum settlement: 13mm

1. Introduction to Combined Pile Raft Foundations (CPRF)2. Foundation design procedure / Case study3. Existing buildings on CPRF in Dubai4. Why carry out a preliminary test pile early and to failure?5. Benefits of using BS8004:20156. Common ME practice VS Atkins geotechnical approach7. Conclusions8. References

Foundation design for tall buildings

16 November 2016 14

From Pile Groups to Piled Rafts – Contents

Existing tall buildings with CPRF in Dubai

16 November 2016 15

A few numbers from published literature

Building HeightFoundation System

(dimensions)

Predicted Settlement

Bui

lt

Emirates Twin Towers 355mCombined Piled Raft Foundation(1.2mØ, 45m piles & 1.5m thick raft)

140mm

(1/378)

Burj Khalifa 828mCombined Piled Raft Foundation(1.5mØ, 50m piles & 3.7m thick raft)

80mm

(1/250)

On

Hol

d Pentominium Tower 516mCombined Piled Raft Foundation

(1.2-1.5mØ, 32-42m piles & 5m thick raft)92mm

Nakheel Tall Tower 1,000mCombined Piled Raft Foundation

(2.8mx1.2m, up to 58m barrettes & up to 8m thick raft)

Up to 100mm

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

Existing CPRF arrangements in Dubai 2/2

16 November 2016 17

Pentominium Tower (on hold) Nakheel Tall Tower (on hold)

1. Introduction to Combined Pile Raft Foundations (CPRF)2. Foundation design procedure / Case study3. Existing buildings on CPRF in Dubai4. Why carry out a preliminary test pile early and to failure?5. Benefits of using BS8004:20156. Common ME practice VS Atkins geotechnical approach7. Conclusions8. References

Foundation design for tall buildings

16 November 2016 18

From Pile Groups to Piled Rafts – Contents

Recommendations – PTP to failure

16 November 2016 19

Recommendations – PTP to failure

16 November 2016 20

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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

1. Introduction to Combined Pile Raft Foundations (CPRF)2. Foundation design procedure / Case study3. Existing buildings on CPRF in Dubai4. Why carry out a preliminary test pile early and to failure?5. Benefits of using BS8004:20156. Common ME practice VS Atkins geotechnical approach7. Conclusions8. References

Foundation design for tall buildings

16 November 2016 22

From Pile Groups to Piled Rafts – Contents

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

• 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 toICE 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?

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

DefaultValue

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 SAVINGMinimum 25%

1. Introduction to Combined Pile Raft Foundations (CPRF)2. Foundation design procedure / Case study3. Existing buildings on CPRF in Dubai4. Why carry out a preliminary test pile early and to failure?5. Benefits of using BS8004:20156. Common ME practice VS Atkins geotechnical approach7. Conclusions8. References

Foundation design for tall buildings

16 November 2016 26

From Pile Groups to Piled Rafts – Contents

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

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STANDALONEGROUND

INVESTIGATION CONTRACTOR

CONSERVATIVE / UN-ECONOMIC / RISKY SOLUTION

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

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By GI Contractor

1. Foundation design for tall buildings is complex and requires sophisticated analyses2. 3D FE analyses encompass soil-structure interactions3. CPRF are economical and proven in Dubai4. Preliminary test piles with O-cells help reduce cost and risk5. BS8004:2015 is recommended for the design of foundations6. Ignoring project specific data raises the foundation risk7. Employing a specialist geotechnical consultant is recommended to maximise

opportunities

Conclusions

16 November 2016 29

• 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

Foundation design for tall buildingsFrom Pile Groups to Piled Rafts

Benoît LatapieTechnical Manager – Ground Engineeringbenoit.latapie@atkinsglobal.com16 November 2016 31

Underground Infrastructure and Deep Foundations UAE

QUESTIONS?