Frances Badelow.pdf

7/27/2019 Frances Badelow.pdf

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PILED RAFT DESIGN

Frances Badelow


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Outline of Presentation

Advantages of piled rafts

Describe a design process for piled raft foundations

Present two case studies


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Why a Piled Raft ?

Raft foundation providesstability in strength case(ultimate)

Pile foundation able tosupport large loads without

significant displacement(serviceability)

Piled raft Ultimate loadcapacity and total anddifferential settlementperformance improved

Performance of raft enhancedby strategic placement of pilesin areas of high load


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Advantages of Piled Rafts

Potential for substantial savings

Strategic location of piles to control differential

settlements

Different length and/or diameter piles to optimize design

Varying raft thickness to optimize foundation design

Piles designed to carry load approaching their ultimate

load


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Where Piled Rafts are Most Effective

Raft provides significant stiffness and load capacity

stiff clays near the surface

stiffer "crust" overlying weaker soil

relatively dense sand

A piled raft concept can significantly reduce the number

of piles

May be some increase in raft thickness or reinforcement


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Where Piled Rafts May Not Be Effective

Very soft clays at or near the surface (raft contributes a

small proportion of ultimate load capacity)

Profiles which may be subjected to long-term

consolidation settlement

Profiles which may be subjected to expansive (upward)

movements


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Foundation Design Process Key Issues

Ultimate capacity and global stability

Influence of cyclic loading (wind & earthquake)

Overall foundation settlements

Differential settlements Effects of externally imposed ground movements

Earthquake effects including liquefaction

Dynamic response to wind-induced forces

Structural design of the foundation system


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Design Steps

Desk study of ground & groundwater conditions

Site investigation to assess stratigraphy & site variation

In-situ & laboratory testing

Preliminary assessment of foundation requirements Refinement of design using more accurate structural

layout, loading information & geotechnical conditions

Detailed design stage

In-situ foundation testing

Monitoring of building performance


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Case Study 1 - Artique

30 storey residential tower + basements Alluvial sediments (sands & clays) overlying weathered

rock profile

Original foundation design

> 140 bored piles (0.7m ), > 35m in length founding on weathered rock

0.7m thick slab on ground


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Design Process

Stage 1 Geotechnical site characterization

development of geotechnical model

selection of raft & pile design parameters

Stage 2 - Preliminary assessment approximate analysis of load-settlement behaviour

Methods (Poulos, 2002) & (Randolph, 1994)


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Results of Feasibility Assessment

Load - Settlement

0

100

200

300

400

500

600

700

800

900

0 100 200 300 400 500

Settlement mm

Load

MN Raft only

Raft + 50 piles

Raft + 70 piles

Raft + 140 piles


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Effect of No. of Piles

0

50

100

150

200

250

300

350

400

50 75 100 125 150

Number of Piles

Settlement

mm

Serviceability Load

2*Serviceability Load

3*Serviceability Load

No piles-Serviceability

No piles -

2*serviceability

No piles -

3*Serviceability


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Assessed a piled raft foundation provides cost-effectivesolution

Stage 3 Detailed foundation design using GARP8

0.8m thick raft supported on 18m long, 0.7m CFA piles

Optimise layout for SLS loading

Complete ULS loading analysis


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Serviceability Case GARP Analysis

123 piles

18m long

Max settlement =

44mm Max differential

settlement = 10mm

(1/400)


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Case Study 1 - Conclusions

Number of piles reduced by 10%

Pile length reduced from 35m to 18m

Total pile length reduced by about 2.5km Settlement criteria (both total and differential) satisfied

Potential variations in pile stiffness compensated for by

raft


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Case Study 2 - Incheon 151 Tower Project

151 storey (600m) tower in Songdo Korea


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Incheon 151 Tower - Overview

Site within a reclamation area (20m of marine

clay)

Regional geology metamorphic, granitic &

volcanic rocks

Piled raft foundation

172 No. 2.5m dia. bored piles

5.5m thick raft


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Incheon 151 Tower

Ground Conditions

23 boreholes to over 100m depth

In-situ testing including geophysical logging

Extensive laboratory testing for rock characterisation Detailed interpretation to:

Assess ground conditions

Develop geotechnical properties for various strata

Develop geotechnical design parameters


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Incheon 151 Tower Ground Conditions

Approx 30m

Approx 20m


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Incheon 151 Tower Interpreted Ground Conditions

Gneiss roof pendant

Sheared/crush zone

Granite

Granodiorite


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Incheon 151 Tower

Preliminary Design Stage

Aim to establish foundation system & evaluate approximate

foundation behaviour

Based on simplified ground model

Following details provided to structural designer Pile capacities for a range of pile

Lateral & vertical pile stiffness (single pile & group)

Pile layout options

Raft size

Pile group efficiency & adequacy of building behaviour


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Incheon 151 Tower

Detailed Design

Load Component

Vertical = 6622MN

Lateral (wind) = 112MN

Lateral (seismic) = 105MN

Moments

Mx = 12578MNm

My = 21173MNm

Mz = 1957MNm 24 wind loading combinations


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Incheon 151 Tower

Detailed Design

Foundation Components

Raft size & thickness by structural

engineer

Pile size, number & layout via trial

analyses by geotechnical &

structural designers


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Incheon 151 Tower

Detailed Design

Challenges of Detailed Design included:

Simulation of interaction effects of large pile group

Simulation of interaction between piles and raft Negative skin friction of consolidating marine clays

Lateral stability of foundation

Large variation in pile lengths (45m to 70m)


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Incheon 151 Tower

Overall Stability

Conventional text book methods not applicable

Assessed using CLAP developed by Coffey

Ultimate load combinations applied Ultimate pile capacities reduced by g

Condition satisfied if foundation system does not

collapse

6 critical wind loading cases analysed


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Incheon 151 Tower

Foundation Settlement

Assessment using GARP

Considers varying ground conditions

Considers varying pile lengths Considers pile-pile and pile-raft interactions

Multiple load combinations considered

All 172 piles modelled

5.5m thick raft modelled


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Incheon 151 Tower


Load CaseWind Load

Combination

Settlement (mm)Maximum Angular

Rotation of the RaftMaximum Minimum

DL + LL - 67 28 1:790

0.75(DL + LL + WL) 1 52 18 1:730

0.75(DL + LL + WL) 4 52 18 1:730

0.75(DL + LL + WL) 7 53 18 1:740

0.75(DL + LL + WL) 11 55 19 1:570

0.75(DL + LL + WL) 15 54 19 1:570

0.75(DL + LL + WL) 20 52 20 1:870


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Incheon 151 Tower


Independent assessment using PLAXIS 3D Foundation

Assumes uniform ground conditions

All piles modelled and founded at EL-55m


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Incheon 151 Tower


Max. total settlement for (DL + LL) condition = 68mm

Differential settlement ~ 19mm

50% less than CLAP predictions PLAXIS does not consider variation in ground conditions

and pile lengths across tower footprint


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Incheon 151 Tower Foundation Stiffness

Pile head stiffness critical input for 3-D structural model

Vertical pile head stiffness computed for each pile using GARP

Outer piles stiffer than central piles

Concentration of loads on outer piles a real phenomenon

More accurate foundation behaviour simulated by using individual

pile stiffness values

900 1300MN/m

550 750MN/m


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Incheon 151 Tower

Foundation Stiffness

Lateral foundation behaviour critical issue

Several analyses completed

3D finite element analysis

DEFPIG & CLAP

HorizontalLoad (MN) Direction

Pile GroupDisp. (mm)

Lateral PileStiffness(MN/m)

Lateral RaftStiffness(MN/m)

Total LateralStiffness(MN/m)

149 X 17 8760 198 8958

115 Y 14 8210 225 8435


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Incheon 151 Tower

Post Design Studies

Pile Load Test Objectives

Assess constructability and integrity of piles

Comparison of measured pile performance with design

expectations

Assess possible variability due to changing ground

conditions within site


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Piled Raft Design Conclusions

Importance of understanding & modelling site variationsin design

Close collaboration between structural & geotechnical

designers vital

A suite of computer programs is required to fully assessperformance of the foundation system

Foundation design process has been successfully used

by Coffey on some of the worlds tallest buildings (e.g.

Burj Kalifa) Process can be applied to high-rise and super tall

buildings


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THANK YOU

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