Negative Skin Friction on Large Pile Group New Wembley Stadium. Mei Cheong

Transcription

1 Negative Skin Friction on Large Pile Group New Wembley Stadium Mei Cheong

2 PRESENTATION OUTLINE An Introduction NSF Back analysis from monitoring Design Implications Conclusion

3 An Introduction NSF Rigid Floating Pile Compressible Clay

4 An Introduction NSF =Drag down =Primary Consolidation d =? NSF/Drag force Neutral plane Key question Assumption: : Depth No of drag neutral force plane? Ground settlement = pile group settlement

5 Indicative section Fill Arch base Eastern Arch Base 9m clay fill NSF Eastern Arch Base

6 Base dia = 19.5m Total piles = 19 Pile dia = 1.5m Pile length = 27m Clay fill = 9m NSF Eastern Arch Base

7 PRESENTATION OUTLINE An Introduction NSF Back analysis from monitoring Design Implications Conclusion

8 Settlement, mm Back analysis-ground settlement with time Start of monitoring 26 April '03 Primary Consolidation t = 19 years p=~120mm Monitoring End of fill 03 Aug '03 Start of arch lift 21 May 04 End of arch lift 26 June 04 Current time 28 Mar 07 12m bbf 6m bbf Base of fill Time, days

9 Back analysis-ground settlement with depth 50 Base of fill 40 Elevation, mod Monitoring Monitoring Settlement, mm

10 Settlement, mm Back analysis-pile Group settlement with time End of arch lift 26 June 04 Start of arch lift 21 May 04 Monitoring Rezeroed monitoring Load on arch Last monitoring point 14 March Load, MN Time, days -100

11 Settlement, mm Back analysis Neutral plane End of arch lift 26 June 04 Monitoring Start of arch lift Ground settlement = pile group Rezeroed settlement monitoring 21 May 04 Pile Group Settlement Assumption: Neutral Plane Ground Settlement Load on arch 10m bbf 6m bbf Last monitoring point 14 March Time, days Neutral plane about 8m bbf Load, MN

12 PRESENTATION OUTLINE An Introduction NSF Back analysis from monitoring Design Implications Conclusion

13 Day to day design Ground settlement with depth Crucial Allowable pile/pile group settlement Identify Neutral Plane Negative Skin Friction/ Drag Force No Yes Job Check Acceptable FoS and Settlement done!

14 Ground settlement with time 0 20 Start of monitoring 26 April '03 12m bbf Settlement, mm Δ=35mm Primary Consolidation t = 19 years p=~120mm Monitoring Neutral Plane 8m bbf End of fill 03 Aug '03 Start of arch lift 21 May 04 End of arch lift 26 June 04 Base of fill Current time 28 Mar 07 6m bbf Time, days

15 Ground settlement with depth Pile head level = 43.5mOD Elevation, mod m Δ = 35mm 28m Linear Elastic Settlement, mm Non-Linear Elastic Pile toe level = 16.5mOD Linear Elastic Non Linear Elastic

16 Non Linear Elastic better solution Analysis type Depth of Neutral Plane (m) Backanalysis () ~8 Non Linear Elastic ~10 Linear Elastic ~28

17 Drag down, mm Varying Depth of Neutral Plane Drag down Total Stress Method Effective Stress Method Drag force Non-Linear Elastic Linear Elastic Ko varying with depth Ko constant Drag force, kn Depth of Neutral Plane, m 0

18 Conclusion NSF Large pile group back analysis Depth of Neutral Plane CRITICAL for NSF Non linear elastic comparable to back analysis Linear elastic grossly overestimate Neutral Plane

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20 Model m 43.5 Fill 41mOD Elevation, mod Upper Weathered London Clay Lower Weathered London Clay Upper Unweathered London Clay Lower Unweathered London Clay 5.0 Lambeth Group

21 Input Parameters Soil Layer Layer thickn ess (m) Unit weig ht, γ sat (kn/ m 3 ) Horizontal & Vertical Permeab ility, k (m/s) Poisson s Ratio, υ Young s Modulus, E (MN/m 2 ) z=depth from top of London Clay Cohesion, c (kpa) Friction angle, φ (degre es) Interface Param eter R Clay Fill z Upper Weathered London Clay Lower Weathered London Clay z 6+0.2z z 7+0.6z Upper Unweathered London Clay Lower Unweathered London Clay z z z z Lambeth Group z z 22 1

22 NLS input parameters Elevation, mod Depth, m Initial Mean Effect ive Stres s, p (kpa) Young s Modulus, E at 0.1% strain (MPa) E /E Strain

23 VDisp Input Elevation, (mod) Thickness, (m) Undrained shear strength U 100, s u (kpa) 0-4m, s u =60+17z >4m, s u = z z = depth from top of London Clay Young s Modulus, E =200s u, (MPa)

24 Soil Structure Interaction 50 Elevation, mod Extensometer [ME4R] at Point A Plaxis Clay Fill at Point A Single Pile Pile Group Settlement, mm

25 A better solution! 0 Down drag, mm Linear Elastic Non Linear Elastic Non Linear Elastic Linear Elastic Depth of neutral plane, m

26 Drag force, kn Sensitivity test Total Stress Effective Stress Total stress Effective stress Thickness of NSF zone, m