Negative Skin Friction on Large Pile Group New Wembley Stadium Mei Cheong
PRESENTATION OUTLINE An Introduction NSF Back analysis from monitoring Design Implications Conclusion
An Introduction NSF Rigid Floating Pile Compressible Clay
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
Indicative section Fill Arch base Eastern Arch Base 9m clay fill NSF Eastern Arch Base
Base dia = 19.5m Total piles = 19 Pile dia = 1.5m Pile length = 27m Clay fill = 9m NSF Eastern Arch Base
PRESENTATION OUTLINE An Introduction NSF Back analysis from monitoring Design Implications Conclusion
Settlement, mm Back analysis-ground settlement with time 0 20 40 60 80 100 120 140 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 10 100 1000 10000 Time, days
Back analysis-ground settlement with depth 50 Base of fill 40 Elevation, mod 30 20 10 0 Monitoring Monitoring -10 0 10 20 30 40 50 60 70 80 Settlement, mm
Settlement, mm Back analysis-pile Group settlement with time 10 5 0-5 -10 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 06 20 0-20 -40-60 -80 Load, MN -15 0 100 200 300 400 500 600 700 800 Time, days -100
Settlement, mm 10 5 0-5 -10-15 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 06 0 100 200 300 400 500 600 700 800 900 Time, days Neutral plane about 8m bbf 20 0-20 -40-60 -80-100 Load, MN
PRESENTATION OUTLINE An Introduction NSF Back analysis from monitoring Design Implications Conclusion
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!
Ground settlement with time 0 20 Start of monitoring 26 April '03 12m bbf Settlement, mm 40 60 80 100 120 140 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 10 100 1000 10000 Time, days
Ground settlement with depth 50 45 Pile head level = 43.5mOD Elevation, mod 40 35 30 25 20 15 10 5 0 10m Δ = 35mm 28m Linear Elastic 0 50 100 150 200 250 Settlement, mm Non-Linear Elastic Pile toe level = 16.5mOD Linear Elastic Non Linear Elastic
Non Linear Elastic better solution Analysis type Depth of Neutral Plane (m) Backanalysis () ~8 Non Linear Elastic ~10 Linear Elastic ~28
Drag down, mm Varying Depth of Neutral Plane 0 50 100 150 200 Drag down Total Stress Method Effective Stress Method Drag force Non-Linear Elastic Linear Elastic Ko varying with depth Ko constant 7000 6000 5000 4000 3000 2000 1000 Drag force, kn 250 0 5 10 15 20 Depth of Neutral Plane, m 0
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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Model 52.5 19.5m 43.5 Fill 41mOD Elevation, mod 37.5 32.5 21.0 13.0 Upper Weathered London Clay Lower Weathered London Clay Upper Unweathered London Clay Lower Unweathered London Clay 5.0 Lambeth Group
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 9 19 10-9 0.25 10+0.6z 1 20 1 Upper Weathered London Clay Lower Weathered London Clay 6 20 10-8 0.15 5+3z 6+0.2z 20 0.85 5 20 10-9 0.15 25+5z 7+0.6z 20 0.85 Upper Unweathered London Clay Lower Unweathered London Clay 11.5 20 10-10 0.15 50+13.6z 10+0.4z 22 0.9 8 20 10-9 0.15 220+26z 15+0.4z 22 0.8 Lambeth Group 8 20.5 10-8 0.2 400+80z 26+0.4z 22 1
NLS input parameters Elevation, mod Depth, m Initial Mean Effect ive Stres s, p (kpa) Young s Modulus, E at 0.1% strain (MPa) 43.5 0 80 23 E /E 0.1 0.5 0.75 1.0 1.5 2.7 Strain 1.0 0.2 0.1 0.05 0.02 38.5 5 130 38 33.5 10 185 54 23.5 20 305 88 13.5 30 460 133-3.0 40.5 710 206
VDisp Input Elevation, (mod) Thickness, (m) Undrained shear strength U 100, s u (kpa) 0-4m, s u =60+17z >4m, s u =115+3.3z z = depth from top of London Clay Young s Modulus, E =200s u, (MPa) 43.5 0 60 12 39.5 4 129 26 16.5 27 204 41 5 38.5 242 48
Soil Structure Interaction 50 Elevation, mod 40 30 20 10 0-10 Extensometer [ME4R] at Point A Plaxis Clay Fill at Point A Single Pile Pile Group 0 10 20 30 40 50 60 70 80 Settlement, mm
A better solution! 0 Down drag, mm 20 40 60 80 100 120 140 160 180 Linear Elastic Non Linear Elastic Non Linear Elastic Linear Elastic 0 5 10 15 20 Depth of neutral plane, m
Drag force, kn Sensitivity test 5000 4000 3000 2000 1000 Total Stress Effective Stress Total stress Effective stress 0 0 5 10 15 20 Thickness of NSF zone, m