lecture 3 prof harry on nsf pile response due to vertical soil movement
TRANSCRIPT
CE5107 Lecture 3 NSF Pile due to Settling Soils by Plaxis FEM Study –Settling Soils by Plaxis FEM Study Validation of Fellenius Unified Pile
D i CDesign Concepts
Prof Harry TanJan 2011Jan 2011
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OutlineOutline
• 2D Axi‐symmetric Pile Model2D Axi symmetric Pile Model• Problem Definition
2• Parameters Input 2D FEM• Parametric Studies• Conclusions
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2D Axi‐symmetric Pile Model2D Axi symmetric Pile Model
Load
Interface for soil‐slip
Solid concrete of E=30 GPa for pilep
Dummy paper beam to get axial f i il ilforce in pile easily;EA and EI= 1e‐6 times of pile EA and EI respectively
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p y
Problem DefinitionProblem Definition
Reclaimed Fill cause soil long‐term settlements GWT at ground surface
Soft Clay consolidates slowly over very long time (like Singapore Marine Clay)g p y)
Pile socketed in non‐settling Dense hard soil (like OA)
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Plaxis InputR_inter = 1.0
• Pile is elastic with E=30 GPa• Dummy pile is paper plate ith EA 1E 6 times of real pilewith EA=1E‐6 times of real pile
• Pile in model is L=20m with 5m socket into Dense soil
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Pile in model is L 20m with 5m socket into Dense soil• Pile radius=0.565m so that pile cross‐section area = 1m2 for convenience to get load in kPa same as in kN in Plaxis plot of Load vs movement of pile
Parametric Studies• Impact of Soil Settlements on Pile load vsmovement responsemovement response– Produce different ground settlements by fictitious values of weight of 3m fill above soft clayvalues of weight of 3m fill above soft clay
– Vary ground settlements from 0 mm to 200 mm
• Impact of Bitumen Coated pile• Impact of Bitumen Coated pile– Simulate bitumen coat by reducing R_inter in Fill and Soft Clay from 1 0 to 0 1 (assume coating isand Soft Clay from 1.0 to 0.1 (assume coating is 90% effective as observed in real projects)
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Impact of Soil Settlements
Pile installed and simulate drained consolidation
Fill Wt [kN/m3]
So [mm]
10 0
So=215 mm
Fill Soil Reclaimed land
10 0
12 22
15 54
20 108
30 215
So is gro nd s rfaceSo is ground surface settlement due to reclaimed fill loading after full consolidation is
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completed (Drained)
Load Test SimulationLoad Test Simulation
• Pile installed with reclaimed land (Drained)• Load Tests (UnDrained)
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Load Tests Results
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Pile movement [mm]
10
30
50
LOAD [kN]2000 4000 6000
• NSF do not affect Ultimate Pile Resistance (about 6500 kN in above cases)
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• Soil settlements (So) produce drag‐loads (NSF) on piles• Larger So showed softer pile response; and larger pile settlements
Model Pile in long‐term Working Load Condition
P=2000 kN
•Install Pile before Reclaim Land (Drained)•Load Pile to WL = 2000 or 4500 kN (Drained)•Switch on Reclaim Fill (Drained ie assume full soil consolidation is
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•Switch on Reclaim Fill (Drained ie assume full soil consolidation is completed)
Compare piles at various So (P=2000 kN)So=0mm So=22mm So=215mmSo 0 So 22mm So 215mm
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• At P=2000 kN; Reclaim land with Drained Analysis (full consolidation)
Compare piles at various So (P=2000 kN)So=0mm So=22mm So=215mmSo 22mm
Pile move 3.99 mm Pile move 4.86 mm Pile move 8.55 mm
NP at 12.5mNP at 15m NP at 15.0m
• At P 2000 kN Neutral Plane NP is deeper as So increases
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• At P=2000 kN, Neutral Plane, NP is deeper as So increases
Compare piles at various So (P=2000 kN)So=0mm So=22mm So=215mmSo 22mm
NP at 12.5m
Large transition zone from Nsf to Psf
Small transition zone from Nsf to Psf
NP at 15m NP at 15.0m
A P 2000 kN NP i d S i
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• At P=2000 kN, NP is deeper as So increases• Smaller So produces large transition zone (small toe movement)• Larger So produces small transition zone (large toe movement)
Compare piles at various So (P=2000 kN)So=0mm So=22mm So=215mmPil 3 99 l
P=2000 kN P=2000 kN P=2000 kN
Pile move 3.99 mm Pile move 4.86 mm Pile move 8.55 mm
Large transition zone from Nsf to Psf
Small transition zone from Nsf to Psf
NP at 12.5mPnsf=0 kN
Pnsf=430 kN
from Nsf to PsfNP at 15 m
Pnsf=1340 kN
• At P=2000 kN; Drag load Pnsf increases as So increases
Pt=520 kN Pt=602 kN Pt=856 kN
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• At P=2000 kN; Drag‐load Pnsf increases as So increases• Position of NP is deeper as So increases• End bearing resistance Pt, increases as So increases
Compare piles at various So (P=4500 kN)So=0mm So=22mm So=215mmSo 0 So 22mm So 215mm
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• At P=4500 kN; Reclaim land with Drained Analysis (full consolidation)
Compare piles at various So (P=4500 kN)So=0mm So=22mm So=215mmSo 22mm
Pile move 77mmPile move 14mmPile move 10mm
NP at 7.5mNP at 7.5m
NP at 10.5m
NP at 15m
• At P=4500 kN, NP is deeper as So increases
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, p• Compare slides #12 and #16, for same So; larger load P means higher NP, and will produce larger vertical movement of piles
Compare piles at various So (P=4500 kN)So=0mm So=22mm So=215mmSo 22mm
NP at 7.5m Large Small t iti
NP at 10.5mtransition zone from Nsf to Psf
transition zone from Nsfto Psf
NP at 15m
• At P 4500 kN NP is deeper as So increases
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• At P=4500 kN, NP is deeper as So increases• Smaller So produces large transition zone (small toe movements)• Larger So produces small transition zone (large toe movements)
Compare piles at various So (P=4500 kN)So=0mm So=22mm So=215mm
Pile move 77mmPil 14Pil 10
P=4500 kN P=4500 kN P=4500 kN
Pile move 77mmPile move 14mmPile move 10mm
Large S ll
Pnsf=250 kN
transition zone from Nsfto Psf
Small transition zone from Nsf
P f
NP at 7.5m
NP at 10.5m
Pnsf=0 kN Pnsf=990 kN to Psf
Pt=946 kN Pt=1132 kN Pt=1786 kN
• At P=4500 kN; Drag load Pnsf increases as So increases
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• At P=4500 kN; Drag‐load Pnsf increases as So increases• Position of NP is deeper as So increases• End bearing resistance increases as So increases
Compare piles at So=22 mmPile move 4.86 mm Pile move 14mm
P=4500 kNP=2000 kN P=4500 kN
NP at 7.5m
NP at 12.5m Pnsf=250 kN
Pt=1132 kNPt 602 kN
Pnsf=430 kN
Pt 1132 kNPt=602 kN
• For same So=22 mm; as P increases from 2000 to 4500 kN:• NP rises from 12 5m to 7 5m level
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NP rises from 12.5m to 7.5m level• Drag‐load, Pnsf reduces from 430 to 250 kN•End bearing resistance Pt increases from 602 to 1132 kN
Compare piles at So=215 mmPile move 8.55 mm Pile move 77mm
P=4500 kNP=2000 kN P=4500 kN
Pnsf=990 kN
NP at 10.5m
NP at 15.0m
Pt=1786 kNPt=856 kN
Pnsf=1340 kN
• For same So=215 mm; as P increases from 2000 to 4500 kN:• NP rises from 15 0m to 10 5m level
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NP rises from 15.0m to 10.5m level• Drag‐load, Pnsf reduces from 1340 to 990 kN• End bearing resistance Pt increases from 856 to 1786 kN
Unified Design at So=215 mm
P=2000 kN Pult=6400 kNP=4500 kN Pult=6400 kN
Ultimate pile load test
NP at 15.0m
load test
NP at 10.5m
Pnsf=1340 kN
Pt=856 kN
• NP can be determined by intersection of resistance vs loading plots at P=4500 kN when skin friction (fs) along pile is fully mobilized
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• At P=2000 kN, the fs below NP is not yet fully mobilized, so the NP is not readily obtained from the force vs load equilibrium plots. It is quite likely that NP is at interface between soft clay and stiff soil layers in this instance
Bitumen Coated PileBitumen Coated Pile
• Effects is produced by choosing R inter to be 0.1 in Fill and p y g _Soft clay soils
• Install pile first and simulate full consolidation of reclaimed fill t d i d f ttl t f S f 0to produce various ground surface settlements of So from 0 mm to 215 mm (Drained analysis)
• Simulate top‐down load test of bitumen coated piles until p pfailure is reached
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Results of load tests on bitumen coated pilesPil t [ ]
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Pile movement [mm]
30
50
LOAD [kN]2000 4000 6000
• Coating reduces total resistance of pile from 6500 kN to 5300 kN
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g p• But the external ground settlements influence on pile movement is almost insignificant compared to uncoated pile
Bitumen piles at P=4500 kNSo=22 mm So 215 mmSo=22 mm So=215 mm
Pnsf = 170 kN Pnsf = 250 kN
d h f d bl O
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• NP at 15m depth on top of dense stable OA• Small drag‐loads in the piles (Pnsf = 170 to 250 kN)• Both axial force plots are nearly identical
Conclusions• NSF do not affect the geotechnical capacity of pile• FEM analysis showed agreement with Unified Pile design
principles and conceptsprinciples and concepts• The neutral plane (NP) is defined by the point along the pile
where soil and pile settle together• The NP can be identified by the intersection of force curve
and the resistance curve by the Unified Pile design method• For the same top load (P), larger soil settlements (So) resultsFor the same top load (P), larger soil settlements (So) results
in deeper NP, larger drag‐load (Pnsf), and larger mobilized end bearing (Pt)
• For the same soil settlement (So) larger top load (P) results in• For the same soil settlement (So), larger top load (P) results in shallower NP, smaller drag‐load (Pnsf), and larger mobilized end bearing (Pt)
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Conclusions• The ultimate axial load capacity of a pile is not influenced by
NSF, but its load vs movement response can be significantly affected by NSFy
• The settlement of pile will increase as soil settlement becomes larger. This increase is larger at larger loads on pile headhead
• The CP4 method of design by capacity is too conservative; but it will prevent excessive settlementsTh i i f NSF il d i h ld b l d• The criterion for NSF pile design should be settlement and not capacity based analysis
• Bitumen coating of pile may lead to improved design ie higher allowable loads or smaller settlements than uncoated pile. Coating of pile is a good alternative to increasing pile length for NSF designg
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Singapore Case History at PSA Port
NP at base of MC
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Singapore Case History at PSA Port
NP at base of MC
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