BRIDGE FOUNDATION DESIGN
Siva
Theivendrampillai Sivakumar
Principal Engineer (Geotechnical)
Overview
Brief Discussion on:
• Foundation Type • Foundation Design • Pile Load Testing
3
TMR-Specifications
• Cast-in-Place Piles – MRTS63 and 63A
• Driven PSC Piles – MRTS65
• Driven Steel Piles –MRTS66
• Dynamic Testing of piles—MRTS68
• Project Specific-
Geotechnical Design
Basic Foundation Types
• Shallow Foundations
¾ Bearing strata at shallow depths
• Deep Foundation (Piles)
¾ Deeper bearing strata Driven Piles
5
Basic Foundation Types
7
When can we use Shallow Foundations?
When Surface strata are:
• Strong ( Adequate bearing capacity and no settlement issues).
• Not vulnerable to Scour • Non-expansive
Shallow Foundation Design –
Things to Consider• Concentric / Eccentric Loading • Overturning moment
• Sliding
• Global Stability ( esp. footing on / adjacent to slope)
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Basic Foundation Types
When do we need piles?
• When surface strata are
¾ Weak
¾ Compressible ¾ Erodable
¾ Expansive
• To resist flood, earth pressures
¾ Lateral loads ¾ Uplift loads
11
Pile Use: Transfer load through surface strata which may be weak, compressible, expansive etc.
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Pile Use: Support against scour or lateral loading due to excavation
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Pile Use – Further example of lateral support for deep excavation induced lateral loading
Deep Foundations - Pile Types
• Driven piles
¾ Displacement piles
¾ Soil is ‘displaced’ within the adjoining soil mass
(displaced volume ≈ pile volume)
• Cast-in-place piles or Bored piles
¾ Non-Displacement piles ¾ Soil is removed
17
Driven Piles - Types and basic
requirement in design
• Types
¾ Octagonal Prestressed Concrete
(PSC)
¾ Reinforced Concrete (RC) ¾ Steel “H Pile”
¾ Timber Piles
DRIVEN PILES
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SITE INVESTIGATION FOR DRIVEN PILES
1. Soil strength and stiffness
2. Soil chemical analysis
⇒
corrosion/aggressiveness
3. Possible obstructions to installation
4. Potential for damage to adjoining
structure due to “ground heave”
5.
Vibrations
Vibrations
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Driven Piles
• Will refuse in SPT N>50 material
• Loads: e.g.,550mm PSC working 1500kN
• Settlement: ~ 10 mm
• Vulnerable to:
¾ Lateral movement / Negative skin friction ¾ Excess vertical settlement
• Drive after construction of approach
embankments
Example of Negative Skin friction
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Bored or Cast-in-place Piles
• Types
¾ Short bored piers ¾ Cylinders on rock
¾ Cylinders socketed into rock** ¾ Belled sockets
• Bored piles
¾ Could be up to 4 x cost of driven pile
Bored Piles -
Construction• Bored piles are cast in place cylindrical piles • Excavated by
Augers Buckets
Large drill bit (for hard rock)
Chisel grab and casing oscillator for bouldery
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Bored Pile Excavation - Bucket
Cleaning Bucket
Excavation Bucket Drilling Rig
Rock Sockets
Bored Piles – Cylinders
Socketed into rock
Rock Sockets
• High compression loads
• Greater resistance to lateral movement
• Socket length 2 to 5 x diameter
• Diameter from 900mm to 1800mm
• High strength rock
¾ Point Load (Is50 > 1 MPa)
¾ Rock anchors preferred to resist large uplift
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Rock Sockets
• May need
casing
in overburden soils and
XW rock (SPT N<50)
• Sealing/control of
groundwater
important
• Capacity to take heavy loads dependent
on extremely
clean
socket bases –
inspection important (WH&S)
• More expensive - so
fewer
, larger piles
Loads on Bridge Foundations
Structural Engineer to advise, consists of but not limited to • Vertical Compressive (Dead + imposed) loads
¾ Imposed Loads
¾ + ½ Dead Load – highway bridges ¾ + 2/3 Dead Load – railway bridges
• Vertical Uplift
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Loads on Bridge Foundations
• Horizontal Loads
¾ braking force of vehicle in longitudinal
direction
¾ flood loads in transverse direction ¾ Earthquake
• Horizontal Loads create Bending
Moments
Selection of Foundation Type
What influences the decision for
driven
or bored piles
?
The following factors will influence the
choice of foundation type:
9 Loads
9 Environment 9 Logistics and 9 Geology
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Selection of Foundation Type:
Loads
• Structural Loads
¾ Heavy compressive loads from large spans
• Hydraulic Issues
¾ Lateral and uplift loads from flood loading ¾ Scour in loose sands and silts
Selection of Foundation Type:
Environment
• Vibration
¾ proximity to people ¾ vulnerable structures ¾ damage to services
• Aggressiveness due to groundwater
• Obstructions
35
Selection of Foundation Type:
Logistics
• Transporting fresh concrete in western
Queensland
¾ Distance and temperature
• Availability/Transporting PSC piles
¾ Max length around 25 – 27m
• Quality of access roads
• Accessibility at foundation locations
Selection of Foundation Type:
Geology
• Depth to competent strata
• Obstructions to pile driving
¾ Coffee rock (Indurated Sand)
• Steeply dipping bearing strata
¾ Basalt flows
• Interbedded rock types with different
properties
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Selection of Foundation Type:
Geology
• Compressible deposits
• Defects with soft infills
• High head of groundwater
¾ Sealing issues ¾ Hole stability ¾ Concreting
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PILE DESIGN
THEORY EMPIRICISM EXPERIENCE FIELD LOADING TESTS
Engineering Geology Soil Mechanics Rock Mechanics Structural Mechanics To account for various methods of pile installation Regional (geology + local construction practices) Static Dynamic Design Stage Construction Stage
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PILES
PILES
-
-
design
design
The following aspects should be considered in design:
1. Load carrying capacity (Geotechnical Engineer) - strength and stiffness ⇒ “serviceability”
2. Pile material strength (Structural Engineer) 3. Pile material durability (Structural Engineer)
Pile Design - Geotechnical
• Foundations: ¾ Load capacity ¾ Settlements ¾ Lateral Fixity ¾ Uplift resistance • Scour Issues ¾ Land/water structures• Approaches
¾ Stability ¾ Settlements• Interaction
¾ Abutments ¾ Widening/ duplicationThe following DESIGN ELEMENTS should be accounted for in design:
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Pile Capacity
• Q
= Pile Capacity
• Q
end= End Resistance
• Q
shaft= Shaft Resistance
• Q =
Q
end+ Q
shaftQ
Qshaft
End versus Shaft Bearing Piles
• Pile in Clay • Pile in
Sand End Bearing Pile
Qshaft Qend = 5-10% Qshaft Qshaft Qend Qshaft Qend
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Low load Ultimate load
fs = τ max fs = τ max for the full length fs << τ max Base resistance, fb, mobilized
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Design of Piles
Traditional Approach
Ultimate Geotechnical Capacity =
Ult. Skin Friction + Ult. End Resistance Allowable Geotechnical Capacity =
Ult. Skin Friction/1.5 + Ult. End Resistance/3.0
OR
Allowable Geotechnical Capacity =
Ultimate Geotechnical Capacity/2.5
The allowable geotechnical capacity should be compared with
Design of Piles
Limit State Design (e.g AS2159)
Rug (Ultimate Geotechnical Capacity) =
Ult. Skin Friction + Ult. End Resistance Rg* (Design Geotechnical Capacity) =
Ф
x RugRg* >= N* or S* (Design Action Effect or Ultimate Design Load)
Rg* should be compared with ultimate design load (not driving capacity or structural capacity)
Load and Settlement-
(idealized)PILE DESIGN – WIDELY ACCEPTED BEHAVIOUR Pile NONDISPLACEMENT Drilled shafts Micropiles in soils CFA (Auger cast) PARTIAL DISPLACEMENT H-Piles
Open-ended pipe piles (in some soils)
FULL DISPLACEMENT
Precast concrete Closed-ended pipe piles
Open-ended pipe piles (in some soils)
Franki
Spectrum of soil displacement caused by pile installation and Its relationship to
bearing capacity.
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2nd Session
• Pile Load Testing• Site Investigation – Need to get it right
• Design Elements – Stability and Settlement at Bridge Approaches
• Selection of Design Parameters
• Design Charts – for estimating shaft resistance and settlement of piles
Pile Load Test
• Why Pile Load Test¾ Derivation of design parameter
¾ Verification of design load or pile carrying capacity
• MRTS63 Requires that at least 10% of piles at a site to be tested
• Common methods of pile load test
¾ Static Load Test (Kentledge or Reaction Piles) ¾ Dynamic Test (PDA with CAPWAP)
53 Static Load Test
Reaction Piles Kentledge
55 Static Load Test – Further example of
Dynamic Load Test – Pile Driving Analyser (PDA)
• The PDA system consists of
¾ Two strain transducers (to measure strain/force)
¾ Two accelerometers (to measure velocity)
Attached to opposite sides of the pile (near the top of the pile).
• The measured force and velocity at the pile top provide necessary information to estimate soil
57
59 Force & velocity wave traces recorded during initial driving and restriking
Site Investigation - Need to get it right
• What can go wrong?
• How can we manage undue contractual
claims as well as save construction time
• Limited investigation can be disastrous as
this could lead to undue claims
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65
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Design Element – Stability and Settlement at
Bridge Approaches
• Stability
69 Different Origins that could Lead to Formation of
Abutment Stability and Settlement
• Compression of Natural Soil Due to
Embankment Load
• What are compressible Soils?
¾ Soft clays (SPT N = HW to 6 or Su
<25kPa)
• Where can we find soft clays
(compressible soils)?
¾ Old River Channels
Paleo-channels
• GUP, near
Schultz canal
• From old
topography
maps and
airphotos
•
Paleochannels
Old buried channels from previous creek
routes
Deposits of softer younger alluvium Can be difficult to identify
Create a sudden change in ground conditions
Paleo-channels – Long Section
Abutment Stability and Settlement
• Risks associated with soft clays
¾Embankment stability and settlement ¾Structures (damage, bumps)
¾Pavements Deterioration - unevenness ¾Retaining wall foundations
¾Construction delays ¾Construction access
75 Abutment Stability: Soft Clay Issue
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Abutment Stability and Settlement: Soft Clay Issue, Bump at Bridge Approach
Abutment Stability and Settlement: Soft Clay Issue, Differential Settlement
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Abutment Stability and Settlement: Typical Examples on Projects in South East Queensland
• Gateway Arterial @ Bald Hills Creek
• East – West Arterial @ Pound Drain
• Ipswich Motorway – BR340 @
Gateway Arterial - Bald Hills Creek
• 3m high
embankment
• 100m failure
during
construction
• Boreholes
150m apart
Bald Hills Creek - Mitigation Strategy
• Stability failure reinstated with timber piled raft • Abrupt differential settlement between
embankment sections
Embankment on piles didn’t settle
Bald Hills Creek, Settlement
≈ 800 mm by Jul 98 ≈ 150 mm predicted in 1986 by consultant85
East – West Arterial @ Pound Drain
• Damaged by lateral loading on piles from
the approach embankment
• Differential settlement also
¾Loads on abutment piled foundations
¾Interaction effects on adjacent structures ¾Functionality of drainage structures
Ipswich Motorway
Ipswich Motorway -- Bridge Bridge BR340, StabilityBR340, Stability
•
• Number of Spans = 3Number of Spans = 3 •
• Span Length = 13m, 18m & 13mSpan Length = 13m, 18m & 13m •
• Bridge Bridge SpillthroughSpillthroughEmbankment Embankment 9m high with batter Slopes
9m high with batter Slopes
1(H):1(V)
1(H):1(V)
•
• Number of Piles at Abutments = 3Number of Piles at Abutments = 3 Spaced at 6.5m
Spaced at 6.5m c/cc/c •
• Number of Piles at Piers = 5Number of Piles at Piers = 5 Spaced at 3.3m
87 Ipswich Motorway - 2009
Approach
Approach embankment failed. embankment failed. Cracks in embankment
Cracks in embankment
plus Pier piles displaced.
Risks Associated with Soft Clays – Managing Stability and Settlement
• How can we manage stability and
settlement
89
Overview of Management Strategies
Light-weight Fill Stone Columns Embankment on Piles Vacuum Preload Partial Replacement Total Replacement Temporary Surcharge Height reduction. Counter Berms Stage Construction Vertical Drains Reinforced Embankment
SELECTION OF DESIGN PARAMETERS
• SOILS • ROCKS
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Soils
SAND CPT SPT CLAY Oedometer Consolidation Stiff Soft UU CPT CPTu UU SPT: Standard Penetrometer CPT: Cone Penetrometer CPTu: Piezocone UU: TriaxialVS: Vane Shear Test
Selection of Design Parameters : CPT
CPT
Sands / Stiff Clays
fs qc
Shaft
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Selection of Design Parameters : CPTu
CPTu
Soft Clays
qc u
Su (Undrained Strength for stability)
Cv (Rate of settlement)
Drainage lenses Fs/qc/u
Selection of Design Parameters : Su
Undrained Strength
Soft clay ClayStiff
Stability Shaft Resistance End Bearing
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Selection of Design Parameters: Rock
XW/HW
Visual SPT Point Load
MW/SW
Visual USC Point Load
Pressure -meter
96
Selection of Design Parameters:
Rock Tests
UCS Pressuremeter Point Load (Is)50 HW/MW/ SW/Fr Settlement of Sockets Shaft Resistance End Bearing CNS MW/SW/Fr Shaft Resistance97
