Transcript
BRIDGE FOUNDATION DESIGNSiva
TheivendrampillaiSivakumar
Principal Engineer (Geotechnical)
Geotechnical Branch
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OverviewBrief Discussion on:
• Foundation Type
• Foundation Design
• Pile Load Testing
• Approach Embankment to Bridge
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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 Standard – Minimum Requirements
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Basic Foundation Types
• Shallow FoundationsBearing strata at shallow depths
• Deep Foundation (Piles)Deeper bearing strata
Driven PilesCast-in-Place Piles
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Basic Foundation Types
SHALLOW FOUNDATIONS
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When can we use Shallow Foundations?
When Surface strata are:
• Strong ( Adequate bearing capacity and no settlement issues).
• Not vulnerable to Scour
• Non-expansive
• Low ground water level
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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
DEEP FOUNDATIONS - PILES
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When do we need piles?• When surface strata are
WeakCompressibleErodableExpansive
• To resist flood, earth pressuresLateral loadsUplift loadsOverturning loads
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Pile Use: Transfer load through surface strata which may be weak, compressible, expansive etc.
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Pile Use: For resisting lateral loading
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Pile Use: For resisting uplift
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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
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Deep Foundations - Pile Types
• Driven pilesDisplacement piles
Soil is ‘displaced’ within the adjoining soil mass (displaced volume ≈ pile volume)
• Cast-in-place piles or Bored pilesNon-Displacement pilesSoil is removedThe excavation may or may not be supported
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Driven Piles - Types and basic requirement in design
• TypesOctagonal Prestressed Concrete(PSC)Reinforced Concrete (RC)Steel “H Pile”Timber Piles
• Limitations on maximum length
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DRIVEN PILES
PSC Piles in use at Wetheron Creek Bridgesite
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Pile Driving Frame
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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. VibrationsVibrations
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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:
Lateral movement / Negative skin frictionExcess vertical settlement
• Drive after construction of approach embankments
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Example of NegativeSkin friction
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Bored or Cast-in-place Piles
• TypesShort bored piersCylinders on rockCylinders socketed into rock**
Belled sockets
• Bored pilesCould be up to 4 x cost of driven pile
Bedrock
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Bored Piles - Construction
• Bored piles are cast in place cylindrical piles
• Excavated byAugers
Buckets
Large drill bit (for hard rock)
Chisel grab and casing oscillator for boulderyground, etc.
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Bored Pile Excavation- Augering
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Bored Pile Excavation - Bucket
Cleaning Bucket
Excavation Bucket
Drilling Rig
Rock Sockets
Bored Piles – Cylinders Socketed into rock
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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
Point Load (Is50 > 1 MPa)Rock anchors preferred to resist large uplift loads
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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 may be more economical
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Loads on Bridge Foundations
Structural Engineer to advise, consists of but not limited to
• Vertical Compressive (Dead + imposed) loads
Imposed Loads+ ½ Dead Load – highway bridges+ 2/3 Dead Load – railway bridges
• Vertical Upliftflood loads in transverse direction
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Loads on Bridge Foundations
• Horizontal Loadsbraking force of vehicle in longitudinal direction
flood loads in transverse direction
Earthquake
• Horizontal Loads create Bending Moments
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Selection of Foundation Type
What influences the decision for driven or bored piles?The following factors will influence the choice of foundation type:
LoadsEnvironmentLogistics andGeology
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Selection of Foundation Type: Loads
• Structural LoadsHeavy compressive loads from large spans
• Hydraulic IssuesLateral and uplift loads from flood loading
Scour in loose sands and silts
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Selection of Foundation Type: Environment
• Vibration proximity to people vulnerable structuresdamage to services
• Aggressiveness due to groundwater • Obstructions
overhead power lines / headroom
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Selection of Foundation Type: Logistics
• Transporting fresh concrete in western Queensland
Distance and temperature
• Availability/Transporting PSC pilesMax length around 25 – 27m
• Quality of access roads
• Accessibility at foundation locationsCrane pads, piling rig pads
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Selection of Foundation Type: Geology
• Depth to competent strata• Obstructions to pile driving
Coffee rock (Indurated Sand)
• Steeply dipping bearing strataBasalt 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
Sealing issuesHole stabilityConcreting
• Rock excavatability
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Coffee Rock (Indurated Sand)
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Steeply Dipping Bearing Strata
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PILE DESIGN
THEORY EMPIRICISM EXPERIENCE FIELD LOADING TESTS
Engineering GeologySoil Mechanics
Rock MechanicsStructural Mechanics
To account for various methods of
pile installation
Regional (geology + local construction
practices)
StaticDynamic
Design Stage
Construction Stage
Pile Design - Approaches
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PILES PILES -- designdesign
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)
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Pile Design - Geotechnical
• Foundations:Load capacitySettlementsLateral FixityUplift resistance
• Scour IssuesLand/water structures
• ApproachesStabilitySettlements
• InteractionAbutmentsWidening/ duplication
The following DESIGN ELEMENTS should be accountedfor in design:
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Pile Capacity
• Q = Pile Capacity
• Qend = End Resistance
• Qshaft = Shaft Resistance
• Q = Qend + Qshaft
Q
Qshaft
Qend
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End versus Shaft Bearing Piles• Pile in Clay • Pile in
SandEnd 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
lengthfs << τ max
Base resistance, fb, mobilized
Driven Pile Capacity
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Design of PilesTraditional ApproachUltimate 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 load (unfactored) from the structure.
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Design of PilesLimit State Design (e.g AS2159)Rug (Ultimate Geotechnical Capacity) =
Ult. Skin Friction + Ult. End Resistance
Rg* (Design Geotechnical Capacity) = Ф x Rug
Rg* >= 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)
(600 mm, 10 m long bored pile in stiff clay)
PILE DESIGN – WIDELY ACCEPTED BEHAVIOUR
Pile
NONDISPLACEMENTDrilled shafts
Micropiles in soils
CFA(Auger cast)
PARTIAL DISPLACEMENTH-Piles
Open-ended pipe piles(in some soils)
FULL DISPLACEMENTPrecast concrete
Closed-ended pipe pilesOpen-ended pipe piles
(in some soils)Franki
Spectrum of soil displacement caused by pile installation and Its relationship to
bearing capacity.
Increasing unit base or shaft resistance
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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
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Pile Load Test
• Why Pile Load TestDerivation of design parameter
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 testStatic Load Test (Kentledge or Reaction Piles)
Dynamic Test (PDA with CAPWAP)
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Static Load Test
Reaction Piles
Kentledge
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Kentledge Set up for Static Pile Load Test
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Static Load Test – Further example of Kentledge
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Dynamic Load Test – Pile Driving Analyser (PDA)
• The PDA system consists ofTwo strain transducers (to measure strain/force)
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 resistance and its distribution.
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PDA – Set Up
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Typical arrangement of PDA - Schematic
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Force & velocity wave traces recorded during initial driving and restriking
Load-settlement Behaviour
Test Pile: Predicted versus Measured Performance
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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• Example – Six Mile Creek, Central Qld
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Six Mile Creek, Central Qld
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Six Mile Creek – Footing Plan Area
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Six Mile Creek: Additional Investigation-DCP
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Six Mile Creek - Footing Excavation
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Six Mile Creek: Footing re-design
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Design Element – Stability and Settlement at Bridge Approaches
• Stability• Settlement
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Different Origins that could Lead to Formation of Bump at the Approaches to a Bridge
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Abutment Stability and Settlement
• Compression of Natural Soil Due to Embankment Load
• What are compressible Soils?Soft clays (SPT N = HW to 6 or Su <25kPa)
• Where can we find soft clays (compressible soils)?
Old River ChannelsPaleo-channels (very dangerous)
Paleo-channels
• GUP, near Schultz canal
• From old topography maps and airphotos
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• 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
Abutment Stability and Settlement
Paleo-channels – Long Section
10 – 15m soft clay
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Abutment Stability and Settlement
• Risks associated with soft claysEmbankment stability and settlementStructures (damage, bumps)Pavements Deterioration - unevennessRetaining wall foundationsConstruction delays Construction access
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Abutment Stability: Soft Clay Issue Slip Failure - Schematic
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Abutment Stability and Settlement: Soft Clay Issue
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Abutment Stability and Settlement: Soft Clay Issue, Bump at Bridge Approach
Vertical Settlement
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Abutment Stability and Settlement: Soft Clay Issue, Differential Settlement
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Abutment Stability and Settlement: Typical Exampleson Projects in South East Queensland
• Gateway Arterial @ Bald Hills Creek• East – West Arterial @ Pound Drain• Ipswich Motorway – BR340 @
Dinmore
Gateway Arterial – Bald Hills Creek, Stability
Gateway Arterial - Bald Hills Creek
• 3m high embankment
• 100m failure during construction
• Boreholes 150m apart
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Bald Hills Creek - Mitigation Strategy
• Stability failure reinstated with timber piled raft• Abrupt differential settlement between
embankment sectionsEmbankment on piles didn’t settleEmbankment on natural did (4-5mm /month)
Bald Hills Creek, Settlement
≈ 800 mm by Jul 98
≈ 150 mm predicted in 1986 by consultant
East – West Arterial @ Pound Drain
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East – West Arterial @ Pound Drain
• Damaged by lateral loading on piles from the approach embankment
• Differential settlement alsoLoads on abutment piled foundations
Interaction effects on adjacent structures
Functionality of drainage structures
Problems at relieving slab and pavement
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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 SpillthroughSpillthrough Embankment 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 Spaced at 3.3m c/cc/c
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Ipswich Motorway - 2009
Approach Approach embankment failed. embankment failed. Cracks in embankmentCracks in embankmentplus Pier piles displaced.plus Pier piles displaced.
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Risks Associated with Soft Clays – ManagingStability and Settlement
• How can we manage stability and settlement
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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
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SELECTION OF DESIGN PARAMETERS
• SOILS• ROCKS
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Soils
SAND
CPT SPT
CLAY
OedometerConsolidation
Stiff Soft
UU CPT CPTu UU
SPT: Standard PenetrometerCPT: Cone PenetrometerCPTu: PiezoconeUU: TriaxialVS: Vane Shear Test
VS
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Selection of Design Parameters : CPT
CPT
Sands / Stiff Clays
fs qc
Shaftresistance
End bearing resistance
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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
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Selection of Design Parameters : Su
Undrained Strength
Soft clayStiff Clay
StabilityShaft 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
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Selection of Design Parameters: Rock Tests
UCS PressuremeterPoint Load (Is)50
HW/MW/SW/Fr
Settlement of Sockets
Shaft Resistance
End Bearing
CNS
MW/SW/Fr
Shaft Resistance
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Design Charts (after Poulos)
• Design charts for the estimation of shaft resistance and settlement of pilesDriven PilesBored Piles
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Shaft Resistance
Settlement (Poulos 1989)
Settlement (Poulos 1989)
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