Bored Pile Construction -Chris

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GTC Presentation

Bored Pile and Driven H-Pile Construction

(24 June 2011)

Christopher Leung(Geotechnics)

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Secondment at Tysan Foundation Limited

Oct 2011 – Jan 2011 (3 months)Construction of Public Rental Housing at Kai Tak Site 1B(~1100 Nos. of mixed driven H-pile and sockettedH-pile)

Jan 2011 – Jun 2011 (6 months)Piling Works for the Proposed Industrial Building at No. 6-28 Chai Wan Kok Street, Tsuen Wan(33 Nos. of bored piles and 53 Nos. of driven H-pile)

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Bored Pile Construction- Procedures, plants and equipment

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Bored Pile Construction – Typical Procedures

1. Pre-drilling (at each pile location) 2. Setting out3. Casing driving through soft layer and excavation4. Rock socket and bell-out forming (by RCD)5. Air-lifting6. Koden test7. Steel cage installation8. Concreting9. Interface/ full-depth concrete coring

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Determination of rockhead level before constructionAt each pile locationContinuous 5m of Grade III or better rock with TCR over 85%Double or triple-tube (for fragmented rock) core-barrelT2/TN size barrel

Bored Pile Construction – Pre-drilling

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General site arrangementCasing driving RCDSteel cage fabricationAir liftingConcreting

Chai Wan Kok Street (Feb 2011)

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Casing drivingWelded Splice/ Bolted JointCommon size: 2.0m, 2.5m & 3.0mCommonly by oscillatorDown to bedrock/hard stratum

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Other common methods for casing driving

VibratorRotator

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By crawler crane simultaneously with casing drivingBy hammer grab and rock chisel

Excavation

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Rock socket forming

Reverse Circulation Drill (RCD)Vertical drill-bitRock sample inspection at rockhead, founding andpile base levels

11Cutter

Drum Stabilizer

Stabilizer

Drill Rod

Swivel Head

Drilling PlatformClamping Collar

Drill Bit

Water Tank Air Comp Power Pack

Rock socket forming by RCD

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Bell-out tools

Reverse Circulation Drill (RCD)Replaced with bell-out bitNo further drilling downwards but only extending the arms“Travel” calibration

Founding Level

Pile Base Level

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Air-lifting

Tremie Pipe

Air Hose

Recycle water / Fresh Water

Flush out to Sedimentation Tank

Air Compressor

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Koden Test

Common acoustic device to verify verticality & bell-out sizeRequire sufficient clarity of waterDO NOT expect very exact values

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Steel cage installationAware of possible cage collapse

Use crawler crane with sufficient powerAware of design with heavier top reinforcement

Ensure sufficient lapping length

Sufficient nos. of reservation pipes (150mm dia.)

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Concreting

Ramp

Skip

Tremie Pipe

Concreting Platform

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Concreting

Simultaneous with casing extraction and removal and tremie pipeCasing supported securely by oscillator/crawler crane

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Quality Problems in Association with Construction Technology and Design

Quality Problems

Pile Inclination and Off-CentreNecking Pile ShaftThermal Cracking of ConcreteAir Voids and Honeycomb in ConcreteSediments at Pile Base

Unknown underground condition!

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Design ConsiderationDimension

Avoid Very Large, Extremely Long Piles (e.g. 3m diameter more than 80m deep) and Large Bell-Out (e.g. 4.8m)

Concrete and reinforcementUse High Grade Concrete with Care (e.g. Grade 50)Maintain Sufficient Clear Spacing between Reinforcing BarsEnsure Sufficient Stiffness of the Reinforcement Cage

Ground conditionUse Permanent Casing in Very Soft Ground or Subject to Tidal Effect and Underground Stream

Quality Problems in Association with Construction Technology and Design

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Tests on Bored Pile ConstructionInterface Coring

Coring through reservation pipesOn each bored pile after concreting for a certain periodVisual inspection on the contact of the pile base and the founding rock

Example of good contact

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Tests on Bored Pile ConstructionFull-depth Concrete Coring

Coring from the top of pile into the founding rockNos. subject to criteria of completion (e.g. 5%)Concrete cores selected for compressive testAs alternative to interface coring

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Interface and full-depth concrete coring

Common imperfectionsConcrete core

Thermal jointAir voids and honeycomb in concrete

Interface contactNo recovery (inferred as soil inclusion) due to sediment at pile toeLoss of contact at the interface due to inaccessibility of concrete mix into rock fragments

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Process control to minimize the occurrence of segregation

Introduce grout/bentonite in advance of concrete dischargingUse high slump concreteUse of additional retarder to enhance workability

Interface and full-depth concrete coring

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1. Additional coreholes2. Clean the pile toe with high pressure water jet3. Pressurized cement grout4. Verification cores

As specified by RSE to which extent of remedial works should be performed

Remedial Actions for Pile Base Imperfections

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Remedial Actions for Pile Base Imperfections

Caution!!!This method may only work in intact rocksituation with confined defects

In highly fractured rock zones, high-pressure water jetting and grouting may induce more collapses and aggravate the situation⇒ progressive creation of cavity ⇒ induce further collapses at pile toe

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Driven H-Pile Construction- Procedures, plants and equipment

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Driven H-Pile Construction – Typical Procedures

1. Pre-drilling (piles covered by 5m radius) 2. Setting out3. H-pile section inspection4. Pitching of 1st section5. Splice more sections and welded joint inspection

(Visual, some with non-destructive test)6. Final set (some with PDA)7. Load test

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H-pile inspectionMill cert, stocklist, s/n recordLength measurement (VERYimportant especially for sections cut from other piles)Pile tip inspection

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Pile pitching/drivingHydraulic HammerOffset control

Limit power in the first few blowsClose monitoring by pile workerPile extraction if necessary

Noise permit

Hydraulic hammer

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Better co-ordination and recording for large site• KT1B Site with over 1000 nos. of H-piles of average 5,6 sections

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Splice sections by welding

Better arrangement of welding period/ weldersAllow certain time of cooling before drivingVisual inspection by RE

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Non-destructive test on welds

Ultrasonic testRequired for a certain ratio of pilesBy pulse reflection of transducerDiscontinuity of weld

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Final set

Use of drop hammer of specified weightSpecified drop height for 10 blowsFinal set graph

Cp+Cq

sFinal set process

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Calibration

Weight of drop hammer, helmet & followerTemporary compression of helmet (Cc)Coefficient of restitution (e)Hammer efficiency (k)

Hiley’s formula

)(*)(

*)(5.0

* 2

PWWWePWWW

CCCsEkRu

fhr

fhr

qpc +++

+++

+++=

Depending on the specification requirement

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Pile Driving Analysis (PDA)

For a certain ratio of piles specified (e.g. 5%)Can be done simultaneously with final setWave signal analysis during drivingWave equation modelInduced pile capacity as additional verification

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Post-construction drilling

In case the pile founding level is higher than designed level from pre-drillingMinimum nos. as specifiedProcess similar to pre-drilling

Rockhead/SPT

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Load TestLoading to ultimate pile capacityKentledge with sufficient loadingFour dial gauge extensometerAllowable maximum and residual settlement

Kentledge and hydraulic jack

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Conclusion

Typical procedures for Bored Pile and Driven H-Pile constructionExamples from the projects involved

END