Design of Piled Foundations - hkieged.org

1 Design of Piled FoundationsSammy CheungSenior Geotechnical EngineerGEO, CEDD GEO, CEDD 20 April 2013 OUTLINE OF PRESENTATION Vertical Load Horizontal LoadPile Group Pile Group Negative Skin Friction Instrumented Pile Test ResultsObjectives To appreciate the interaction between pile construction and pile Design To appreciate what can go wrong with different piling To appreciate what can go wrong with different piling techniques To understand the empirical nature of pile Design and the role of precedents (load tests and monitoring) To understand the role of rational Design approach and proper geotechnical inputproper geotechnical inputGeneral Perspective Ground conditions in Hong Kong are complex and can pose major challenge to piling Design and construction ( corestone-bearing weathered profiles, karstic marble, deep and/or steeply inclined rock head) Piling Design in Hong Kong is always criticized for overly Piling Design in Hong Kong is always criticized for overly conservative Design Short pile scandals in Hong Kong (magic tape, etc.)

2 Borehole BBorehole ABorehole log Simplified geologyBorehole log Simplified Borehole BBorehole ABorehole log ASimplified geologyBorehole log BSimplified geologyVIVIP otential risk of using an overly simplified VVoverly simplified geological model( layered-model in IVcorestone-bearing saprolites)IIIIIIIIIIIINote :(1) Refer to Geoguide 3 (GCO, 1988) for classification of rock decomposition grade I to grade Pile Design in Hong Kong Many Hong Kong-specific deemed-to-satisfy rules are stipulated by the Authorityy Rules were derived through experience & have been applied without geological considerationsgeological considerations Some rules are not conservative and are not based on soil mechanics principlesprinciples Unnecessarily long piles may encounter major problems during i ( ld d b i ff!)

3 Construction (so could end up as being worse off!)Common Pile Design in Hong Kong Submissions for private and housing projects Building (Construction) Regulations Building (Construction) Regulations Code of Practice for Foundations , 2004Pi N f AP/RSE/RGE Practice Notes for AP/RSE/RGE Submission for public projects GEO Publication No. 1/2006 GEO Publication No. 1/2006 Specifications (Arch SD)Ei di i d i dd f i Engineer s discretion on adopting standards for private submissionFOUNDATION Design FOR PRIVATE PROJECTS Buildings (Construction) Regulations AP/RSE Notes Code of Practice for Foundations (2004)() deemed-to-satisfy rules more economic Design may be gyfeasible by rational Design methodRelevant PNAP for Foundation Submission for Private Projects Key PNs include.

4 APP18 (PNAP 66) (At it i f il t ti) APP-18 (PNAP 66) (Acceptance criteria for pile testing) APP-61 (PNAP 161) (Scheduled Area for karstic marble) APP-103 (PNAP 227) (Structures On Grade on Newly Reclaimed Land) APP-16 (PNAP225) Ground Investigation Works in Scheduoled Areas Approval and Consent APP-134 (PNAP 283) (Designated Area of Northshore Lantau) APP-137 (PNAP 289) (Ground-borne Vibrations Arising from Pile Driving and Similar Operations)Foundation Design for Public Projects Promote use of rational Design Promote use of rational Design First edition was published in 1996 Consolidate good Design and Consolidate good Design and construction practice for pile Foundations , with special reference to ,pHong Kong s ground conditionsGEO Publication No. 1/96 Foundation Design for Public Projects Updated experience cumulated in t recent years Piling data obtained from the instrumented piling load tests instrumented piling load tests programme for the rail projects expanded scope to include shallow expanded scope to include shallow Foundations and recent advancesGEO Publication No.

5 1/2006 Other Useful ReferencesINTRINSIC PROBLEMS ABOUT PILING Design The piling process changes the ground behaviour, for good or worse compacting, loosening the soilspgg It is the behaviour of the ground afterpile installation that controls pile performance (pile soil interaction)performance (pile soil interaction) Varying ground conditions involve uncertainty and risk opportunity C ltd k bid bti d ii di th Completed works are buried; observations and supervision during the installation process are important In some cases, there may be time-dependent effects that could influence the development of pile capacity in the long termCOMMON PILE TYPES IN HONG KONGPile TypesTypical range of pile i (kN)Geotechnical load i icapacity (kN)carrying capacityDisplacement PilesDriven H-piles2000 kN to 3500 kNShaft friction and end bearingDriven prestressed1950 kNto 3500 kNgDriven prestressedprecast concrete piles1950 kNto 3500 kNJacked Steel H Pile2950 kNJacked Steel H Pile2950 kNCOMMON PILE TYPES IN HONG KONGPile TypesTypical range of pile capacity (kN)

6 Geotechnical load carrying capacityReplacement PilesSocketedH-piles3500 kNto 5300 kNShaft friction on rockSocketedHpiles3500 kNto 5300 kNShaft friction on rockAuger piles1500 kNShaft friction on soilMi iil1400 kNSh ft f i ti kMini-piles1400 kNShaft friction on rockMini-bored piles2000 kNShaft friction on rock and end bearingBarrettesUp to 20,000 kNShaft friction on soil and end bearing Bored pilesUp to 80,000 kN( Shaft friction on soil/rock Bored pilesUp to 80,000 kN( m bell-out)Shaft friction on soil/rock and end bearingTRADITIONAL PILE Design IN HONG KONGNd t id t h i l it d ttl it f Need to consider geotechnical capacity and structural capacity of piles Driven piles piles usually driven to a set based on dynamic drivingformula to match the structural capacity ( fyfor steel H piles ))

7 Bored piles & socketed H-piles piles are usually designed as end-bearingandlimitedshaftfrictiononrock Ifdepthofweatheringissignificant, ,the piles behave as friction piles INSTALLATION Displacement piles hi tl t i t th d ith hammering steel or concrete into the ground with sufficient energy to refusal" Replacement piles dig a hole and fill with steel and concrete" dig a hole and fill with steel and concreteSounds simple, but not so! Pile installation can affect pile material (damage), the ground (disturbance) & surrounding facilities(damage), the ground (disturbance) & surrounding facilitiesEFFECTS OF PILE CONSTRUCTION ON GROUND Displacement piles (driven piles)- akin to cavity expansion pp(p)ypproblems, with the horizontal stresses increased and granular soils subject to densification and compactionjp Bored piles stress relief effect due to hole formation horizontal Bored piles -stress relief effect due to hole formation.

8 Horizontal stresses in the ground reduced and ground is subject to looseningPILE DESIGNPILE DESIGNPILE Design Deem-to-satisfy rules Simplified rules Code of Practice for Foundations (2004)() Rational Design method Rational Design method Based on soil/rock mechanic principles Consider geotechnical capacity and settlement May require instrumented pile loading tests to confirm Design assumption More economical Design can be achieved! More economical Design can be achieved!RATIONAL PILE Design APPROACH An alternative to use of default values [ presumed bearing pressure, hf fi i ]zero shaft friction] Adequate ground investigation to assist in formulation of appropriate ground model Characterization of ground properties by means of appropriate insitu and ppyppplaboratory tests Proper geotechnical + engineering geological input Proper geotechnical engineering geological input Design analysis to be based on principles of mechanics, and/or an established empirical correlationsestablished empirical correlations Pile testing programme to verify Design assumptionsDesign of Axially Loaded Pile (Geotechnical Capacity)

9 P = Qs+ QB PsB Soil type 1 Qs= shaft capacitySl 2 Soil type 2 QB = base capacityDESIGN OF AXIALLY LOADED PILE (STRUCTURAL CAPACITY) Structural strength of piles to be determined in accordance with appropriate limitations of Design stresses Permissible stresses given in Code of Practice for Structural Use of Concrete & Code of Practice for Structural Use of Steel For bored piles, reduce concrete strength by 20% where groundwater is likely to be encountered during concreting, or where concrete is placed underwaterUltimate Pile Shaft CapacityQ = x AQs sx As = Ultimate shear stress in each soil stratum s= Ultimate shear stress in each soil stratumfflhfhlAs=Surface area of pile shaft in each soil stratumFACTORS AFFECTING SHAFT FRICTIONFACTORS AFFECTING SHAFT FRICTIONFACTOR AFFECTING SHAFT FRICTION v r r Changes of radial effective stress affects the Changes of radial effective stress affects the skin friction Displacement piles increases in radial Pile Shaft Displacement piles increases in radial stress Replacement piles decrease in radial Pile Shaft Replacement piles decrease in radial stressFactor Affecting Shaft Friction =( ho + h )tan =( hf )

10 Tan ho is the locked-in effective horizontal stress afterpile constructionhop h is the change of horizontal stress after pile construction istheeffectivehorizontalstressatfailurea ndwillbeaffectedby: hfistheeffectivehorizontalstressatfailur eandwillbeaffectedby: interface dilation/compression under constant stiffness conditionduringpileloadingwhichcanincrea se(duetodilationofadenseduringpileloadin gwhichcanincrease(duetodilationofadenses oil), or reduce (due to compression of a loose soil)SHAFT FRICTION IN GRANULAR SOILSTwo common Design approaches as follows:Mthd 1 Eff ti t th dMethod 1 : Effective stress method = K tan [c is usually taken as zero]_ s= Ks. v. tan [c is usually taken as zero]The above may be simplified to: s= . v _[ method, where = Ksx tan ]Method 2 : Correlation with SPT N values s= fs.)