Practice Note 28 Screw Piles: Guidelines For Design .

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www.ipenz.nzPractice Note 28Screw Piles: Guidelines for Design,Construction & InstallationEngineering PracticeISSN 1176-0907October 2015

PrefaceThe purpose of the Practice Note Screw Piles: Guidelines for Design, Construction and Installation is to highlightmany critical elements of the design, installation and testing of screw piles. It provides recommendations for goodpractice in New Zealand, and should be used as a technical reference to inform engineers, developers, contractorsand local authorities.Practice Note DevelopmentThis Practice Note has been facilitated by the Institution of Professional Engineers New Zealand (IPENZ), withsupport from principal sponsors Piletech, and authorship provided by Beca Ltd.The IPENZ Engineering Practice Advisory Committee has given the lead author the task of preparing a documentto be adopted by the engineering industry that reflects a national perspective.The Practice Note has been prepared in accordance with standard IPENZ Practice Note procedures, which includesreporting on progress to the Engineering Practice Advisory Committee, peer review and general Membership review.This review and reporting process ensures the delivery of a robust, good-practice technical document.IIPractice Note 28: Screw Piles: Guidelines for Design, Construction and Installation Version 1, October 2015

AcronymsASAustralian StandardB1/VM4New Zealand Building Code, Structure Foundations, Verification MethodBCABuilding Consent AuthorityCPTCone Penetrometer testCHSCircular Hollow SectionDHelix DiameterIANZInternational Accreditation New ZealandIPENZInstitution of Professional Engineers New ZealandLPILE and GROUPSoftware program for analysing either single or pile groups under lateral loadingMBIEMinistry of Business, Innovation and EmploymentNDTNon-destructive testingNZGSNew Zealand Geotechnical SocietyNZSNew Zealand StandardPS1Producer Statement DesignPS2Producer Statement Design ReviewPS-CProducer Statement – ConstructionPS4Producer Statement Construction ReviewSLSServiceability Limit StateSPDScrew Pile DesignerSPTStandard Penetrometer TestULSUltimate Limit StatetTonnePractice Note 28: Screw Piles: Guidelines for Design, Construction and Installation Version 1, October 2015III

GlossaryIVBearing capacityThe capacity of the soil to resist loadCohesive soilA sticky soil such as clay or clayey silt having a strength that depends on the surface tensionof capillary waterCohesionless soilsAny free-running type of soil such as sand or gravels having a strength that depends on thefriction between particlesCompression capacityThe maximum amount of downward force (from the load of the structure or soil) that a screwpile can resist before failing (may be limited by either the geotechnical capacity of the soil orstructural capacity of the pile).DeflectionMovement of the installed pile in either – or both – the lateral or vertical plane(s)Founding layerThe layer of soil in which the helix plate is seated (also called the ‘bearing layer’).Founding levelThe exact place or level where the lower plate of the bearing helix sitsInstallation torqueThe rotating force required to install the screw pile into the groundLateral capacityThe maximum amount of horizontal force that a screw pile can resist, such as earthquakeshear loads before failing (may be limited by either the geotechnical capacity of the soil orstructural capacity of the pile).Negative skin frictionThe downward movement of a pile as a result of soft or liquefiable soils producing a downdrag as they compress from an additional load. (For design purposes NSF can either be takeneither as an addition to structural capacity requirements or as a reduction in geotechnicalcapacity.)Punch throughExcessive downwards deflection of the pile seat (under compression) into a weaker soil belowwhen the geotechnical capacity of the stronger, but relatively thin, founding layer is exceeded.Sensitive soilsSoils that have a high loss of strength following a disturbance, such as volcanic ash or highlyweathered ignimbriteSoil consolidation(settlement)Lowering of the ground surface over time following deformation (over time) of a weaksoil layer beneath when pore water pressures (generated after structural loading) havedissipated.Strength reduction factorA factor (of less than one) that provides a margin of safety to reduce the risk of failure appliedto the ultimate soil capacity or pile capacity to cover variations and uncertainties.Tension capacityThe maximum amount of upwards pulling force that a screw pile can resist before failing (maybe limited by either the geotechnical capacity of the soil or structural capacity of the pile).True helixA helix having perfect symmetry with a uniform pitch all round and parallel leadingand trailing edgesUndrained shear strengthThe maximum amount of shear stress that may be placed on a cohesive soil before it yieldsor fails.Practice Note 28: Screw Piles: Guidelines for Design, Construction and Installation Version 1, October 2015

Contents1. Introduction12. Overview of Screw Pile Technology22.1 What is a screw pile?22.2 Applications of screw piles32.3 How screw piles are installed53. Advantages, Disadvantagesand Suitability of Screw Piles63.1 Advantages3.2 Disadvantages663.3 Suitability of screw piles in certain groundconditions4. Factors Influencing Screw Pile Capacity788. Materials, Durabilityand Manufacturing248.1 Durability248.2 Materials248.3 Manufacture248.3.3 Shaft258.3.4 Helix259. Construction Practice269.1 Installation269.2 Pile construction record card2710. Testing Requirementsand Practices285. Geotechnical Investigation Requirementsfor Screw Pile Foundations910.1 Purpose of testing2810.2 General testing requirements296. Geotechnical Factors in determiningScrew Pile Capacity10.3 Other considerations291011. Producer Statements306.1 Seismic considerations1111.1 Producer statements – pile designer306.2 Compression capacity1111.2 Producer statements – pile contractor306.2.1 Coarse grained soils1212. Procurement316.2.2 Fine grained soils13Bibliography326.3 Tension capacity146.4 Lateral capacity156.5 Installation torque and capacity166.6 Vertical deflection186.7 Pile spacing and group effects196.8 Strength reduction factors197. Screw Pile Structural Design207.1 General structural design requirements207.2 Assessment of loadings207.2.1 Loadings during installation207.2.2 In-service loads207.3 Screw pile componentsAppendix A: Considerations forGeotechnical Investigations for Screw Piles 33Appendix B: Pre-Design, Designand Installation Checklist36Appendix C: Technical Specification forthe Design, Construction, Installationand Certification of Screw Piles38Appendix D: Screw Pile ShaftSpecification and Design45Appendix E: Example Screw PileRecord Card47217.3.1 Shaft217.3.2 Helix217.3.3 Connections and splices22Practice Note 28: Screw Piles: Guidelines for Design, Construction and Installation Version 1, October 2015V

1. IntroductionScrew piles are a type of foundation system now widely used in the construction industry. They can offersignificant advantages in terms of speed and ease of installation and by offering a reduction in the offsitedisposal of drilling spoil.New Zealand has recently seen a substantial increase in their popularity, especially following the 2010/2011earthquakes in Christchurch. The Australian Standard AS 2159 Piling Design and Installation [1] can be used as aguideline in the design,testing and installation of screw piles and is referred to extensively in this Practice Note.However, the lack of any published New Zealand code of practice means that significant reliance is placed onindividual practitioners to ensure that piles meet the performance requirements of each structure. An increasingnumber of designers and installers find themselves having to work within a largely unregulated marketplace.Concerns have been expressed that New Zealand might experience similar problems to those encountered in Australiaa decade ago when the evolution of screw piling technology saw large numbers of practitioners entering a marketwhere standards were not well defined. This lack of standards and Codes of Practice resulted in performance issuesand litigation. Adding to these concerns is the fact that any issues with screw pile designs are unlikely to presentthemselves until the piles experience significant additional loadings, such as during a large seismic event.This Practice Note highlights many critical elements of the design, certification, installation and testing of screwpiles. It provides recommendations for good practice in New Zealand, and should be used as a reference to informengineers, developers, contractors and local authorities.Piletech, a specialist designer, supplier and installer of screw piles, has led the way in establishing the screw pilemarket in New Zealand and has sponsored the writing of this Practice Note.1Practice Note 28: Screw Piles: Guidelines for Design, Construction and Installation Version 1, October 2015

2. Overview of Screw Pile Technology2.1What is a screw pile?Screw piles are a type of piled foundation, or retaining wall anchor, that have been in use since the 1830s.They are made of circular hollow steel sections with one or more helices welded to the shaft that providea self-tapping mechanism during installation. The hollow stem may be filled with reinforced concrete followinginstallation and is structurally connected to the building substructure.Shaft diameters range from 50mm up to 600mm and helix diameters range from 150mm up to 1200mm dependingon capacity requirements.Figure 2.1: Components of a Screw PilePractice Note 28: Screw Piles: Guidelines for Design, Construction and Installation Version 1, October 20152

2.2Applications of Screw PilesScrew piles are an option where the ground near the surface is, or has become too weak, to support a structure.Screw piles can also be used where the shape, size and location of the structure cannot be supported by alternativefoundation methods.Screw piles can be used for the following:New foundationsSupport to new structures including: residential, commercial and industrial buildings bridges, wharves and jetties transmission towers and wind turbines transportable buildings signal gantries pipes.Re-levelling and Strengthening Existing Foundations permanent or temporary support for re-levelling of existing structures that have settled seismic/gravity capacity improvements responding to building damage from hazards such as earthquakes, particularly in constrained work areasand around existing structures, (as shown in Figure 2.2)Anchoring permanent support for new or existing retaining structures, slope stabilization and slip remediation (shown in Figure 2.3), particularly if it is required in a short time frame temporary anchoring of machinery particularly against uplift. Piles can be removed and reused.Pile Load Testing as an alternative to the Kentledge system of weights (load testing using pre-cast concrete blocks or steel plates),especially when large forces are required.3Practice Note 28: Screw Piles: Guidelines for Design, Construction and Installation Version 1, October 2015

Figure 2.2: Screw Piles used for Re-levelling and Strengthening existing Foundations after Quake DamageFigure 2.3: Screw Piles used for AnchoringPractice Note 28: Screw Piles: Guidelines for Design, Construction and Installation Version 1, October 20154

2.3How screw piles are installedScrew piles are wound into the ground, much like a wood screw. The helices cut into the soil following a constantpitch, as opposed to auguring through it. The helical flights and shafts are specifically shaped and designed to suitthe ground conditions.Hydraulic powerheads are used to apply the large torque that is required to screw a screw pile into the ground.The powerheads are fastened onto handling machines that range in size from ½ tonne Bobcats, 20 tonne Excavators,and up to 100 tonne mast mounted crane rigs. The combination of hydraulic powerhead and handling machinerequired is determined by: Installation torque requirements Torsional capacity of the shaft Site access limitations (height/width/clearance to obstructions) Soil profile and site ground conditions Positional tolerances.For permanent applications once the pile has reached the target depth the shaft is typically filled with concrete.In some cases this can increase the capacity of the pile. Embedded and protruding reinforcing, or using a top plate,provides a connection to the building structure above.Because screw piles can be easily installed and removed they are often used in temporary situations.5Practice Note 28: Screw Piles: Guidelines for Design, Construction and Installation Version 1, October 2015

3. Advantages, Disadvantagesand Suitability of Screw PilesDuring the option evaluation and detailed design stages, it is important to understand the relative advantages,disadvantages and suitability of screw piles as pile foundations or anchors.3.1Advantages Shorter set-up and installation times that need less labour than other pile options, with some specialisedmachines only requiring one certified installer Easy to extend and splice in-situ Simple to remove if required for a temporary situation with no damage to the screw pile, enabling re-use Possible to install in areas with poor accessibility, for example, where there is low headroom or other types ofconfined spaces Can be installed at an inclination, primarily for anchoring slopes Minimal vibration and construction noise during installation compared to other pile methods – important whenputting in foundations near vibration and noise sensitive structures or underground services Can be installed in almost any climatic condition (only the most extreme ground temperatures or atmosphericweather conditions will affect proper installation) Reduced disposal costs, particularly where ground is contaminated, due to very little spoil created by the helixduring installation Can be loaded (including for load testing) immediately after installation (unless concrete setting time is requiredafter infilling) Installation torque provides an indication of ground strength at each pile Temporary supports, such as casings, are not required in soft or unstable ground.3.2Disadvantages The lack of soil produced post-installation prevents the confirmation of founding materials The relatively narrow shaft diameter may compromise capacity in the event of large (seismic) lateral loadsor overturning moments Obstructions or hard layers may prevent embedment of the screw pile Soil corrosion factors may limit life (as with all buried steel piles).Practice Note 28: Screw Piles: Guidelines for Design, Construction and Installation Version 1, October 20156

3.3Suitability of screw piles in certain ground conditionsHard rockInstalling into hard rock can result in spin without penetration forming a void below the helix (or helices) anddisplacement of material above the helix. If the hard rock cannot be penetrated, alternative solutions will need to befound such as using high penetration heads, changing to conventional bored piles or grouting the seat of the pile toe.Screw piles are best suited to ground conditions with gradually increasing soil strength or where there is a gradualweathering profile at the top of the rock. In these conditions it is much more likely that the helix will be fully seated,that is, the full face (lower surface) of the helix will be in contact with the founding layer. Where soft soils overlay hardrock it may not be possible to fully seat the helix and there is a risk that the helix plate will buckle.Weak and/or liquefiable soilsScrew pile shafts are generally more slender than other types of pile foundation making them more prone tobuckling in poorly supporting soils. When piling is required through deep deposits of weak soils and/or liquefiablesoils, such as peat, very loose sands, and soft clay, there is an increased risk of shaft buckling.Screw piles, or indeed any other type of deep pile, are considered unsuitable for sites with the potential for majorlateral spreading ( 300mm) originating from deep and thick liquefiable layers, or for sites without any densenon-liquefiable bearing layer.Layered soilsIf weaker soils are present either immediately above or below the helix position at founding level (once installationhas been completed) installation torque values may indicate a greater pile capacity than that actually available.Monitoring of torque during installation for the last 3D in depth prior to founding may be required to verifythe strength of the material immediately above the helix to ensure tension capacities (see Section 6.5).Either Cone Penetrometer tests (CPT) or boreholes extending below the founding level are recommendedat critical pile locations to confirm that ground beneath the helix is competent.Sensitive soilsLarge pile deflections may occur in highly sensitive soils when resisting uplift forces. This is due to the potentiallydisturbed soil matrix above the helices. In such soils the displacement tolerances of the structure to be supported willbecome a critical factor in the foundation design.Soils with obstructionsPrecise placement of screw piles in soils with a variety of large particle sizes, such as very gravelly soils or bouldersmay not be possible. In this situation, positional tolerances with respect to the structure and load transfer into the pilecap will be a critical design factor.7Practice Note 28: Screw Piles: Guidelines for Design, Construction and Installation Version 1, October 2015

4. Factors Influencing Screw PileCapacityGeotechnical conditions and the structural design of the screw pile affect pile capacity. Screw piles are designed somost of the capacity of the pile is generated through the bearing of the helix plates against the soil rather than shaftfriction. This is why ground conditions and the pile design itself are important.A number of factors affect pile capacity. These are listed below and discussed in more detail in Section 6 (GeotechnicalFactors in Determining Screw Pile Capacity) and Section 7 (Screw Pile Structure Design): Types of soils and their engineering properties Depth, thickness and the strength of soil overlying and underlying the founding layer Whether the load is compression, tension, lateral or a combination Choice of strength reduction factors Screw pile design and manufacture: Shaft size Number of helices Size and spacing of helices Concrete in-filled or not Helix-to-shaft weld Quality of “true helix” form Pile spacing and installation angle Installation torque Structure to pile deflection tolerance Structure to pile connection.Practice Note 28: Screw Piles: Guidelines for Design, Construction and Installation Version 1, October 20158

5. Geotechnical InvestigationRequirements for Screw PileFoundationsAppropriate geotechnical investigations must be carried out on all sites to determine the ground conditions andenable identification of potential issues such as the presence of obstructions, hard rock, liquefaction, instability,and corrosiveness of soil/groundwater.The number of investigation locations will vary and will be greater where ground conditions are expected to be highlyvariable or when there are larger, more complex foundation requirements. Any investigations for screw pile designshould extend below proposed founding depths to confirm the adequacy of the proposed founding material. SeeAppendix A: Considerations for Geotechnical Investigations for Screw Piles for a summary of considerations whendetermining the scope of site investigations for screw pile design.Field classification and description of soil and rock should be undertaken in accordance with the New ZealandGeotechnical Society (NZGS) Guideline for Field Description of Soil and Rock [2].At the time of writing this Practice Note (October 2015) the Ministry of Business, Innovat

IV Practice Note 28: Screw Piles: Guidelines for Design, Construction and Installation Version 1, October 2015 Glossary Bearing capacity The capacity of the soil to resist load Cohesive soil A sticky soil such as clay or clayey silt having a strength that depends on the surface tension of capillary water Cohesionless soils Any free-running type of soil such as sand or gravels having a .

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