Transcription

Engineering Design Manual

Table of ContentsHelical Anchors Design Manual ----5Introduction ------------------------------------------------- ----------- 6Unique Features ------------------------------------------- 6Description -------------------------------------------------- 7Installation -------------------------------------------------- 8Applications ------------------------------------------------ 8Design Guide ----------------------------------------------- 8Predicting Capacity ------ 8Individual Bearing Capacity -------------------------------------------- 9Non-Cohesion Soil ------ 10Cohesive Soil -------------- 12Projected helical plates areas ----------------------------------------- 12Torque Anchor Capacity ----------------------------------------------- 14Ultimate Skin Resistance ----------------------------------------------- 15α Method ------------------- 16λ Method ------------------- 16β Method ------------------- 16Helical Tie Back Anchor Considerations ------------------------- 17Design Example 1 – Cohesive Soil ----------------------------------- 19Design Example 2 – Non-cohesive Soil ----------------------------- 21Helical Anchors Operational Guide ------------------------------------------------ 23Procedure -------------------------------------------------- 24Length ---- 25Connection to the Structure --------------------------- 25Load Testing ---------------------------------------------- 25Torsional Resistance ------------------------------------ 25Clean up - 26Push Piers Design Manual ---------- 27Introduction ----------------------------------------------- 28Push Pier Advantages ---------------------------------- 28Description ------------------------------------------------ 28Installation ------------------------------------------------ 29Applications ---------------------------------------------- 29Design Guide --------------------------------------------- 30Predicting Capacity ---- 30Page 2EDM Rev 02 2014, Helical Anchors Inc.

Push Pier Operational Guide ------- 32Foundation Exposure --------------------------------- 33Bracket Installation ----------------------------------- 33Under-footing 2-piece Pier Bracket ------------------------------------ 332-Piece Plate Pier Bracket ------------------------------------------------ 33Driving Pier Sections --------------------------------- 34Cutting Final Pier ---------------------------------- 34Transferring Loads to Piers ------------------------ 34Clean Up -------------------------------------------------- 35Corrosion Overview ------------------- 36Introduction --------------------------------------------- 37Soil Resistivity ------------------------------------------ 37Soil ----- 38Zinc Galvanization ------------------------------------ 38Appendix A - Products Strength Rating ------------------------------------------- 40Helical Anchor Technical Data -------------------- 41Table A1: Helical Anchors Product Rating -------- 49Table A2: General Properties of Helical Anchors Shaft ----------------------------------------- 50Table A3: Ultimate Capacities of Helical Anchors Helices ------------------------------------- 50Appendix B - Evaluation Report --- 51Page 3EDM Rev 02 2014, Helical Anchors Inc.

Definition of TermsResistance - A force developed in opposition to a load applied to a structural system.Capacity – The maximum resistance a system can mobilize; usually used in relation tothe ability of a soil/structure system to support load. Also known as ultimate resistance.Strength – The maximum resistance a structural member or assembly can mobilizewithout regard to the ability of soil to support load.Design Load – The specified load for which a structural system is to be designed. It isrelated to, but may not be the same as, the maximum load the system is expected toexperience during its lifetime.Allowable Load – The maximum load that may be applied to a structural system whilestill maintaining an appropriate margin of safety to handle overloads, material andconstruction variations, environmental factors and other unspecified but reasonablyforeseeable detrimental conditions.Margin of Safety (Factor of Safety) – Margin of Safety is the difference between theactual performance capability of a system and its design requirement. Factor of Safety isthe ratio of actual performance capability to design requirement.Loads – Forces or other actions that result from the weight of building materials,occupants and their possessions, environmental effects, differential movement andrestrained dimensional changes.Ultimate – Describes the maximum load that can be supported by a structural member orsoil/structure system under specific conditions, or the deflection that corresponds withthat load.Yield – Describes a load, strength or deflection, at which a structure departs from elasticbehavior.Page 4EDM Rev 02 2014, Helical Anchors Inc.

Helical AnchorsDesign ManualSection 1Page 5EDM Rev 02 2014, Helical Anchors Inc.

IntroductionThe earliest recorded uses of helical anchors were by an Irish engineer, AlexanderMitchell, who used them to support a lighthouse. Screw piles were not popular becausethe force of many men was required to produce the necessary torque. Through the yearsscrew piles were improved, and increased, especially when torque motors becameavailable. Screw piles have become ideal for many applications and their popularity hasrisen to the point where any trained contractor can install them quickly and easily thesedays. Screw piles, also known as helical anchors, are convenient for easy access onconstruction sites that are inaccessible by larger equipment.Helical Anchors Inc. with its 30 years of experience in the earth boring industry bringsnew solutions to the soil stabilization and foundation industry. Helical Anchors Inc, afamily owned and operated company applies its state of the art technology and expertiseto change the way the foundation industry installs foundation piles/anchors. Our superiorchoice of raw materials, “new” design and fabrication techniques make our anchors theanchors of the future.AdvantagesHelical anchors have presented many solutions for different types of projects. Thissection will summarize the advantages of their use. They produce small or no vibration during installation, decreasing possibledamage to the structures from soil movement.They can be installed in any weather conditions and may be loaded immediatelyafter installation; there is no cure time as with concrete foundations.Easy installation; there is no need for excavation and they can be installed inlimited access areas.They are quick to install and there is no need of big equipment in comparisonwith other types of deep foundation construction.They can be installed in soft surface and high water table conditions.Prediction of capacity is found after installation from torque to capacityrelationship, useful to verify theoretical capacity.Installation produces no spoils to remove or remediate.Unique FeaturesHelical Anchors, Inc. has developed new ways of fabrication with superior raw materialsthat make our products the best in the market.Page 6EDM Rev 02 2014, Helical Anchors Inc.

Seamless tube shafts with high tensile strength give our products higher torsionstrength than our competitors. This allows installation into stronger soil stratafor higher load capacity. Helical Anchors, Inc. products have the highest torqueratings in the industry.Our telescoping connections are precision CNC machined for a strongerconnection.The connections are inertia welded allowing a streamlined one piece design.High-strength helical bearing plates combine with the high-torsion-strengthshafts to allow higher compression loads than our competitors.Quality galvanizing (hot-dipped process) for enhanced underground corrosionresistance.DescriptionA helical anchor is a steel shaft with one or more helical plateswelded around it. Helical anchors are considered deepfoundations and may be used to support any type of load. Insimple words, it is a screw with a discontinuous thread and auniform pitch.The central shaft is fabricated from seamless Grade 80 steeltubing, giving our products higher strength than our competition.Helical Anchors, Inc. offers a wide variety of shaft sizes for anykind of application. Shaft and helical bearing plate sizes availableare shown in Appendix A. Shaft sections may be fabricated invarious lengths ranging from 36” to 240” depending on jobrequirements. Couplings are inertia welded to the shaft ends toallow attachment of extensions for deeper penetration into theground when needed.Helical plates are fabricated with Grade 50 steel. They vary from6” to 16” in diameter and have a thickness of 3/8” to 1”depending on job requirements. The number and sizes of helicalFigure 1: Leadplates may be varied to match soil conditions to the requiredSectionanchor capacity. When multiple helix plates are provided on asingle anchor they are positioned so that no plate is smaller than any preceding plate.The nominal pitch of each helix is three inches and to ensure that each of them developsfull capacity, each succeeding plate is located above the preceding plate a distance equalto three times the diameter of the preceding plate.Page 7EDM Rev 02 2014, Helical Anchors Inc.

The top of the helical anchor connects to the foundation or structure with different typesof connectors depending on the application. These connectors allow the loads from thefoundation to be transferred to the helical anchor and then to the soil at a deeper level.InstallationA helical anchor is similar to a wood screw, one obvious difference being that the helicalanchor has widely-spaced discontinuous threads. Helical anchors are screwed into theground making sure they penetrate at a rate of about one pitch length (3 inches) perrevolution. There are two ways helical anchors can be installed; one of them is usingmachine-mounted equipment and the other is hand-held. For an extensive detailedinstallation procedure, see the operations guide section of this manual.ApplicationsThe main purpose of a helical anchor is to transfer structural loads to soil. Nowadays,helical anchors are used for a variety of applications in tension, compression and lateralloads. Typical tension applications of helical anchors include guy anchors for poles andtowers, tiebacks for temporary or permanent retaining walls and foundation tiedowns.They can also be used for underpinning to lift sinking foundations, deep foundationelements to support walkways and boardwalks, and tilt-up wall braces. Also, helicalanchors have become a foundation of choice for lateral load applications including slopestabilization, poles, towers and fences.Design GuidePredicting CapacityCapacity is defined herein as the maximum load a foundation /soil system can support.The bearing capacity of a helical anchor varies depending on many factors such as soilproperties and conditions, anchor design characteristics, installation parameters, and loadtype (tension, compression, shear and/or overturning).The equations used to predict capacity of helical anchors in tension and/or compressionare based on the assumption that the anchors act as deep foundations. This requires thatthe bearing plates be embedded some minimum distance below the ground surface.However, after many years of study, researchers still haven’t come to an agreement onjust what the depth requirements are. Helical Anchors, Inc. recommends each helix beembedded at least three feet vertically and six times its own diameter measured along theshaft from the ground surface.Page 8EDM Rev 02 2014, Helical Anchors Inc.

Though there are several theories for calculating the ultimate capacity of a helical anchor,Helical Anchors, Inc. products are designed to fit the assumptions of the two mostcommon methods best. These are the individual bearing method, which assumes abearing failure of the soil supporting each helix, and the torque correlation method whichmakes use of the empirically-derived relationship between installed capacity and thetorsional resistance encountered during installation.Individual Bearing CapacityIn the individual bearing method, capacity is determined by calculating the ultimatebearing resistance of the soil at each helix and multiplying it times the projected area ofthe helix. The total capacity of a multi-helix system is then the sum of the individualcapacities.The general equation used to calculate the bearing capacity of a single helix plate is thefollowing:Equation 1Where:Qu Ultimate Capacity (lbs)Ah Projected Helical Plate Area (ft2)c Soil Cohesion (lb/ft2)Nc Bearing capacity factor for cohesion (dimensionless)q' Effective Overburden Pressure (lb/ft2)Nq Bearing capacity factor for overburden (dimensionless)γ’ Effective unit weight of the soil (lb/ft3)B Footing width (ft)Nγ Bearing Capacity factor (dimensionless)According to Bowles (1988) concerning Equation 1 (above); the base width term 0.5 γ’BNγ can be neglected with little error where B 3 to 4m. Since the width of a helicalanchor will never exceed the limit mentioned above, the resulting equation for anindividual helix then is the following:Equation 2Page 9EDM Rev 02 2014, Helical Anchors Inc.

When dealing with a multi-helix anchor,the same Equation 2 above is used withthe difference that instead of being justone projected area, it will be all of theprojected areas. Spacing between eachhelix for multi-helix anchors is of highimportance since they need to be farenough apart so each helix plate candevelop full capacity without overlapping.Helical Anchors Inc. has determined thathelix plates must be spaced three times thediameter of the lower helix. (shown inFigure 2)Friction along the shaft is commonlyignored when dealing with solid shaftsbecause the small surface area per foot oflength will create an insignificantresistance force to affect the total capacity.However, friction resistance might betaken into consideration for circularhollow shafts since they will have moresurface area per foot of length developinggreater resistance than solid shafts.Commonly, soil is not homogeneousFigure 2: Bearing Capacitythrough the entire required depth ofinstallation; a soil behavior analysis mustResistance Diagrambe done in order to be able to calculate thetheoretical ultimate capacity. The soil behavior varies quite a lot depending on the siteconditions. Typically, soil can be simplified into non-cohesive and cohesive soil.Non-Cohesive SoilCohesion is the term we use for shear strength that exists in the absence of compressivestress. In non-cohesive soils like sand and gravel, shear strength exists only in thepresence of compressive stress. Due to the fact cohesion is zero (c 0) the ultimatecapacity can be determined with the following equation where the first term of equation 2is eliminated.Page 10EDM Rev 02 2014, Helical Anchors Inc.

Equation 3Effective overburden pressure, q’, is the sum of the effective unit weights of the soil inoverlying strata multiplied by the thicknesses of the strata.Table 1: General Properties of Non-Cohesive Soil(ASCE 1996)Unit Weight (lb/ft3)Soil DensityDescriptionRelativeDensity“%”SPTBlow Count“N”Angle ofInternalFriction ΦMoist “γm”Very Loose0-150–4 28 100Submerged“γsub” 60Loose16-355 – 1028 – 3095 - 12555 – 65Medium Dense36-6511 – 3031 – 36110 – 13060 – 70Dense66-8531 – 5037 – 41110 – 14065 – 85Very Dense86-100 51 41 130 75Note: When soil is below theground water table, buoyancyforces reduce the contact forcesbetween the soil particlesreducing the overburden effect.The effective unit weight of soilbelow the ground water table isthe saturated unit weight of thesoil less the unit weight ofwater.350300Nq Values25020015010050001020304050Angle of Friction, Ø (degres)60Bearing capacity factor Nqvaries as a function of the soil’sangle of internal friction whichcan be found in Table 1 abovealong with the soil densitydescription. Helical Anchors Increcommends the relationshipgiven in Figure 3 for Nq valueswhen using its helical anchors.Figure 3: Bearing Capacity Factor, NqPage 11EDM Rev 02 2014, Helical Anchors Inc.

Cohesive SoilCohesive soil, such as clay, is composed of particles that adhere to each other even in theabsence of compressive stress. When working with this type of soil under short-termloading the second term in equation 2 is eliminated because under such conditions theangle of internal friction, φ, of the soil is very small.Table 2: General Properties of Cohesive Soil(after ASCE 1996)Soil ConsistencyDescriptionSPTBlow Count“N”UndrainedShear Strength“c” – lb/ft2SaturatedUnit Weight(psf)Very Soft0–2 250 100 - 110Soft3–4250 – 500100 – 120Firm5–8500 – 1,000110 – 125Stiff9 – 161,000 – 2,000115 – 130Very Stiff16 – 322,00