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  • Engineered foundation technologyfor fast Telecom Tower build-out! Speed your sites to market

    Nothing faster than our all-steel piles and guys No excavation spoils; torque indicates capacity

    Immediately apply loadsNo start/stop delays of formed concrete

    Lower foundation costsLabor-competitive at most sites, simpler, easier

    Utilize inaccessible sitesModular system installs by low-psi machines

    Solve weak soil situationsHelical Pulldown Micropiles developultra-high loads through soft surfaces

    Exclusive services Exclusive services Installed-cost estimates Installer training & certification

    New!New!HeliCAPHeliCAP

    EngineeringEngineeringSoftwareSoftware

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  • Telecom Industry Applicationsof Helical Foundationsand Anchors for the full range!

    Telecom TowersGuyed Towers Self-Supported Monopoles

    Site Augmentations EquipmentPlatforms & Shelters

    For platform piles: HS Series, page 8 T/C Series, page 8 Non-Extendable Series, page 8

    For prefabricatedbuildings: HS Series, page 8 Helical Pulldown

    Micropiles, page 9

    For guy wires:For guy wires: SS Anchor Series, SS Anchor Series, page 7 page 7 Adjust-A-Grip Adjust-A-Grip

    Deadend Grips,Deadend Grips, page 10 page 10

    For center masts:For center masts: HS Pile Series page 8 HS Pile Series page 8 HS/SS Combo Piles, HS/SS Combo Piles, page 8 page 8 Helical Pulldown Helical Pulldown

    Micropiles, page 9Micropiles, page 9 Grillages, page 10 Grillages, page 10

    For leg piles:For leg piles: HS Pile Series page 8 HS Pile Series page 8

    HS/SS Combo Piles, HS/SS Combo Piles, page 8 page 8

    Helical Pulldown Helical Pulldown

    Micropiles, page 9Micropiles, page 9

    For base piles:For base piles: HS Pile Series page 8 HS Pile Series page 8

    HS/SS Combo Piles, HS/SS Combo Piles, page 8 page 8

    Helical Pulldown Helical Pulldown

    Micropiles, page 9 Micropiles, page 9

    Monopole grillages, Monopole grillages, page 10 page 10

    For radialpiles: HS Pile Series,

    page 8 HS/SS Series,

    page 8 Helical Pulldown

    Micropiles,page 9

    For outrigger anchors: SS Anchor Series,

    page 7

  • THEORY OF FOUNDATION ANCHOR DESIGN

    Soil mechanics

    Throughout this discussion wewill concern ourselves with thetheories of soil mechanics as as-sociated with foundation anchordesign. The mechanical strengthof the foundations will not be con-sidered in this section as we ex-pect foundations with properstrengths to be selected by thedesign professional at the time ofdesign. For this discussion, weassume the mechanical proper-ties of the foundations are ad-equate to fully develop the strengthof the soil in which they are in-stalled. Although this discussiondeals with the foundation anchor,the design principles are basicallythe same for either a tension orcompression load. The designersimply uses soil strength param-eters above or below a helix, de-pending on the load direction.

    Bearing capacity theory

    This theory suggests that thecapacity of a foundation anchor isequal to the sum of the capacitiesof individual helices. The helix ca-pacity is determined by calculat-ing the unit bearing capacity of thesoil and applying it to the indi-vidual helix areas. Friction alongthe central shaft is not used indetermining ultimate capacity.Friction or adhesion on extensionshafts (but not on lead shafts) maybe included if the shaft is roundand at least 312" (8.9 cm) in diam-eter.

    A necessary condition for thismethod to work is that the helicesbe spaced far enough apart toavoid overlapping of their stresszones. A.B. Chance manufacturesfoundations with three-helix-diam-eter spacing, which has histori-cally been sufficient to preventone helix from significantly influ-encing the performance of another.

    The following reflects the state-of-the-art for determining deep

    multi-helix foundation capacitiesas practiced by A.B. Chance.

    Ultimate theoretical capacityof a multi-helix foundation equals

    Load

    Graphic representationof individualcompressionbearingpressureson multi-helixfoundation anchor.

    Equation A:

    Qt = Qh

    Where:Qt = total multi-helix anchor

    capacityQh = individual helix bearing

    capacity

    Equation B:

    Qh = Ah (9c + q Nq) Qs

    Where:Qh = Individual helix bearing

    capacityAh= projected helix areac = soil cohesionq = effective overburden

    pressureNq= bearing capacity factor

    (from the graph, nextpage)

    Qs= upper limit determinedby helix strength

    Projected helix area (Ah) isthe area projected by the helixon a flat plane perpendicularto the axis of the shaft.

    Qh3

    Qh2

    Qh1

    the sum of all individual helix ca-pacities, see Equation A. To de-termine the theoretical bearingcapacity of each individual helix,use Equation B.

    For additional information, seeTechnical Manual, Bulletin 01-9601.

    Cohesiveand non-cohesive soils

    Shear strength of soils is typicallycharacterized by cohesion (c) andangle of internal friction phi (),given in degrees. The designationgiven to soil that derives its shearstrength from cohesion is cohe-sive and indicates a fine-grain(e.g., clay) soil. The designationgiven to soil that derives its shearstrength from friction is non-co-hesive or cohesionless and in-dicates a granular (e.g., sand) soil.

    The product 9c from Equa-tion B is the strength due to cohe-sion in fine grain soils, where 9 isthe bearing capacity factor for co-hesive soils. The product qNqfrom Equation B is the strengthdue to friction in granular, cohe-sionless soils. The bearing capac-ity factor for cohesionless soils(Nq) may be determined from Fig-ure 1. This factor is dependentupon the angle of internal friction(). The curve is based onMeyerhoff bearing capacity fac-tors for deep foundations and hasbeen empirically modified to re-flect the performance of founda-tion anchors. Effective overbur-den pressure (q) is determined bymultiplying a given soils effectiveunit weight () times the verticaldepth (d) of that soil as measuredfrom the surface to the helix.

    For multiple soil layers abovea given helix, effective overbur-den pressure may be calculatedfor each layer and then addedtogether.

    When c and for a given soilare both known, (9c + q Nq) can besolved directly. However, soil re-

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  • ports often do not contain enoughdata to determine values for bothc and . In such cases, EquationB must be simplified to arrive at ananswer.

    The design professional mustdecide which soil type (cohesiveor cohesionless) is more likely tocontrol ultimate capacity. Once thisdecision has been made, the ap-propriate part of the (9c + q Nq)term may be equated to zero,which will allow solution of theequation. This approach gener-ally provides conservative results.When the soil type or behaviorexpected cannot be determined,calculate for both behaviors andchoose the smaller capacity.

    Tension anchor capacities arecalculated by using average pa-rameters for the soil above a givenhelix. Compression capacities maybe calculated similarly, howeversoil strength parameters shouldbe averaged for the soil below agiven helix.

    We recommend the use of fieldtesting to verify the accuracy oftheoretically predicted foundationanchor capacities.

    Bearing Capacity Factorfor Cohesionless Soils

    The patented HelicalPulldown Micropile is a compos-ite end-bearing/friction pile de-signed to both stiffen the pile shaftand develop additional capacityvia skin friction along its groutedcolumn. It consists of a steel screwanchor with a grout column abovethe helical end-bearing plates.The formation of this pile involvesthe use of circular lead displace-ment discs and extension displace-ment discs. The lead discs haveangled plates that when rotatedduring the installation process dis-place the soil laterally outwardaway from the pile shaft. There isno soil removed during the instal-lation of a Helical Pulldown

    Micropile. The remolded soil atthe outside of the displaced annu-lus would naturally tend to flowback around the lead disc and fillthe void created by the disc. How-ever, the use of high-density,

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    Angle of Internal Friction, degrees

    Nq

    Val

    ues

    Theory of the Helical Pulldown Micropile

    flowable grout acting under hy-drostatic pressure prevents thesoil from filling this voided annu-lus. The grout is pulled down asthe installation of the anchor con-tinues, thus forming a cast-in-placeconcrete pile. This concrete col-umn, not only stiffens the pile shaftto allow for the full mobilization ofthe end bearing capacity of thehelices, but also provides addi-tional capacity through the devel-opment of skin friction along theformed concrete column.

    Bearing capacity theory

    The end bearing capacity of aHelical Pulldown Micropile is de-termined by the methods as previ-ously described in this article.Namely, the bearing capacity ofeach individual helix is calculatedusing the bearing capacity equa-tion (Equation B). The sum of theseindividual bearing capacities isequal to the ultimate theoreticalbearing capacity of the multi-helixfoundation (Equation A).

    Friction capacity theory

    Friction piles in clays developtheir capacity via adhesion be-tween the pile and the soil. Theultimate friction capacity (Qf) isassumed to be equal to the unitadhesion times the embeddedarea of the pile. In very soft to softclays, the unit adhesion is nor-mally assumed equal to the cohe-sive strength of the clay. In me-dium and stiff clays, the unit adhe-sion is typically smaller than thecohesive strength of the clay. Theunit adhesion will also vary by thematerial type of pile, i.e., concrete,steel, timber,