CHANCE® Underpinnings Technical Manual

1

®®

POWER SYSTEMS, INC.

PROGRAMPARTICIPANT Report No. 94-27

I.C.B.O. ListedReport No. ER5110

®

™

LISTED®

ReportNo.9504

HELICAL PIER® Foundation SystemsU.S. Patents 5,011,336; 5,120,163;5,213,448

TechnicalManual

Contents• HELICAL PIER foundation systems

• History• Research and Development• Advantages

• Theory of Foundation Anchor Design• Soil Mechanics• Shallow and Deep HELICAL PIER foundation systems• Bearing Capacity Theory• Cohesive and Non-Cohesive Soils

• Installation Torque vs. Anchor Capacity

• Product Specification• Lead and Extension Section Lengths• Helix Areas• Helix Configuration• HELICAL PIER foundation systems ratings (table)• Corrosion• Slenderness Ratio/Buckling• Application Guidelines• Design Example• Specification References

See our catalog in Sweet’s,on Sweet’s CDand website,

McGraw-Hill, Inc.A. B. CHANCE COMPANY,HUBBELL POWER SYSTEMSCertificate Number 001136SIC Numbers 3499, 3429, 5063

Original Registration: July 1, 1992Current Registration: Oct. 23, 2003

This product was manufactured in a plant whose Quality Management Systemis certified/registered as being in conformity with ISO 9001:2000.

www.abchance.com

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HELICAL PIER FOUNDATION SYSTEMS

to enter the ground) may beused with one or more helices(generally, four is themaximum) with varyingdiameters in the range from 6to 14 inches (15 to 36 cm).Extensions, either plain or withadditional helices, may be usedto reach deep load-bearingstrata. Generally, eight is themaximum number of helicesused on a single screw pilefoundation. The shaft size mayvary from 11⁄2" (3.8 cm) squaresolid bar material to 10" (25cm) diameter pipe material.The number and size of helicesand the size and length of shaftfor a given application aregenerally selected based onthe in-situ soil conditions andthe loads that are to beapplied.

Advantages

The screw pile foundationsystem is known for its easeand speed of installation.Installation generally requiresno removal of soil, so there areno spoils to dispose of.Installation causes adisplacement of soils for themost part. However, in thecase of a foundation with apipe shaft, some soil will enterthe interior of the pipe until itbecomes plugged. Installationequipment can be mounted onvehicles when required. Theinstallation of a screw pilefoundation is for practicalpurposes vibration free. Thesefeatures make the screw pilefoundation attractive on sitesthat are environmentallysensitive. Installations nearexisting foundations or footingsgenerally cause no problems.However, the screw pilefoundation generally cannot beinstalled into competent rock orconcrete. Penetration willcease when materials of thisnature are encountered.

History

The earliest known use ofan anchor foundation was forthe support of lighthouses intidal basins around England. Ablind English brickmaker,Alexander Mitchell, is creditedwith design of a “screw pile” forthis purpose in 1833. The useof the “screw pile” wasapparently successful, butadvancement of the helix-platefoundation did not progress.

In the 1950s, the A.B.Chance Company introducedthe Power-Installed ScrewAnchor (PISA®) for resistingtension loads. The anchorfound favorable, widespreadacceptance. This anchorconsists of a plate or plates,formed into the shape of a helixor one pitch of a screw thread.The plate is attached to acentral shaft. The helix platehas its characteristic shape tofacilitate installation.Installation is accomplished byapplying torque to the anchorand screwing it into the soil.The effort to install the anchoris supplied by a torque motor.

Research and development

With the development ofthe tension screw anchor,came the use of the same orsimilar devices to resistcompression loads. Thus,screw pile foundations cameinto greater use. Various sizesand numbers of helices havebeen used with shafts ofvarying sections to providefoundations for differentapplications. In the past 40years, projects that haveutilized screw pile foundationsinclude electric utilitytransmission structures,Federal Aviation Administrationflight guidance structures,pipeline supports, buildingfoundations, remedial

underpinning, streetlights,walkways in environmentallysensitive areas and manyothers.

Torque capacities ofavailable installation equipmenthave increased over the pastyears. Hydraulic torque motorsin the 3,000 to 5,000 ft.-lb. (4.0to 6.8 kN-m) range haveincreased to the 12,000 to15,000 ft.-lb. (16 to 20 kN-m)range. Mechanical diggers nowextend the upper range to50,000 ft.-lb. (68 kN-m) ormore. “Hand-held” installershave expanded the availableequipment in the lower rangeof torque, with a capacity up to2,500 ft.-lb (3.4 kN-m). Thoughcalled “hand-held,” theseinstallers are hand-guidedwhile a torque bar or otherdevice is used to resist thetorque being applied to thescrew pile foundation.

As suggested earlier, thescrew pile foundation may beutilized in various forms. Thelead section (i.e., the first part

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THEORY OF FOUNDATION ANCHOR DESIGN

Soil mechanics

Throughout this discussionwe will concern ourselves withthe theories of soil mechanics asassociated with foundation an-chor design. The mechanicalstrength of the foundations willnot be considered in this sectionas we expect foundations withproper strengths to be selectedby the design professional at thetime of design. For this discus-sion, we assume the mechani-cal properties of the foundationsare adequate to fully develop thestrength of the soil in which theyare installed. Although this dis-cussion deals with the founda-tion anchor, the design principlesare basically the same for eithera tension or compression load.The designer simply uses soilstrength parameters above orbelow a helix, depending on theload direction.

Shallow and deepfoundation anchors

Two modes of soil failuremay occur depending on helixdepth: One is a shallow failuremode and the other is a deepfailure mode. Foundations ex-pected or proven to exhibit a spe-cific mode are often referred toas “shallow” or “deep” founda-tions. The terminology “shallow”or “deep” refers to the locationof the bearing plate with respectto the earth's surface. By defini-tion, “shallow” foundations ex-hibit a brittle failure mode withgeneral eruption of the soil all theway to the surface and a suddendrop in load resistance to almostzero. With “deep” foundations,the soil fails progressively, main-taining significant post-ultimateload resistance, and exhibits littleor no surface deformation. Thedividing line between shallowand deep foundations has beenreported by various investigatorsto be three to eight times the

foundation diameter. ChanceCompany uses five diameters asthe break between shallow anda deep foundation anchors. Thefive-diameter depth is the verti-cal distance from the surface tothe top helix. The five-diameterrule is often simplified to 5 feet(1.5 m), minimum.

Any time a foundation anchoris considered, it should be ap-plied as a deep foundation. Adeep foundation has two advan-tages over a shallow foundation:

1. Provides an increased ul-timate capacity.

2. Failure will be progressivewith no sudden decrease in loadresistance after the ultimate ca-pacity has been achieved.

Bearing capacity theory

This theory suggests that thecapacity of a foundation anchoris equal to the sum of the capaci-ties of individual helices. Thehelix capacity is determined bycalculating the unit bearing ca-pacity of the soil and applying itto the individual helix areas. Fric-tion along the central shaft is notused in determining ultimate ca-pacity. Friction or adhesion onextension shafts (but not on leadshafts) may be included if theshaft is round and at least 31⁄2"(8.9 cm) in diameter.

A necessary condition for thismethod to work is that the heli-ces be spaced far enough apartto avoid overlapping of theirstress zones. Chance Companymanufactures foundations withthree-helix-diameter spacing,which has historically been suf-ficient to prevent one helix fromsignificantly influencing the per-formance of another.

The following reflects thestate-of-the-art for determining

Load

Graphic representationof individual compres-sion bearing pressureson multi-helix founda-tion anchor.

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THEORY OF FOUNDATION ANCHOR DESIGN (continued)

deep multi-helix foundation ca-pacities as practiced by Chance.

Ultimate theoretical capacityof a multi-helix foundation equalsthe sum of all individual helix ca-pacities, see Equation A. To de-termine the theoretical bearingcapacity of each individual helix,use Equation B.

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.

Bearing Capacity Factor for Cohesionless Soils

Figure 1

Cohesiveand non-cohesive soils

Shear strength of soils is typi-cally characterized by cohesion(c) and angle of internal friction“phi” (Ø), given in degrees. Thedesignation given to soil thatderives its shear strength fromcohesion is “cohesive” and indi-cates a fine-grain (e.g., clay) soil.The designation given to soil thatderives its shear strength fromfriction is “non-cohesive” or “co-hesionless” and indicates a

be solved directly. However, soilreports often do not containenough data to determine valuesfor both c and Ø. In such cases,Equation B must be simplified toarrive at an answer.

The design professionalmust decide which soil type (co-hesive or cohesionless) is morelikely to control ultimate capac-ity. Once this decision has beenmade, the appropriate part of the(9c + q Nq) term may be equatedto zero, which will allow solutionof the equation. This approachgenerally provides conservativeresults. When the soil type orbehavior expected cannot be de-termined, calculate for both be-haviors and choose the smallercapacity.

Tension anchor capacitiesare calculated by using averageparameters for the soil above agiven helix. Compression ca-pacities may be calculated simi-larly, however soil strength pa-rameters should be averaged forthe soil below a given helix.

We recommend the use offield testing to verify the accuracyof theoretically predicted founda-tion anchor capacities.

granular (e.g., sand) soil.

The product “9c” from Equa-tion B is the strength due to co-hesion in fine grain soils, where9 is the bearing capacity factorfor cohesive soils. The product“qNq” from Equation B is thestrength due to friction in granu-lar, cohesionless soils. The bear-ing capacity factor for cohesion-less soils (Nq) may be deter-mined from Figure 1. This factoris dependent upon the angle ofinternal friction (Ø). The curve isbased on Meyerhoff bearing ca-pacity factors for deep founda-tions and has been empiricallymodified to reflect the perfor-mance of foundation anchors.Effective overburden pressure(q) is determined by multiplyinga given soil’s effective unit weight(γ) times the vertical depth (d) ofthat soil as measured from thesurface to the helix.

For multiple soil layers abovea given helix, effective overbur-den pressure may be calculatedfor each layer and then addedtogether (see Design example,pages 8 and 9).

When c and Ø for a given soilare both known, (9c + q Nq) can

Angle of Internal Friction, degrees

Nq —

Val

ues

10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 440

10

20

30

40

50

60

70

80

90

100

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INSTALLATION TORQUE VS. ANCHOR CAPACITY

Type SS HELICAL PIER® Foundation SystemsInstallation Torque vs. N-Value

in Sand

Figure 2

Inst

alla

tion

Torq

ue

N-Value

Figures 2 and 3 show graphs depicting how installation torque varies with respect to SPTresults (N-values per ASTM D-1586) indicating the in-situ soil strength.

Figure 2 shows the relationship between installation torque and N-values for sands. Theenvelope of curves depicts increasing torque for a given N value with increasing depth. Watertable position directly affects installation torque and ultimate capacity by causing a reduction inthe effective unit weight of the soil below the table. This in turn will cause a reduction ininstallation torque and ultimate capacity.

For cohesive soil (Figure 3), a straight-line relationship is provided as soil strength orcohesion is the only factor affecting installation torque and ultimate capacity.

Type SS HELICAL PIER® Foundation SystemsInstallation Torque vs. N-Value

in Clay

Figure 3

Inst

alla

tion

Torq

ue

N-Value

the subject is in the paper“Uplift Capacity of HelicalAnchors in Soil” by R.M. Hoytand S.P. Clemence (Bulletin 2-9001). It gives the formula forthe torque/anchor capacity as:

Qu = Kt x TwhereQu = ultimate uplift capacity

[lb. (kN)]Kt = empirical torque

factor [ft.-1 (m-1)]T = average installation

torque [ft.-lb. (kN-m)]

The value of Kt mayrange from 3 to 20 ft.-1 (10 to

Holding strength relatedto installing torque

The idea that the amount oftorsional force required toinstall a foundation anchorrelates to the ultimate capacityof the foundation in tension orcompression has long beenpromoted by the Chance Co.Precise definition of therelationship for all possiblevariables remains to beachieved. However, simpleempirical relationships havebeen used for a number ofyears.

Recommended reading on

Increasing Depth

66 m-1), depending on soilconditions and anchor design(principally the shaft size). ForType SS foundation anchors, ittypically ranges from 10 to 12(33 to 39) with 10 (33) beingthe recommended defaultvalue. For Type HS foundationanchors, the recommendeddefault value is 7 (23). Thesame values of Kt are used forboth tension and compressionloading. Torque monitoringtools are available fromChance. Their use provides agood method of productioncontrol during installation.

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*SS150 shafts have a paint stripe at top to distinguish from Type SS5.(1)For 14" (36 cm)-dia. foundation anchors, reduce allowable capacity by 20% per building code requirements. Not applicable to HS.(2)For 14" (36 cm)-dia. helices, reduce ultimate capacity by 20%.3)Determined by bracket and haunch design.(4)The capacity of Chance HELICAL PIER foundation systems is a function of many individual elements including the capacity of thefoundation, bracket, anchor shaft, helix plate and bearing stratum, as well as the strength of the foundation-to-bracket connection andthe quality of anchor installation. This row of the table shows typical achievable capacities under normal condtions. Actual achievablecapacities could be higher or lower depending on the above factors.

SystemRatings Table

Minimum Ultimate TorqueCapacity [ft.-lb. (kN-m)]

Ultimate Strength [kips (kN)]for Axially Loaded FoundationTorque Limited

Working Capacity [kips (kN)]with 2.0 Safety FactorTorque Limited

Ultimate Strength per Helix -Tension/Compression [kips (kN)]

Working Capacity per Helix -Tension/Compression [kips (kN)]with 2.0 Safety Factor

Bracket C150-0121

Min. Ultimate Strength [kips (kN)]

Working Capacity [kips (kN)]with 2.0 Safety Factor

Typical Achievable InstalledCapacity [kips (kN)](4)

Bracket C150-0298




Bracket C150-0299




Bracket C150-0147




HELICAL PIER Foundation Systems Family

HS Pipe Shaft

31⁄2" (8.9 cm) OD

Column 4

11,000 (15)

100 (440)

77 (340)

50 (220)38.5 (170)

(2)50 (220)

(2)25 (110)

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A(3)

N/A(3)

N/A(3)

*SS150 Square Shaft

11⁄2" (3.8 cm)

Column 2

7,000 (9.5)

70 (300)

70 (300)

35 (150)35 (150)

(2)40 (180)

(2)20 (90)

40 (180)

20 (90)

25 (110)

80 (360)

40 (180)

25 (110)

N/A

N/A

N/A

N/A

N/A

N/A

SS175 Square Shaft

13⁄4" (4.4 cm)

Column 3

10,000 (13.5)

100 (440)

100 (440)

50 (220)50 (220)

(2)50 (220)

(2)25 (110)

N/A

N/A

N/A

N/A

N/A

N/A

80 (360)

40 (180)

30 (130)

80 (360)

40 (180)

40 (180)

SS5 Square Shaft

11⁄2" (3.8 cm)

Column 1

5,500 (7.5)

70 (310)

55 (240)

35 (160)27.5 (120)

(2)40 (180)

(1)(2)20 (90)

40 (180)

20 (90)

20 (90)

80 (360)

40 (180)

20 (90)

N/A

N/A

N/A

N/A

N/A

N/A

Row E

Row D

Row C

Row B

Row A

Row J

Row I

Row N

Row M

Row L

Row K

Row H

Row G

Row F

Row Q

Row P

Row O

PRODUCT SPECIFICATION

7

Note: This chart uses a factor ofsafety vs. ultimate capacity = 2.

Minimum HELICAL PIER®

Foundation SystemsAnchor

Required

SS5

SS150

SS175 or HS

DesignLoad,kips(kN)

0 to 25(0 to 110)

25 to 35(110 to 150)

35 to 50(150 to 220)

Minimum anchor type requiredbased on mechanical ratings

Lead and extension section lengthsone helix arranged in increas-ing diameters from the founda-tion tip to the uppermost helix.The nominal spacing betweenhelix plates is three times thediameter of the next lowerhelix. For example, a HELICAL

PIER foundation systemsanchor with an 8-, 10-, and 12-inch (20, 25 and 30 cm) helixcombination has a 24-inch (61cm) space between the 8- and10-inch (20 and 25 cm) helixand a 30-inch (76 cm) spacebetween the 10- and 12-inch(25 and 30 cm) helix. Exten-sions with helix plates can beadded to the foundation if morebearing area is required. Theyshould be installed immediatelyafter the lead section.

Capacities listed in theRatings Table on the page 6are mechanical ratings. Onemust be aware that the actualinstalled load capacities aredependent on actual soilconditions at each specificproject site. Therefore, thedesign professional should usethe bearing capacity method indesigning anchor foundations.The number of helices, theirsize, and depth below grade isdetermined by obtaining soil

shear strength factors, cohe-sion (c) and angle of internalfriction (Ø), and applying themas outlined in Theory of Foun-dation Anchor Design. Theanchor family specified isbased on the rated load carry-ing capacities for the specificfoundation shaft size andinstallation torque required toinstall the foundation. The shaftsizes are 11⁄2- or 13⁄4-inch (3.8or 4.5 cm) square solid steel or31⁄2-inch (8.9 cm) OD heavy-wall steel pipe.

Chance is available to aidthe design professional indetermining the best helixcombination/foundation anchorfamily for a given application.Additional design consider-ations are as follows:

CorrosionCorrosion of foundation

anchors is a major consider-ation in permanent structures.That is why foundation compo-nents are hot-dip galvanizedper ASTM A153. The zinccoating will add between 5%and 20% to the life of HELICAL

PIER foundation systemsanchors. The Federal HighwayAdministration (FHWA-SA-96-072) has established, from anextensive series of field testson metal pipes and sheet steelburied by the National Bureauof Standards, maximum corro-sion rates for steel buried in

soils exhibiting theelectrochemicalindex propertiesshown in the table:

The corrosionrates shown beloware suitable for

designs for screw anchorfoundations. These rates ofcorrosion assume a mildlycorrosive in-situ soil environ-ment having the electrochemi-cal property limits that arelisted in table below. Thedesign corrosion rates, perFHWA-SA-96-072, are:

For Zinc15 µm/year (first 2 years)4 µm/year (thereafter)

For Carbon Steel12 µm/year

For example, in a soil

Test Method

AASHTO T-288-91

AASHTO T-289-91

AASHTO T-291-91

AASHTO T-290-91

AASHTO T-267-86

Criteria

>3000 ohm-cm

>4.5<9

<100 PPM

<200 PPM

1% max.

Property

Resistivity

pH

Chlorides

Sulfates

Organic Content

HELICAL PIER foundation sys-tems standard lead-sectionlengths are 5, 7, and 10 ft. (1.5,2 and 3 m). The standardextension section lengths are31⁄2, 5, 7, and 10 ft. (1, 1.5, 2and 3 m). These combinationsof leads and extensions pro-vide for a variety of installedfoundation anchor lengths.

Helix areasStandard diameters for helicesmanufactured by Chance are:

6 in. = 26.7 sq. in.(15 cm = 0.0172 m2)

8 in. = 48.4 sq. in.(20 cm = 0.0312 m2)

10 in. = 76.4 sq. in.(25 cm = 0.0493 m2)

12 in. = 111 sq. in.(30 cm = 0.0716 m2)

14 in. = 151 sq. in.(35 cm = 0.0974 m2)

Helix configurationStandard helices are 3⁄8 inch(0.95 cm) thick steel plates withouter diameters of 6, 8, 10, 12and 14 inches (15, 20, 25, 30and 35 cm). The lead section,or first section installed into thesoil always contains helixplate(s). Extensions may beplain or helixed. Multihelixfoundations have more than

PRODUCT SPECIFICATION (continued)

8

environment that meets theelectrochemical propertieslisted in the table, the allowablestrength of the galvanized SS5screw anchor foundation for adesign life of 75 years, is 37kips (160 kN). For soils with

corrosion potential differentthan that stated above, oneshould consult a corrosionengineer.

Chance Bulletin 01-9204contains extensive data takenfrom NBS circular 579, April

1957, by Melvin Romanoff. Thereader is encouraged to obtainthis bulletin for more referenceand examples on corrosion,including methods of additionalcorrosion protection.

Slenderness ratio/bucklingIt is intuitively obvious that

HELICAL PIER foundation sys-tems anchors have slendershafts. Very high slendernessratios (Kl/r) can be expecteddepending on the length of thefoundation. This conditionwould be a concern if thefoundation were a column in airor water and subjected to acompressive load. However,the foundations are not sup-ported by air or water, but bysoil. Therein lies the reason

that foundations can be loadedin compression up to theirrated load capacities.

As a practical guideline,when a specific soil’s StandardPenetration Test blow countdata per ASTM D-1586 isgreater than 4, buckling of thefoundation shaft has beenfound not to occur when loadedto the rated capacities.

Buckling analysis for soilshaving lower blow counts canbe done by hand calculationswith the *Davisson (1963)

Method, or by computer solu-tion with a finite differencemethod such as that used inthe program LPILE (ENSOFT,Austin, TX). Research byChance Co. and others (†Hoyt,et al, 1995) has shown thatbuckling is a practical concernonly in the softest soils (verysoft and soft clay, very loosesands) and this is in agreementwith past analyses and experi-ence on other types of pilefoundations. Refer to ChanceBulletin 01-9605 for moredetails.

Design exampleAn existing two-story brick-

veneer residence has experi-enced settlement. The designerhas calculated a foundationload of 1500 pounds per linearfoot (22 kN per meter). Inaddition, the designer hasdetermined the best foundationanchor spacing is 6 feet (1.8m)on centers. Thus, the designload is 9 kips (40 kN) peranchor. Soil properties arelisted below. Determine thenumber and size of helix(es)required, their depth belowgrade, and the foundationanchor family needed to carrythe design load of 9 kips (40kN). Use a safety factor (SF) of

2.

Soil Properties (as determinedfrom soil boring data):

51⁄2 feet (1.7 m) of sandy clay filloverlying homogeneous sandmaterial having soil parametersof:

phi (Ø) of sand = 34°

unit weight (γ) of sand = 120lb./ft.3 (19 kN/m3); sandy clay =103 lb./ft.3 (16 kN/m3)

Water table at 18 ft. depth.

• Using the standard bearingequation:

Qh = Ah (9c + qNq)

For sand, the bearing equa-tion reduces to:

Qh = Ah (qNq)

From Figure 1 at the end ofTheory of FoundationAnchor Design, choose thebearing capacity factor:

Nq = 22 For phi (Ø) = 34°

• At this point, an iterativeprocess is required. Select ahelixcombination youbelieve can develop therequired load.

• Trial 1: Select a single helixfoundation anchor [10" (25cm) diameter helix].

Determine vertical depth tothe helix. In this example, it isdesired to install the helix intothe homogeneous sand wellbelow the fill material. Applica-

*Davisson, M.T. 1963. “Estimating Buckling Loads for Piles.” Proc. 2nd Pan-Amer. Conf. on S.M. & F.E., Brazil, vol. 1: 351-371.†Hoyt, R.M., et al 1995. “Buckling of Helical Anchors Used for Underpinning”, Proc. Foundation Upgrading and Repair for InfrastructureImprovement, San Diego.


9

tion guideline No. 4 from page10 requires at least 3 helixdiameters. Therefore, the helix[10" (25 cm) dia.] should be atleast 51⁄2 ft. + 30 inches = 8 ft.(1.7 m + 0.7 m = 2.4 m)

Calculate effective overburdenpressure for the 10" (25 cm)helix.

q = γ x dq10 = (0.103 x 5.5) + (0.120 x 2.5) = 0.867 ksf[q10 = (16 x 1.7) + (19 x 0.76) = 42 kN/m3]

Determine the capacity of thehelix using reduced bearingequation.

Qh = Ah (qNq)A10 = 76.4 in.2 (0.0493 m2 )(see “Helix areas” on page 7)

Q10 = (76.4/144) x 0.867 x 22 = 10.12 kips

Q10 = 0.0493 x 42 x 22 = 45 kN

Ultimate theoretical capacity = 10.12 kips (45kN)

Another trial is requiredbecause the design load of 9kips (40 kN) times a SafetyFactor of 2 = 18 kips (80 kN) >10.12 kips (45kN).

• Trial 2: Select a 12" (30 cm)dia. foundation anchorinstalled 2 feet (0.6 m)deeper into the sand.

d12 = 8 ft. + 2 ft. = 10 ft.

[d12 = 2.4 m + 0.6 m = 3.0 m]q12 = (0.103 x 5.5) + (0.120 x 4.5) = 1.107 ksf[q12 = (16 x 1.7) + (19 x 1.3) = 52 kN/m3]A12 = 111 in.2 (0.0716 m2)Q12 = (111/144) x 1.107 x 22 = 18.77 kips[Q12 = 0.0716 x 52 x 22 = 82 kN]

Ultimate theoretical capacity = 18.77 kips (82 kN)

9 kips (40 kN) times SF of 2 = 18 kips (80 kN)<18.77 kips (82 kN).

Thus, a 12" (30 cm) diam-eter foundation anchor with thehelix 10 ft. (3 m) below thesurface will work.

• Feasibility check:

Application guideline No. 8from page 10 recommends thateconomic feasibility should bechecked if more than one

combination of foundation leadand extension sections can beused. Therefore, an 8"-10" (20cm - 25 cm) two-helix founda-tion will be considered for thisexample: The top helix [10" (25cm) dia.] should be at least 8 ft.(2.4 m) for reasons explainedpreviously. Remember thehelices are spaced 3 diametersapart. So 8" (20 cm) x 3 = 24inches (0.6 m). Thus, thevertical distance to each helixis:d10 = 5.5 ft. + (3x10) in. = 8 ft.[d10 = 1.7 m + 3(25 cm) = 8 ft.]d8 = 8 ft. + (3x8) in. = 10 ft.[d8 = 2.4 m + 3(20 cm) = 3 m]

Calculate effective overburdenpressure for each helix.q = Y x dq10 = (0.103 x 5.5) + (0.120 x 2.5) = 0.867 ksf[q10 = (16 x 1.7) + (19 x 0.76) = 42 kNm2]q8 = (0.103 x 5.5) + (0.120 x 4.5) = 1.107 ksf[q8 = (16 x 1.7) + (19 x 1.3) = 52 kNm2]

Determine the capacity of eachhelix using reduced bearingequation and sum for resultingultimate theoretical anchorcapacity.Qh = Ah (qNq)A8 = 48.4 in.2 (0.0312 m2)Q10 = (76.4/144) x 0.867 x 22 = 10.12 kips[Q10 =0.0493 x 42 x 22 = 46 kN]Q8 = (48.4/144) x 1.107 x 22 = 8.19 kips[Q8 =0.0412 x 52 x 22 = 36 kN]

Ultimate theoretical capacity = 18.31 kips(82 kN)

Thus, an 8"-10" (20 cm - 25cm)diameter foundation anchorwith the top helix 8 ft. (2.4 m)below the surface will alsowork. The best choice will haveto be made based on totalinstalled cost.

• Select the appropriatefoundation anchor family:

Because this is an underpin-ning retrofit job, check theFoundation System RatingsTable to select the appropriate

foundation anchor family. FromRow D, Column 1 of the Table,an SS5 series 11⁄2" (3.8cm)square shaft foundation’srated capacity is 20 kips (90kN), which exceeds the designload of 9 kips (40 kN).

• Check helix ratings:

Based on Row B, Column 1of the Ratings Table, theallowable capacity of an SS5foundation anchor with a singlehelix is 20 kips (90 kN), whichexceeds the design load.Therefore, a single helix foun-dation can be used.

• Check the installationtorque required to ensureadequate capacity:

Torque required = Required Load/Kt

18,000 lb. = 1,800 ft.-lb.10

80 kN = 2.4 kN-m33 m-1

Based on Row A, Column 1of the Ratings Table, theallowable torque capacity ofan SS5 foundation anchor is5,500 ft.-lb. (7.5 kN-m), whichexceeds the 1,800 ft.-lb. (2.4kN-m) required torque.

Per application guideline No.9 from page 10, if a stronger ordenser stratum overlaid thesand, it would be necessary tocheck the installing torque inthis stratum to be sure theanchor could be installed.

For additional reference,Chance Bulletin 31-8901contains example problems fortension anchors in both cohe-sive and cohesionless soils.

Design example (continued)


10All components are hot-dip galvanized to increase product life in aggressive soils.

SS5, SS150, SS175Underpinning BracketsApplied in multiple locations along thefoundation to stabilize and correct problemscaused by poor soil conditions.For seismic uplift loads, the Uplift RestraintBracket may be added.

HeavyDuty BracketFor such higher loads as commercial buildings and largerresidences. Applied in multiples to stop settled areas,resist new movement.

Light DutyBracketPrimarily for correct-ing sagging, lesserloads; affordable “quickfix” outlasts theporches, stairways,decks and patios itrepairs.

Specification ReferencesDetails on the Chance HELICAL

PIER foundation systems areavailable upon request in thethree-part section Manu-Spec®

format (of the Spec-Data® pro-gram copyrighted by The Con-struction Specifications Institute).Filed under the identical 02150designation as Sweet’s, highlightsinclude:

Part 1 — General1.01 Summary: New and reme-dial building foundation reinforce-ment and stabilization, retainingwalls, tieback systems. Relatedsections: Excavating to workinglevel, load tests, cast-in-placeconcrete reinforcement. Pricing.1.02 References: Thirteen ASTMand one SAE standards specifica-tions.1.03 Definition of system1.04 System description1.05 Submittals: Conditions ofthe Contract, Spec-Data®, shopdrawings, certified test reportsand installation instructions.Closeouts: Warranty, projectrecords.1.06 Quality Assurance: Dealercertification, preinstallationmeetings.1.07 Warranty: Project, manufac-turer, period (term).

Part 2 — Products2.01 Shoring and underpinning:Chance; proprietary system.2.02 Product substitutions: None.2.03 Manufactured compo-nents: Screw anchor plate, shaft,bolts, steel bracket.2.04 Source quality: Tests, inspec-tions, verification of performance.

Part 3 — Execution3.01 Manufacturer’s instruc-tions: Comply with technical data.3.02 Preparation: Spare nearbystructures; varying elevations.3.03 Installation: Certifiedinstaller; power units; torquerecording; alignment; adapters;down pressure; rate of rotation;obstructions; minimum depth,torque and cover, A/E approval,connect to structure.

3.04 Field quality requirements:Site tests and inspections.3.05 Protection: From damageduring construction.

Business PracticesBefore each job by contrac-

tors certified to install the ChanceHELICAL PIER foundation systems,a quotation is prepared. Custom-arily, the bid for work is based onthe amount to be billed perfoundation, access and finaldetails required.

Application guidelines1. Foundation anchors should beapplied as deep foundations. Thevertical distance between theuppermost helix and the soil surfaceshould be no less than 5 feet (1.5 m)or 5 times the helix diameter.

2. Installation torque should beaveraged over the last three diam-eters of embedment of the largesthelix. This will provide an indicationof the anchor’s capacity based on theaverage soil properties throughoutthe zone that will be stressed by thefoundation.

3. The uppermost helix should beinstalled at least three diametersbelow the depth of seasonal variationin soil properties.4. The uppermost helix should beinstalled at least three helix diametersinto competent load-bearing soil.5. For a given foundation length, it isbetter to use a few long extensionsthan many shorter extensions. Thisresults in fewer connections in the soil.

6. Foundation anchors should bespaced laterally no closer than threediameters on centers. A better spacingis five diameters. Use the largest helix

diameter in making the spacingdetermination.

The influence of the structure’sexisting foundation on the foundationanchor also should be considered.

8. Check economic feasibility if morethan one combination of foundationlead and extension sections can beused.

9. If any stronger, denser, etc. stratumoverlies the bearing stratum, checkinstallation torque in the stratum toensure anchor can be installed to finalintended depth without torsionaloverstressing.


11

NewConstructionBracketFor support of new structures.Placed on foundation anchors in-stalled between footing forms andtied to reinforcing bars beforepouring concrete.

Slab BracketFor stabilizing uneven or damagedfloors. Bolt adjusts through capfitting on top of foundation sochannel lifts floor.

UpliftRestraintBracketFor seismic conditions andto resist other upwardforces. Shown as applied,assembled to top of Stan-dard-Duty Bracket.

Wall AnchorsTo restrain movement infoundation walls.Through a hole drilled inwall, a rod threads into ananchor plate installed intothe soil bank. A ribbedretainer plate and a nutsecure the rod inside thewall. Either of two meth-ods may be used to stabi-lize, or often to straighten,failing walls.

DURA-GRIP® wall repair systemcross plate anchors tiebackretaining and foundation walls.

At left, screw anchors tiebackretaining and foundation walls.

At right,installationconcept


12

Power-installed screwanchors have proven tobe a reliable and economical

advancement in foundation technology.Chance HELICAL PIER foundation systemsanchors and related hardware areavailable in a wide range of sizes tomeet many job applications. The ChanceCompany also offers such unique prod-uct resources as:

■ Training and field supervision ofcertified installers

■ Geotechnical engineering guid-ance for any job

■ Computer-assisted design capa-bility through interactive softwareprograms and a field manualbringing design theory to practicalfield application

®®

POWER SYSTEMS, INC.

Printed in USA©2004 Hubbell, Inc.

A.B. Chance Company210 North Allen StreetCentralia, MO 65240 USA

DISCLAIMER: The material presented in this bulletin is derived from generally accepted engineering practices. Specific application and plans ofrepair should be prepared by a local structural/geotechnical engineering firm familiar with conditions in that area. The possible effects of soil (suchas expansion, liquefaction and frost heave) are beyond the scope of this bulletin and should be evaluated by others. Chance Company assumes noresponsibility in the performance of anchors beyond that stated in our SCS policy sheet on terms and conditions of sale.NOTE: Because Hubbell has a policy of continuous product improvement, we reserve the right to change design and specifications without notice.

Bulletin 01-9601Revised 3/04

www.abchance.com

Documents

CHANCE® Underpinnings Technical Manual