HELICAL PIER® Foundation Systems

U.S. Patents 5,011,336; 5,120,163; 5,139,368; 5,171,107; 5,213,448

Technical
Manual
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
PROGRAM
PARTICIPANT
• Lead and Extension Section Lengths
Report No. 94-27
®

• Helix Areas
LISTED • Helix Configuration
® • HELICAL PIER® Foundation Systems Ratings (Table)
™
I.C.B.O. Listed Report • Corrosion
Report No. ER5110 No.9504 • Slenderness Ratio/Buckling
• Application Guidelines
See our catalog in Sweet’s, • Design Example
on Sweet’s CD
and website, • Specification References
McGraw-Hill, Inc.
® ISO 9001-1994
® Cert. No. 001136

A. B. Chance Co.
Centralia, MO USA

POWER SYSTEMS, INC. www.hubbell.com/abchance

1
 HELICAL PIER® FOUNDATION SYSTEMS

History to enter the ground) may be

used with one or more helices
The earliest known use of
(generally, four is the
an anchor foundation was for
maximum) with varying
the support of lighthouses in
diameters in the range from 6
tidal basins around England. A
to 14 inches (15 to 36 cm).
blind English brickmaker,
Extensions, either plain or with
Alexander Mitchell, is credited
additional helices, may be used
with design of a “screw pile” for
to reach deep load-bearing
this purpose in 1833. The use
strata. Generally, eight is the
of the “screw pile” was
maximum number of helices
apparently successful, but
used on a single screw pile
advancement of the helix-plate
foundation. The shaft size may
foundation did not progress.
vary from 11⁄2" (3.8 cm) square
In the 1950s, the A.B. solid bar material to 10" (25
Chance Company introduced cm) diameter pipe material.
the Power-Installed Screw The number and size of helices
Anchor (PISA®) for resisting and the size and length of shaft
tension loads. The anchor for a given application are
found favorable, widespread generally selected based on
acceptance. This anchor the in-situ soil conditions and
consists of a plate or plates, the loads that are to be
formed into the shape of a helix applied.
or one pitch of a screw thread.
underpinning, streetlights, Advantages
The plate is attached to a
central shaft. The helix plate walkways in environmentally The screw pile foundation
has its characteristic shape to sensitive areas and many system is known for its ease
facilitate installation. others. and speed of installation.
Installation is accomplished by Installation generally requires
Torque capacities of
applying torque to the anchor no removal of soil, so there are
available installation equipment
and screwing it into the soil. no spoils to dispose of.
have increased over the past
The effort to install the anchor Installation causes a
years. Hydraulic torque motors
is supplied by a torque motor. displacement of soils for the
in the 3,000 to 5,000 ft.-lb. (4.0
most part. However, in the
Research and development to 6.8 kN-m) range have
case of a foundation with a
increased to the 12,000 to
With the development of pipe shaft, some soil will enter
15,000 ft.-lb. (16 to 20 kN-m)
the tension screw anchor, the interior of the pipe until it
range. Mechanical diggers now
came the use of the same or becomes plugged. Installation
extend the upper range to
similar devices to resist equipment can be mounted on
50,000 ft.-lb. (68 kN-m) or
compression loads. Thus, vehicles when required. The
more. “Hand-held” installers
screw pile foundations came installation of a screw pile
have expanded the available
into greater use. Various sizes foundation is for practical
equipment in the lower range
and numbers of helices have purposes vibration free. These
of torque, with a capacity up to
been used with shafts of features make the screw pile
2,500 ft.-lb (3.4 kN-m). Though
varying sections to provide foundation attractive on sites
called “hand-held,” these
foundations for different that are environmentally
installers are hand-guided
applications. In the past 40 sensitive. Installations near
while a torque bar or other
years, projects that have existing foundations or footings
device is used to resist the
utilized screw pile foundations generally cause no problems.
torque being applied to the
include electric utility However, the screw pile
screw pile foundation.
transmission structures, foundation generally cannot be
Federal Aviation Administration As suggested earlier, the installed into competent rock or
flight guidance structures, screw pile foundation may be concrete. Penetration will
pipeline supports, building utilized in various forms. The cease when materials of this
foundations, remedial lead section (i.e., the first part nature are encountered.
2
THEORY OF FOUNDATION ANCHOR DESIGN

Soil mechanics foundation diameter. Chance Load

Company uses five diameters as
Throughout this discussion
the break between shallow and
we will concern ourselves with Graphic representation
a deep foundation anchors. The
the theories of soil mechanics as of individual compres-
five-diameter depth is the verti-
associated with foundation an- sion bearing pressures
cal distance from the surface to
chor design. The mechanical on multi-helix founda-
the top helix. The five-diameter
strength of the foundations will tion anchor.
rule is often simplified to 5 feet
not be considered in this section
(1.5 m), minimum.
as we expect foundations with
proper strengths to be selected Any time a foundation anchor
by the design professional at the is considered, it should be ap-
time of design. For this discus- plied as a deep foundation. A
sion, we assume the mechani- deep foundation has two advan-
cal properties of the foundations tages over a shallow foundation:
are adequate to fully develop the 1. Provides an increased ul-
strength of the soil in which they timate capacity.
are installed. Although this dis- 2. Failure will be progressive
cussion deals with the founda- with no sudden decrease in load
tion anchor, the design principles resistance after the ultimate ca-
are basically the same for either pacity has been achieved. A necessary condition for this
a tension or compression load. method to work is that the heli-
The designer simply uses soil Bearing capacity theory ces be spaced far enough apart
strength parameters above or This theory suggests that the to avoid overlapping of their
below a helix, depending on the capacity of a foundation anchor stress zones. Chance Company
load direction. is equal to the sum of the capaci- manufactures foundations with
ties of individual helices. The three-helix-diameter spacing,
Shallow and deep
helix capacity is determined by which has historically been suf-
foundation anchors
calculating the unit bearing ca- ficient to prevent one helix from
Two modes of soil failure pacity of the soil and applying it significantly influencing the per-
may occur depending on helix to the individual helix areas. Fric- formance of another.
depth: One is a shallow failure tion along the central shaft is not
mode and the other is a deep The following reflects the
used in determining ultimate ca-
failure mode. Foundations ex- state-of-the-art for determining
pacity. Friction or adhesion on
pected or proven to exhibit a spe- extension shafts (but not on lead
cific mode are often referred to shafts) may be included if the
as “shallow” or “deep” founda- shaft is round and at least 31⁄2"
tions. The terminology “shallow” (8.9 cm) in diameter.
or “deep” refers to the location
of the bearing plate with respect
to the earth's surface. By defini-
tion, “shallow” foundations ex-
hibit a brittle failure mode with
general eruption of the soil all the
way to the surface and a sudden
drop in load resistance to almost
zero. With “deep” foundations,
the soil fails progressively, main-
taining significant post-ultimate
load resistance, and exhibits little
or no surface deformation. The
dividing line between shallow
and deep foundations has been
reported by various investigators
to be three to eight times the

3
 THEORY OF FOUNDATION ANCHOR DESIGN (continued)
Bearing Capacity Factor for Cohesionless Soils
deep multi-helix foundation ca- 100
pacities as practiced by Chance Figure 1
Company. 90

Ultimate theoretical capacity 80

of a multi-helix foundation equals
70
the sum of all individual helix ca-
pacities, see Equation A. To de-

Nq — Values
60
termine the theoretical bearing
capacity of each individual helix, 50
use Equation B.
40
Equation A:
Qt = ∑Qh 30

Where: 20
Qt = total multi-helix anchor
10
capacity
Qh = individual helix bearing
0
capacity 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44
Angle of Internal Friction, degrees
Equation B:
granular (e.g., sand) soil. be solved directly. However, soil
Qh = Ah (9c + q Nq) ≤ Qs reports often do not contain
The product “9c” from Equa- enough data to determine values
Where: tion B is the strength due to co-
Qh = Individual helix bearing for both c and Ø. In such cases,
hesion in fine grain soils, where Equation B must be simplified to
capacity 9 is the bearing capacity factor
Ah = projected helix area arrive at an answer.
for cohesive soils. The product
c = soil cohesion “qN q” from Equation B is the The design professional
q = effective overburden strength due to friction in granu- must decide which soil type (co-
pressure lar, cohesionless soils. The bear- hesive or cohesionless) is more
Nq= bearing capacity factor ing capacity factor for cohesion- likely to control ultimate capac-
(from the graph, next less soils (N q) may be deter- ity. Once this decision has been
page) mined from Figure 1. This factor made, the appropriate part of the
Qs= upper limit determined is dependent upon the angle of (9c + q Nq) term may be equated
by helix strength internal friction (Ø). The curve is to zero, which will allow solution
Projected helix area (Ah) is based on Meyerhoff bearing ca- of the equation. This approach
the area projected by the helix pacity factors for deep founda- generally provides conservative
on a flat plane perpendicular tions and has been empirically results. When the soil type or
to the axis of the shaft. modified to reflect the perfor- behavior expected cannot be de-
mance of foundation anchors. termined, calculate for both be-
Effective overburden pressure haviors and choose the smaller
Cohesive capacity.
(q) is determined by multiplying
and non-cohesive soils
a given soil’s effective unit weight Tension anchor capacities
Shear strength of soils is typi- (γ) times the vertical depth (d) of are calculated by using average
cally characterized by cohesion that soil as measured from the parameters for the soil above a
(c) and angle of internal friction surface to the helix. given helix. Compression ca-
“phi” (Ø), given in degrees. The
For multiple soil layers above pacities may be calculated simi-
designation given to soil that
a given helix, effective overbur- larly, however soil strength pa-
derives its shear strength from den pressure may be calculated rameters should be averaged for
cohesion is “cohesive” and indi- for each layer and then added the soil below a given helix.
cates a fine-grain (e.g., clay) soil. together (see Design example,
The designation given to soil that pages 8 and 9). We recommend the use of
derives its shear strength from field testing to verify the accuracy
friction is “non-cohesive” or “co- When c and Ø for a given soil of theoretically predicted founda-
hesionless” and indicates a are both known, (9c + q Nq) can tion anchor capacities.
4
INSTALLATION TORQUE VS. ANCHOR CAPACITY

Holding strength related the subject is in the paper 66 m-1), depending on soil
to installing torque “Uplift Capacity of Helical conditions and anchor design
Anchors in Soil” by R.M. Hoyt (principally the shaft size). For
The idea that the amount of and S.P. Clemence (Bulletin 2- Type SS foundation anchors, it
torsional force required to 9001). It gives the formula for typically ranges from 10 to 12
install a foundation anchor the torque/anchor capacity as: (33 to 39) with 10 (33) being
relates to the ultimate capacity the recommended default
of the foundation in tension or Qu = Kt x T value. For Type HS foundation
compression has long been where anchors, the recommended
promoted by the Chance Co. Qu = ultimate uplift capacity default value is 7 (23). The
Precise definition of the [lb. (kN)] same values of Kt are used for
relationship for all possible Kt = empirical torque both tension and compression
variables remains to be factor [ft.-1 (m-1)] loading. Torque monitoring
achieved. However, simple T= average installation tools are available from
empirical relationships have torque [ft.-lb. (kN-m)] Chance. Their use provides a
been used for a number of good method of production
years. The value of Kt may
control during installation.
range from 3 to 20 ft.-1 (10 to
Recommended reading on

Type SS HELICAL PIER® Foundation Systems Type SS HELICAL PIER® Foundation Systems
Installation Torque vs. N-Value Installation Torque vs. N-Value
in Sand in Clay

Figure 2 Figure 3
Installation Torque

Installation Torque

Increasing Depth

N-Value N-Value

Figures 2 and 3 show graphs depicting how installation torque varies with respect to SPT
results (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. The
envelope of curves depicts increasing torque for a given N value with increasing depth. Water
table position directly affects installation torque and ultimate capacity by causing a reduction in
the effective unit weight of the soil below the table. This in turn will cause a reduction in
installation torque and ultimate capacity.
For cohesive soil (Figure 3), a straight-line relationship is provided as soil strength or
cohesion is the only factor affecting installation torque and ultimate capacity.

5
 PRODUCT SPECIFICATION
HELICAL PIER® Foundation Systems Family
System SS5 Square Shaft *SS150 Square Shaft SS175 Square Shaft HS Pipe Shaft
Ratings Table 11⁄2" (3.8 cm) 11⁄2" (3.8 cm) 13⁄4" (4.4 cm) 31⁄2" (8.9 cm) OD
Column 1 Column 2 Column 3 Column 4

Minimum Ultimate Torque Row A 5,500 (7.5) 7,000 (9.5) 10,000 (13.5) 11,000 (15)
Capacity [ft.-lb. (kN-m)]

Ultimate Strength [kips (kN)] 70 (310) 70 (300) 100 (440) 100 (440)
for Axially Loaded Foundation Row B
Torque Limited 55 (240) 70 (300) 100 (440) 77 (340)
Working Capacity [kips (kN)]
with 2.0 Safety Factor Row C 35 (160) 35 (150) 50 (220) 50 (220)
Torque Limited 27.5 (120) 35 (150) 50 (220) 38.5 (170)
Ultimate Strength per Helix - Row D (2)40 (2)40 (2)50 (2)50
(180) (180) (220) (220)
Tension/Compression [kips (kN)]

Working Capacity per Helix -

(1)(2)20 (2)20 (2)25 (2)25
Tension/Compression [kips (kN)] Row E (90) (90) (110) (110)
with 2.0 Safety Factor

Bracket C150-0121

Min. Ultimate Strength [kips (kN)] Row F 40 (180) 40 (180) N/A N/A
Working Capacity [kips (kN)] Row G 20 (90) 20 (90) N/A N/A
with 2.0 Safety Factor

Typical Achievable Installed Row H 20 (90) 25 (110) N/A N/A

Capacity [kips (kN)](4)

Bracket C150-0298

Min. Ultimate Strength [kips (kN)] Row I 80 (360) 80 (360) N/A N/A
Working Capacity [kips (kN)] Row J 40 (180) 40 (180) N/A N/A
with 2.0 Safety Factor

Typical Achievable Installed Row K 20 (90) 25 (110) N/A N/A

Capacity [kips (kN)](4)

Bracket C150-0299

Min. Ultimate Strength [kips (kN)] Row L N/A N/A 80 (360) N/A
Working Capacity [kips (kN)] Row M N/A N/A 40 (180) N/A
with 2.0 Safety Factor

Typical Achievable Installed Row N N/A N/A 30 (130) N/A

Capacity [kips (kN)](4)

Bracket C150-0147

Min. Ultimate Strength [kips (kN)] Row O N/A N/A 80 (360) N/A(3)
Working Capacity [kips (kN)] Row P N/A N/A 40 (180) N/A(3)
with 2.0 Safety Factor
Typical Achievable Installed Row Q N/A N/A 40 (180) N/A(3)
Capacity [kips (kN)](4)
*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 the

foundation, bracket, anchor shaft, helix plate and bearing stratum, as well as the strength of the foundation-to-bracket connection and
the quality of anchor installation. This row of the table shows typical achievable capacities under normal condtions. Actual achievable
6 capacities could be higher or lower depending on the above factors.
PRODUCT SPECIFICATION (continued)
Lead and extension section lengths Minimum anchor type required
HELICAL PIER® Foundation one helix arranged in increas- based on mechanical ratings
Systems standard lead-section ing diameters from the founda- Design Minimum HELICAL PIER®
lengths are 5, 7, and 10 ft. (1.5, tion tip to the uppermost helix. Load, Foundation Systems
2 and 3 m). The standard The nominal spacing between kips Anchor
extension section lengths are helix plates is three times the (kN) Required
31⁄2, 5, 7, and 10 ft. (1, 1.5, 2 diameter of the next lower 0 to 25
and 3 m). These combinations helix. For example, a HELICAL (0 to 110) SS5
of leads and extensions pro-
PIER® Foundation Systems 25 to 35 SS150
vide for a variety of installed
anchor with an 8-, 10-, and 12- (110 to 150)
foundation anchor lengths.
inch (20, 25 and 30 cm) helix
Helix areas 35 to 50
combination has a 24-inch (61 (150 to 220)
SS175 or HS
Standard diameters for helices cm) space between the 8- and
Note: This chart uses a factor of
manufactured by the Chance 10-inch (20 and 25 cm) helix
safety vs. ultimate capacity = 2.
Company are: and a 30-inch (76 cm) space
6 in. = 26.7 sq. in. between the 10- and 12-inch shear strength factors, cohe-
(15 cm = 0.0172 m2) sion (c) and angle of internal
(25 and 30 cm) helix. Exten-
8 in. = 48.4 sq. in. sions with helix plates can be friction (Ø), and applying them
(20 cm = 0.0312 m2) as outlined in Theory of Foun-
added to the foundation if more
10 in. = 76.4 sq. in. dation Anchor Design. The
(25 cm = 0.0493 m2) bearing area is required. They
should be installed immediately anchor family specified is
12 in. = 111 sq. in. based on the rated load carry-
(30 cm = 0.0716 m2) after the lead section.
ing capacities for the specific
14 in. = 151 sq. in. Capacities listed in the foundation shaft size and
(35 cm = 0.0974 m2) Ratings Table on the page 6 installation torque required to
Helix configuration are mechanical ratings. One install the foundation. The shaft
must be aware that the actual sizes are 11⁄2- or 13⁄4-inch (3.8
Standard helices are 3⁄8 inch
installed load capacities are or 4.5 cm) square solid steel or
(0.95 cm) thick steel plates with dependent on actual soil 31⁄2-inch (8.9 cm) OD heavy-
outer diameters of 6, 8, 10, 12 conditions at each specific wall steel pipe.
and 14 inches (15, 20, 25, 30 project site. Therefore, the
and 35 cm). The lead section, Chance Company is avail-
design professional should use
or first section installed into the able to aid the design profes-
the bearing capacity method in
soil always contains helix sional in determining the best
designing anchor foundations.
plate(s). Extensions may be helix combination/foundation
The number of helices, their
anchor family for a given
plain or helixed. Multihelix size, and depth below grade is
application. Additional design
foundations have more than determined by obtaining soil
considerations are as follows:

Corrosion PIER® Foundation Systems

designs for screw anchor
Corrosion of foundation anchors. The Federal Highway
foundations. These rates of
anchors is a major consider- Administration (FHWA-SA-96-
corrosion assume a mildly
ation in permanent structures. 072) has established, from an
corrosive in-situ soil environ-
That is why foundation compo- extensive series of field tests ment having the electrochemi-
nents are hot-dip galvanized on metal pipes and sheet steel
cal property limits that are
per ASTM A153. The zinc buried by the National Bureau
listed in table below. The
coating will add between 5% of Standards, maximum corro-
design corrosion rates, per
and 20% to the life of HELICAL sion rates for steel buried in
FHWA-SA-96-072, are:
soils exhibiting the
Property Criteria Test Method electrochemical For Zinc
Resistivity >3000 ohm-cm AASHTO T-288-91 index properties 15 µm/year (first 2 years)
pH >4.5<9 AASHTO T-289-91 shown in the table:
4 µm/year (thereafter)
Chlorides <100 PPM AASHTO T-291-91 The corrosion For Carbon Steel
Sulfates <200 PPM AASHTO T-290-91 rates shown below 12 µm/year
Organic Content 1% max. AASHTO T-267-86 are suitable for For example, in a soil
7
 PRODUCT SPECIFICATION (continued)
environment that meets the corrosion potential different April 1957, by Melvin
electrochemical properties than that stated above, one Romanoff. The reader is
listed in the table, the allowable should consult a corrosion encouraged to obtain this
strength of the galvanized SS5 engineer. bulletin for more reference and
screw anchor foundation for a Chance Co. Bulletin 01- examples on corrosion, includ-
design life of 75 years, is 37 9204 contains extensive data ing methods of additional
kips (160 kN). For soils with taken from NBS circular 579, corrosion protection.

Slenderness ratio/buckling that foundations can be loaded Method, or by computer solu-

It is intuitively obvious that in compression up to their tion with a finite difference
HELICAL PIER® Foundation rated load capacities. method such as that used in
Systems anchors have slender As a practical guideline, the program LPILE (ENSOFT,
shafts. Very high slenderness when a specific soil’s Standard Austin, TX). Research by
ratios (Kl/r) can be expected Penetration Test blow count Chance Co. and others (†Hoyt,
depending on the length of the data per ASTM D-1586 is et al, 1995) has shown that
foundation. This condition greater than 4, buckling of the buckling is a practical concern
would be a concern if the foundation shaft has been only in the softest soils (very
foundation were a column in air found not to occur when loaded soft and soft clay, very loose
or water and subjected to a to the rated capacities. sands) and this is in agreement
compressive load. However, with past analyses and experi-
Buckling analysis for soils ence on other types of pile
the foundations are not sup- having lower blow counts can
ported by air or water, but by foundations. Refer to Chance
be done by hand calculations Co. Bulletin 01-9605 for more
soil. Therein lies the reason with the *Davisson (1963) details.

Design example 2. From Figure 1 at the end of

An existing two-story brick- Soil Properties (as determined Theory of Foundation
veneer residence has experi- from soil boring data): Anchor Design, choose the
enced settlement. The designer bearing capacity factor:
51⁄2 feet (1.7 m) of sandy clay fill Nq = 22 For phi (Ø) = 34°
has calculated a foundation overlying homogeneous sand
load of 1500 pounds per linear material having soil parameters • At this point, an iterative
foot (22 kN per meter). In of: process is required. Select a
addition, the designer has helix combination you
determined the best foundation phi (Ø) of sand = 34°
believe can develop the
anchor spacing is 6 feet (1.8 unit weight (γ) of sand = 120 required load.
m)on centers. Thus, the design lb./ft.3 (19 kN/m3); sandy clay =
load is 9 kips (40 kN) per 103 lb./ft.3 (16 kN/m3) • Trial 1: Select a single helix
anchor. Soil properties are foundation anchor [10" (25
Water table at 18 ft. depth. cm) diameter helix].
listed below. Determine the
number and size of helix(es) • Using the standard bearing Determine vertical depth to
required, their depth below equation: the helix. In this example, it is
grade, and the foundation Qh = Ah (9c + qNq)
desired to install the helix into
anchor family needed to carry For sand, the bearing equa- the homogeneous sand well
the design load of 9 kips (40 tion reduces to: below the fill material. Applica-
kN). Use a safety factor (SF) of Qh = Ah (qNq)

*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 Infrastructure

Improvement, San Diego.

8
PRODUCT SPECIFICATION (continued)
Design example (continued)
tion guideline No. 4 from page combination of foundation lead tion anchor family. From Row
10 requires at least 3 helix and extension sections can be D, Column 1 of the Table, an
diameters. Therefore, the helix used. Therefore, an 8"-10" (20 SS5 series 11⁄2" (3.8 cm)square
[10" (25 cm) dia.] should be at cm - 25 cm) two-helix founda- shaft foundation’s rated capac-
least 51⁄2 ft. + 30 inches = 8 ft. tion will be considered for this ity is 20 kips (90 kN), which
(1.7 m + 0.7 m = 2.4 m) example: The top helix [10" (25 exceeds the design load of 9
Calculate effective overburden cm) dia.] should be at least 8 ft. kips (40 kN).
pressure for the 10" (25 cm) (2.4 m) for reasons explained • Check helix ratings:
helix. previously. Remember the
helices are spaced 3 diameters Based on Row B, Column 1
q =γxd of the Ratings Table, the
apart. So 8" (20 cm) x 3 = 24
q10 = (0.103 x 5.5) + (0.120 x 2.5) = 0.867 ksf inches (0.6 m). Thus, the allowable capacity of an SS5
[q10 = (16 x 1.7) + (19 x 0.76) = 42 kN/m3]
vertical distance to each helix foundation anchor with a single
Determine the capacity of the is: helix is 20 kips (90 kN), which
helix using reduced bearing d10 = 5.5 ft. + (3x10) in. = 8 ft.
exceeds the design load.
equation. [d10 = 1.7 m + 3(25 cm) = 8 ft.]
Therefore, a single helix foun-
d8 = 8 ft. + (3x8) in. = 10 ft.
dation can be used.
Qh = Ah (qNq)
A10 = 76.4 in.2 (0.0493 m2 ) [d8 = 2.4 m + 3(20 cm) = 3 m] • Check the installation
(see “Helix areas” on page 7) Calculate effective overburden torque required to ensure
pressure for each helix. adequate capacity:
Q10 = (76.4/144) x 0.867 x 22 = 10.12 kips
q =Yxd Torque required = Required Load/Kt
Q10 = 0.0493 x 42 x 22 = 45 kN
q10 = (0.103 x 5.5) + (0.120 x 2.5) = 0.867 ksf 18,000 lb. = 1,800 ft.-lb.
Ultimate theoretical capacity = 10.12 kips (45kN) [q10 = (16 x 1.7) + (19 x 0.76) = 42 kNm2] 10
Another trial is required q8 = (0.103 x 5.5) + (0.120 x 4.5) = 1.107 ksf
80 kN = 2.4 kN-m
because the design load of 9 [q8 = (16 x 1.7) + (19 x 1.3) = 52 kNm2]
33 m-1
kips (40 kN) times a Safety Determine the capacity of each
Factor of 2 = 18 kips (80 kN) > helix using reduced bearing Based on Row A, Column 1
10.12 kips (45kN). equation and sum for resulting of the Ratings Table, the
ultimate theoretical anchor allowable torque capacity of
• Trial 2: Select a 12" (30 cm) an SS5 foundation anchor is
dia. foundation anchor capacity.
Qh = Ah (qNq)
5,500 ft.-lb. (7.5 kN-m), which
installed 2 feet (0.6 m) exceeds the 1,800 ft.-lb. (2.4
deeper into the sand. A8 = 48.4 in.2 (0.0312 m2)
Q10 = (76.4/144) x 0.867 x 22 = 10.12 kips
kN-m) required torque.
d12 = 8 ft. + 2 ft. = 10 ft.
[Q10 =0.0493 x 42 x 22 = 46 kN] Per application guideline No.
[d12 = 2.4 m + 0.6 m = 3.0 m] Q8 = (48.4/144) x 1.107 x 22 = 8.19 kips 9 from page 10, if a stronger or
q12 = (0.103 x 5.5) + (0.120 x 4.5) = 1.107 ksf [Q8 =0.0412 x 52 x 22 = 36 kN] denser stratum overlaid the
[q12 = (16 x 1.7) + (19 x 1.3) = 52 kN/m3] sand, it would be necessary to
Ultimate theoretical capacity = 18.31 kips
A12 = 111 in.2 (0.0716 m2) check the installing torque in
(82 kN)
Q12 = (111/144) x 1.107 x 22 = 18.77 kips this stratum to be sure the
[Q12 = 0.0716 x 52 x 22 = 82 kN] Thus, an 8"-10" (20 cm - 25 anchor could be installed.
cm)diameter foundation anchor
Ultimate theoretical capacity = 18.77 kips (82 kN)
with the top helix 8 ft. (2.4 m) For additional reference,
9 kips (40 kN) times SF of 2 = 18 kips (80 kN) below the surface will also Chance Co. Bulletin 31-8901
<18.77 kips (82 kN). work. The best choice will have contains example problems for
to be made based on total tension anchors in both cohe-
Thus, a 12" (30 cm) diam- sive and cohesionless soils.
eter foundation anchor with the installed cost.
helix 10 ft. (3 m) below the • Select the appropriate
surface will work. foundation anchor family:
• Feasibility check: Since this is an underpinning
Application guideline No. 8 retrofit job, check the Founda-
from page 10 recommends that tion System Ratings Table to
economic feasibility should be select the appropriate founda-
checked if more than one
9
 PRODUCT SPECIFICATION (continued)

Application guidelines 3. The uppermost helix should be diameter in making the spacing
1. Foundation anchors should be installed at least three diameters determination.
applied as deep foundations. The below the depth of seasonal variation The influence of the structure’s
vertical distance between the in soil properties. existing foundation on the foundation
uppermost helix and the soil surface 4. The uppermost helix should be anchor also should be considered.
should be no less than 5 feet (1.5 m) installed at least three helix diameters
or 5 times the helix diameter. 8. Check economic feasibility if more
into competent load-bearing soil. than one combination of foundation
2. Installation torque should be 5. For a given foundation length, it is lead and extension sections can be
averaged over the last three diam- better to use a few long extensions used.
eters of embedment of the largest than many shorter extensions. This
helix. This will provide an indication 9. If any stronger, denser, etc. stratum
results in fewer connections in the soil. overlies the bearing stratum, check
of the anchor’s capacity based on the
average soil properties throughout 6. Foundation anchors should be installation torque in the stratum to
the zone that will be stressed by the spaced laterally no closer than three ensure anchor can be installed to final
foundation. diameters on centers. A better spacing intended depth without torsional
is five diameters. Use the largest helix overstressing.
Specification References Part 3 — Execution 3.04 Field quality requirements:
Details on the Chance HELICAL 3.01 Manufacturer’s instruc- Site tests and inspections.
PIER® Foundation Systems are tions: Comply with technical data. 3.05 Protection: From damage
available upon request in the 3.02 Preparation: Spare nearby during construction.
three-part section Manu-Spec® structures; varying elevations. Business Practices
format (of the Spec-Data® pro- 3.03 Installation: Certified Before each job by contrac-
gram copyrighted by The Con- installer; power units; torque tors certified to install the Chance
struction Specifications Institute). recording; alignment; adapters; HELICAL PIER® Foundation Sys-
Filed under the identical 02150 down pressure; rate of rotation; tems, a quotation is prepared.
designation as Sweet’s, highlights obstructions; minimum depth, Customarily, the bid for work is
include: torque and cover, A/E approval, based on the amount to be billed
connect to structure. per foundation, access and final
Part 1 — General
1.01 Summary: New and reme- details required.
dial building foundation reinforce-
ment and stabilization, retaining
walls, tieback systems. Related
sections: Excavating to working Light Duty
level, load tests, cast-in-place
concrete reinforcement. Pricing. Bracket
1.02 References: Thirteen ASTM Primarily for correct-
and one SAE standards specifica- ing sagging, lesser
tions. loads; affordable “quick
1.03 Definition of system fix” outlasts the
1.04 System description porches, stairways,
1.05 Submittals: Conditions of decks and patios it
the Contract, Spec-Data®, shop repairs.
drawings, certified test reports
and installation instructions.
Closeouts: Warranty, project
records. SS5, SS150, SS175
1.06 Quality Assurance: Dealer
certification, preinstallation Underpinning Brackets
meetings. Applied in multiple locations along the
1.07 Warranty: Project, manufac- foundation to stabilize and correct problems
turer, period (term). caused by poor soil conditions.
Part 2 — Products For seismic uplift loads, the Uplift Restraint
2.01 Shoring and underpinning: Bracket may be added.
Chance Co.; proprietary system.
2.02 Product substitutions: None.
2.03 Manufactured compo- Heavy
nents: Screw anchor plate, shaft,
bolts, steel bracket.
Duty Bracket
2.04 Source quality: Tests, inspec- For such higher loads as commercial buildings and larger
tions, verification of performance. residences. Applied in multiples to stop settled areas,
resist new movement.
All components are hot-dip galvanized to increase product life in aggressive soils.
10
PRODUCT SPECIFICATION (continued)

At right,
installation
concept

Uplift
Restraint
Bracket
For seismic conditions and
to resist other upward New
forces. Shown as applied,
assembled to top of Stan- Construction
dard-Duty Bracket.
Bracket
Slab Bracket For support of new structures.
For stabilizing uneven or damaged Placed on foundation anchors in-
floors. Bolt adjusts through cap stalled between footing forms and
fitting on top of foundation so tied to reinforcing bars before
channel lifts floor. pouring concrete.

At left, screw anchors tieback

Wall Anchors retaining and foundation walls.
To restrain movement in
foundation walls.
Through a hole drilled in
wall, a rod threads into an
anchor plate installed into
the soil bank. A ribbed
retainer plate and a nut
secure the rod inside the
wall. Either of two meth-
ods may be used to stabi-
lize, or often to straighten,
DURA-GRIP™ Wall Repair System
failing walls.
cross plate anchors tieback
retaining and foundation walls.

11
 P
ower-installed screw
anchors have proven to
be a reliable and economical
advancement in foundation technology.
Chance HELICAL PIER® Foundation Sys-
tems anchors and related hardware are
available in a wide range of sizes to
meet many job applications. The Chance
Company also offers such unique prod-
uct resources as:
n Training and field supervision of
certified installers
n Geotechnical engineering guid-
ance for any job
n Computer-assisted design capa-
bility through interactive software
programs and a field manual
bringing design theory to practical
field application

DISCLAIMER: The material presented in this bulletin is derived from generally accepted engineering practices. Specific application and plans of
repair should be prepared by a local structural/geotechnical engineering firm familiar with conditions in that area. The possible effects of soil (such
as expansion, liquefaction and frost heave) are beyond the scope of this bulletin and should be evaluated by others. Chance Company assumes no
responsibility in the performance of anchors beyond that stated in our SCS policy sheet on terms and conditions of sale.
NOTE: Because Chance has a policy of continuous product improvement, it reserves the right to change design and specifications without notice.
®
®
A.B. Chance Company
210 North Allen Street
Centralia, MO 65240 USA

POWER SYSTEMS, INC.

Printed in USA
©2000 Hubbell, Inc.

Bulletin 01-9601
Revised 1/00
A&J 5M
12