Copyright 2011 – Magnum Piering, Inc. Reliability and Theory of Helical Pile Sizing Howard A....

Copyright 2011 – Magnum Piering, Inc.

Reliability and Theory of Helical Pile Sizing

Howard A. Perko, Ph.D., P.E.Magnum Geo-Solutions, LLC

Copyright 2011 – Magnum Piering, Inc., All Rights reserved. May be distributed

and printed by express written permission of the author only.

MAGNUM Informational Bulletin ISO-MHP3-11A


• Axial Capacity and Helix Spacing• Theoretical Bearing Capacity• Theoretical Pullout Capacity• Helix Sizing Charts• Reliability of Charts• Capacity to Torque Ratio Background• Variation in Capacity:Torque Ratio• Reliability of Torque Method• Combined Reliability• Contact Information

Outline


Axial Capacity

Limit States:

Cylindrical Shear& Individual Bearing

There are two ways to calculate the theoretical capacity of a helical pile. If the helices are far apart, they behave independently and one would use the individual bearing method. If the helices are close together, they would behave as a group and one would use the cylindrical shear method.


Helix Spacing

Most efficient spacingwhere:

Cylindrical Shear= Individual Bearing

Generally occurs at2 to 4 Helix

Diameters


Bearing Capacity

Traditional Soil Mechanics:

Individual Bearing

Q = R2(1.3cNc+Nq)

Cylindrical Shear

Q = 2RL(c+Ktan) +

R2(1.3cNc+ Nq)Limit State: Calculate both, and report whichever is least.

From Perko (2009) Helical Piles


Pullout Capacity

Pullout Capacity is calculated the same way as Bearling Capacity. However, generally in practice, Pullout Capacity = 82% Bearing Capacity

-40

-30

-20

-10

0

10

20

30

40

-4 -3 -2 -1 0 1 2 3 4

Displacement (in)

Tes

t L

oad

(ki

ps)

-180

-160

-140

-120

-100

-80

-60

-40

-20

0

20

40

60

80

100

120

140

160

180

-100 -50 0 50 100

Displacement (mm)

Tes

t L

oad

(kN

)

Compression TestPu=36 kips [160 kN]

Tesnion TestPu=30 kips [130 kN]



Magnum Helix Sizing Charts

From Magnum 2011 Product Catalog

Magnum Piering, Inc. sizing charts were created using individual bearing and cylindrical shear methods. Charts are provided for cohesive soils, non-cohesive soils, and weak bedrock. Based on the SPT Blow Count of the bearing stratum, they yield required ultimate capacity.

In helical piles, engineers typically use a factor of safety of 2.0.


Reliability of Sizing Charts


Test data indicates sizing charts are 90% reliable when used with a F.S. = 2.0.


Hoyt and Clemence (1989)

-91 Full-scale tension load tests

-Torque is shown to be statistically a strong verification of capacity

Capacity:Torque Background


Perko (2000)

-Torque is shown to be theoretically related to capacity

0.0

10.0

20.0

30.0

40.0

0 10 20 30 40

Predicted Capacity/Torque Ratio (1/ft)

Mea

sure

d C

apac

ity/

To

rqu

e R

atio

(1/

ft)

Rupiper and Edwards (1989): 14-indiameter single helix, 1.5-in square hubpier

Ghaly, Hanna, and Hanna (1991): 2-indiameter single helix, multi-pitchlaboratory models

Mitsch and Clemence (1985): 12-indiameter triple helix, square hub pier

Mitsch and Clemence (1985): 4-indiameter single and triple helix laboratorymodels

Hargrave and Thorsten (1992): 10-indiameter single helix, square hub pier

Chance (1994): 10-14 in diameter singleand multiple helix, square hub piers

Chance (1994): 12-14 in single andmultiple helices on 3.5 diameter roundhub

Atlas (1995): 8-in diameter single helix, 1-3/4-in square shaft hub pier

Atlas (1995): 12 and 14 in double helices,2-7/8 in diam round shaft pier

Magnum Piering (2001): 8" & 12" doublehelices cut-out for moment balancing, 3"diameter round-shaft pier

Capacity:Torque Background


Variation of Torque Correlation

Capacity to Torque Ratio Varies with Helical Pile Shaft Diameter.



Ultimate(tons)

Bare / Galv

Allowable(tons)

Bare / Galv

Ultimate(tons)

Allowable(tons)

MHL313 0.13 3.00 3.8 26 / 35 13 / 17 4,000 8.0 16 8 8,10,12,14 0.375 B (1) standard 6, 12, 24

MHL313R 0.13 3.00 3.8 26 / 35 13 / 17 6,000 8.0 24 12 8,10,12,14 0.375 B (1) standard 6, 12, 24

MHL325 0.25 3.00 7.3 60 / 68 30 / 34 8,700 8.0 35 17 8,10,12,14 0.375 B (1) standard 6, 12, 24

MHL325R 0.25 3.00 7.3 60 / 68 30 / 34 12,700 8.0 51 25 8,10,12,14 0.375 B (1) standard 6, 12, 24

MHL425 0.25 4.50 11.6 93 / 105 47 / 53 24,000 5.7 68 34 10,12,14,16 0.625 B (2) optional 7, 10, 24

MHL425R 0.25 4.50 11.6 93 / 105 47 / 53 28,000 5.7 80 40 10,12,14,16 0.625 B (2) optional 7, 10, 24

MHL431 0.31 4.50 14.3 118 / 131 59 / 65 29,000 5.7 83 41 10,12,14,16 0.625 B (2) optional 7, 10, 24

MHL431R 0.31 4.50 14.3 118 / 131 59 / 65 34,000 5.7 97 48 10,12,14,16 0.625 B (2) optional 7, 10, 24

MHL625 0.25 5.72 15.0 120 / 136 60 / 68 40,000 4.6 92 46 12,16,20,24 0.875 W or B (3) optional 6, 9, 18, 24

MHL625R 0.25 5.72 15.0 120 / 136 60 / 68 45,000 4.6 104 52 12,16,20,24 0.875 W or B (3) optional 6, 9, 18, 24





System Ratings & Capacity Specifications

HelixGauge

(in)

Bolted (B)or

Welded (W)Connection

Hot DipGalvanized

ASTMA153

StandardSection Lengths

(custom sizes available)

(ft)

Capacityto

TorqueRatio

(ft-1)

ShaftWall

Gauge(in)

ShaftO.D.(in)

Approx Weight

(plf)

End Bearing CapacityCapacity by Torque (Tension & Comp)

MaximumTorque(ft-lbs)

Helix Sizes(available in

standard & dual cutting edge)

(in)

Capacity-to-Torque RatiosMagnum’s Product Catalog Contains Suggested Capacity:Torque Ratios.

From Magnum 2011 Product Catalog


0

5

10

15

20

25

30

35

40

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4

Popu

lati

on

Measured Axial CapacityPredicted Axial Capacity

Tension

Compression

TensionMean= 1.06Std Dev= 0.46

CompressionMean= 1.14Std Dev= 0.54

Installation Torque Correlation243 Load Tests

Reliability of Torque Method


Test data indicates torque is 98% reliable when used with a F.S. = 2.0.


0.3% with FS=2.0

Statistically Independent

Variables

Combined Reliability


Using Sizng Charts in Combination with Torque Verification yields 99.7% Reliability.


Contact InformationMagnum Piering, Inc.

www.magnumpiering.com800-822-PIER (7437)

6082 Schumacher Park DriveWest Chester, OH 45069

Copyright 2011 – Magnum Piering, Inc. Reliability and Theory of Helical Pile Sizing Howard A....

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