Design of Helical Pier Foundations in Frozen Ground

7/30/2019 Design of Helical Pier Foundations in Frozen Ground

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1 INTRODUCTION

Helical piers create a lightweight foundation that can

be used either in compression or tension. Due to their

light weight, ease of installation and minimal grounddisturbance they are becoming very popular in remote

building sites such as arctic villages in Alaska (Figs 1,

2). Conditions in these places often include perma-nently frozen soils, which changes the dictating

design criteria from bearing capacity and consolida-

tion settlement to creep settlement. Current designmethods are not suitable for helical piers in frozen

ground, which may limit their use. Therefore, the

Alaska Science and Technology Foundation (ASTF)is funding research to develop guidelines for design

of helical piers in frozen ground. The purpose of this

paper is to show how the Finite Element Analysis(FEA) is used to create design curves and aid in

design of helical piers in frozen silt.

2 HELICAL PIERS

The helical piers typically consist of a central shaftmade from square or round sections that can be either

solid or hollow. To this shaft is connected between one

to four spiral plates (Fig. 3) that are designed to mobi-lize more soil resistance under normal loading condi-

tions than a conventional smooth pile.

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Design of helical pier foundations in frozen ground

H.K. Zubeck & H. LiuUniversity of Alaska Anchorage, Anchorage, Alaska, USA

ABSTRACT: Helical piers have been used as foundations for boardwalks, utilidors and fences in rural Alaska

in cold and warm permafrost. The piers are becoming more and more popular due to their light weight, ease of

transportation and installation, and resistance against frost jacking. However, engineers have been hesitant tospecify them as foundations for buildings due to the lack of design guidelines for frozen ground. To increase the

use of helical piers as a cost effective foundation alternative, Alaska Science and Technology Foundation funded

a study to create these guidelines. This paper presents the current experience with helical piers in Alaska and givesa design example using the developed finite element analysis and creep results.

Permafrost, Phillips, Springman & Arenson (eds) 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7

Figure 1. Helical pier stockpile in St. Michael, Alaska.

Figure 2. Insignificant ground disturbance.Figure 3. Helical pier lead sections (A.B. Chance Co.1996).


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3 APPLICATIONS

The helical piers are used for a variety of applications.In compression, they are used as building foundations

for new structures, underpins for existing structures,

and as support for boardwalks, utilidors, fence posts,light posts, etc. In tension, the helical piers, often called

helical anchors, are used as soil anchors for retaining

walls, and to resist wind loads for tall structures.In Figure 4, the utilidors in Selewik, Alaska, are

founded on helical piers, but the boardwalk is founded

by conventional methods. The helical piers are well-suited to resist the frost jacking that is a common

problem with pile foundations in frozen ground.

Figure 5 shows helical piers installed for a utilidor inSt. Michael, Alaska.

4 INSTALLATION

Helical piers are installed by applying a torque on the

pier shaft. The pier then augers itself into the ground.

In frozen ground, additional vertical pressure is oftenrequired. First pier installations into very cold (5C)

permafrost were conducted into 50 mm pilot holes to

ensure straight piers. Installers found, however, thatpiers can be installed correctly even into the cold per-

mafrost without pilot holes.

5 DESIGN METHODS

The current design method for determining the capac-ity of helical piers in thawed soils is based on bearing

capacity theory. According to Ladanyi & Johnston

(1974), it is not appropriate to analyze deep circularpile foundations in frozen soils on the basis of

Prandtl-type bearing capacity equations and a sepa-

rate settlement analysis using Boussinesqs stress-

distribution theory and compressibility of soil. Infrozen soil, the temperature and undrained creep

become predominant in the determination of allowablefoundation pressures. Therefore, Ladanyi & Johnson

(1974) developed an alternative method for predicting

the time and temperature dependent creep settlementand the bearing capacity of frozen soil under deep

circular loads. The asymptotic ultimate pullout resist-

ances calculated with the model seem to agree withfield results, but it is not clear if the model can be used

for long term creep under compressive loads. There-

fore, further analysis using FEA is warranted.

The purpose of the project is to develop a finite ele-ment model that will be used to create design curves

with different soil temperatures and soil properties.The analysis developed includes four models that are

Large Model, Small Model, Installation Failure

Model, and Creep Model (Liu et al. 1999a). The LargeModel analyzes the soil stresses and displacements

immediately after the pier is subjected to its design

load. These data are critical for the development ofmore detailed analysis using sub-modeling tech-

niques. The Small Model is a sub model of the Large

Model. It analyzes the stresses developed within the

spiral structure by using results from the Large Modelanalysis. The Installation Failure Model is a detailed

model of the spiral structure subjected to a torsional

load during installation. This model provides insightinto the failure mechanism of helical piers during

construction. The Creep Model analyses the long-

term displacement and soil stress in frozen ground.Figures 6 and 7 provide examples of the deforma-

tions and stresses in the pier itself and the surrounding

soil obtained from the Large Model. The finite ele-ment analysis shows that the traditional bearing capac-

ity equations are not adequate design tools for helicalpiers with multiple helixes (Liu et al. 1999a, 1999b,2000). The stress distribution in the soil does not

agree with the traditional method (bearing capacity

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Figure 5. Utilidor under construction in St. Michael,Alaska.

Figure 4. Typical uses for helical piers: boardwalks andutilidors.


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equation) that assumes the helixes are not connectedwith each other. Further, the traditional bearing capac-

ity equations do not consider creep in frozen soil. The

analysis and results given in the following sections arebased on the creep model that utilizes the Large

Model. The creep equation used by Morgenstern et al.

(1980) was adopted in the analysis (Equation 1):

(1)

where e strain rate; c reference strain rate;

n experimental creep parameter, se equivalent

stress and cuu temperature dependent total defor-mation modulus, corresponding to the reference strain.

A full-scale loading test was conducted in the U.S.Army Cold Regions Research and Engineering

Laboratory (CRREL) to calibrate the FEA. However,

the FEA models could not be calibrated using theCRREL test results due to uneven temperatures in the

test cell.

5.1 Development of design guidelines

The design guidelines are developed on the basis of the

Creep Model. For frozen ground, the dictating designcriterion is the settlement due to the creep of the ice in

the ground. The guidelines obtainable in the following

sections are results from the Large Model creep analy-

sis. Input parameters for the analysis include materialproperties for the pier and the soil. The soil parameters

include the strength parameters and creep parameters.

These are functions of soil type, water content, unfrozenwater content, and soil temperature. They vary greatly

depending from location and season, and therefore,

general guidelines cannot be provided for typical soiltypes, e.g. for sand, silt or clay. The following sections

describe an example of design of helical piers for a

silty soil at three different temperatures. To analyzeany other soil, the engineer needs to contact the authors

for FEA runs to produce the creep curves.

5.2 Materials and model dimensions

The soil properties for the silty soil are given in Table 1.

The pier has a pipe shaft with 90 mm outer diameterand 13mm thick wall, and a 203mm diameter, 13 mm

thick plate for spirals. For the large-scale model, the

spiral is modeled as a flat plate in order to limit thenumber of elements in the model. The element and

material properties for steel appear in Table 2. The soil

& & e ce

cu

ns

s u

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Figure 7. Vertical stress distribution in soil volume

x 3302mm, z 3556 mm.

Figure 6. Vertical displacement distribution in soil volume

x 3302mm, z 3556 mm.

Table 1. Soil properties for silty soil.

TemperatureUnit 1C 5C 10C

Unit weight kN/m3 19.07 19.07 19.07Cohesion kN/m2 2413 6206 9653

Friction angle 25 25 25Creep 2.04 2.04 2.04

parameter, nCreep h (h MPan)1 3.81E-7 5.49E-8 1.85E-8

parameter, BYoungs kPa 1800 7400 14400

modulusPoissons ratio 0.2 0.2 0.2

B &c

cui

n

1

s


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surrounding the helical piers is modeled as a cylindri-

cal volume. The dimensions used in the f inal analysis

models are given in Figures 8 and 9.

5.3 Analysis of the results

Creep settlement (displacement) deduced from the

FEA as a function of temperature is plotted in Figures1015. Negative values imply settlement rather than

heave. For the given soil, the creep displacementunder various design loads is less than 3mm at tem-peratures from 1 to 10C, which is less than a

typical allowable settlement.

5.4 Design example

In order to design helical piers in the given frozen silt,

the following information is needed: design tempera-ture and allowable displacement after 25 years or less.

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Table 2. Pier properties and FEA parameters.

E r

Material MPa n kN/m3 Element NDF

Steel shaft 200000 0.3 77.0 Beam 6

Steel plate 200000 0.3 77.0 Shell 6

NDF Nodal Degrees of Freedom.

3556

16511651

1270

2286

203

Soil Volume

Figure 8. Model dimensions for a pier with one helix (mm).

4166

610

1270

2286

16511651

203

Soil Volume

Figure 9. Model dimensions for a pier with two heli-

xes (mm).

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.00 10 20 30

Time (years)

Displacement(mm) 28 kN

56 kN

83 kN

111 kN

Figure 10. Creep displacement for a pier with one helix ata design temperature of1C for various axial loads.

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.00 10 20 30

Time (years)

Displa

cement(mm)

28 kN

56 kN

83 kN

111 kN

-0.5

-0.4

-0.3

-0.2

-0.1

0.00 10 20 30

Time (years)

Displace

ment(mm)

28 kN

56 kN

83 kN

111 kN

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.00 10 20 30

Time (years)

Displacement(

mm)

28 kN

56 kN83 kN

111 kN

Figure 13. Creep displacement for a pier with two helixes

at a design temperature of 1C for various axial loads.

Figure 11. Creep displacement for a pier with one helix ata design temperature of -5C for various axial loads.

Figure 12. Creep displacement for a pier with one helix ata design temperature of 10C for various axial loads.


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When this information is obtained, the following stepswill be performed:

1. Select the proper chart matching the designtemperature.

2. Select design life and maximum allowable

displacement.3. Select a design load that yields a predicted dis-

placement that is smaller than the allowable

displacement.

The following example illustrates the design method. A

single helical pier foundation should be designed for a100m long wall with a design load of 20kN/m. A pier

with an allowable capacity of 111 kN should be used.

The design temperature is 1C and the allowable dis-placement after 20 years is a) 25 mm, b) 1.5mm (the

small displacement is selected for the illustration).

5.4.1 Solution: (a)Select the chart matching -1C (Figures 10, 13). Note

that all loads yield smaller displacement than the

allowable value of 25 mm. Then, for the most eco-nomical design, select a design load of 111 kN per

pier. The required spacing between the piers becomes

111kN/(20 kN/m) 5.55 m. If the structural consid-erations allow this, the total number of the piers is

100m/(5.55 m/pier) 1 pier 19 piers.

5.4.2 Solution: (b)Select the chart matching 1C for a single pier

(Figure 10). If the piers were loaded to their capacity

of 111 kN, the allowable displacement of 1.5 mm

would be exceeded in 11 years. Consequently the

piers need to be loaded by a smaller load. After20 years a load of 83 kN per pier would yield 1.3 mm

displacement and this is smaller than the allowablemaximum displacement and is therefore acceptable.

To obtain a load of 83 kN per pier, the spacing between

the piers becomes 83 kN/(20 kN/m) 4.15m and thetotal number of the piers is 100m/(4.15 m/ pier)

1 pier 25 piers. Again, check if the 4.15 m spacing

meets the structural requirements. See Figure 16 foran illustration of this design example.

Check if piers with a double helix would yield a

more economical design. Select the chart matching

1C for a double pier (Fig. 13). Again, all loads

yield smaller displacement than the allowable value of

25 mm. Then, for the most economical design, select

a design load of 111 kN per pier. The required spacingbetween the piers becomes 111 kN/(20 kN/m)

5.55 m. If the structural considerations allow this, the

total number of the piers is 100 m/(5.55 m/pier)

1 pier 19 piers. Check if 19 double helix piers are

less expensive than 25 single helix piers. See Figure 17

for an illustration of this design example.

5.5 Conclusions and recommendations for design

Even if specific design guidelines for frozen groundhave not been in existence before, the piers are used

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-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.00 10 20 30

Time (years)

Displacement(mm)

28 kN

56 kN

83 kN

111 kN

-0.4

-0.3

-0.2

-0.1

0.00 10 20 30

Time (years)

Displacement(mm)

28 kN

56 kN

83 kN

111 kN

Figure 14. Creep displacement for a pier with two helixesat a design temperature of5C for various axial loads.

Figure 15. Creep displacement for a pier with two helixes

at a design temperature of10C for various axial loads.

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.00 10 20 30

Time (years)

Di

splacement(mm) 28 kN

56 kN

83 kN

111 kN

Figure 16. Design example for a pier with one helixat 1C.

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.00 10 20 30

Time (years)

Displacement(mm)

28 kN

56 kN

83 kN

111 kN

Figure 17. Design example for pier with two helixes

at 1C.


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successfully as a foundation for boardwalks, utilidorsand other structures. However, engineers have been

hesitating to specify helical piers as building founda-

tions, because of the lack of design guidelines. Thedesign method suggested here is very simple and

helpful in designing piers for the given frozen soil.

The design curves do not apply to any other soil,and even if the given creep displacements are insig-

nificant, other soil may produce larger settlements. To

properly design any foundations in frozen ground, theengineer still needs the creep parameters for the foun-

dation soil. These parameters are not readily available

for the engineering community. Therefore, actual soiltesting needs to be conducted for various soils to cre-

ate a library or database of soils encountered in cold

regions. These tests would include strength parame-ters in thawed conditions, and creep and strength

parameters at several frozen temperatures under vari-ous loads. Currently, to analyze any other soil, theengineer needs to contact the authors for FEA runs to

produce the creep curves.

REFERENCES

A.B. Chance Co. 1996. Helical Pier Foundation SystemTechnical Manual, Bulletin 0196.

Ladanyi, B. & Johnson, G.H. 1974. Behavior of CircularFootings and Plate Anchors Embedded in Permafrost.

Canadian Geotechnical Journal11: 531553.Liu, H., Zubeck, H., Schubert, D., Baginski, S. & Hsieh, Y.

1999a. Behavior of Helical Piers in Frozen Ground.NAFEMS World Conference 99. Proc. April 1999,

Rhode Island, USA: 1271

1280. East Kilbride, UK:The International Association for the Engineering

Analysis Community.

Liu, H., Zubeck, H. & Baginsky, S. 1999b. Evaluation ofHelical Piers in Frozen Ground. Proc. Tenth Inter-

national Conference on Cold Regions Engineering,Lincoln, NH, 16-19 August 1999: 232242. Reston,

VA, USA: American Society of Civil Engineers.Liu, H., Schubert, D., Zubeck, H., & Baginski, S. 2000.

Application and Analysis of Helical Piers in FrozenGround. The 5th Annual Alaska Water and Wastewater

Management Association International Research andDevelopment Conference on Rural Sanitation, Proc.

April 25

26, 2000, Fairbanks, Alaska: 139

151.Anchorage: AWWWMA.

Morgenstern, N., Roggensack, W. & Weaver, J. 1980. The

Behavior of Friction Piles in Ice and Ice-Rich Soils.Canadian Geotechnical Journal17: 405415.

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Design of Helical Pier Foundations in Frozen Ground

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