A New Experimental Procedure to Investigate the Torque Correlation Factor of Helical Anchors

8/9/2019 A New Experimental Procedure to Investigate the Torque Correlation Factor of Helical Anchors

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A New Experimental Procedure to

Investigate the Torque Correlation

Factor of Helical Anchors

João Manoel Sampaio Mathias dos Santos Filho,

Thaise da Silva Oliveira Morais,

Cristina de Hollanda Cavalcanti Tsuha* Department of Geotechnical Engineering, University of São Paulo at São Carlos,

Av. Trabalhador Sãocarlense, 400, São Carlos, Brazil

*Corresponding author

e-mail : [email protected]

ABSTRACTThe uplift capacity of helical anchors is correlated to the torque recorded during anchorinstallation. The torque measurement is a typical practice used to control the anchor capacity.

This procedure is based on the empirical torque factor K T , which relates the uplift capacity tothe torque required to install helical anchors. During the anchor installation, the torqueregistered is the sum of the torque resisted at the helices surface and surrounding soil, and atthe shaft surface. Similarly, the pullout capacity of multi-helix anchors, with widely spacedhelices, is composed of two parts, helix bearing capacities and shaft resistance. However,although the torque factor has been investigated several authors, there are no experimental

field studies that examine the individual fractions of this ratio between resistance forces

mobilized during anchor installation and loading. The present research was carried out toobtain the fractions contained in the torque factor of a multi-helix anchor. Field tests were performed on instrumented anchors installed in a residual soil site. The portions of installationtorque were registered separately, as the fractions of pullout capacity. The results show that thelead section diameter, the load distribution along the helical anchor, and the number of helicesinfluence the torque factor K T . However, more tests are necessary to confirm the presentedresults.

KEYWORDS: helical anchors, full-scale field testing, load-transfer mechanism,installation torque, uplift capacity

INTRODUCTIONHelical anchors have been widely used to resist tensile loads in supporting structures such

a guyed towers, transmission towers, buried pipelines, retaining wall systems, etc. The use of

helical anchors as tower foundation has being increased significantly in Brazil during the last five

years. These anchors are made out of helical steel plates welded to a steel shaft at a given spacing.

The components of a helical anchor are the lead and extensions sections. The lead section is the

first section to be installed into the ground, and contains the helical plates. The extensions are used

to insert the lead section into the soil at the desired depth.

The helical anchor is installed by applying a torque to the upper end of the shaft using

hydraulic motors. During the installation, the torsional resistance to the anchor penetration is

recorded because the final torque needed to install the anchor is empirically correlated to its uplift

capacity. Zhang (1999) cited that this correlation is simple to use and provides a process to

evaluate if the predicted loads have been reached at the site location.

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This on-site monitoring procedure to control helical anchor capacity assumes that the effort

necessary to install the anchor into the ground is proportional to its capacity. Perko (2009)

commented that the common sense states that the torque necessary to advance a helical plate

would be indicative of soil consistency and strength. Therefore, it is coherent that the installation

torque should offer an indication of bearing and pullout pressure.

A number of theoretical correlations between installation torque and uplift capacity of helical

anchors are reported in the literature (Narasimha Rao et al., 1989; Ghaly et al., 1991; Ghaly &

Hanna, 1991; Perko, 2000; and Tsuha & Aoki 2010). However, the empirical correlations, based

on the experience and/or field-testing in different locations and soil types, supported by statistical

analysis, are commonly used in the industry to predict the capacity of helical anchors. In this case,

the relationship between anchor pullout capacity and the final installation torque is represented by

the empirical torque factor KT.

Hoyt and Clemence (1989) expressed the pullout capacity of helical anchors, Qu, calculated

from installation torque as:

= .

(1)

where, T is the average installation torque (averaged for the final penetration equivalent to three

times the diameter of the largest helix). These authors assumes that the KT factor depends

primarily on shaft diameter, and suggested values of KT equal to 33 m–1 for all square-shaft

anchors and round-shaft anchors less than 89 mm in diameter, 23 m–1 for 89 mm diameter round-

shaft anchors, and 9.8 m–1 for anchors with 219 mm diameter extension shafts.

Perko (2009) presented an empirical expression that relates the KT factor to the effective shaft

diameter. This expression was obtained from several load tests on helical anchors, and for the case

of anchors in tension, the coefficient of determination (R-squared value) for the best-fit

relationship is around 0.7. This suggests that there are probably other factors besides the shaft

diameter that influence the KT factor. For this reason, the presented research is focused on verify

an experimental procedure to investigate the parameters that might affect this correlation factor.

The aim of this experimental work is to obtain data of the fractions (related to the shaft and to

the lead section with helical plates) of torque and pullout capacity of a multi-helix anchor, to

evaluate how these parts of the helical anchors influence the KT factor. To achieve these

objectives, pullout load tests were performed on instrumented and non-instrumented full-scale

anchors. During the installation of the instrumented anchor, the fractions of torque resisted by the

shaft and by the lead section with helices were registered separately, as occurred in the load tests.

Details of the testing program are described in the next section.

The individual torque measurements of this investigation, to verify the torque resisted along

the anchor length during installation, is an innovative technique to assist the current understandingof the relationship between pullout capacity and installation torque of helical anchors.

TESTING PROGRAMME

Three helical anchors were tested for this study. Two tests were carried out on instrumented

anchors (A1 and A2), and one test on a non-instrumented anchor (B1). This instrument-

ation was designed for measuring the torque distribution along the anchor length during

installation and loading (pullout tests).

Anchor configurations

Two different anchor configurations were tested in this study (Figure 1). The lead section ofthe instrumented anchors A1 and A2 were fabricated with a reduced diameter in relation to the

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extension sections diameter. The aim of these anchor configurations was to compare the KT

factor of multi-helix anchors with reduced lead section and with lead and extensions sections of

same diameter (anchor B1).

Figure 1: Configurations of the tested anchors.

The tests on anchors with the lead section of reduced diameters were performed also to verify

the possible reduction of the installation torque due to this configuration, compared to the typical

configuration of helical anchors (lead and extensions sections of same diameter), as the contactarea between the soil penetrated and the anchor surface during installation is reduced.

The lead sections of the instrumented anchors A1 and A2 were composed of a cylindrical

shaft with a diameter of 73 mm, and four welded helices (thickness of 12.5 mm, and pitch of 75

mm) of diameters of 254 mm, 305 mm, 356 mm, and 356 mm (Figure 2). The extension sections

with a diameter of 101.6 mm were connected to these anchors to penetrate the lead section at the

desired depth. Differently, the anchor B1 (non-instrumented) was constructed with the lead

section with diameter of 101.6 mm (same diameter of the extensions).

Figure 2: Instrumented lead section of the anchors A1 and A2.

The test anchors were manufactured by Vercon Industrial (a Brazilian company). The four-

helix anchors were fabricated with the inter-helix spacing of thee helix diameter to avoid the

interaction of the bearing resistances between helices. In this investigation, is assumed that during

the anchor loading the individual helices act independently of each other.



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Two sections of the anchor lead section, S1 and S2, were instrumented as showed in Figure 2.

Strain gauges were fixed to the outside of the anchor shaft (Figure 3a) to determine the

distribution of torque and load within the anchor during installation and load testing. The cables

containing the wires for the strain gauges run up the inside of the anchor shaft. Steel threaded

sleeves were installed to the outside of the shaft to protect the gauges from damage during

transportation and installation. The instrumented lead section of the anchors A1 and A2 are

showed in Figure 3b.

Figure 3: (a) Strain gauges for axial force and torque measurements; (b) Covered

instrumented sections S1 and S2.

Site investigation

The test site is located in Betim, Minas Gerais State, Brazil. The residual soil of this region is

predominantly comprised of acid rock such as granites, gneisses, and migmatites.

Standard penetration tests (SPT) were performed in two boreholes (SP-01 and SP-02). This

site investigation indicated that the site at the location of the test anchors consists of a clayey

sandy silt crust of around 5.0 m in thickness. The crust is underlain by around 20 m thick layer ofsandy silt residual soil with N60-indices (of standard penetration test) ranging from 5 to 37 blows

per 300 mm of penetration.

The plan view of the site with the exact location of the tests performed and of the borehole

location (SPT tests) is showed in Figure 4. This figure shows that the anchors A1 and B1 were

installed close to the borehole SP-01, and the anchor A2 close to the borehole SP-02. Figure 5

shows the details of the soil profile, SPT tests, and the anchors tested in this study.

Anchor installation

Helical anchors are installed through the application of mechanical torque at the anchor head.

The torque applied, and the portions of torque resisted at the instrumented sections (anchors A1,

and A2) during installation were continuously recorded and the penetration depth was measured.

During the installation of the non-instrumented anchor B1, only the torque applied at the anchor

head was registered. The anchors A1 and B1 were installed with the anchor tip at a depth of 15

meters, and the anchor A2 at 12.5 meters, as illustrated in Figure 5.



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Figure 4: Plan view of the tested anchors and boreholes (out-of-scale).

Figure 5: Distribution of SPT N-values at boreholes SP-01 and SP-02.

The installation of the instrumented section used for the anchors A1 and A2 is illustrated in

Figure 6a. After the installation of the instrumented section, the extensions were connected to

them by a sleeve (Figure 6b) to provide the transition of the lead section of 73 mm diameter with

the extension section with a diameter of 101.6 mm. The measurements of the torque resisted

during installation at the instrumented sections were registered by using the Vishay Micro-

Measurements Model P3 (Figure 6c).



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Figure 6: (a) Installation of the instrumented section of Anchor A1; (b) Sleeved

connection of the instrumented lead with the extension section; (c) Wires of the

instrumented section connected to the data acquisition systems.

AXIAL LOAD TESTS

The axial tension load tests were carried out on the three helical anchors of this investigation.

The load test setup is presented in Figure 7. A hydraulic jack with 450 kN capacity, a load cell of

500 kN capacity, a reaction beam of five meters, and wood cribbing for reaction were used for thetests performed on the instrumented and non instrumented anchors.

Figure 7: Axial tension load test setup.



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The loads were applied in increments of 10% of the estimated anchor uplift capacity in 5 min

time intervals in conformity with the Brazilian standard ABNT-NBR 12131 (Associação

Brasileira de Normas Técnicas 2006). The anchor head displacements were monitored at four

points during the test using independently supported dial gauges (0.01 mm accuracy, 50 mm

travel).

During the load tests on the instrumented anchors, the applied axial load along the length of

the anchor was monitored by the same data acquisition systems used to register the individual

portions of installation torque resisted during the anchor installation into the ground.

RESULTS AND DISCUSSION

Torque measurements of anchors with the lead section of reduced diameter (A1, A2)

The results of the measured torque at the anchors head (total torque) and along the length are

shown in Figure 8. In this figure, from the results obtained in the instrumented section, the

portions of installation torque of each anchor are presented as: (1) total torque measured at thehead of the anchor T total; (2) torque resisted by the shaft, T shaft ; (3) torque resisted by the top helix

H4, T H4; and (4) torque resisted by the three bottom helices H1+H2+H3, T H1+H2+H3.

Figure 8: Torque measurement versus tip penetration depth: (a) anchor A1, and (b)

anchor A2.

The torques results T H4 and T H1+H2+H3 are related to the resistance of the helices penetration

and also of interface shear resistance along inter-helix shaft. However, as the shaft diameter of the

lead section is reduced, the authors suppose that the portion resisted by the helices is more

significant in this case.



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Table 1 and Figure 8 show the individual results of installation torque resisted by the shaft and

by the lead section with the helices. This table presents the results of torque for each meter

penetrated by the anchor tip, after the total penetration of the lead section.

Table 1: Measurements of installation torque of anchors A1 and A2.

Tested

anchor

Tip depth

(m)

Ttotal Tshaft Tlead section

TH4 TH1+H2+H3

kN.m kN.m Tshaft/Ttotal kN.m TH4/Ttotal kN.m TH1+H2+H3/Ttotal

A1

4.0 3.0 0.0 0.00 1.0 0.33 2.0 0.67

5.0 3.3 0.3 0.09 1.4 0.42 1.6 0.48

6.0 4.5 1.0 0.22 1.3 0.29 2.1 0.47

7.0 8.0 3.1 0.39 1.0 0.13 3.9 0.49

8.0 8.1 2.4 0.30 0.5 0.06 5.1 0.63

9.0 8.9 1.4 0.16 3.2 0.36 4.2 0.47

10.0 6.8 2.4 0.35 1.7 0.25 2.8 0.41

11.0 6.5 2.5 0.38 1.6 0.25 2.4 0.37

12.0 6.6 2.7 0.41 0.9 0.14 3.0 0.45

13.0 7.7 4.5 0.58 0.9 0.12 2.3 0.30

14.0 7.1 3.7 0.52 1.0 0.14 2.4 0.34

15.0 (final) 7.9 3.9 0.49 1.2 0.15 2.8 0.35

A2

- - - Tlead (kN.m) T leadsection /Ttotal

3.5 4.2 0.6 0.14 3.6 0.86

4.5 5.0 1.0 0.20 4.0 0.80

5.5 6.5 1.7 0.26 4.8 0.74

6.5 6.0 1.0 0.17 5.0 0.83

7.5 9.1 3.2 0.35 5.9 0.65

8.5 9.2 2.1 0.23 7.1 0.77

9.5 11.0 3.9 0.35 7.1 0.65

10.5 12.7 4.9 0.39 1.4 0.11 6.4 0.50

11.5 12.4 4.1 0.33 2.9 0.23 5.4 0.44

12.5 (final) 13.6 5.7 0.42 3.1 0.23 4.8 0.35

The results of Table 1 indicate that as the helical anchor penetrates into the ground, the

percentage of the total torque resisted by the extension section (shaft above the helices) increases,

and the percentage of total torque resisted by the lead section (with helices) decreases. Figure 9

illustrates this tendency observed. This observation indicates the influence of the shaft length on

the torque measured at the anchor head.



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Figure 9: Fraction of torque resisted by the shaft during the installation of anchors A1

and A2.

Axial tensile load test results of the instrumented anchors A1

and A2

The load-displacement responses of the anchors A1 and A2 obtained from the axial tension

load tests are shown in Figure 10. The pullout capacity, Qu , of these tests was taken as the load

producing a relative displacement of 10% of the helix average diameter. Table 2 shows the results

of pullout capacity of the tested anchors (0.1Dhelix), and also the fractions of capacity related to the

shaft resistance and to the helical plates.

Table 2: Fractions of the total uplift capacity of the instrumented anchors A1 and A2.

Anchor

Qu-

total

(kN)

Qshaft

(kN)

Qhelix (KN)QH4/Qhelix

(%)

QH1+H2+H3/Qhelix(%)

Qshaft / Qu-

total

(%)

Qhelix/ Qu-

total

(%)QH4 QH1+H2+H3

A1 116 57 10 49 17 83 49 51

A2 142 14 25 103 20 80 10 90



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Figure 10: Applied load at anchor head versus displacement during the axial tension load

test performed on the anchors A1 and A2.

Table 2 illustrates that the portion of shaft resistance above the helices is equivalent to 50% of

the total uplift capacity of the anchors A1, and, in the case of anchor A2 this fraction is only 10%.

This difference is due to the fact that the shaft of the anchor A1 is 2.5 meters deeper than the shaft

of A2, and also because the helices of the anchor A2 are installed in a soil layer of greater SPT N-values compared to anchor A1, as shown in Figure 5. Also, the initial parts of the curves presented

in Figure 10 shows the superiority of the shaft resistance of the anchor A1 compared to A2. This

fact is also observed by the load distribution along the anchors illustrated in Figure 11.

Figure 11: Load distribution for each load applied to the anchor head: (a) A1; (b) A2.

0

10

20

30

40

50

60

70

80

90

0 50 100 150 200 250 300

D i s p l a c e m e n t ( m m )

Tension load (kN)

A1

A2

failure = 10%Dhelix



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The load distribution along the two tested anchors is considerably different, however, for both

anchors the helix bearing capacity of the top helix Q H4 is equivalent to around 20% of the total

uplift capacity of related to the helices Qhelix. This observation indicates that the contribution of the

top helix to the total capacity of the lead section is not affected by the soil layer of the helices

installation.

In addition, the measured ultimate pressure on the top helix (H4) is around a half of the value

found for the bottom helices (H1, H2, and H3). Although the top helix is installed in a position

less deep than the lower helices, this difference on the helix efficiency is due to the disturbance

caused by the anchor installation on the soil penetrated by the helices. This effect is more

significant above the upper helices, because in this case the soil above them is penetrated and

disturbed more times.

Figure 12: Installation torque measured at the anchor head versus tip depth of A1 and B1.

Torque factor KT

Table 3 shows the results of torque factor K T of the anchors A1 and A2. The final installation

torque used to calculate this factor is the average torque equivalent to the final penetration of three

times the diameter of the largest helix. This table also presents the results of torque factor related

to the shaft above the helices (K T shaft = Qshaft / T shaft ) and related to the anchor section with helices

(K T helix = Qhelix / T lead section). Also, the torque factor of the top helix (K T H4 = Q H4 / T H4) , and of the

bottom helices (K T H1+H2+H3 = Q H1+H2+H3 / T H1+H2+H3) are shown in Table 3.



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Table 3: Fractions of torque factor K T of the instrumented anchors A1 and A2.

Anchor K T-total (m-1) K T shaft (m

-1) K T helix section (m-1)

K T helix section

K T H4 (m-1

) K T H1+H2+H3 (m-1

)

A1 15.9 16.2 15.6 8.6 18.8

A2 10.7 2.7 15.5 8.2 19.7

The results of Table 3 show that these two multi-helix anchors (A1 and A2) although have

identical lead section and shaft diameter, the results K T-total are significantly different. 1. This

result does not agree with the suggestions of empirical K T factors found in the literature (only

dependent of the shaft diameter).

The shaft contribution of the anchor A2 to the total anchor pullout capacity (10%) is less

important compared to the case of the anchor A1 (shaft resistance is 50% of total anchor pullout

capacity). Also, the K T value of the anchor A2, is considerably inferior to the one measured for

the anchor A1. This observation indicates that the K T value, for this case studied, is also dependent

on the load distribution along the anchor length (for multi-helix anchors with the shaft resistance

appreciably low, the K T value tends to decrease).

In the case of the anchor A1, in which the shaft resistance is equal to 50% of the total anchor

capacity, the K T-total value is similar to the K T helix section and to the K T shaft (Table 3).

In addition, Table 3 illustrates that the K T related to the helices (K T helix section) is similar for the

two tested anchors of identical lead sections that provided different results of helices pullout

capacities (59 and 128 kN). It shows that the K T helix section is more dependent on the geometry of the

lead section than on the final installation soil of the helices.

Table 3 also shows that the K T value of the top helix (K T H4) is less than a half of the K T value

of the bottom helices (K T H1+H2+H3). This fact demonstrates that the ratio between the top helixuplift capacity, and the torque resisted by this helix during installation is inferior compared to the

case of the bottom helices. Therefore, the addition of one more top helix in this case has caused an

increase in the installation torque more important than in the pullout capacity. This remark

indicates that the addition of new helices to the anchor shaft apparently reduces the K T value of the

helical anchor.

Comparison between anchor A1 and B1

One object of this investigation was to evaluate the efficiency of a helical anchor

configuration with the lead section of reduced diameter inrelation to the shaft part above the

helices. The anchors A1 and B1 (Figures 1) were installed at the same final depth of 15 meters as

shown in Figure 5. Figure 12 presents the results of the installation torque recorded at the anchorhead of the anchors A1 e B1. This figure shows that the final torque to install the anchor B1 is

33% larger than the necessary to install the anchor A1. Therefore, the shaft diameter of the inter-

helix space of the lead section influences the total torque.

This torque reduction is an advantage of the A1 configuration, because, if necessary, in this

case the anchor could penetrate deeper into the soil compared to the anchor of lead section with

larger diameter (anchor B1). Also, it could reduce the costs of the foundation.

The load x displacement curves of the tension load tests conducted on the anchors A1 and B1

are shown in Figure 13. The uplift capacity of the both anchors is similar. However, the first part

of the curves shows a superior shaft resistance of the anchor A1 (of lead section with reduced

diameter). Also, after the unloading procedure of the load tests, the permanent displacement of the

anchor A1 is around 10 mm inferior to the case of anchor B1. Probably, the gain in the shaft



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resistance of the anchor A1 is due to the reduction of the effect of the lead section installation on

the shaft capacity of the extension section (above the helices).

Figure 13: Load at anchor head versus displacement during the tension load test on A1

and B1.

Also, as the uplift capacity is the same for both anchors, and the final torque is 33% greater

for the anchor B1, the K T value of the anchor B1 (12.0 m-1) is smaller than the one of the anchorA1 (15.9 m

-1). This fact indicates that the shaft diameter of the lead section also influences the K T

value.

The anchor A1 provides similar capacity to the anchor B1, however the torque necessary to

install this anchor is inferior compared to anchor B1. These results show the better performance of

the configuration used to the anchor A1 compared to the anchor B1. However, more investigations

are needed to confirm this observation.

CONCLUSIONS

An innovative procedure to measure the installation torque resisted by the shaft and by the

lead section with helices was used for this investigation. This procedure allowed us to examine thevariables that affect the torque correlation factor of helical anchors, but future studies are needed

to validate these first findings presented in this paper.The following are the major conclusions of

this study.

1.

During the helical anchor installation, the torque resisted by the shaft part above the

helices increases as the anchor advances into the soil.

2. The ultimate pressure related to the top helix is a half of the results found for the bottom

helices. This occurs due to the more significant installation effect of the anchor on the soil

above the upper helices.

3.

The two instrumented multi-helix anchors, with identical lead section and shaft diameter,

but different embedment depth and surrounding soil, provide results of KT factorconsiderably different.



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4. The K T factor is much greater in the case in which the anchor shaft resistance is 50% of

the total anchor pullout capacity compared to the anchor case of very low shaft resistance.

5. This investigation showed that for this case of multi-helix anchor, the KT factor varies

with: (i) the load distribution along the anchor length (percentage of shaft resistance); (ii)the inter-helix shaft diameter; and (iii) the number of helices.

6. The anchor configuration with the lead section of reduced diameter is advantageous

compared to the case of the lead section with the same diameter of the extension shaft.

ACKNOWLEDGEMENTS

This research was supported by the Vercon Industrial, Brazil, and the Brazilian Agency

CAPES (Ministry of Education).

REFERENCES

1.

Associação Brasileira de Normas Técnicas (2006) “ Piles - Static load test - Methodof test ” ABNT NBR-12131, Rio de Janeiro, Brazil (in Portuguese).

2. Ghaly, A., and Hanna, A (1991) “Experimental and theoretical studies on installation

torque of screw anchors,” Canadian Geotechnical Journal, Vol. 28, No 3, pp. 353–

364.

3.

Ghaly, A., Hanna, A., and Hanna, M. (1991) “Installation torque of screw anchors in

sand,” Soils and Foundations, Vol.31, No 2, pp. 77–92.

4. Hoyt, R.M., and Clemence, S.P. (1989) “Uplift capacity of helical anchors in soil,” In

Proceedings of the 12th International Conference on Soil Mechanics and Foundation

Engineering, Rio de Janeiro, 13–18 August 1989. A.A. Balkema, Rotterdam, the

Netherlands. Vol. 2, pp. 1019–1022.

5.

Narasimha Rao, S., Prasad, M.D., Shetty, M.D., and Joshi, V.V. (1989) “Uplift

capacity of screw pile anchors,” Geotechnical Engineering, Vol. 20, No 2, pp. 139–

159.

6.

Perko, H.A. (2000) “Energy method for predicting the installation torque of helical

foundations and anchors,” In New technologiesl and design developments in deep

foundations. Edited by N.D. Dennis, Jr., R. Casteli, and M.W. O’Neill. ASCE Press,

Reston, Va. pp. 342–352.

7.

PERKO, H.A. (2009) “Helical Piles: a practical guide for design and installation,”

John Wiley & Sons, New York.

8.

Tsuha, C.H.C., and Aoki, N. (2010) “Relationship between installation torque anduplift capacity of deep helical piles in sand,” Canadian Geotechnical Journal, Vol.47,

No 6, pp. 623-647.

9.

Tsuha, C.H.C., Aoki, N., Rault, G., Thorel, L., and Garnier,J. (2012) “Evaluation of

the efficiencies of helical anchor plates in sand by centrifuge model tests,” Canadian

Geotechnical Journal, Vol.49, No 9, pp. 1102-1114.

10.

Zhang, D.J.Y. (1999) “Predicting capacity of helical screw piles in Alberta soils,”

M.Sc.E. thesis, Department of Civil and Environmental Engineering, University of

Alberta, Edmonton, Alta.

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A New Experimental Procedure to Investigate the Torque Correlation Factor of Helical Anchors

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