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Prediction of the installation torque resistanceof large diameter helical piles in dense sand

CONFERENCE PAPER · AUGUST 2013

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Ken Gavin

University College Dublin

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Paul Doherty

University College Dublin


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Giovanni Spagnoli

BAUER Maschinen GmbH


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Prediction of the Installation Torque Resistance ofLarge Diameter Helical Piles in Dense Sand

by

Kenneth G. Gavin, Lecturer, University College Dublin ([email protected]), Paul Doherty, Post-Doctoral Researcher, University College Dublin and Giovanni Spagnoli, Adjunct Lecturer, University

College Dublin

ABSTRACT: Whilst helical piles have long been used to resist tension loads, more recently they are being used more in practice to resist compression forces and thus the diameter of helical piles used inindustry has increased significantly. This paper presents data from the installation of large diameter,single helix piles installed at two sites. Five 400 mm diameter helical piles were installed at the

University College Dublin, sand test bed site in Ireland. The sand at the test bed site is very dense, withCone Penetration Test (CPT) values in the range 15 to 20 MPa. The results of a tension load test on aninstrumented pile load test allowed a simple model to be proposed to predict the torque required toinstall helical piles. This design method is used to predict the installation response of helical piles froma second site, where helical piles with diameters of up to 1.5 m were installed in medium-dense todense sand with Standard Penetration Test (SPT) N values varying 24 to 59. The simple model provedto predict the very high installation resistance of the large diameter helical piles.

INTRODUCTION

Helical piles which were invented by the Irish Engineer Alexander Mitchell saw their firstapplication in the middle of the 19 th century to support Maplin Sands lighthouse in the estuary of theriver Thames (Lutenneger 2013). Whilst their use declined with the invention of pile driving hammers,recent years have seen a resurgence in their popularity due to the development of hydraulic unitscapable of generating very large torque. Whilst traditionally helical piles were used to resist tension(uplift) loading, piling contractors are now exploring the possibility of using large diameter single ormulti-helix piles to carry both compression and tension loads. Design methods which correlate the

piles axial pull-out capacity (Q u) to the installation torque (T), required to install the pile through anempirical factor, K t are popular in practice:

[1] Q u = K t T

Stephenson (1997) notes that K t values which range from 33m -1 for small diameter shafts (D < 89 mm)to 9.8m -1 for shafts of D = 219 mm suggested by Hoyt and Clemence (1989) covers the range of valuesused most in industry practice. There is some debate as to whether the same K t factors can be used for

both compression and tension loading, with Pack (2009) arguing that the values are identical, whilstLivneh and Naggar (2008) and Sakr (2009) reported field tests indicating that K t factors in tensionwere as low as 50% of the values mobilized in compression load tests.

Design methods based on conventional bearing capacity theory e.g. Canadian Geotechnical Society(2006) are also in widespread use. However, the effect of installation, assumed failure mechanism,

https://www.researchgate.net/publication/237152381_Axial_testing_and_numerical_modeling_of_square_shaft_helical_piles_under_compressive_and_tensile_loading?el=1_x_8&enrichId=rgreq-de89b081-834e-48cd-949d-d0d2af670ac6&enrichSource=Y292ZXJQYWdlOzI2MTA2NTMyNjtBUzoxMDM1NjA2NDk5MDQxMzNAMTQwMTcwMjE5MTc3Mg==https://www.researchgate.net/publication/237152381_Axial_testing_and_numerical_modeling_of_square_shaft_helical_piles_under_compressive_and_tensile_loading?el=1_x_8&enrichId=rgreq-de89b081-834e-48cd-949d-d0d2af670ac6&enrichSource=Y292ZXJQYWdlOzI2MTA2NTMyNjtBUzoxMDM1NjA2NDk5MDQxMzNAMTQwMTcwMjE5MTc3Mg==


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dilation and stress level effects make the choice of appropriate design friction angle and bearingcapacity factors difficult. This paper explores the potential use of cone penetration test data todetermine the axial resistance and installation torque of large diameter, single helix piles installed insand.

INSTRUMENTED LOAD TESTS ON PILES

Test site

The load test described in the paper was performed at the University College Dublin (UCD)geotechnical test bed site at Blessington, County Wicklow. The sand at this site is heavily over-consolidated with an estimated pre-consolidation pressure of 700 to 1,000 kPa at the piling platformlevel. The sand is partially saturated with a moisture content of 10% and fine grained with D 50 ranging from 0.1 to 0.15 mm. The cone penetration test (CPT) end resistance q c, is seen in Figure 1 toincrease from approximately 10 MPa at ground surface to 20 MPa at 6 m bgl. Detailed description ofthe in-situ and laboratory test properties of Blessington sand are available in Gavin and Lehane (2007),Igoe et al. (2011) and Tolooiyan and Gavin (2011).

Figure 1 CPT q c and f s values at the test site

Pile Details

In total, five piles with a single 400 mm diameter helix were installed at the test site. One pile(Pile 1) was instrumented with strain gauges bonded to the surface at five levels, see Figure 2a. Fouradditional piles with no instrumentation were installed in order to act as anchors/reaction piles for the


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compression and tension load test which were performed on the instrumented pile. The test pileconsisted of a 3.07 m long lead section, with a 400 mm helix, with a pitch of 100 mm mounted on a110 mm outer diameter pile shaft. Strain gauges were bonded to the pile surface at multiple locationsalong the pile shaft to determine the distribution of load during field load testing. Five separate loadlevels were established (L#1 to L#5), which ranged from 80 mm above the helix to 470 mm from the

pile head. For each level, four 120 Ohm resistance axial strain gauges were bonded to the pile surfaceat 90 degree offsets, at locations North, South, East and West. The gauges were coated with anabrasive resistant ceramic based epoxy resin to protect the gauges during installation and reducefiction. The lead cables were brought up the inside of the pile shaft. These were protected fromtwisting during pile installation by fixing them in place using an expansive foam agent. The straingauges on the East and West axis followed the line of the holes used to couple the lead section to thedrive unit and load the pile during the tension load test. The gauges were coated with an abrasiveresistant ceramic based epoxy resin to protect the gauges during installation. The cables were protectedfrom twisting during pile installation by fixing them in place using an expansive foam agent.

The piles were installed using a drive unit capable of applying a maximum torque of approximately 30kNm. The installation torque readings are illustrated in Figure 2b. The piles had relatively uniforminstallation torque values with an average value of 14kN-m at ground surface and increased with depthuntil the reaction piles locked-out at a depth of 4.5m below ground level. The instrumented pile, whichinstalled to a depth of 2.6 m depth (in order to keep the L5 gauges above ground level – See Figure 2a)at which point the torque reading was 18.7 kN-m.

3 0 7 0 m m

1 0 0 m m

D=110 mm

500 mm

1 3 0 m m

N

S

W E

4 StrainGauges at

Each Level

8 0 m m 4

8 0 m m

6 8 0 m m

5 7 0 m m

6 3 0 m m

30 mm

Helix=400 mm

L#1

L#2

L#3

L#4

L#5

(a) (b)

Figure 2 Details of the instrumented test pileand installation torque values


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LOAD TEST

Test Programme

Following installation of the instrumented test pile and the four anchor piles, a load frame wasassembled to provide reaction for the load tests, See Figure 3. The test load was applied on the day ofinstallation using a hand pump connected to a hydraulic jack, with a locking valve used to maintain aconstant load during holding intervals. The load was measured at the pile head and recorded by both adigital load cell and a gauge connected to the calibrated hydraulic jack. The pile head displacementswere measured using three linear variable displacement transducers (LVDTs) mounted on anindependent reference frame. A compression load test was performed on the pile, the details of whichare discussed in Gavin et al. (2013). This paper considers the tension load test response only. The loadwas applied in loading increments of approximately 20kN and maintained at each level until creep

became negligible, after which the next increment was applied.

Figure 3 Load test set-up

Test Results

The load-displacement response measured during the tension load test is shown in Figure 4.The pile developed a maximum tension load resistance of 240 kN at a pile head displacement of 40mm (10% of the pile diameter). Thereafter, the pile exhibited a brittle, strain softening response withthe tension capacity reducing to approximately 160 kN after a pile head displacement of 110 mm. Thestrain gauge response was particularly important in determining the load distribution along the pileshaft and to determine how the helical pile was carrying the tension load applied to the pile head.Unfortunately, the strain gauges at Level 2 seized to function after installation due to mechanicaldamage at the cable terminal connections. The strain gauges at the remaining levels were used todetermine the load distribution in the pile shown in Figure 4b. These indicated that even from the


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earliest load increments, the majority of the load was transferred to the helix. At the failure load of 240kN, approximately 25% of the load was shed along the pile shaft with the remaining 75% of the loadresistance being provided by the helix.

0

50

100

150

200

250

300

0 40 80 120

L o a

d ( k N )

Displacement (mm)

0.1D

0

0.5

1

1.5

2

2.5

0 100 200 300

D e p

t h ( m )

Load (kN)

Figure4. (a) Load – Displacement and Strain Gauge Response during Load Test

The average shaft resistance q sav developed during the tension load test is shown in Figure 5a. The q sav

value increased to a maximum of 90 kPa at a pile head displacement of 37 mm, before reducing to 64kPa at large displacements. The uplift resistance (q up) developed on the helix during the tension test isshown in Figure 5b. The uplift resistance reached a peak value of 1400 kPa at a pile head displacementof 42 mm before reducing to 1050 kPa at the end of the test.

Figure5. (a) Shear stress and (b) Uplift resistance on Helix Mobilized during Load Test


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Gavin et al (2013) explored correlations between the compression resistace of helical piles and theCone Penentration Test end resistance q c. Since the installation torque resistance depends on the upliftresistance of the pile, an established framework for estimating the torque resistance is adopted in this

paper, with tentaive proposals for linking the pile shaft and helix resistance to the CPT q c value beingexplored.

De Hollanda et al. (2010) suggest that the installation torque resistance of a helical pile, T is linked tothe uplift resistance of the pile through an expression of the form:

[1]

Where Q s and Q h are the shaft and helix uplift load resistance, D and Dh are the shaft and helixdiameter, is the helix angle, and r is the residual interface friction angle between the sand the pile. Inthis paper, the average shaft resistance is correlated to the CPT f s value and the uplift resistance on thehelix developed during the tesnion load tests are linked to the CPT resistance using the following

expressions:

[2a] q sav = 0.6 f s [2b] q up = 0.065 q c

and

[3a] Q s = q sav D L[3b] Qh = q up Dh2

Equations 3 were used to predict the installation resistance of the helical piles at Blessington, See

Figure 6.


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Figure6. Measured and Predicted Torque for the 400 mm diameter Helical PilesThe method was applied using average f s and q c values from ten CPT tests performed at the test siteand is seen to provide reasonably good estimates of the measured torque over most of the installeddepth. However, the simple expression fails to capture the sudden increase in torque resistancemeasured at a depth of 4.5 m on Pile 2 and Pile 5.

PREDICTION OF THE TORQUE RESISTANCE OF LARGE DIAMETER PILES

Site Details

A trial installation was performed using single helix piles with a shaft diameter of 368 mm. Two helixdiameters were tested, 880 mm and 1500 mm. The 1500 mm diameter pile had a pitch of 750 mm,whilst two version of the 880 mm diameter pile were tested, one with a pitch of 200 mm and one of440 mm. The soil conditions at the test site consisted of medium-dense to dense sand with StandardPenetration Test (SPT) N values varying 24 to 59, See Figure 7a. CPT data for the site was derived

(See Figure 5b) based on the following correlation:

[4] q c (kPa) = N

Where is a correlation factor which depends on the particle size distribution of the sand, a factor of400 was assumed for this site. CPT f s values were derived based on the assumption that the frictionratio f s/qc was 1%.

Figure7. Measured SPT N Values and Predicted CPT qc values at the Test Site


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The installation torque was predicted using Equation 3 and the data from Figure 7. The pile shaftdiameter and pitch were taken as 0.3 m. The torque resistance profiles predicted are shown in Figure 8.The following points are noteworthy:

1. Both piles developed very high torque resistance values at shallow depths. The piles wereinstalled with standard torque drive units with a capacity of either 200 or 400 kNm, the smallerunit would be able to install the 880 mm pile to a penetration depth of 10 m. However, it wouldnot be able to install the 1500 mm diameter pile.

2. The larger drive unit would be able to install the 880 mm piles to a maximum depth of 16 mand the 1500 mm pile to ≈ 5 m.

3. The effect of varying the pitch had little effect on the predicted torque for the 880 mm diameter pile until the pile embedment exceeded 11 m.

4. Because of the low relative installation depths achieved the large diameter piles were not viableat the site.

5. Gavin et al. (2013) present finite element analyses which show that the helix uplift resistance isconcentrated in a zone which extends up to three helix diameters. Given that the torque islargely mobilised as uplift resistance, the practice of reversing the torque motor to reduce theinstallation torque resistance is not likely to be successful for helical pile installation in densesand.

6. Alternative solutions such as the use of rotary units and hydraulic osscilators could beconsidered to achieve installation torque values of up to 5,000 kNm would make the largediameter piles a viable solution.

Figure 8 Predicted Torque for 1.5 m Diameter Pile


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CONCLUSIONS

The results of a tension load test on an instrumented helical pile and a published theoretical frameworkfor estimation of installation torque were used to produce a tentative correlation between the torquerequired to install a helical pile and the CPT test data. The correlation was shown to provide areasonable estimate of the installation torque of five piles installed in dense sand. The correlationswere then used to predict the installation torque required to install 880 to 1500 mm diameter piles inmedium-dense to dense sands. The correlations suggested that standard rigs would be unable to installthe 1500 mm pile more than 5 m into the natural sand deposit.

ACKNOWLEDGEMENTSThe work described was funded by Enterprise Ireland through an Innovation Voucher and ScrewpileIreland Ltd. The Cone Penetration Tests were performed by In-situ Site Investigations Ltd. The authorsthank the following students in University College Dublin, Tim Hennessey, Patricia Friend-Perriera,Weichao Li and Dr. David Igoe who assisted with the fieldwork. We acknowledge also Roadstone Ltd,for the continued unhampered use of the quarry at Blessington.

REFERENCES

Lutenegger, A.J. (2013), Historical Application of Screw-Piles and Screw-Cylinder Foundations for19th Century Ocean Piers, Proceedings of the International Conference on case Histories inGeotechnical Engineering, Chicago.

Hoyt and Clememce (1997) Hoyt, R.M., and Clemence, S.P. 1989. Uplift capacity of helicalanchors in soil. In Proceedings of the 12th International Conference on Soil Mechanics and FoundationEngineering, Rio de Janeiro, Brazil, 13 – 18 August 1989. Vol. 2, pp. 1019 – 1022.

Stephenson, R.W. (1997) Helical foundations and tie back: A state of the Art. Available athttp://nees.org/data/download/NEES-2006 0149/Documentation/References/Stephenson_1997.pdf,Downloaded on 21/05/2011

Pack, J.S. (2003). Helical foundation and tiebacks: quality control, inspection and performancemonitoring. Proceedings of the 28th Annual Conference on Deep Foundations, Miami Beach, Florida.Deep Foundations Institute, Hawthorne, N.J. pp. 269 – 284.

Livneh, B., and El Naggar, M.H. 2008. Axial testing and numerical modeling of square shaft helical piles under compressive and tensile loading. Canadian Geotechnical Journal, 45(8): 1142 – 1155.doi:10.1139/T08-044.

Sakr, M. 2009. Performance of helical piles in oil sand. Canadian Geotechnical Journal, 46(9): 1046 – 1061. doi:10.1139/T09-044.

Canadian Geotechnical Society (2009) Canadian foundation engineering manual. Canadian


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Geotechnical Society (CGS).

Gavin, K.G. and Lehane. B.M (2007), Base Load-Displacement Response of Piles in Sand, CanadianGeotechnical Journal, Vol. 44, No. 9, September 2007, pp 1053-1063.

Igoe, D., Gavin, K.G. and O’Kelly, B. (2011) , The shaft capacity of pipe piles in sand, ASCE Journalof Geotechnical and Geoenvironmental Engineering (2011). Vol 137, No.10, pp 903-912 (DOI10.1061/(ASCE)GT.1943.5606.0000511Tolooiyan, A. and Gavin, K.G. (2011), Modelling the Cone Penetration Test in Sand Using CavityExpansion and Arbitrary Lagrangian Eulerian Finite Element Methods. Computers and Geotechnics,2011, Volume 38, No.4, pp 482-490.

Gavin, K.G., Doherty, P. and Tolooiyan, A. (2013), Field investigation of the axial resistance of helical piles in dense sand, Submitted to the Canadian Geotechnical journal, under review.

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