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Helical Piles for Power Transmission Lines: Case Study inNorthern Manitoba, CanadaMohammed SakrAlmita Manufacturing Ltd., Ponoka, Alberta, CanadaIntroductionHelical piles have been used with great success to supportpower transmission lines. This is mainly due to the factthat helical piles offer significantly higher uplift resistancecompared to other deep foundation options combined withtheir ease in installation in remote areas with relativelysmall equipment. Moreover helical piles can be loadedimmediately after installation. This paper summarizes acase study for helical pile foundations supporting powertransmission line located in Northern Manitoba, Canada, inwhich the site conditions, pile installation, and performanceof foundations are described.Subsurface StatigraphyPile load tests were carried out in two different soilconditions including either soft to firm clay or stiff highplastic clay with silt varves that extended along the entireembedded depth of piles. The groundwater level wasmeasured at the existing ground surface. The undrained shearstrength parameters obtained from undrained compression(CIUC) triaxial tests are summarized in Table 1. Residualundrained shear strength values were used to estimate theaxial capacities of helical piles.Screw Pile ConfigurationFour pile load tests were carried out, including twocompression and two uplift load tests. The helical pileconfigurations used for the pile load test program consistedof two piles with triple helixes spaced at either 2.5D or 3D,Table 1. Soil design parameters.Soil TypeUndrained Shear Strength,C u(kPa)Peak ResidualSoft to firm clay 30 18Stiff clay 60 30where D is the helix diameter, for guy anchors (i.e., to resistuplift loads) and two piles with either triple or four helixesfor tower support (i.e., to support compression loads). Helicalpile configurations are summarized in Table 2.Pile Installation and Test SetupThe helical piles tested in this study were manufactured andinstalled by ALMITA Manufacturing Ltd. of Ponoka, Alberta.Helical piles were installed through the use of mechanicaltorque applied at the pile head. Torque applied at the pilehead during pile installation was continuously recorded, andpenetration depth was measured. Final measured torque atthe end of pile installation and total embedment depths arealso summarized in Table 2. The maximum torque measuredduring installation for triple helix and four helix piles C1 andC2 installed in stiff clay and soft to firm clay were similar.However, the maximum torque for guy anchor, T2, washigher than that of T1 by about 15% due to higher spacingratio. The embedment depth for triple helix pile was 7.5 m,while the embedment depth for four helix pile was 10.8 m.For guy anchors, the embedment depths were 7.9 m for bothpiles.Typical pile load test setup consisted of two reactionpiles and a test pile. The reaction piles were positioned atLoad (kN)1000.0800.0600.0400.0200.00.0Triple Helixes Pile C1, Stiff ClayFour Helixes Pile C2, Soft to Firm Clay0 5 10 15 20 25 30 35 40 45 50 55 60Settlement (mm)Figure 1. Load vs. settlement curves for axial compression loadtests.Table 2. Summary of pile installation.PileNoPile Configuration Test Type Soil Type InstallationTorquekN.m (ft.Ibs)(Dia. (m) ×Length (m) × No. of helixes × Helixthickness (mm) × Helixes Dia. (m)Spacingbetweenhelixes (m)EmbedmentDepthmC1 Triple helixes (0.324 × 8.5 × 3× 25.4× 0.813) 2.134 Compression Stiff clay 94.9 (70,000) 7.5T1 Triple helixes (0.219 × 8.5 × 3× 19× 0.711) 1.778 Uplift Stiff clay 52.9 (39,000) 7.9C2 Four helixes (0.324 × 11.6 × 4× 25.4× 0.864) 2.286 Compression Soft to firm 94.9 (70,000) 10.8T2 Triple helixes (0.219 × 8.5 × 3× 19× 0.711) 2.134 Uplift Soft to firm 61.0 (45,000) 7.9261
Ni n t h In t e r n at i o n a l Co n f e r e n c e o n Pe r m a f r o s tspacing of 6 m (about 4 helix diameter from the tested pile).Axial pile load tests were conducted according to the ASTMD-1143 Quick Load Test Method for Piles under Static AxialCompressive Load and ASTM D-3689 Quick Load TestMethod for Piles under Static Axial Tensile Load. Loadswere applied in increments of approximately 10% of theestimated pile capacity in 10-minute time intervals.Load Test ResultsAxial compressive pile capacitiesThe results of compressive load tests are presented inFigure 1 in the form of load settlement curves. It can benoted from Figure 1 that load settlements were linear at theinitial part of the load-settlement curve up to a settlementof about 3 mm and corresponding loads of about 200 kN.At higher settlement levels, piles showed a nonlinear loadsettlementfollowed by plunging failure at settlement ofabout 50 mm and corresponding loads of 677 kN and 772 kNfor piles C1 and C2, respectively. The ultimate compressiveload capacities for piles C1 and C2 are presented in Table 3.Cycles were carried out at loads of about 245 kN, and 490kN indicated that the effect of cyclic loading had a minoreffect on the response.It is noted from Figure 1 that pile C2 with four helixesinstalled in stiff clay offered higher resistance comparedto pile C1 with triple helixes installed in soft to firm clay.This behaviour suggested that the cylindrical shear failuremechanism is mobilized. In cylindrical shear failuremechanism, the load at pile head is resisted by threeLoad (kN)500.0400.0300.0200.0100.00.0Triple Helixes Pile T1, Stiff Clay,S/D = 2.5Triple Helixes Pile T2, Soft to Firm Clay, S/D = 30 2 4 6 8 10 12 14 16 18 20Settlement (mm)Figure 2. Load vs. displacement curves for axial tension (uplift)load tests.Table 3. Pile load test results.PileNoSoil ConditionsCompression Test ResultsUltimate Pile Capacity(Q ult)kNUltimateSettlementmmC1 Stiff clay 677 50C2 Soft to firm 772 50Tension (Uplift) Test ResultsT1 Stiff clay 370 15T2 Soft to firm 445 15components including the skin friction along the shaft, thedeveloped cylindrical shear resistance between the helixes,and the surrounding soil and end bearing of the bottom helix(for compression load tests) or the top helix(for uplift loadtests). For pile C2 with four helixes, the cylindrical shearresistance component was considerably higher than that ofpile C1 with triple helixes due to the larger surface area.Axial tension (uplift) pile capacityThe results of tensile (uplift) load tests are presented inFigure 2 in the form of load displacement curves for piles T1and T2. Both piles had the same configurations, except pileT1 installed in stiff clay had a triple helixes spaced at 2.5D(1.77 m), where D is the diameter of the helix, while pile T2installed in soft clay helixes were spaced at 3D (2.134 m).It can be seen from Figure 2 that load displacement curvesfor both piles were linear at the initial part of the uplift loadup to a settlement of about 1 mm and load of about 100 kN.It should be noted that pile T1 installed in stiff clay showedsofter response manifested by the larger displacement at thesame load level compared to pile T2. However at higherdisplacement levels, the load-displacement curves werehighly nonlinear, and pile T2 reached a plunging failure ofabout 445 kN at displacement level of about 15 mm (i.e.,2.5% of the helix diameter). The uplift load test for pile T1was stopped at a load level of about 300 kN, while pile T2was tested till failure. The ultimate capacity of pile T1 wasextrapolated based on the shape of load-displacement curve,and the results are presented in Table 3.ConclusionsThe results of the axial compression and tension loadtests performed in soft to firm or stiff clays demonstratedthe suitability of helical pile foundations for the powertransmission lines in Northern Manitoba. The results ofthe load testing program confirmed that the helical pile isa viable deep foundation option for construction of powertransmission towers in remote areas and demonstrated theiradvantages.The results of the full-scale load tests are also used tovalidate the theoretical model used for helical pile designinstalled in soft and stiff high plastic clays. The resultsindicated that the cylindrical shear failure mechanismcontrols the behavior of helical piles with spacing ratio up to3 installed in clay materials.ReferencesASTM D 1143-81. 1981. Standard Test Method for PilesUnder Static Axial Compressive Load (Reapproved1994). Annual Book of ASTM Standards 1997, Vol.04.08: 95-105.ASTM D 3689-90. 1990. Standard Test Method for IndividualPiles Under Static Axial Tensile Load; (Reapproved1995). Annual Book of ASTM Standard, 1997, Vol.04.08: 366-375.262
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NICOP 2008 Ninth <
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ContentsPreface ...................
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Mapping and Modeling the Distributi
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Satellite Observations of Frozen Gr
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xiiIce Wedge Thermal Regime in Nort
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xivPermafrost Response to Dynamics
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NICOP SponsorsUniversitiesUniversit
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Deep Permafrost Studies at the Lupi
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Effect of Fire on Pond Dynamics in
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Cryological Status of Russian Soils
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Acoustical Surveys of Methane Plume
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Permafrost Delineation Near Fairban
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Preparatory Work for a Permanent Ge
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A Provisional Soil Map of the Trans
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Martian Permafrost Depths from Orbi
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Time Series Analyses of Active Micr
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Impact of Permafrost Degradation on
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DC Resistivity Soundings Across a P
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Modeling Thermal and Moisture Regim
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A Provisional Permafrost Map of the
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Alpine Permafrost Distribution at M
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Cryogenic Formations of the Caucasu
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Modeling Potential Climatic Change
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A Hypothesis: A Condition of Growth
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Modeled Continual Surface Water Sto
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Freeze/Thaw Properties of Tundra So
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Discontinuous Permafrost Distributi
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Thermal Regime Within an Arctic Was
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Seasonal and Interannual Variabilit
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Twelve-Year Thaw Progression Data f
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Continued Permafrost Warming in Nor
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A Permafrost Model Incorporating Dy
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Seasonal Sources of Soil Respiratio
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Greenland Permafrost Temperature Si
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The Importance of Snow Cover Evolut
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The Combined Isotopic Analysis of L
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Adaptating and Managing Nunavik’s
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Human Experience of Cryospheric Cha
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HiRISE Observations of Fractured Mo
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A Soil Freeze-Thaw Model Through th
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Mapping and Modeling the Distributi
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First Results of Ground Surface Tem
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Historical Changes in the Seasonall
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Snowpack Evolution on Permafrost, N
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Climate Change in Permafrost Region
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Maximizing Construction Season in a
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