Screw Piles:Screw Piles:Use and DesignUse and Design

Kristen M. TappendenKristen M. Tappenden

November 2006November 2006

ObjectivesObjectives

► What are screw piles? What are screw piles? ► geometrygeometry► fabrication fabrication ► installationinstallation► common usescommon uses

► Why use screw piles? Why use screw piles? ► advantages over conventional pile typesadvantages over conventional pile types

► How do we design screw piles? How do we design screw piles? ► axial failure modelsaxial failure models► direct pile design approach: LCPC methoddirect pile design approach: LCPC method► empirical approach: correlates installation effort to axial empirical approach: correlates installation effort to axial

capacitycapacity

What are Screw Piles?What are Screw Piles?► Deep foundations: Deep foundations:

carry tensile, carry tensile, compressive, and compressive, and lateral loadslateral loads

► Constructed of helical Constructed of helical plates welded to hollow plates welded to hollow steel pipesteel pipe

Emergence of Screw PilesEmergence of Screw Piles► No related engineering literature exists prior to 1950s/1960sNo related engineering literature exists prior to 1950s/1960s

► First use of screw piles: Maplin Sands light house in the First use of screw piles: Maplin Sands light house in the Thames estuary in 1838Thames estuary in 1838

Screw Pile GeometriesScrew Pile Geometries

TerminologyTerminology

Inter-Helix Spacing RatioInter-Helix Spacing Ratio == SS//DD

18 cm diameter shaft18 cm diameter shaft

35 cm diameter helix35 cm diameter helix

5 meter length5 meter length

Shaft diameters: 11 cm to 32 cm (4 ½ to 12 ¾ inches) Shaft diameters: 11 cm to 32 cm (4 ½ to 12 ¾ inches)

Helix diameters: Helix diameters: Commonly 2-3 times the shaft diameterCommonly 2-3 times the shaft diameter

30 cm to 91 cm (12 to 36 inches)30 cm to 91 cm (12 to 36 inches)

InstallationInstallation

► Turning moment applied to the head of Turning moment applied to the head of screw pile shaft, and pile “twisted” into the screw pile shaft, and pile “twisted” into the groundground

► Desirable rate of penetration is one helix Desirable rate of penetration is one helix pitch per revolutionpitch per revolution

► Video Clip: courtesy of ALMITA Video Clip: courtesy of ALMITA ManufacturingManufacturing

Installation EquipmentInstallation Equipment

Screw Pile AdvantagesScrew Pile Advantages

► Rapid installation (typ. < 30 min per pile)Rapid installation (typ. < 30 min per pile)► Little installation noise or vibrationLittle installation noise or vibration► No casing or dewatering requiredNo casing or dewatering required► Lightweight installation equipment:Lightweight installation equipment:

soft terrainsoft terrain areas of restricted accessareas of restricted access

► Sustain load immediately after installationSustain load immediately after installation► May be removed and re-usedMay be removed and re-used

temporary structurestemporary structures

► Resistant to frost heaveResistant to frost heave

Screw Pile LimitationsScrew Pile Limitations

► Not for use in very hard or rocky soilsNot for use in very hard or rocky soils may sustain damage to the helical platesmay sustain damage to the helical plates piles may be removed and helices checkedpiles may be removed and helices checked

► Lack of acceptance/understanding in the Lack of acceptance/understanding in the engineering communityengineering community

Typical Screw Pile Uses:Typical Screw Pile Uses:► Tower foundationsTower foundations

Ft. McMurray, Alberta: 27 cm (10 ¾ in) shaft, one or two 76 cm (30 in) helices, 6 m length► Pipeline foundationsPipeline foundations

► Earth retention systemsEarth retention systems► Guy wire anchorsGuy wire anchors

► Building Foundations:Building Foundations: WarehousesWarehouses Multi-family HousingMulti-family Housing Commercial BuildingsCommercial Buildings Modular HomesModular Homes

Hythe, Alberta: 22 cm (8 5/8 in) shaft, single 40 cm (16 in) helix, 8 m

length

►Oil Field FoundationsOil Field Foundations Temporary BuildingsTemporary Buildings Pump JacksPump Jacks CompressorsCompressors TanksTanks

Typically 18 cm (7 in) shaft, single 40 cm (16 in) helix, 7.5 m deep

Screw Pile Failure ModelsScrew Pile Failure Models

►Cylindrical Shear ModelCylindrical Shear Model

► Individual Plate-Bearing ModelIndividual Plate-Bearing Model

Choice of the most representative model Choice of the most representative model depends on the screw pile geometry, in depends on the screw pile geometry, in particular the particular the Inter-Helix Spacing Ratio (S/D)Inter-Helix Spacing Ratio (S/D)

Cylindrical Shear ModelCylindrical Shear Model

After Narasimha Rao et al. (1991)

Effect of Inter-Helix Spacing Effect of Inter-Helix Spacing Ratio (S/D)Ratio (S/D)

1:1: S/D S/D ≈≈ 1.5 1.5 Cylindrical surface fully formsCylindrical surface fully forms

2: 2: S/D S/D ≈ 2 ≈ 2 Cylindrical surface begins to Cylindrical surface begins to

deterioratedeteriorate

3: 3: S/D ≈ 4.5 S/D ≈ 4.5 Cylindrical surface nearly non-Cylindrical surface nearly non-

existentexistent

1 2 3

After Narasimha Rao et al. (1991)

Individual Plate Bearing Individual Plate Bearing ModelModel

Summary: Failure ModelsSummary: Failure Models

►Cylindrical Shear Model:Cylindrical Shear Model: Multi-helix screw piles, generally most Multi-helix screw piles, generally most

representative for S/D <2representative for S/D <2

► Individual Plate Bearing Model:Individual Plate Bearing Model: Single-helix screw pilesSingle-helix screw piles Multi-helix screw piles, applicable for S/D>2Multi-helix screw piles, applicable for S/D>2

Axial Capacity PredictionAxial Capacity Prediction

► Theoretical Design MethodsTheoretical Design Methods► Application of relevant soil strength parameters (sApplication of relevant soil strength parameters (su u ,,αα, , ΦΦ,,γγ,,

NNqq, N, Nququ))

► Direct Design Approach: LCPC MethodDirect Design Approach: LCPC Method► Directly relates results of cone penetration test to ultimate Directly relates results of cone penetration test to ultimate

axial screw pile capacity, with no intermediate axial screw pile capacity, with no intermediate determination of soil strength parametersdetermination of soil strength parameters

► Empirical ApproachEmpirical Approach► Directly correlates measured installation torque to Directly correlates measured installation torque to

ultimate axial screw pile capacityultimate axial screw pile capacity

Direct Design: LCPC MethodDirect Design: LCPC Method

► Established design method for predicting the axial Established design method for predicting the axial capacity of conventional piles, based on site-specific capacity of conventional piles, based on site-specific CPTCPT

► LCPC method developed in France by the LCPC method developed in France by the LLaboratoire aboratoire CCentral des entral des PPonts et onts et CChauseeshausees, based on results of , based on results of many full-scale pile load tests (Bustamante and many full-scale pile load tests (Bustamante and Gianeselli, 1982)Gianeselli, 1982)

► Use of the CPT is advantageous because the test is Use of the CPT is advantageous because the test is fast, repeatable, and provides continuous profile of fast, repeatable, and provides continuous profile of soil informationsoil information

Direct Design: LCPC MethodDirect Design: LCPC Method

►Basic premise of LCPC method is to Basic premise of LCPC method is to apply scaling (reduction) factors to apply scaling (reduction) factors to CPT profile of tip resistance to CPT profile of tip resistance to calculate appropriate components of calculate appropriate components of bearing resistance and bearing resistance and friction/adhesionfriction/adhesion

QQtotaltotal = Q = Qbearingbearing + Q + Qshaftshaft + Q + Qcylindercylinder

Direct Design: LCPC MethodDirect Design: LCPC Method

Soil Type Average CPT tip resistance over layer i

Bearing capacity factor

Skin friction factor

Maximum unit skin friction

qc kc α qs

(kPa) (kPa)

Soft clay and mud <1,000 0.50 30 15

Moderately compact clay 1,000 to 5,000 0.45 40 35

Silt and loose sand ≤ 5,000 0.50 60 35

Compact to stiff clay and compact silt > 5,000 0.55 60 35

Soft chalk ≤ 5,000 0.30 100 35

Moderately compact sand and gravel 5,000 to 12,000 0.50 100 80

Weathered to fragmented chalk > 5,000 0.40 60 120

Compact to very compact sand and gravel 12,000 0.40 150 120

LCPC CalculationLCPC Calculation► Two 36 cm helicesTwo 36 cm helices► Spacing = 3DSpacing = 3D► 21 cm shaft21 cm shaft

► qqs s = 35 kPa = 35 kPa► qqb1b1 = 811 kPa = 811 kPa► qqb2b2 = 990 kPa = 990 kPa

Calculated Capacity in Compression:Calculated Capacity in Compression:► Cylindrical Shear: 188 kNCylindrical Shear: 188 kN► Individual Plate Bearing: 209 kNIndividual Plate Bearing: 209 kN

Calculated Capacity in Tension:Calculated Capacity in Tension:► Cylindrical Shear: 160 kNCylindrical Shear: 160 kN► Individual Plate Bearing: 180 kNIndividual Plate Bearing: 180 kN

► Measured Capacity: 210 kN in Measured Capacity: 210 kN in both both tension and tension and compressioncompression

0

1

2

3

4

5

6

7

8

0 1000 2000 3000

qc (kPa)D

ep

th (

m)

after Zhang (1999)

LCPC Method—Compression LCPC Method—Compression

0.00

1.00

2.00

3.00

4.00

5.00

0 50 100 150 200 250

Axial Capacity (kN)

Dep

th (

m)

QLP, Cylindrical Shear Model QL, Cylindrical Shear Model

QLP, Individual Plate Bearing Model QL, Individual Plate Bearing Model

LCPC MethodLCPC Method

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

C1 C2 C3 T1 T2 T3 C4 C5 C6 T4 T5 T6 C7 C8 C9 C10 C11 C12 T7 T8 T9 C16 C17

Test Pile Designation

Qpr

edic

ted

/ Qm

easu

red

Predicted to Measured Capacity, Cylindrical Shear Predicted to Measured Capacity, Individual Plate Bearing

26 axial load tests, 7 test sites: clay, sand, clay shale, glacial till

LCPC MethodLCPC Method

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

C1 C2 C3 T1 T2 T3 C4 C5 C6 T4 T5 T6 C7 C8 C9 C10 C16 C17

Test Pile Designation

Qpr

edic

ted/

Qm

easu

red

Predicted to Measured Capacity, Cylindrical Shear Predicted to Measured Capacity, Individual Plate Bearing

Empirical Torque CorrelationEmpirical Torque Correlation

► Direct empirical relationship between torque required to Direct empirical relationship between torque required to install a given screw pile and the pile’s ultimate axial capacityinstall a given screw pile and the pile’s ultimate axial capacity

QQultimateultimate = K = Ktt∙∙ T T (after Hoyt and Clemence, (after Hoyt and Clemence,

1989)1989)

► Analogous to relationship between pile driving effort and pile Analogous to relationship between pile driving effort and pile capacity used for driven steel pilescapacity used for driven steel piles

► Can only predict capacity once pile is installed– best used for Can only predict capacity once pile is installed– best used for field-level verification of expected design capacitiesfield-level verification of expected design capacities

Torque CorrelationTorque Correlation

0

500

1000

1500

2000

2500

3000

3500

0 50 100 150 200 250 300

Installation Torque (kN-m)

Ult

imat

e A

xial

Pil

e C

apac

ity

(kN

)

Measured Data (11.4 cm shaft piles)Linear Regression, 11.4 cm shaft piles (Kt = 16.9 m-1)Measured Data (14.0 to 40.6 cm shaft piles)Linear Regression, 14.0 to 40.6 cm shaft piles (Kt = 9.19 m-1)

Torque CorrelationTorque Correlation

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

C1 C2 C3 T1 T2 T3 C4 C5 C6 T4 T5 T6 C7 C8 C9 C10 C11 C12 T7 T8 T9 C13 C14 C15 C16 C17 C18 C19 C20

Test Pile Designation

Qpr

edic

ted/

Qm

easu

red

29 screw pile axial load tests, 10 test sites: sand, clay, glacial till, clay shale, sandstone

SummarySummary

► Screw piles have many advantages, such as Screw piles have many advantages, such as ease of installation, immediate load-bearing ease of installation, immediate load-bearing capacity, no casing/dewatering requiredcapacity, no casing/dewatering required

► LCPC method provides good axial capacity LCPC method provides good axial capacity prediction in clay and sand, but not suitable prediction in clay and sand, but not suitable for glacial till soilsfor glacial till soils

► Torque correlation factors provide good Torque correlation factors provide good capacity prediction for screw piles in a capacity prediction for screw piles in a variety of soil typesvariety of soil types

Thank YouThank You

► Research Partners:Research Partners: Dr. Dave SegoDr. Dave Sego Gerry CyreGerry Cyre Peace Land Piling / Peace Land Power Ltd.Peace Land Piling / Peace Land Power Ltd. ALMITA Manufacturing Ltd.ALMITA Manufacturing Ltd. ATCO ElectricATCO Electric ConeTec Inc.ConeTec Inc.

► Funding Providers:Funding Providers: Natural Sciences and Engineering Research Council (NSERC)Natural Sciences and Engineering Research Council (NSERC) Alberta Ingenuity FundAlberta Ingenuity Fund University of AlbertaUniversity of Alberta

ReferencesReferences

Bustamante, M. and Gianeselli, L. 1982. Pile bearing capacity prediction Bustamante, M. and Gianeselli, L. 1982. Pile bearing capacity prediction by means of static penetrometer CPT. by means of static penetrometer CPT. In In Proceedings of the Second Proceedings of the Second European Symposium on Penetration Testing, ESOPT-II. Amsterdam. European Symposium on Penetration Testing, ESOPT-II. Amsterdam. Balkema Publisher, Rotterdam, Vol. 2, pp. 687-697.Balkema Publisher, Rotterdam, Vol. 2, pp. 687-697.

Narasimha Rao, S., Prasad, Y.V.S.N, and Shetty, M.D. 1991. The Narasimha Rao, S., Prasad, Y.V.S.N, and Shetty, M.D. 1991. The behavior of model screw piles in cohesive soils. Soils and behavior of model screw piles in cohesive soils. Soils and Foundations, Foundations, 3131(2):35-50.(2):35-50.

Zhang, D. 1999. Predicting capacity of helical screw piles in Alberta Zhang, D. 1999. Predicting capacity of helical screw piles in Alberta soils. M.Sc. Thesis, Department of Civil and Environmental soils. M.Sc. Thesis, Department of Civil and Environmental Engineering, University of Alberta, Edmonton, Alberta.Engineering, University of Alberta, Edmonton, Alberta.

Questions?Questions?