Screw piles

Screw PilesScrew PilesUse and DesignUse and Design

Muhammad FarooqMuhammad Farooq2007-MS-GEO-212007-MS-GEO-21

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 capacityempirical approach: correlates installation effort to axial capacity

► ConclusionsConclusions

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

tensile, compressive, and tensile, 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 Thames First use of screw piles: Maplin Sands light house in the Thames estuary in 1838estuary in 1838

Screw Pile GeometriesScrew Pile Geometries

TerminologyTerminology

Inter-Helix Spacing RatioInter-Helix Spacing Ratio == S S//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 screw pile Turning moment applied to the head of screw pile shaft, and pile “twisted” into the groundshaft, and pile “twisted” into the ground

► Desirable rate of penetration is one helix pitch per Desirable rate of penetration is one helix pitch per revolutionrevolution

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 equipmentLightweight 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 UsesTypical 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 systemEarth retention system

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

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

1 2 3

After Narasimha Rao et al. (1991)

Individual Plate Bearing ModelIndividual Plate Bearing Model

Summary: Failure ModelsSummary: Failure Models

►Cylindrical Shear Model:Cylindrical Shear Model: Multi-helix screw piles, generally most representative for S/Multi-helix screw piles, generally most representative for S/

D <2D <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, etc, etc))

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

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

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

screw pile capacityscrew pile capacity

Direct Design: LCPC MethodDirect Design: LCPC Method

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

► 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 many , based on results of many full-scale pile load tests (Bustamante and Gianeselli, 1982)full-scale pile load tests (Bustamante and Gianeselli, 1982)

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


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

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


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 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


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

QQultimateultimate = K = Ktt∙∙ T T (after Hoyt and Clemence, 1989)(after Hoyt and Clemence, 1989)

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

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


Qpr

edic

ted/

Qm

easu

red

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

ConclusionsConclusions

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

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

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

ReferencesReferences► Bustamante, M. and Gianeselli, L. 1982. Pile bearing capacity prediction by Bustamante, M. and Gianeselli, L. 1982. Pile bearing capacity prediction by

means of static penetrometer CPT. means of static penetrometer CPT. In In Proceedings of the Second European Proceedings of the Second European Symposium on Penetration Testing, ESOPT-II. Amsterdam. Balkema Publisher, Symposium on Penetration Testing, ESOPT-II. Amsterdam. Balkema Publisher, Rotterdam, Vol. 2, pp. 687-697.Rotterdam, Vol. 2, pp. 687-697.

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

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

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