Screw Piles

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Screw PilesUse and Design : 

Screw PilesUse and Design Muhammad Farooq 2007-MS-GEO-21

Objectives : 

Objectives What are screw piles? geometry fabrication installation common uses Why use screw piles? advantages over conventional pile types How do we design screw piles? axial failure models direct pile design approach: LCPC method empirical approach: correlates installation effort to axial capacity Conclusions

What are Screw Piles? : 

What are Screw Piles? Deep foundations: carry tensile, compressive, and lateral loads Constructed of helical plates welded to hollow steel pipe

Emergence of Screw Piles : 

Emergence of Screw Piles No related engineering literature exists prior to 1950s/1960s First use of screw piles: Maplin Sands light house in the Thames estuary in 1838

Screw Pile Geometries : 

Screw Pile Geometries

Terminology : 

Terminology Inter-Helix Spacing Ratio = S/D

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18 cm diameter shaft 35 cm diameter helix 5 meter length

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Shaft diameters: 11 cm to 32 cm (4 ½ to 12 ¾ inches) Helix diameters: Commonly 2-3 times the shaft diameter 30 cm to 91 cm (12 to 36 inches)

Installation : 

Installation Turning moment applied to the head of screw pile shaft, and pile “twisted” into the ground Desirable rate of penetration is one helix pitch per revolution

Installation Equipment : 

Installation Equipment

Screw Pile Advantages : 

Screw Pile Advantages Rapid installation (typ. < 30 min per pile) Little installation noise or vibration No casing or dewatering required Lightweight installation equipment soft terrain areas of restricted access Sustain load immediately after installation May be removed and re-used temporary structures Resistant to frost heave

Screw Pile Limitations : 

Screw Pile Limitations Not for use in very hard or rocky soils may sustain damage to the helical plates piles may be removed and helices checked Lack of acceptance/understanding in the engineering community

Typical Screw Pile Uses : 

Typical Screw Pile Uses Tower foundations Ft. McMurray, Alberta: 27 cm (10 ¾ in) shaft, one or two 76 cm (30 in) helices, 6 m length Pipeline foundations Earth retention system

Building Foundations: Warehouses Multi-family Housing Commercial Buildings Modular Homes Hythe, Alberta: 22 cm (8 5/8 in) shaft, single 40 cm (16 in) helix, 8 m length

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Oil Field Foundations Temporary Buildings Pump Jacks Compressors Tanks Typically 18 cm (7 in) shaft, single 40 cm (16 in) helix, 7.5 m deep

Screw Pile Failure Models : 

Screw Pile Failure Models Cylindrical Shear Model Individual Plate-Bearing Model Choice of the most representative model depends on the screw pile geometry, in particular the Inter-Helix Spacing Ratio (S/D)

Cylindrical Shear Model : 

Cylindrical Shear Model After Narasimha Rao et al. (1991)

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

Effect of Inter-Helix Spacing Ratio (S/D) 1: S/D ≈ 1.5 Cylindrical surface fully forms 2: S/D ≈ 2 Cylindrical surface begins to deteriorate 3: S/D ≈ 4.5 Cylindrical surface nearly non-existent 1 2 3 After Narasimha Rao et al. (1991)

Individual Plate Bearing Model : 

Individual Plate Bearing Model

Summary: Failure Models : 

Summary: Failure Models Cylindrical Shear Model: Multi-helix screw piles, generally most representative for S/D <2 Individual Plate Bearing Model: Single-helix screw piles Multi-helix screw piles, applicable for S/D>2

Axial Capacity Prediction : 

Axial Capacity Prediction Theoretical Design Methods Application of relevant soil strength parameters (su ,α, Φ,γ, Nq, etc) Direct Design Approach: LCPC Method Directly relates results of cone penetration test to ultimate axial screw pile capacity, with no intermediate determination of soil strength parameters Empirical Approach Directly correlates measured installation torque to ultimate axial screw pile capacity

Direct Design: LCPC Method : 

Direct Design: LCPC Method Established design method for predicting the axial capacity of conventional piles, based on site-specific CPT LCPC method developed in France by the Laboratoire Central des Ponts et Chausees, based on results of many full-scale pile load tests (Bustamante and Gianeselli, 1982) Use of the CPT is advantageous because the test is fast, repeatable, and provides continuous profile of soil information

Direct Design: LCPC Method : 

Direct Design: LCPC Method Basic premise of LCPC method is to apply scaling (reduction) factors to CPT profile of tip resistance to calculate appropriate components of bearing resistance and friction/adhesion Qtotal = Qbearing + Qshaft + Qcylinder

Direct Design: LCPC Method : 

Direct Design: LCPC Method

LCPC Method : 

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

LCPC Method : 

LCPC Method

Empirical Torque Correlation : 

Empirical Torque Correlation Direct empirical relationship between torque required to install a given screw pile and the pile’s ultimate axial capacity Qultimate = Kt∙ T (after Hoyt and Clemence, 1989) Analogous to relationship between pile driving effort and pile capacity used for driven steel piles Can only predict capacity once pile is installed– best used for field-level verification of expected design capacities

Torque Correlation : 

Torque Correlation

Torque Correlation : 

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

Conclusions : 

Conclusions Screw piles have many advantages, such as ease of installation, immediate load-bearing capacity, no casing/dewatering required LCPC method provides good axial capacity prediction in clay and sand, but not suitable for glacial till soils Torque correlation factors provide good capacity prediction for screw piles in a variety of soil types

References : 

References Bustamante, M. and Gianeselli, L. 1982. Pile bearing capacity prediction by means of static penetrometer CPT. In Proceedings of the Second European Symposium on Penetration Testing, ESOPT-II. Amsterdam. Balkema Publisher, Rotterdam, Vol. 2, pp. 687-697. 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, 31(2):35-50. Zhang, D. 1999. Predicting capacity of helical screw piles in Alberta soils. M.Sc. Thesis, Department of Civil and Environmental Engineering, University of Alberta, Edmonton, Alberta.

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