Screw Piles: Use and Design

Transcription

1 Screw Piles: Use and Design Kristen M. Tappenden November 2006

2 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 city

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

4 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

5 Screw Pile Geometries

6 Terminology Inter-Helix Spacing Ratio = S/D

7 18 cm diameter shaft 35 cm diameter helix 5 meter length

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

9 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 Video Clip: courtesy of ALMITA Manufacturing

10 Installation Equipment

11 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

12 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

13 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 systems Guy wire anchors

14 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

15 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

16 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)

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

18 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 After Narasimha Rao et al. (1991)

19 Individual Plate Bearing Model

20 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

21 Axial Capacity Prediction Theoretical Design Methods Application of relevant soil strength parameters (s( u,α, Φ,γ, N q, N qu qu ) 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 al screw pile capacity

22 Direct Design: LCPC Method Established design method for predicting the axial capacity of conventional piles, based on site-specific 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

23 Direct Design: LCPC Method Basic 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 Q total = Q bearing + Q shaft + Q cylinder

24 Direct Design: LCPC Method Soil Type Average CPT tip resistance over layer i Bearing capacity factor Skin friction factor Maximum unit skin friction q c k c α q s (kpa) (kpa) Soft clay and mud <1, Moderately compact clay 1,000 to 5, Silt and loose sand 5, Compact to stiff clay and compact silt > 5, Soft chalk 5, Moderately compact sand and gravel 5,000 to 12, Weathered to fragmented chalk > 5, Compact to very compact sand and gravel 12,

25 0 q c (kpa) LCPC Calculation 1 2 Two 36 cm helices Spacing = 3D 21 cm shaft Depth (m) q s = 35 kpa q b1 = 811 kpa b2 = 990 kpa q b2 Calculated Capacity in Compression: Cylindrical Shear: 188 kn Individual Plate Bearing: 209 kn Calculated Capacity in Tension: Cylindrical Shear: 160 kn Individual Plate Bearing: 180 kn 8 after Zhang (1999) Measured Capacity: 210 kn in both tension and compression

26 LCPC Method Compression 0.00 Axial Capacity (kn) Depth (m) QLP, Cylindrical Shear Model QL, Cylindrical Shear Model QLP, Individual Plate Bearing Model QL, Individual Plate Bearing Model

27 LCPC Method 26 axial load tests, 7 test sites: clay, sand, clay shale, glacial till Qpredicted / Qmeasured 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 Predicted to Measured Capacity, Cylindrical Shear Predicted to Measured Capacity, Individual Plate Bearing

28 LCPC Method Qpredicted/ Qmeasured C1 C2 C3 T1 T2 T3 C4 C5 C6 T4 T5 T6 C7 C8 C9 C10 C16 C17 Test Pile Designation Predicted to Measured Capacity, Cylindrical Shear Predicted to Measured Capacity, Individual Plate Bearing

29 Empirical Torque Correlation Direct empirical relationship between torque required to install a given screw pile and the pile s ultimate axial capacity Q ultimate = K t T (after Hoyt and (after Hoyt and Clemence,, 1989) Analogous to relationship between pile driving effort and pile capacity c used for driven steel piles Can only predict capacity once pile is installed best used for field-level level verification of expected design capacities

30 Torque Correlation Ultimate Axial Pile Capacity (kn) Installation Torque (kn-m) 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)

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

32 Summary 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

33 Thank You Research Partners: Dr. Dave Sego Gerry Cyre Peace Land Piling / Peace Land Power Ltd. ALMITA Manufacturing Ltd. ATCO Electric ConeTec Inc. Funding Providers: Natural Sciences and Engineering Research Council (NSERC) Alberta Ingenuity Fund University of Alberta

34 References Bustamante, M. and Gianeselli,, L 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 Narasimha Rao,, S., Prasad, Y.V.S.N, and Shetty,, M.D The behavior of model screw piles in cohesive soils. Soils and Foundations, 31(2):35 (2): Zhang, D Predicting capacity of helical screw piles in Alberta soils. M.Sc. Thesis, Department of Civil and Environmental Engineering, University of Alberta, Edmonton, Alberta.

35 Questions?