Agenda. April 9, pm MDT. Introduction. Helical Piles in Practice Examples Closing Poll 10/1/2014

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1 Helical lpiles in Practice (1Hr Webinar) Presented by Dr Gamal Abdelaziz, P.Eng. Sponsored by SAGA Engineering sagaengineering.ca April 9, pm MDT gic edu.com Agenda Introduction Upcomng Courses Helical Piles in Practice Examples Closing Poll gic edu.com 1

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3 GIC Poll Online automated registration process 10 years proven track record in educational services Over 300 live courses offered per year Portfolio of over 1000 available courses Courses offered in all across Canada and Internationally Public & Private Classroom, In house, Distance, Online, Webinar Courses Ability to customize courses for organizations Exam Preparation Services Certification programs gic edu.com Special for Attendees Today s attendees will receive a special gift for attending today s webinar. We will reveal at the end of today s presentation gic edu.com 3

4 Introductions Dr. Gamal Abdelaziz, P.Eng, MSc. has a Ph.D. in Geotechnical Engineering from Concordia University, Montreal, Canada. Dr. Abdelaziz has over 30 years of experience in geotechnical and structural engineering, foundation design, teaching, research and consulting in Canada and overseas. EoR for Stony Plain ring road, Calgary, AB. Currently he is a senior geotechnical engineer with SAGA Engineering, Edmonton, Alberta. SAGAengineering.ca gic edu.com Basic Helical Pile Design Dr. Gamal Abdelaziz, Ph.D., P.Eng. Senior Geotechnical Engineer SAGA Engineering Edmonton, Alberta Former Visiting Professor, Ryerson University, Adjunct Professor, University of Western Ontario 1. Basics of Design 8 4

5 Introduction Screw piles have been in use for more than 160 years. In 1838 a lighthouse designed by an Irish engineer, Alexander Mitchell. In 1863, Eugenius Birch designed the Brighton West Pier in Brighton, 9 England. Introduction These piers are still in use 140 years later. The original screw piles were installed at 10 feet per hour using eight 20 foot long torque bars and the force of 32 to 40 men. 10 5

6 Sporadic use of screw piles has been documented throughout the 19th and early 20th centuries mainly for supporting structures and bridges over weak or wet soil. 11 Advantages of Helical Piles Ease of Installation Little to No Vibration Immediate Load Transfer upon Installation Installed Torque Correlates to Capacity Easily Load Tested to Verify Capacity Installs Below Active Soils All Weather Installation Little to No Disturbance to Jobsite 12 6

7 Ragged Point Light was constructed in March 1910 a It was the last lighthouse built in the Chesapeake Bay area. Piney Point Lighthouse in Maryland is on the opposite side of the river. Ragged Point Light was one of the first lighthouses to be dismantled. It was deactivated in Carysfort Reef Light, four miles east of Key Largo, Florida, was built in 1852 and is the oldest screw- pile (with disk) lighthouse still in service in the United States

8 In foundation restoration and stabilization applications, foundation brackets are available that attach between the helical l screw pile and the foundation beam or footing. Transferring the load from the soil below the footing to the helical screw pile restores the structure. 15 Hydraulic torque motors became available in the 1960's 16 8

9 Configura tions of Typical ECP Torque Anchor'" Brand of Helcial Screw Piles 17 Current uses for screw pile foundations include foundations for commercial and residential structures, light standards, retaining walls tieback anchors, failed foundation restorations, pipeline and pumping equipment supports, elevated walkways, bridge abutments, and numerous uses in the electric utility industry. 18 9

10 Screw Pile Components Helical screw piles consist of a shaft fabricated from either solid square steel bar or tubular steel. Welded to the shaft are one or more helical plates

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12 Each lead section of a helical steel pier has provisions at the top for a connection to an extension, and has an earth-penetrating pilot at the bottom. Each extension has provisions for 23 The plates can vary in diameter from 6" to 14" and have a thickness of 3/8" or 1/2" depending upon the soil and the application. Typically the plate diameters increase from the bottom of the shaft upward and are spaced a distance of three times the diameter of the plate directly below, unless specified otherwise 24 12

13 The standard thickness for helical plates is 3/8 inch, but in high load applications a plate thickness of 1/2" may be specified. The pitch of the helical plate is three inches, which means that the anchor advances into the soil a distance of three inches during one revolution of the shaft. The number of the plates per screw pile is limited only by the capacity of the shaft to transmit the torque required to advance the helical screw pile into the soil

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15 winnipegscrewpiles.com 29 Screw Piles Limitations locations where subsurface material may damage the shaft or the helices. Soils containing cobbles, large amounts of gravel, boulders, construction debris, and/or landfill materials are usually unsuitable for helical products

16 Screw Piles Limitations Helical piles have slender shafts, buckling may occur in extremely soft soil, which cannot exert sufficient lateral force on the narrow shaft. When extremely soft soils are present, generally having a Standard Penetration Test - "N" <5blowsper foot, one must take into consideration the axial stiffness of the anchor shaft in the design. 31 Screw Piles Limitations The slender shafts also render the typical screw pile ineffective against large lateral loads or overturning moments

17 Capacities of ECP Helical Torque Anchor' Brand of Helical Screw Piles The capacities listed above are mechanical ratings 33 Design Criteria The bearing capacity of a helical screw pile (P w ) can be defined as the load which can be sustained by the pile without producing objectionable settlement, either initially or progressively, which results in damage to the structure or interferes with the use of the structure

18 Screw Piles Bearing Capacity Bearing Capacity is dependant upon many factors: Kind Of Soil, Soil Properties, Surface and/or Ground Water Conditions, Screw Pile Configuration (Shaft Size & Type, Helix Diameter, and Number Of Helices), Depth to Bearing, Installation Angle, Pile Spacing, Installation Torque, Type of Loading - Tension, Compression, Alternating Loads, etc. 35 The most accurate design requires knowledge from field testing using the Standard Penetration Test (SPT) standardized to ASTM D1586 plus laboratory evaluations of the soil shear strength, which is usually given as soil cohesion - "c", soil density - and granular friction angle -" " 36 18

19 Helical Pile Load and Reaction Diagram 37 Each design requires specific information involving the structure and soil characteristics at the site. Each design should involve geotechnical and engineering input

20 Standard and V-style cut plates

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26 51 Preliminary Design Guidelines Helical screw pile systems must be considered as deep foundation elements as a rule of thumb, screw pile to be installed to a depth of at least six times the diameter of the largest helix. The measurement is from the surface to the upper most helical plate of the screw pile

27 The capacity of a multi-helix deep foundation system assumes that the ultimate bearing capacity is the sum of the bearing support from each plate of the system. Testing has shown that when the helical plates are spaced at three times the diameter away from the adjacent lower helical plate, each plate will develop full efficiency in the soil. 53 Spacing the helical plates at less than three diameters is possible, however, each plate will not be able to develop full capacity and the designer will have to include a plate efficiency factor in the analysis when conducting the design

28 Equation 1: Ultimate Theoretical Capacity: Eq 1 Where: P u = Ultimate Capacity of Screw Pile A H = Sum of Projected Helical Plate Areas c = Cohesion of Soil (Ib Ib/ft 2 ) N c = Bearing Capacity Factor for Cohesion q = Soil Overburden Pressure to Mid-Plate Depth - Ib/ft 2 N q = Bearing Capacity Factor for Granular Soil. 55 The ultimate capacity is defined here as the working capacity at a factor of safety of 2.0 that results in a deformation of one inch. In all cases, it is highly recommended that field testing to verify the accuracy of the preliminary design load capacities

29 Soil Behavior Cohesive Soil (Clays) Cohesive soil is soil that is generally classified as a fine grained clay soil. By comparison, granular soils like sands and gravels are sometimes referred to as non-cohesive or cohesionless soil. 57 Clays or cohesive soils are defined as soils where the internal friction between particles is approximately zero. This internal friction angle is usually referred to as " "" or "phi". Cohesive soils have a rigid behavior when exposed to stress

30 Stiff clays act almost like rock. They remain solid and inelastic until they fail. Soft clays act more like putty. The soft clay bends and molds around the anchor when under stress. 59 Undrained Shear Strength - "c": The undrained shear strength of a soil is the maximum amount of shear stress that t may be placed on the soil before the soil yields or fails. This value of "c" only occurs in cohesive soils where the internal friction " of the fine grain particles is zero or nearly zero

31 The value of "c generally increases with soil density; therefore, one can expect that stiff clays have greater undrained shear strength than soft clay soil. It is easy to understand that when dealing with cohesive soils; that the greater the shear strength "c" of the soil, the greater the bearing capacity. It also follows that the shear strength of the soil tends to increase with depth. 61 Cohesion Bearing Capacity Factor - "N c ": The bearing capacity factor for cohesion is an empirical value proposed by Meyerhof in the Journal of the Geotechnical Engineering Division, Proceedings of ASCE,

32 Cohesion Bearing Capacity Factor - "N c ": For small shaft screw piles with helical plate diameters under 18 inches, the value of "N c " = 9 is generally accepted as a reasonable value to use when determining capacities of helical piles and anchors. When determining the ultimate capacity for a helical screw pile in cohesive soil, Equation 1 may be simplified because the internal friction of the soil particles " can be assumed to be zero and the cohesive bearing factor "N c " = Table 2. Cohesive Soil Classification 64 32

33 Where: Ultimate Theoretical Capacity (Equation 1) P u = Ultimate Capacity of Helical Screw Pile A H = Sum of Projected Helical Plate Areas c = Cohesion of Soil (Ib Ib/ft 2 ) N c = Bearing Capacity Factor for Cohesion = 9 N q = Bearing Capacity Factor for Granular Soil = 0. q = Soil Overburden Pressure to Mid-Plate 65 Depth (When multiplied by N q = 0) Equation 2: Ultimate Capacity -- Cohesive Soil Where: (Use With Clay Soil Only) P u = Ultimate Capacity of Helical Screw Pile in Clay AA H = Sum of Projected Helical l Plate Areas c = Cohesion of Soil (lb/ft 2 ) 66 33

34 Table 3. Properties of Cohesive Soil 67 Cohesionless Soil (Sands) Particles of sand in cohesionless soil act independently of each other. This type of soil has fluid-like like characteristics. When cohesion less soils are placed under stress they tend to reorganize into a more compact configuration

35 Cohesionless soils achieve their strength and capacity in several ways: The unit weight of the soil above the Torque Anchor, The internal friction angle " ", ", The adhesion or skin friction. 69 In sandy soil, the grains are independent and there is no cohesion, therefore "c" may be assumed to be zero, therefore N c = 0 One can simplify Equation 1 when analyzing cohesionless (sandy) soil by eliminating the values relating to cohesive (clay) soil that become zero when the screw pile is founded only in sand and gravel. Equation 1 is repeated below and simplified

36 Ultimate Theoretical Capacity (Equation 1) Where: P u = Ultimate Capacity of Helical Screw Pile A H = Sum of Projected Helical Plate Areas c = Cohesion of Soil (lb/ft2) N c = Bearing Capacity Factor for Cohesion = 0 N q = Bearing Capacity Factor for Granular Soil. q = Soil Overburden Pressure to Mid-Plate Depth 71 Table 4. Cohesionless Soil Classification 72 36

37 Equation 3: Ultimate Capacity -- Cohesionless Soil (Use with Sand & Gravel Only) Where: P u = Ultimate Capacity of Helical Screw Pile A H = Sum of Helical Plate Areas q = Soil Overburden Pressure from the surface to mid-plate depth - lb/ft 2 N q = Bearing Capacity Factor for Granular Soil. 73 Soil Overburden Pressure - "q": The soil overburden pressure at a given depth is the sum of the soil density " (lb/ft 3 - kn/m3) multiplied by the depth of the soil. When calculating the value of "q" for a given soil layer above the water table, the dry density of the soil is used

38 Below the water table, the buoyancy effect of the water must be taken into consideration by reducing the dry density of the soil by 62 lb/ft kn/m 3 The general equation for calculating "q" is presented in Equations 4 & Equation 4: Soil Overburden Pressure (Above Water Table) Where: q = Soil Layer Overburden Pressure (lb/ft 2 ) = Dry Density of the Soil Layer (lb/ft 3 ) h = Thickness of the Soil Layer (ft) 76 38

39 Equation 5: Soil Overburden Pressure (Below Water Table) To arrive at the total soil overburden pressure on a helical screw pile, the values of "q" of each stratum of soil from the surface to the point midway between the upper helical plate and the lowest helical plate must be determined and then added together. 77 Cohesionless Bearing Capacity Factor - "N q ": Zhang proposed the ultimate compression capacity of the helical screw pile in a thesis for the University of Alberta in From this work the dimensionless empirical ii value "N q " was introduced. d "N q " is related to the friction angle of the soil " "" as shown in Table

40 Table 5. Properties of Cohesionless Soil 79 Helical Screw Pile Design Considerations Projected Areas of Helical Plates: When determining the capacity of a screw pile in a given soil, knowledge of the projected total area of the helical plates is required. This projected area is the summation of the areas of the helical plates in contact with the soil less the cross sectional area of the shaft

41 Table 6 provides projected areas in square feet of bearing for various plate diameters on the different shaft configurations of ECP Torque Anchors. The projected areas for helical plates may be slightly different for other manufacturers of screw piles as some manufacturers stamp the plates from flat steel bars. 81 Table 6. Projected Areas* of ECP Helical Torque Anchor Brand Helical Plates 82 41

42 Allowable Capacity per Helix: When conducting a preliminary design, one must also be aware of the mechanical capacity of a helical plate when welded to the shaft. The capacities of the ECP Torque Anchor plates are given in Table Table 7. Helical Plate Capacities 84 42

43 Preventing "Punch Through" When designing the helical screw pile to achieve bearing in the competent soil situated above a weaker soil, we must consider the possibility that the screw pile could "punch through" to the weaker soil when fully loaded. 85 When designing an axial compression pile in such a situation, it is recommended that a distance greater than 5 times the diameter of the lowest helical plate (55 x d 1 ) exist to prevent the helical l screw pile from "punching through" to the stratum of weak soil

44 List of References 1- Basic Helical Screw Pile Design, Donald J. & Clayton, PE, 2005, Earth Contact Products, LLC. 87 Special for Attendees Thank you for attending today s webinar on Helical Piles in Practice. Here is today s promotional code to save $150 on any upcoming public or webinar course in Geotechnical Engineering. WEBGEOTECHAPR Valid until May 1 st, 2014 gic edu.com 44

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