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Helical Pile Design Technical Specifications

Helical pile designHelical Pile Design

The axial capacity of the helical pile is produced through the bearing of the helix bearing plates against the soil. The helix blades are spaced three diameters apart along the pile shaft, to remove additional stress influence from the other bearing plates. (Space varies dependant upon soil) The helix blade is manufactured according to the ICC-ES AC358 criteria for geometry. The leading and trailing edges of true helix blades are within one-quarter inch of parallel to each other and any other radial measurement across the blade is perpendicular to the pile shaft.

Each blade acts independently in bearing along the pier shaft. By spacing the helix-shaped bearing plates, the stress is limited to a bulb-shaped section of soil, which is approximately two, lateral helix diameters from the center of the pile shaft.

In accordance with ICC-ES AC358, when installing multiple piles the center-to-center spacing at the helix depth must be at least four times the diameter of the largest helix bearing plate. The helical pile tops may be closer to each other near the ground surface however they must be installed at a batter away from the other piles in order to meet the spacing requirement.

Capacity Calculations

The ultimate capacity of the helical pile can be calculated using the bearing capacity equation: Qu = ∑ [Ah(cNc + qNq)]

A safety factor of 2 is traditionally used to determine the allowable soil bearing capacity, especially if the torque is monitored during installation. Effective stress parameters should be used for long-term, permanent load applications. Total stress parameters should be used for transient load and short-term applications.

Helical pile capacity can also be estimated using correlation to installation torque: Qu = KT

The capacity to torque ratio is not constant, varying depending upon soil conditions and the size of the pile shaft. ICC-ES AC358 provides default K-values for pile shaft diameters, which can be used conservatively in most soil conditions. However, to properly determine project specific K-values, it’s best to load test using the helical pile and helix bearing plate configuration.

The default value for the FSI 288 Helical Piles: (2 7/9-inch diameter) is K= 9 ft-1
Qu = Ultimate Pile Capacity (lb)
Ah = Area of Individual Helix Bearing Plate (ft2)
c = Effective Soil Cohesion (lb/ ft2)
Nc = Dimensionless Bearing Capacity Factor = 9
q = Effective Vertical Overburden Pressure (lb/ ft2)
Nq = Dimensionless Bearing Capacity Factor
K = Capacity to Torque Ratio (ft-1)
T = Installation Torque (ft-lb)

When installing deep foundation stabilization solutions it’s best to consult with a geotechnical engineer to take into account the myriad of factors not included in the equations. 

Helical Pile DesignNew Construction Applications

FSI 288 helical piles can be used within grade beams and pile caps in commercial, industrial and residential new construction applications. Installation equipment can be sized according to the project, including hand-held equipment, mini-excavators, skid steers, trackhoes or backhoes.

 

 

Pile Capacity

Mechanical Axial Capacities i
Allowable Compression = 70.5 kipsii
Allowable Tension = 39.8 kipsii
The torque limited axial design capacities are based on:
Ultimate Torsional Resistance of pile shaft = 9,180 ft-lbs.iii
Ultimate Soil Capacity = 82.6 kips iii (with K = 9ft-1)i
Allowable Soil Capacity = 41.3 kips iii

i. ICC-ES AC358 has determined K = 9 ft-1 as the default value. This is often determined to be conservative, and high capacities can be achieved with site specific load testing. The allowable soil capacities shall not exceed the Mechanical Axial Capacities.
ii. The capacity is based on black, uncoated steel with a loss in steel thickness due to corrosion over the period of 50 years. The design period and corrosion loss rates are in accordance with ICC-ES AC358. Mechanical compression limits are based on continuous lateral soil confinement. Piles with exposed, un-braced lengths should be evaluated by the project engineer.
iii. The Ultimate Torsional Resistance and the Torque Limited Capacities are based on lab test results, which were conducted in an IAS accredited facility and may only be approached in idealized conditions.

Plastic torsional deformations can begin in the pile shaft when near 7,500 ft-lbs. This value may be reached and exceeded in applications by maintaining alignment between the pile and the drive head, limiting the impact force and torque reversal, and reducing the tendency to push down on the pile. Installation through soils with obstructions or high variability may result in impact loading on the pile. In these cases achieving high torque values becomes more difficult and further reduction of the Design Torque Limit may be appropriate.

Technical Specifications

Outside diameter = 2.875″
Wall thickness = 0.276″
Pile shaft yield strength = 60 ksi (minimum)
Coupling hardware: (3)3/4″ grade 8 bolts with nuts
Available helix blade diameters = 8″, 10″, 12″ and 14″
Helix bearing plate thickness = 0.375″
New construction bracket: 3/4″ x 6″ square A36 plate (for allowable compression capacities up to 60 kips)
New construction bracket hardware: (2) 3/4″ grade 8 bolts with nuts

Helical Pile Design: Retrofit Applications

FSI 288 helical piles can permanently stabilize sinking or settling foundations. The helical pile steel brackets are heavy-duty, L shaped brackets which are attached to the tops of the piles and installed against the foundation footing. After the helical piles are installed, the weight of the structure will be transferred to the piles.

helical pile design - allowable capacities table

 

Pier Testing Procedure

The FSI Helical Pier systems have be designed and tested in accordance with the International Code Council Evaluation Service, Inc. (ICC-ES) Acceptance Criteria for Helical Foundation Systems and Devices (AC358). You can review the guidelines and criterion for AC358 at www.icc-es.org.

AC358 testing is the most rigorous, accurate and thorough testing standard in the industry, and is also the only testing procedure that replicates real life applications.

The results from the testing process may be considered conservative because the full benefit of the external sleeve is not realized until it’s installed in the soil and testing is conducted without the sleeve. The criterion used can more accurately determine the failure loads while identifying failure mechanisms of the pier system and its components.

The AC358 sets precise criteria for testing side-load retrofit piering systems. Figure 1 is in acceptance with criteria illustrating the appropriate laboratory set up. The exposed, unsupported length of the pier from the bearing surface of the bracket to the point of the fixity on the test frame is 60 inches. The test does not show failure loads in excess of those expected in actual application.

The bracket is mounted to a concrete block of known strength and the test sample is loaded until failure occurs in the pier system’s steel components or the concrete interface. The test does not take into consideration the additional support given by the external sleeve when supported by confining soils.

Design

External Sleeve

The FSI Helical Pier system utilizes an external sleeve preserves the axial compressive capacity of the pier. The external sleeve creates a 30-inch long bracket that resists the bending forces generated by the eccentric loading on the side-load bracket.

The external sleeve was designed to minimize the bend moment of the pier, even though it’s localized within a short distance of the side-load bracket. The bending force quickly dissipates against the confining soil, however to improve performance and safety, the external sleeve was designed to negate this factor.

The external sleeve is easy to install. It’s driven at the same time as the starter tube.

Provides additional strength during the installation. The external sleeve is in place while the pier tube is being driven, which is when the pier tube experiences maximum load.

Efficient solution for a local problem. The sleeve is installed in the location where it’s needed, providing a cost effective solution for a localized problem. The sleeve protects the pier tube during install. The sleeve supports the bracket against kinking and guides the pier tubing into the ground at the recommended angle.

The sleeve reduces the friction during install. The external sleeve reduces the friction between the pier tube and side-load bracket which reduces the hydraulic pressure during driving and lifting applications.

Eccentric Loading

When installed in retrofit applications, the pile is not located directly under the structure’s footing, and is eccentrically loaded. The external sleeve aides in resisting the bending forces e created as a result of eccentric loading.

In most applications the dimensions of the pier cross section are typically less than four inches. Because of the dimensions these sections are sensitive to bending moments created by eccentricity, which in turn reduces the capacity of the pier to carry axial load.

Bending moments in the pier sections are minimized by the soil. The bending moment dissipates into the surrounding soil within the first couple of feet. Soil type is a factor in the bending moment: softer soils require the bending forces to dissipate over a longer length than stiffer, more compact soils.

helical pile design - helical pier retrofit bracketBracket Specifications

Bracket: Weldment manufactured from 0.25″, 0.375″ and 0.5″ thick steel plate.
Yield strength = 36 ksi (minimum)
Tensile strength = 58 ksi (minimum)

External sleeve: 3.5″ outside diameter x 0.216″ wall x 30″ long with sleeve collar welded to one end.
Yield strength = 50 ksi (minimum)
Tensile strength = 62 ksi (minimum)

Bracket cap: 5.0″ wide x 9.0″ long x 1″ thick plate with confining ring welded to one side.
Yield strength = 50 ksi (minimum)
Tensile strength = 62 ksi (minimum)

All-thread rod: 0.75″ diameter x 16″ long, zinc plated. Grade B7 Tensile strength = 125 ksi (minimum)

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