Avalon Structural :: For Engineers: Typical Helical Specifications

FOR ENGINEERS: TYPICAL HELICAL SPECIFICATIONS

A. HELIX PILE AND ANCHOR TERMINOLOGY
  1. Piles and Anchors - Helical members used primarily in compression are referred to as piles. Helical members used primarily in tension are referred to as anchors. Helical piles and anchors differ significantly in design, application, capacity, and installation criteria.

  2. Torque Theory of Installation - This is the determination of helical pile or anchor capacity by installation torque. The torque is monitored during installation and multiplied by the capacity-to-torque ratio.

  3. Capacity-to-Torque Ratio (Kt) - the relationship of installation torque to capacity is expressed by the equation PU = Kt * T, where Kt is the capacity-to-torque ratio, PU is ultimate axial capacity, and T is installation torque. The derivation of Kt is discussed below and is a function of the helical shaft shape and size, as well as soil characteristics.

  4. Site-Specific Capacity-to-Torque Ratio - Kt may be determined by an on-site empirical loading test to failure (unacceptable movement) to directly calculate the relationship between torque and capacity for a given helical shaft in the site soils.

  5. Default Capacity-to-Torque Ratio - In lieu of an empirical derivation of the site-specific Kt, the manufacturer's default Kt value is commonly used. This specification itself is usually given as a range. The exact value to be chosen within that range is at the engineer's discretion. This may be based on soil type (cohesive vs. non-cohesive) and/or helical application (pile vs. anchor).

  6. Empirical Observations About Kt Values - (1) Cohesive soils generally have lower Kt values than granular soils; (2) larger shafts have lower Kt values than smaller shafts due to more friction on the larger shaft to neglect; (3) round shafts have lower Kt values than square shafts due to more friction on the round shaft to neglect; (4) given the same helical shaft, plate configuration, and torque, compression capacity is generally higher than tension capacity due to disturbed soil above the plates and generally more stiff/dense soils below the plates.

  7. Design Load - The sum of all applicable dead and live loads with no factors of safety. Design load is synonymous with un-factored load.

  8. Ultimate Capacity - The capacity required of the helical pile, commonly referred to as PU. It is calculated from the design load multiplied by the applicable factor of safety, typically two (2). Ultimate capacity is synonymous with ultimate load, factored load, but is best expressed as Ultimate Capacity as it is a capacity and not an actual load. The Ultimate Capacity should be clearly stated on the plans, either by each helical or in a table. This is the target for the Contractor to reach by the Torque Theory of Installation.

  9. Identification on Plans - Helical piles and anchors are numbered on the plans for identification. Example of pile identification: "P1" or "HP1" for Helical Pile 1. Examples of anchor identification: TB3 for Tieback 3 (or HA3 for Helical Anchor 3 or D3 for Diagonal 3). The contractor should keep an installation log refering to these identifications.

  10. Further Reading - (1) AC358 (ICC-ES, 2016); (2) Perko, Ph.D., PE, Howard A., Helical Piles: a Practical Guide to Design and Installation (2009), John Wiley & Sons, Inc., Hoboken, New Jersey

B. SAMPLE HELIX PILE AND ANCHOR SPECIFICATIONS
  1. MANUFACTURER - HELICAL PILES AND HELICAL ANCHORS SHALL BE MANUFACTURED BY EARTH CONTACT PRODUCTS OF OLATHE, KS (ICC-ES 3559), IDEAL FOUNDATION SYSTEMS OF EAST ROCHESTER, NY (ICC-ES 3750), OR APPROVED EQUAL.

  2. CERTIFICATION - HELICAL PILES AND ANCHORS SHALL BE INSTALLED BY A CONTRACTOR AUTHORIZED BY THE HELICAL MANUFACTURER.

  3. SAFETY - ALL APPLICABLE SAFETY CODES SHALL BE ADHERED TO DURING ALL WORK DESCRIBED HEREIN.

  4. BUILDING CODE - HELICAL PILES AND ANCHORS SHALL CONFORM TO THE 2016 CALIFORNIA BUILDING CODE.

  5. HELICAL TYPE - HELICAL PILES AND ANCHORS SHALL BE SQUARE-SHAFT OR ROUND-SHAFT AS SHOWN ON THE PLANS.

  6. CORROSION PROTECTION - ALL HELICAL PILES AND ANCHORS SHALL BE CORROSION PROTECTED BY HOT DIP GALVANIZATION PER ASTM A153 / ASTM A123. PILE CAPS ENCASED IN CONCRETE NEED NOT BE GALVANIZED.

  7. PLATE CONFIGURATION - HELICAL PLATE CONFIGURATION SELECTION IS AT DISCRETION OF CONTRACTOR. CONTRACTOR IS ADVISED TO CONSULT WITH MANUFACTURER TO DETERMINE PLATE CONFIGURATION.

  8. EQUIPMENT - MOTORIZED INSTALLATION EQUIPMENT SHALL HAVE FORWARD AND REVERSE CAPABILITY AND SHALL HAVE TORQUE CAPACITY 20 PERCENT GREATER THAN REQUIRED TO ACHIEVE THE ULTIMATE CAPACITIES LISTED IN THE PLANS.

  9. CROWD - INSTALLATION EQUIPMENT SHALL BE CAPABLE OF APPLYING CONTINUOUS DOWN PRESSURE DURING THE INSTALLATION PROCESS.

  10. DETERMINATION OF CAPACITY OF PILES - THE CAPACITY OF PILES MAY BE DETERMINED USING THE TORQUE THEORY OF INSTALLATION.

  11. TORQUE MONITORING - TORQUE SHALL BE MONITORED THROUGHOUT THE INSTALLATION PROCESS BY A DIFFERENTIAL PRESSURE GAUGE OR A DIRECT TORQUE-MONITORING DEVICE AT THE DISCRETION OF THE CONTRACTOR. ANY DIFFERENTIAL PRESSURE GAUGE MUST FIRST BE VERIFIED ON-SITE BY SIMULTANEOUS READINGS FROM A DIRECT TORQUE-MONITORING DEVICE SUCH AS A SHEER-PIN LIMITER OR STRAIN GAUGE.

  12. CAPACITY-TO-TORQUE RATIO (Kt) - THE ULTIMATE CAPACITY IS TO BE DETERMINED BY INSTALLATION TORQUE AND THE CAPACITY-TO-TORQUE RATIO (Kt). THE CONTRACTOR HAS DISCRETION TO USE THE MANUFACTURER DEFAULT Kt VALUE OR DETERMINE A SITE-SPECIFIC Kt VALUE OF THE SITE SOIL. ANY SITE-SPECIFIC Kt VALUE SHALL BE DETERMINED FROM A TEST TO FAILURE AND SHALL BE WITNESSED BY THE SPECIAL INSPECTOR.

  13. REDUCTION OF PENETRATION - RATE OF PENETRATION SHALL BE OBSERVED WHEN SETTING HELICAL PILES. LOSS OF TORQUE WITHOUT LOSS OF COMPRESSION CAPACITY MAY OCCUR WHEN RATE OF PENETRATION DROPS BELOW 85% AS THE PLATES SCRAPE ON VERY STIFF/DENSE SOIL. FOR FURTHER CLARIFICATION, SEE CHAPTER 6 OF PERKO, PH.D., PE, HOWARD A., HELICAL PILES: A PRACTICAL GUIDE TO DESIGN AND INSTALLATION (2009)

  14. TESTING - TEN PERCENT OF ALL HELICAL ANCHORS (TENSION MEMBERS) SHALL BE PULL-TESTED TO 133 PERCENT OF THE DESIGN LOAD IN THE PRESENCE OF THE SPECIAL INSPECTOR. TESTING OF HELICAL PILES (COMPRESSION MEMBERS) SHALL BE AT THE DISCRETION OF THE ENGINEER. IF A HELICAL ANCHOR OR PILE FAILS A PULL TEST, IT MAY EITHER BE INSTALLED DEEPER OR REPLACED AT THE DISCRETION OF THE CONTRACTOR AND MUST BE RETESTED.

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