Avalon 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 piles are generally installed vertically. Helical members used primarily in tension are referred to as "anchors." Helical anchors are generally installed diagonally. Helical piles and anchors differ significantly in design, application, capacity, and installation criteria.

  2. Design Load (DL) - The sum of all applicable dead and live loads before any factors of safety are applied. The design load is synonymous with the working capacity, allowable capacity and un-factored load. In the literature this is commonly termed DL for design load or Pa where P is for vertical force and a is for allowable.

  3. Ultimate Capacity (UC) - The axial capacity required of the helical member. In the literature this is commonly termed UC for Ultimate Capacity or Pu where P is for vertical force and u is for ultimate. It is calculated from the design load multiplied by the applicable factor of safety, typically two for helical piles and helical anchors. Ultimate capacity is synonymous with ultimate load, or factored load, but is best expressed as "ultimate capacity" because it is a capacity as distinguished from a load. The ultimate capacity should be clearly stated on the plans, usually in a table.

  4. Torque Theory of Installation - The method of using installation torque to determine the capacity of the helical anchor or pile. The torque is the mechanical rotational force measured in foot-pounds required to advance the helical shaft into the earth. This torque is monitored during installation and multiplied by the capacity-to-torque ratio to determine the capacity of the helical member.

  5. Capacity-to-Torque Ratio (Kt) - The relationship of installation torque to helical capacity is expressed by the equation Pu = Kt * T, where PU is ultimate axial capacity of the helical, Kt is the capacity-to-torque ratio, 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. The higher the Kt value the more capacity is achieved per pound foot of torque.

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

  7. Default Torque-to-Capacity - In lieu of an empirical derivation of the site-specific Kt, the manufacturer's default "K value" is commonly used. This specification is usually given as a range where 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) or helical application (pile vs. anchor).

  8. Empirical Observations About Kt Values - (1) cohesive soils have 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.

  9. 85% Penetration Rule - The plates of typical helical members is three inch. Therefore, 100% penetration rate is three inches advancement per revolution. It is a best practice to observe rate of penetration continuously while installing helical members. With proper downward force applied, when the rate of penetration drops below 85% the torque reading no longer correlates with soil strength and is not valid for the the torque-to-capacity theory. This may be due to a physical impediment such as a boulder or debris or helical plate scraping against very hard soils. In the case of the helical plate scraping against bedrock, compression capacity will be greater than torque suggests. Tension capacity will suffer where the helical has lost grip and is churning the soil, reflected by drop in torque.

  10. Refusal - When the helical pile or anchor will not penetrate farther into the soil after sustained downward force and applied torque, this is termed the refusal condition. In this case the engineering professional must determine if the pile or anchor meets the design criteria without relying on the Torque-to-Capacity theory because the torque is no longer a reliable measure. While the refusal condition is almost always unacceptable for helical anchors, if soil conditions are appropriate the refusal condition may be acceptable for piles used as end-bearing members.

  11. Source and Further Reading - (1) AC358 (ICC-ES, 2024); (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. Design Recommendations
  1. Label by Number. To assist communication between all parties, we recommend helical members be identified by number. For example, "P1" or "HP1" are common names for the first helical pile; "A1" or “T1” is a common name for the first helical anchor or tieback. This allows the designer to compile each helical member’s ultimate capacity into a table shown on the plans. From from this table, the contractor can build an installation log listing torque achieved and length of embedment for each numbered member. With numbered helical members, all parties from the engineer to the installer can identify and discuss each one with clarity and ease.

  2. Pile Placement and Design. Helical piles (as opposed to anchors) are typically installed vertically with an allowable deviation range such as 5 degrees. We recommend the term "battered pile" be reserved for piles set purposely 5 to 10 degrees off vertical, usually in a pile group with angles offsetting each other. If a pile is battered more than about 10 degrees it may lead to excessive deflection under vertical load unless part of an integrated pile group.

  3. Pile Load Calculations. For simple single-story situations it is often workable to calculate a worst-case scenario pile load and apply that to all piles as an additional factor of safety. For two-story situations it is almost always worth the time to calculate the load on each pile individually for a more cost-effective design. It is especially important to locate point loads for two-story situations and to avoid applying such high worst-case loading conditions to all piles.

  4. Anchor Load Calculations. Helical anchors are frequently used to support lateral loads in foundations. Often the skin friction of the grade-beam is adequate by itself or will significantly reduce the lateral loads on any helical anchors. If the near surface soils are unreliable for skin friction, the lateral loads required of the anchor may be higher. The helical anchors are typically installed 30 degrees from horizontal, subject to adjustment influenced by site topography and soils conditions. It is advisable to state an allowable range such as 30 degrees plus or minus 5 degrees. The tighter the angle, the less the resultant force, and the lower the required capacity and required installation torque of the tieback. Additionally, it is advisable to place the helical anchor in line with the lateral loads resisted by that foundation line instead of at an angle or orthogonal to that foundation line. This will reduce the load on the anchor further. For hillside applications, anchors aligned orthogonal to the foundation line may be necessary.

  5. Anchor Placement and Design. Depending on the direction of the horizontal force, the resultant force in the tieback can cause an uplift force or an additional downward force on the foundation, so any diagonal helical anchor must be close to a vertical helical pile to handle this load. To facilitate the avoidance of property lines and installation obstacles, or to move to a new location if the initial placement fails, we recommend the plan include language such as “anchors may be installed anywhere along the line it is anchoring providing the pile cap of the anchor is within two feet of a helical pile.”

  6. Retaining Wall Anchors. Helical anchors are frequently used in the construction of retaining walls to reduce the load on the soldier pile or cantilevered footing. In the case of a shotcrete plate, such as a landslide remediation plan, the anchors take the full lateral load. In this case, the helical anchor or helical soil nail is typically installed between 5 and 30 degrees off horizontal depending upon the slope behind the wall or plate and the embedment desired.

  7. Retaining Wall Piles. Helical members can be combined as the sole method of support for retaining walls in limited access sites with poor soils. Three helical members are used: (i) a vertical helical pile at the heel, (ii) an additional vertical helical anchor at the toe to resist overturning, (iii) a helical anchor driven into the hillside near the base of the wall or at footing elevation to resist sliding.

  8. Lifting Bracket vs. Pile Cap. Helical manufacturers produce brackets which bolt onto existing foundations. These are primarily for lifting sunken foundations back to their prior elevation. Where helical piles are employed to fortify an existing foundation in preparation for additional loads (such as adding a second story to an existing one-story home) we recommend use of the helical manufacturer’s new construction pile cap, with rebar and concrete, for a superior and less expensive connection.

  9. Plate Configuration. It is generally unnecessary for the design engineer to calculate and specify the helical plate configuration. It is the Contractor’s responsibility to achieve the design capacities. The engineering department of the helical manufacturer is available to provide recommendations concerning pile and anchor design. As site soils may vary from one helical pile location to another, and may differ from the soils borings, field adjustments may be necessary. As an example, the Contractor may be required to use a narrower configuration to achieve the required depth without over-torquing the helical pile or anchor.

  10. Liquefaction. Problems with liquefiable soils can often be overcome by increasing shaft diameter and shaft wall thickness. The shaft must be analyzed for buckling over the unbraced length of soil liquefaction, which will reduce the axial capacity of the pile. The manufacturer’s engineering department can provide calculations and analysis for Euler buckling. Since the pile capacity is reduced as the square of the size of the liquefiable zone, if the soils have a very large liquefiable layer (about 20 feet or more), even very large and stout helical members may not be well suited and a redesign may be required. For layers about 10 to 20 feet, larger or thicker round-shaft helical members will often do the trick. For soils with layers up to about 10 feet of liquefaction, this will often simply rule out the use of square-shaft members in favor of more stout round-shaft members.

C. 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.

Avalon Structural, Inc.
181 Ridgeview Drive
Aptos, CA 95003
(831) 479-4389 (office)
info@avalonstructural.com

CA License 677116
Classifications: B, C-8
Avalon is a general building
contracting firm and
does not employ engineers.