Terms

Design Considerations

Like other deep foundation alternatives, there are many factors to be considered in designing a helical foundation. Supportworks recommends that helical pile and tieback design be completed by an experienced geotechnical engineer or other qualified design professional.

This program is intended to assist design professionals with determining capacity of axially and concentrically-loaded helical piles and tiebacks. For allowable mechanical capacities of side-load or retrofit bracket systems, please refer to the latest version of Supportworks' Technical Manual.

Although the program informs the User when low strength soils are present, it is the responsibility of the design professional to evaluate buckling potential or calculate the critical buckling load with any of several methods available. This allows the design professional to choose the method and the level of conservatism to best fit project conditions. Chapter 18 of the International Building Code states, “Any soil other than fluid soil shall be deemed to afford sufficient lateral support to prevent buckling of deep foundation elements and to permit the design of the elements in accordance with accepted engineering practice and the applicable provisions of this code.”

This program provides an estimate of capacity for a single pile or tieback without reductions due to close spacing. Appropriate reduction factors should be considered when center-to-center spacing of helical piles and tiebacks is less than three times the diameter of the largest helix plate at bearing depth.

This program estimates the ultimate helical pile or tieback capacity given the input parameters by the User. For standard, ungrouted helical piles and tiebacks, a minimum factor of safety of 2.0 is typically used in the determination of allowable capacity when torque is monitored during installation. With a factor of safety of 2.0, it is generally assumed for most soil conditions that net deflection of the pile or tieback (total deflection minus elastic compression/elongation) will be less than one-half inch at the nominal or service load. Field load tests typically show less movement.

The Helicast^™ Grouted Helical Pile System (Helicast pile) generates capacity through both end bearing of the helix plates and skin friction of a grout column above. For Helicast piles, a minimum factor of safety of 2.0 is typically used for determination of allowable end bearing capacity when torque is monitored during installation. A minimum factor of safety of 2.5 is recommended for determination of allowable skin friction resistance unless a load test is performed to verify pile capacity, whereas a minimum factor of safety of 2.0 may then also be considered for determination of allowable skin friction resistance. The available skin friction used to determine pile capacity should not exceed the nominal axial resistance of the grout column determined by generally accepted methods of analysis. When available skin friction is limited by this requirement, then the end bearing component at the helix plates should also be neglected in the calculated pile capacity.

The design professional may select higher or lower factors of safety based upon tolerable movements of the structure. Load testing may also be considered to determine actual deflections. Helical tiebacks are generally pre-tensioned to a specific load following installation to reduce anticipated deflections and to fully engage the soil surrounding the helix plates.

HelixPro will generate pile and tieback capacities even when some or all of the helix plates are embedded within weak or weight of hammer (WOH) soils. The User must evaluate the soil profile to identify appropriate pile and tieback depths and adequate bearing conditions above, below and surrounding the helix plates to prevent excessive movement under load. As a general guideline, the User should not design for conditions where all helix plates bear within medium stiff clay, loose sand, or weaker soils. Even if the program identifies that adequate capacity can be achieved, there would still be a significant risk of long-term creep movement. In some cases where marginal soils exist, a higher factor of safety may be considered to minimize the anticipated movement/settlement.