Helical pile

A single helical pile (screw pile) with one or more helices on a round or square steel shaft, in compression and tension: the geotechnical capacity by the individual bearing and cylindrical shear methods, the installation torque from the capacity-to-torque ratio Kt, the shaft in compression with buckling in soft soil and in tension, the helix plates at the manufacturer's strength, and the IBC 2024 1810.3.3.1.9 limits. The methods follow the Iowa DOT Helical Pile Foundation Guide for Bridge Structures (TR-800, 2023), which draws on Perko (2009) and DFI (2019).

Inputs of the helical pile #

• Pile: round hollow shaft (outside diameter and wall) or square solid bar, Fy and E from the manufacturer's evaluation report, the corrosion loss per exposed face (ICC-ES AC358 sets it by exposure and design life), the depth of the lead helix, the helix diameters from the lead up, the helix spacing (2.4 to 3.6 helix diameters is common, about 3 is efficient) and whether the shaft adhesion above the top helix counts (often ignored).
• Soil: the water table and the layers top down: clay with its undrained strength su, sand with φ'.
• Loads: axial per load type, compression positive, combined by ASCE 7-22 2.4.1 (ASD) or 2.3.1 (LRFD).
• Capacity and torque: the factor of safety (IBC 1810.3.3.1.9: 2) or the engineer's resistance factor (no code value; DFI 2019 suggests driven-pile factors), Kt in 1/ft (ICC-ES AC358 defaults about 10 for 1-1/2 and 1-3/4 in. square, 9 for 2-7/8 in., 7 for 3-1/2 in., 6 for 4-1/2 in. round shafts, or the manufacturer's report), the shaft's installation torque rating, a recorded final torque and a load-test capacity when they exist, and the helix plate nominal strength with its φ or Ω.
• Shaft buckling: the horizontal modulus kh of the weakest soil the shaft passes, the unbraced length above firm soil (exposed, scoured, or N < 5) and K.

Auto-design varies the number of helices (the entered sizes, repeating the largest, up to six) and the lead depth (1-ft steps). See Auto-design.

Checks performed #

The drawing shows the pile in its soil profile with the helices at their depths, the depths and spacing dimensioned, and the shaft and helix sizes called out; its notes state the final installation torque.

Limitations #

• The bearing factors (Nc = 9, the deep Nq expression), the cylinder shear strength in sand (K0 = 1 − sinφ) and the buckling expressions are written from knowledge: the CHANCE Technical Design Manual, Perko (2009), DFI (2019), ICC-ES AC358 and IBC 2024 are not on disk (to confirm against the code text).
• No grouted shafts, battered piles or lateral capacity (use the laterally loaded pile types), no group effects (AC358 asks for 4 helix diameters between piles), couplings or cap brackets.
• Bearing on rock and torque correlation in rock are outside the methods.

Verification examples #

Verification status: 7 values compared in 5 examples (0 published, 5 hand calculations from the cited equations): 7 within tolerance, 0 recorded deviations. No published example: every value is a hand calculation. That checks the program against this reading of the code, not the reading itself. Code basis: partial. Only some provisions were read in the target edition; the rest rests on older editions, examples or summaries.

No published helical pile design example is on disk. Hand calculations from the equations (backend/tests/test_verify_helical_pile.py):