Helical Pile Shaft Optimization

The shaft type/size of a helical pile is critical to both the axial and lateral capacity – especially for compression in soft/loose overburden soils where lateral stability of the shaft must be considered.  The following is a brief summary of the 4 different shaft types commonly used for helical piles and their relative advantages and disadvantages based on site conditions and application.  It is very important to understand that helical pile installation must be considered in the design process.

Type 1 - Square Shaft 
Square shaft helical piles are foundation elements that range in size from 1-1/2” solid round-cornered-square (RCS) to 2-1/4” solid RCS.  They are compact sections but have relatively large cross-sectional areas since they are solid bars.  They are more efficient than pipe shaft helical piles in regard to axial capacity derived from installation energy.   A square shaft helical pile provides more axial capacity than a pipe shaft helical pile installed with the same amount of torsional energy into the same soil profile.  Therefore, square shaft helical piles are better at penetrating dense material than pipe shaft helical piles.

Square shaft disadvantage is their slender cross sections. They do not have a large cross section to resist lateral load via passive earth pressure along the side of the shaft.  In addition, their low section modulus/axial stiffness reduces their ability to resist buckling under compressive loads in soft/loose soils.  As a general rule, if the soil profile has an ASTM D-1586 SPT N60 blow count value of 5 or greater, there is sufficient lateral support to prevent the square shafts from buckling at the compressive loads that they are rated for.  If SPT N60 values are 4 or less, then square shaft buckling may be a practical concern.  The designer is encouraged to use square shaft helical piles as much as possible due to their advantages with torque correlation efficiency and better penetration in dense soil.  In addition, square shaft helical piles have the lowest cost per kip of capacity at load ranges 50 kip or less.

Some ask if square shaft helical piles should be used for compression loads – especially for foundation repair.  Seider (1993) – Eccentrically Loaded Helical Pier Systems and Hoyt, et al (1996) – Buckling of Helical Anchors Used for Underpinning demonstrated that square shaft helical piles are suitable for foundation repair – assuming they are properly designed and installed.

Type 2 - Pipe Shaft 
Pipe shaft helical piles (also known as round shaft piles) are foundation elements that range in size from 2-7/8” OD pipe shaft to greater than 8” OD pipe shaft with various wall thicknesses and material strengths.  Low displacement size range is 2-7/8” to 4/5” OD; medium displacement is > 4.5” to 8” OD; large displacement > 8” OD.  Pipe shaft piles have larger section properties and projected area compared to square shaft, so they are used to resist lateral load, or to provide stability when columnar buckling or potential unsupported length is a concern.   The designer may ask why not use pipe shaft helical piles exclusively?  The answer is because square shaft helical piles offer greater axial capacity for a given amount of installation energy due to their greater efficiency, i.e. lower cost per kip.  In addition, pipe shaft helical piles do not penetrate dense material as effectively as square shaft. Therefore, the designer must size the helical pile shaft large enough to transfer/resist all loads, but no larger than necessary.

Helical piles, whether they are square shaft or pipe shaft, are generally considered to be slender members, expect for pipe shafts > 8” OD.  The lateral capacity is dependent on the effective projected area of the pile shaft, the flexural stiffness of the pile, and the resistance of the soil as the pile deflects laterally under load. Due to their slender size, low and medium displacement helical pile shafts have relatively small effective projected area for the soil to bear against. Therefore, helical piles with shaft diameter ≤ 4.5” have about 6 kip lateral resistance; shaft diameters ≤ 8” have about 15 kip lateral resistance; and shaft diameters ≥ 8” have about 20 kip or greater lateral resistance at typical allowable lateral displacements of 1” or less.  As mentioned previously, square shaft helical piles don’t have any significant lateral capacity.

Type 3 - Combo Pile 
A combo pile (Combination Pile) is a compression helical pile that has the advantages of both square shaft and pipe shaft.  A combo pile has a square shaft lead section that is better at penetrating dense material and generating bearing capacity; and is then transitioned to a pipe shaft for the plain extensions where over- burden soils are softer/less dense and a larger section modulus is desired for buckling resistance; or when lateral load resistance is required.  Another advantage provided by combo piles is the torque correlation factor (K_t) is increased compared to the straight pipe shaft pile per the tables below.  Note as the overall shaft length increases, the Kt factor decreases.

Table 1 - Combo Pile Length Less than 30’-0

Table 2 - Combo Pile Length Greater than 30’-0

Type 4 – Helical Pulldown® Micropile 
Commonly known as a grouted shaft helical displacement pile, a Helical Pulldown Micropile has the shaft section encased in a small diameter grout column, typically 5” – 7” diameter. Both square shaft and pipe shaft helical piles can be encased in a grout column, but square shaft is much more common.  It has the advantage of the square shaft lead section to penetrate dense material for end-bearing. The added grout column provides greater section properties for shaft stability and lateral resistance in soft soils. Lateral load resistance with grouted shafts requires a steel case – typically extending 5’-0 to 10’-0 from the pile head. The grout in contact with the soil will develop side resistance via a bond zone in suitable soil stratum. This can greatly increase the total axial capacity of the pile (end-bearing and side resistance) as well as stiffen the axial load response of the pile. The grout column also provides additional corrosion protection to the steel shaft.

Grouted shaft helical piles are recommended for square shaft piles in soft soils, when additional capacity via side resistance is needed,  or when working loads exceed about 60 kip.  To-date, Helical Pulldown Micropiles have achieved 450 kip ultimate compression resistance.

The following table summarizes all the design factors to be considered for helical pile shaft optimization.  The shaft types marked with a “star” are the best choice.  The shaft types marked with a “checkmark” are good choices, but not the best choice.

To learn more, please refer to Appendix C of the CHANCE® Technical Design Manual – 4^th Edition.