Transmission And Substation Foundations - Technical Design Manual (TD06088E)

• Type 1 - Square Shaft: Square shaft 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, meaning they have rela- tively low section properties, but relatively large cross-sectional area since they are solid bars. They are more efficient than pipe shaft helical piles in regards to axial capacity derived from installation energy. A square shaft helical pile will have 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 piles have slender cross sections. Therefore, they do not have a large cross section to resist much lateral load via passive earth pressure along the side of the shaft. In addition, they do not have much section modulus/ stiffness to resist buckling under compressive loads without support from the surrounding soil. As long as there is sufficient soil confinement around the pile to prevent buck- ling, square shaft piles are suitable for compressive loads. As a general rule, if the soil profile has ASTM D-1586 SPT N 60 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 N 60 values are 4 or less, then square shaft buckling may be a practical concern. A rigorous analysis can be done if enough reliable soil data is available, but the problem is best solved by selecting either a pipe shaft or Helical PULL- DOWN Micropile as described in the following sections. The designer is encouraged to use square shaft helical piles as much as possible due to their advan- tages with torque correlation efficiency and better penetration in dense soil. • T ype 2 – Pipe Shaft: Pipe shaft piles are foundation elements that range in size from 2-7/8” OD pipe shaft to 10-3/4” OD pipe shaft with various wall thicknesses and material strengths. Pipe shaft piles have larger section properties 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 square shaft helical piles offer greater axial capacity for a given amount of installation energy due to their greater efficiency (see the torque correlation table C-1 below). 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 evaluated per ICC-ES AC358 comply with the requirement of International Building Code (IBC) Section 1810.3.3.1.9 for the use of “well docu- mented” correlations with installation torque. Helical piles, whether they are square shaft or pipe shaft, are generally considered to be slender mem- bers. 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, helical pile shafts have relatively small effective projected area for the soil to bear against. Therefore, helical piles with shaft diameter ≤ 4” have about 4 kip lateral resistance; shaft diameters ≤ 8” have about 10 kip lateral resistance; and shaft diameters ≤ 10” have about 20 kip 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.

HELICAL PILES AND ANCHORS

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