Transmission And Substation Foundations - Technical Design Manual

A Basic Guideline For Designers APPENDIX C: HELICAL PILES & ANCHORS

correlations with installation torque. Helical piles, whether they are square shaft or pipe shaft, are generally considered to be slender members. 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.

be used to resist lateral loads by components of the axial capacity on the battered pile. The induced shear and moment in battered piles often dictates the shaft size and batter angle. If a geotechnical report is not available, the lateral capacity of a vertical shaft must be determined from load tests per Appendix B of the TDM [Method 3]. P4 SHAFT Type and Size: The shaft type/size 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 for helical piles. • 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 relatively 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 buckling, square shaft piles are suitable for compressive loads. As a general rule, if the soil profile has ASTM D1586 SPT N60 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. 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 Pulldown Micropile as described in the following sections. 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. • Type 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 in Table C-1). 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 documented”

Table C-1 Product Series

Description

Evaluated Per Ac358 Kt

SS125

1.25” Round Cornered Square Bar

10

SS5

1.50” Round Cornered Square Bar

Yes

10

SS150 SS175 SS200 SS225

1.50” Round Cornered Square Bar 1.75” Round Cornered Square Bar 2.00” Round Cornered Square Bar 2.25” Round Cornered Square Bar 2.875” OD, 0.203 Wall Pipe 2.875” OD, 0.276 Wall Pipe 3.500” OD, 0.300 Wall Pipe 4.500” OD, 0.237 Wall Pipe 4.500” OD, 0.337 Wall Pipe 6.625” OD, Varying Wall Pipe 8.625” OD, Varying Wall Pipe 10.750” OD, Varying Wall Pipe

10 10 10 10

Yes

RS2875.203 RS2875.276 RS3500.300 RS4500.237 RS4500.337

Yes Yes Yes

9 9

7

6

Yes

5.6

RS6625 RS8625 RS1075

5 4

2-3

• 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 lateral stability and/or buckling resistance, or when lateral load resistance is required. Another advantage provided by combo piles is the torque correlation factor (Kt) is increased compared to the straight pipe shaft pile per Table C-2 and Table C-3. Note as the overall shaft length increases, the Kt factor decreases.

Table C-2 - Combo Pile Length Less than 30’-0 COMBO PILE TYPE K t , SAND

K t , CLAY

K t , COMBINED

SS5/150/RS2875

10

9.5

10

SS175/RS3500

9.5

9

9

SS200/RS3500

9.5 7.5 9.5

9 7

9 7

SS200/225/RS4500

SS175/RS2875

9.5

9.5

C-4 | www.hubbell.com/hubbellpowersystems