Chance Technical Design Manual

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 correla tion efficiency and better penetration in dense soil. • Type 2 – Pipe Shaft: Pipe shaft piles are foundation el ements 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 cor relation 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 docu mented” 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 ef fective 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 lat eral resistance at typical allowable lateral displacements of 1” or less. As mentioned previously, square shaft helical piles don’t have any significant lateral capacity.

The axial capacity can also be determined from full-scale load tests per Appendix B of the TDM [Method 3]. Full-scale tests are often used to verify Method 1 capacity and Method 2 torque correlation. If a geotechnical report is available, the lateral capacity of a vertical shaft can be determined with various methods includ ing the Finite Difference method (LPILE & GROUP by Ensoft®) and the Broms’ Method (1964a) and (1964b) as detailed in Sec tion 5 of the TDM [Method 1]. Each of these methods may be applied to Round Shaft helical piles or Pulldown® Micropiles. Lateral resistance can also be provided by passive earth pres sure against the structural elements of the foundation. The re sisting elements of the structure include the pile cap, grade beams and stem walls. The passive earth pressure against the structural elements can be calculated using the Rankine Meth od. Battered or inclined piles can 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 Appen dix B of the TDM [Method 3]. P4 SHAFT TYPE AND SIZE: The shaft type/size is critical to both the axial and lateral ca pacity – 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-cor nered-square (RCS) to 2-1/4” solid RCS. They are compact sections, meaning they have relatively low section prop erties, but relatively large cross-sectional area since they are solid bars. They are more efficient than pipe shaft heli cal piles in regards to axial capacity derived from installa tion 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 penetrat ing 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 addi tion, 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 suit able for compressive loads. As a general rule, if the soil profile has ASTM D1586 SPT N 60 value of 5 or greater, there is suffi cient lateral support to prevent the square shafts from buckling at the compressive loads that they are rated for. If SPT N 60 val ues 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

HELICAL PILES & ANCHORS

TABLE C-1 Product Series

Evaluated Per Ac358

Description

K t

SS125

1.25” Round Cornered Square Bar 1.50” Round Cornered Square Bar 1.50” Round Cornered Square Bar

10 10 10 10 10 10

SS5

Yes

SS150

SS175

1.75” Round Cornered Square Bar

Yes

SS200

2.00” Round Cornered Square Bar

SS225

2.25” Round Cornered Square Bar

RS2875.203 2.875” OD, 0.203 Wall Pipe RS2875.276 2.875” OD, 0.276 Wall Pipe RS3500.300 3.500” OD, 0.300 Wall Pipe RS4500.237 4.500” OD, 0.237 Wall Pipe RS4500.337 4.500” OD, 0.337 Wall Pipe

Yes Yes Yes

9 9

7

6

Yes

5.6

RS6625 RS8625

6.625” OD, Varying Wall Pipe 8.625” OD, Varying Wall Pipe 10.750” OD, Varying Wall Pipe

5

4

RS1075

2-3

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