Transmission And Substation Foundations - Technical Design Manual (TD06088E)

P4 Other Considerations: There are several design considerations that should be taken into account when choosing the required shaft type. This is often the most important aspect of specifying a helical pile and too often receives the least amount of attention prior to installation. 1. Is the shaft section sufficient to carry the intended axial load? This will have a great deal to do with the selection of the shaft type. Refer to Table 7-4 of the TDM as a good place to start. It lists torque cor- related capacities for shaft diameters up to 4.5” OD [Method 2]. Large diameter pipe shaft ( ≥ 6”) and PULLDOWN Piles can achieve higher capacities than those listed in Table 7-4. Allowable load upper limit for CHANCE helical piles up to 10” nominal diameter is 100 ton. Tension capacity is controlled by the structural strength of the couplings as detailed in P2 below. 2. The helix plates must generate the downward thrust required to advance the shaft through the soil. Helical piles (i.e. screw piles) are displacement piles that have the advantage of no spoils. The soil that is displaced by the shaft during installation is displaced to the side. The smaller the shaft size relative to the diameter of the helical plates (higher aspect ratio), the more efficient the pile will be in regards to capacity derived from the same installation energy. A helical pile that has a smaller shaft size rela- tive to the size of the helical plates will be better at penetrating dense soil than one with a larger shaft size relative to the size of the helical plates (lower aspect ratio). Displacing more soil will require more installation energy, i.e. additional installation torque and down pressure. The greater the installation en- ergy, the larger the required equipment to install the pile. For example, a 25 ton allowable load square shaft helical pile can be installed with a mini-excavator or skid-steer. However, an 8” diameter pipe shaft helical pile requires a 20 to 25 ton track-hoe excavator. 3. If a soil stratum is too dense, or the shaft too large relative to the size of the helix plates, the pile could “spin-out”. “Spin-out” means that the pile is still being rotated but is not advancing, and installation torque drops dramatically. This is similar to “stripping” a screw. The capacity-to-torque correlation is no longer valid for spun-out piles. (Note: see Section 6 – Installation Methodology of the TDM for a complete explanation of torque correlation for helical anchors and piles). A spun-out pile is just an end bearing pile that was advanced to depth via a screw mechanism. This does not mean that the pile has no capacity, but rather that the capacity cannot be estimated by torque correlation as is normally done for a normally installed helical pile. The pile’s capacity will depend on the type of material the helical plate(s) are in, how much the soil was disturbed, and whether or not the shaft tip, or pilot point, contributes to the capacity in end bearing. High capacities can be possible if the shaft tip is sitting on rock. 4. Lateral resistance requires either pipe shaft or Helical PULLDOWN Micropiles. A Helical PULLDOWN Micropile with a steel casing at the top of the pile will offer the stiffest pile section and the most resis- tance to lateral loads. Lateral capacity ranges from 2 to 4 kip for 3” to 4” diameter piles, 10 kip for 6” to 8” diameter helical piles, and up to 20 kip for 10” diameter piles at allowable lateral displacements of 1” or less. The use of battered (inclined) piles can be utilized to resist lateral loads if needed and are discussed in Section 5 of the TDM. 5. For tension only foundation elements, square shaft is always the logical choice. As noted above, square shaft helical anchors are more efficient in regards to load capacity versus installation energy (torque correlation), are better at penetrating dense soils, and have less surface area for corrosion potential. The size and strength of the square shaft section is governed by the required installation torque, not the tension capacity. There is more steel section available than is required to carry the rated axial ten- sion load. The reason for this is because the steel in the shaft is subjected to more stress during instal- lation than it will ever see while in service. Once the helical anchor is installed, the tension strength is governed by the shear strength of the coupling bolt – see Section 7 of the TDM.

HELICAL PILES AND ANCHORS

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