Chance Technical Design Manual

ten the most important aspect of specifying a helical pile and too often receives the least amount of attention prior to instal lation. 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 correlated capacities for shaft diameters up to 4.5” OD [Method 2]. Large diameter pipe shaft ( ≥ 6”) and Pulldown Piles can achieve higher capaci ties 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 re quired to advance the shaft through the soil. Helical piles (i.e. screw piles) are displacement piles that have the ad vantage 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 en ergy. A helical pile that has a smaller shaft size relative 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. addition al installation torque and down pressure. The greater the installation energy, 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 rela tive 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 dra matically. This is similar to “stripping” a screw. The capac ity-to-torque correlation is no longer valid for spun-out piles.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 ca pacity, 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 pi lot 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 Pull down Micropiles. A Helical Pulldown Micropile with a steel casing at the top of the pile will offer the stiffest pile sec tion and the most resistance to lateral loads. Lateral ca pacity 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”

• 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 advan tage provided by combo piles is the torque correlation factor (K t ) 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 K t 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 9 9 SS200/225/RS4500 7.5 7 7 SS175/RS2875 9.5 9.5 9.5 TABLE C-3 - COMBO PILE LENGTH GREATER THAN 30’-0 COMBO PILE TYPE K t , SAND K t , CLAY K t , COMBINED SS5/150/RS2875 9.5 9.5 9.5 SS175/RS3500 9 8.5 8.5 SS200/RS3500 8.5 8 8 SS200/225/RS4500 7 7 7 SS175/RS2875 9.5 9.5 9.5 • Type 4 - A Helical Pulldown Micropile is a helical pile that has the shaft section encased in a small diameter grout column, typically 5” – 7” in diameter. Both square shaft and pipe shaft helical piles can be encased in a grout col umn, 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 suit able 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 Pulldown Micropiles 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 resistance. P4 OTHER CONSIDERATIONS: There are several design considerations that should be taken into account when choosing the required shaft type. This is of

HELICAL PILES & ANCHORS

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