![]() ![]() The International Building Code specifies that non-fluid soil provides enough lateral bracing to prevent buckling modes of failure. ![]() Similar to most other deep foundation systems, helical piles typically do not need to be considered in buckling failure, unless placed in fluid conditions. For piles with shaft diameters between 4.5 inches to 10.8 inches, x is equal to 0.56, meaning that piles with larger shaft diameters tend to deflect less (Perko 2009). x is a fitting constant, equal to 0.37 for piles with shaft diameters between 2.875 inches to 3.5 inches. ![]() Where λ δ is a fitting constant of 4.33 inches, and N 70 is SPT blow count with an energy ratio of 70. The deflection, δ, of helical piles in compression is given by the best-fit power function The remaining terms remain the same as that described in the individual bearing equation (Perko 2009). Where A 1 is the area of the lowermost bearing plate, T is the soil shear strength, L is length of the mobilized soil between the uppermost and lowermost bearing plates, and D is the diameter of the bearing plates. Following this methodology, the equation is given by q ult can be calculated from a variety of methods, like the Terzaghi, Meyerhof, or Hansen and Vesic equations (Perko 2009).įor cylindrical shearing, the ultimate bearing capacity is calculated from the sum of the bearing force applied to the bottom bearing plate, the shear force on the mobilized soil between the bearing plates, and the adhesion resistance on the shaft above the uppermost bearing plate. Where A n is the area of the nth plate, q ult is the ultimate bearing pressure of the soil, α is the adhesion between the soil and the shaft, H is the distance from the uppermost bearing plate to the top of the shaft, and d is the diameter of the shaft. The soil supporting the helical piles can fail in bearing in two fashions: either the soils fail locally at each plate (individual bearing) or the soils between plates fail and move as a group (cylindrical shearing), as seen in Figure 2.įor individual bearing, the ultimate bearing capacity, P u, is the sum of the bearing capacity on the plates plus the adhesion resistance on the shaft above the uppermost bearing plate. Fig 2: Compression failure modes (Perko 2009) ![]()
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