Chance Technical Design Manual

Bearing Capacity Factor Curve for Cohesionless Soils

100

90

80

70

60

50

(N q )

40

Bearing Capacity Factor

30

20

10

0

10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 Angle of Internal Friction (degrees)

RECOMMENDED BEARING CAPACITY FACTOR (N q ) FOR DEEP HELICAL PILES/ANCHORS IN SAND FIGURE 5-8

5.2.3 MULTI-HELIX HELICAL PILES AND ANCHORS—DEEP INSTALLATION The ultimate capacity of deep multi-helix helical piles and an chors depends on the geometry of the helical section, namely the size and number of helical plates and the spacing between the plates. As shown in Figures 5-3b and 5-3d, if the spacing of helix plates is close, the helix plates interact with each other. The capacity is developed along the failure surface of the pe rimeter shear zone and by end bearing of the end helix plate (the bottom plate for compression loading or the top plate for tension loading). If the spacing of the helix plates is adequate, the capacity is the sum of the capacities developed by the in dividual helix plates, as shown in Figures 5-3a and 5-3c. There is no interaction between helix plates, and no capacity is devel oped along the shaft between the helix plates. In the US, most manufacturers of helical piles and anchors use a standard helix spacing of 3 times the helix diameter. This spacing was originally used in Chance® products over 35 years ago and is assumed to allow individual helix plates to develop full capacity with no interaction between helix plates. Most Chance helical piles and anchors use interhelix spacing that is based on the diameter of the lower helix. For example, the distance between a 10-inch (254 mm) and a 12-inch (305 mm) helix is three times the diameter of the lower helix, or 10 x 3 = 30 inches (762 mm). The first section, called the lead or starter, contains the helix plates. A lead section typically includes up to four helix plates. Additional helix plates can be added, if required, with the use of helical extensions. Standard helix sizes and projected

REPORTED VALUES OF BEARING CAPACITY FACTOR (N q ) FOR DEEP FOUNDATIONS IN SANDS [WINTERKORN & FANG (1983)] FIGURE 5-7

saturated sands, the cohesion is normally considered to be zero, reducing the ultimate uplift capacity to: Q ultU = A h (q’N q + 0.5 g ’BN g ) Also, because the area of the plate is small, the contribution of the width term to ultimate capacity is very small and the width term is often ignored, leaving: 5.2.2.2.c Mixed Soils ( φ ’ > 0; c > 0) The ultimate capacity of a deep helical anchor in mixed-grain soils can be calculated from traditional bearing capacity theory using Equation 5-12: Q ultU = A h (cN c + q’N q + 0.5 g ’BN g ) Note: The term 0.5 g ’BN g is typically ignored for helical anchors because the helix plate is small. Q ultU = A h q’N q

DESIGN METHODOLOGY

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