Chance Technical Design Manual

ULTIMATE TENSION CAPACITY (USING BEARING CAPACITY APPROACH) EQUATION 8-32 Q t = ( A 8 x q 8 + A 10 x q 10 + A 12 x q 12 ) x (N q ) A 8 , A 10 , A 12 = Projected area of helical plates (8”, 10” and 12”) N q = Bearing capacity factor related to j of residual soil (31°) A 8 = 0.336 ft 2 A 10 = 0.531 ft 2 A 12 = 0.771 ft 2 N q = 15 (from Equation 5-20) q h = g x D h (depth of helix below ground line, ft) q 8 = 118 pcf (5’ + 25’ sin(25°)) = 1836 psf q 10 = 118 pcf (5’ + 23’ sin(25°)) = 1736 psf q 12 = 118 pcf (5’ + 20.5’ sin(25°)) = 1612 psf Q t = [(0.336 x 1836) + (0.531 x 1736) + (0.771 x 1612)] x 15 Q t = 41,725 lb CHECK ULTIMATE ANCHOR CAPACITY (T U ) Compare Q t to field load tension tests if required by specifications.

1 ½’

⅓ H = 5’

25°

10’ Dia

8’ Dia

H = 15’

12’ Dia

L = 25’

D = 2’

6’

HELICAL TIEBACK ANCHOR FIGURE 8-8

DESIGN EXAMPLES

SOIL BORING LOG

USCS Symbol

SPT - N Blows/ft

Graphic Log Soil Classification Depth

TIEBACK SPACING

Topsoil

OH

EQUATION 8-33

5

17

Spacing N = (Q t / FS) / DL t

= (41,725 / 2) / (3,200) = 6.5 ft (use 6’-6 center to center spacing)

Silty Sand

SM

where FS = 2.0

10

30

ESTIMATE INSTALLATION TORQUE

EQUATION 8-34

Silty Coarse Sand g = 118 pcf ϕ = 31°

15

SM

32

T = (DL t x Spacing x FS) / K t = (3,200 x 6.5 x 2.0) / 10 = 4,200 ft∙lb

where

20

34

K t = Empirical torque factor (default value = 10 for Type SS series)

SOIL BORING LOG FIGURE 8-9

CHECK INSTALLATION TORQUE RATINGS The rated installation torque of the Type SS5 series is 5,700 ft∙lb, which is greater than the required installation torque (T) of 4,200 ft∙lb. MINIMUM TIEBACK LENGTH The distance from the assumed “active” failure plane to the 12” helix must be at least 5 times its diameter or 5’-0. Both the minimum length and estimated installation torque must be sat isfied prior to the termination of tieback installation.

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