Chance Technical Design Manual

CHANCE® HELICAL ANCHOR SELECTION • Wall height ≥ 15 ft; use two rows of tiebacks

TIEBACK SPACING Spacing N = (Q tN / FS) / DL N = (51,600 / 2) / (4,025) = 6.4 ft Spacing M = (Q tM / FS) / DL M = (84,640 / 2) / (6,040) = 7.0 ft (use 6’-6” center to center spacing for both rows of tiebacks) where FS = 2.0

• Try Type SS150 series, C1500169 (8”-10”-12” lead) for DL N . • Try Type SS175 series, C1100247 (8”-10”-12”-14” lead) for DL M . ULTIMATE TENSION CAPACITY (USING BEARING CAPACITY APPROACH) EQUATION 8-29 Q tN = ( A 8 + A 10 + A 12 ) x (c N c ) Q tN = (0.336 + 0.531 + 0.771) x 3,500 x 9 Q tN = 51,600 lb where A 8 , A 10 , A 12 = Projected area of helical plates (8”,10”, and 12”) N c = Bearing capacity factor related to the residual soil, clay A 8 = 0.336 ft 2 A 10 = 0.531 ft 2 A 12 = 0.771 ft 2 N c = 9 c = N / 8 = 28 / 8 = 3.5 ksf or 3,500 psf (see Equation 5-41) Q tM = ( A 8 + A 10 + A 12 + A 14 ) x (cN c ) Q tM = (0.336 + 0.531 + 0.771+ 1.049) x 3,500 x 9 Q tM = 84,640 lb where A 8 , A 10 , A 12 , A 14 = Projected area of helical plates (8”,10”,12”, and 14”) A 14 = 1.049 ft 2 CHECK ULTIMATE ANCHOR CAPACITY (T U ) Compare Q tN and Q tM to field load tension tests if required by specifications. STRUCTURE TYPE • Cast concrete retaining wall • Granular backfill for wall j = 35° g = 120 pcf • Height = 15 ft, thickness =1-1/2 ft • Anchor Height = H/3 = 5 ft • Residual soils: silty coarse sand; medium to dense j = 31° g = 118 pcf. No ground water table present. • Tieback installation angle = 25° EQUATION 8-30

ESTIMATE INSTALLATION TORQUE

EQUATION 8-31

T = (DL x Spacing x FS) / K t

T N = (DL N x Spacing N x FS) / K t = (4,025 x 6.5 x 2) / 10 = 5,300 ft∙lb T M = (DL M x Spacing M x FS) / K t = (6,040 x 6.5 x 2) / 10 = 7,850 ft∙lb

DESIGN EXAMPLES

where

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

CHECK INSTALLATION TORQUE RATINGS The rated installation torque of the Type SS150 series is 7,000 ft∙lb, which is greater than the required installation torque (T N ) of 5,300 ft∙lb. The rated installation torque of the Type SS175 series is 10,500 ft∙lb, which is greater than the required installation torque (T M ) of 7,850 ft∙lb. MINIMUM TIEBACK LENGTH The distance from the assumed “active” failure plane to the 12” helix must be at least 5 x its diameter or 5’-0. The distance from the assumed “active” failure plane to the 14” helix must be at least 5 x its diameter or 6’-0. Both the minimum length and estimated installation torque must be satisfied prior to the termination of tieback installation.

DESIGN EXAMPLE 9: HELICAL TIEBACK ANCHORS IN SAND

• K a = (1 - sin( j )) / (1 + sin( j )) = 0.27 • DL/ft = (1/2 g H 2 K a ) / cos(25°) • = 4,000 lb/lin ft • Assume tieback carries 80%; therefore, DL t /ft = 0.80 x 4,000 = 3,200 lb/lin ft •

= [1/2 (120) (15) 2 (0.27)] / cos(25°)

CHANCE HELICAL ANCHOR SELECTION • Wall height ≤ 15 ft; use single row of tiebacks • Try Type SS5 series, C1500007 (8”-10”-12” Lead)

STRUCTURAL DESIGN LOADS • Use backfill j = 35°

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