Chance Technical Design Manual

DESIGN EXAMPLE 6: HELICAL PILES FOR BOARDWALKS

SOILS A helical pile foundation is proposed to support a pedestrian walkway. The soil profile consists of 7’-0 (2.1 m) of very soft clay with a reported Standard Penetration Test (SPT) blow count “N” equal to weight of hammer (WOH) and a unit weight of 65 lb/ft 3 (10.2 kN/m 3 ). Below the very soft clay is a thick layer of medium-dense sand with a SPT blow count value of 17. The correlated friction angle is 32° and the unit weight is 107 lb/ft 3 (16.8 kN/m 3 ). The water table is located at the surface. The pro posed helical pile is connected to the walkway with a Chance® Walkway Support Bracket. The helical piles must be checked for lateral stability in the very soft clay. WALKWAY • The helical piles are spaced 5 ft (1.5 m) apart and are ex posed 2 ft (0.61 m) above grade as shown in Figure 8-5. • The walkway is 7 ft (2.1 m) wide; each pile group or “bent” is spaced 10’-0 apart.

STRUCTURAL LOADS • The dead and live vertical load is 100 lb/ft 2 (4.8 kN/m 2 ). Lateral loads are negligible. • The required compression load per helical pile (P w ) is 100 lb/ft 2 x 7’-0 x 10’-0 = 7000 lb/2 helical piles = 3500 lb (15.6 kN) per pile. • Using a Factor of Safety (FS) of 2, the required ultimate capacity (UC r ) per helical pile is 3500 lb x 2 = 7000 lb (31.1 kN). CHANCE HELICAL PILE SELECTION • Try a twin-helix configuration with 10” (254 mm) and 12” (305 mm) diameters. • Try either Type SS5 1-1/2” (38 mm) Square Shaft or Type RS2875.203 2-7/8” (73 mm) Round Shaft material. ULTIMATE PILE CAPACITY The top-most helix should be at least three diameters into a suitable bearing soil; which in this example is the medium dense sand starting 7 ft (2.1 m) below grade. The spacing be tween helix plates is also three diameters; which is 3 x 10” = 2.5 ft (0.8 m) for a 10”-12” (254 mm – 305 mm) configuration. Finally, the distance from the bottom-most helix to the pile tip is 0.5 ft (0.15 m). Therefore, the minimum overall length for a 10”-12” helix configuration in this soil profile is 7 ft + (3 x 12 inch) + 2.5 ft + 0.5 ft = 13 ft (4 m). The effective unit weight is the submerged unit weight in this case, because the water table is at the ground surface. The general bearing capacity equation (simplified for cohesionless soils) is: EQUATION 8-20 Q h = AD g ’N q Q h = Ultimate capacity of helix plate A = Projected area of helix plate D = Vertical depth to helix plate g ’ = Effective unit weight of soil = 2.6 lb/ft 3 (0.4 kN/ m 3 ) for the very soft clay and 44.6 lb/ft 3 (7.1 kN/ m 3 ) for the medium-dense sand N q = Bearing capacity factor for cohesionless soils = 17 for 32° sand For a 10”-12” configuration, the bearing capacity equation is: EQUATION 8-21 S Q h = A 10 D 10 g ’N q + A 12 D 12 g ’N q S Q h = 0.531 ft 2 [(7 ft x 2.6 lb/ft 3 ) + (5.5 ft x 44.6 lb/ ft 3 )]17 + 0.771 ft 2 [(7 ft x 2.6 lb/ft 3 ) + (3 ft x 44.6 lb/ft 3 )]17 S Q h = 4371 lb (19.4 kN) where

DESIGN EXAMPLES

7 ft (2.1 m)

100 lb/ft 2 (4.8 kN/m) Dead load and Live load

2 ft (0.61 m) Above Grade

Very Soft Clay 0-7 ft (2.1m) SPT N=WOH Cohesion = 0 Unit weight = 65 pcf (10.2 kN/m 3 )

15.5 ft (4.7 m) Below Grade

5 ft (1.5 m) Spacing

Medium-Dense Sand 7+ ft (2.1+m) SPT N=17 Friction Angle = 32“ Unit weight = 107 pcf (17 kN/m 3 )

HELICAL PILES FOR BOARDWALKS FIGURE 8-5

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