Transmission And Substation Foundations - Technical Design Manual

SECTION 7: DESIGN EXAMPLES

Design Example 8

Helical Pile Foundation for Remediation of Substation Bus Support

Soil Profile: Soil Description Silty Sand Loose γ=90lb/ft3 ɸ =27°

SPT -N Blows/ft

Depth ft.

0

4

Purpose This design example is intended to assist with the remediation design of a substation bus support that has settled. The plan of repair is to replace the central drilled concrete shaft beneath each leg with two type RS (Round Shaft) piles with a steel grade beam. The central concrete shaft will be demolioshed as well during pile install. This example will show how to calculate the axial capacity of the type RS piles and the lateral capacity of the type RS pile, using LPILE. After the loads for the structure have been determined, it is possible to design the piles. For this example the loading is as follows: Loads

Sand γ=120lb/ft3 Φ=30°

5

10

Note: Water Table = 0 FT

Figure 7-28

CHANCE® Helical Pile Selection RS6625.280 with a single 16” diameter helix

Lateral Capacity Analysis using LPILE Moment load about the longitudinal axis is assumed to be transferred to the piles in tension andcompression. The piles will require passive resistance of the resultant shear load of (1,200 2 + 1,200 2 ) 1/2 = 1,698 lbs with a maximum deflection under this load of 1/2 ” that is typical for substation structures. LPILE yields the following output for this shaft that is within the design parameters and requires a minimum embedment of 14 feet.

• Compression: 5,000 lbs • Tension: 2,000 lbs

• Shear Long Direction: 1,200 lbs • Shear Trans Direction: 1, 200 lbs • Moment About Long Axis: 10,000 ft-lbs • Moment About Trans Axis: 0 ft-lbs

Lateral Deflection (in)

LEG OF BUS SUPPORT

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

12 9 8 7 6 5 4 3 2 1 0 14 13

16” DIA

DEMOED CONCRETE FOUNDATION

Depth (ft)

Figure 7-28

Figure 7-29

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