Transmission And Substation Foundations - Technical Design Manual

SECTION 7: DESIGN EXAMPLES

Design Example 3 When looking at the GROUP® results, the important things to look for in Figure 7-10 are: Is the resultant deflection greater than acceptable and is the moment greater than allowable. For a pulldown pile, the acceptable moment is either 0.6 Mp of the casing or the cracking moment. In this case, the cracking moment is 10.6 kip-in.

The minimum depth of grout for the piles is where the resultant shear becomes very close to 0 (less than 50 lbs). This is the location where the square shaft no longer requires the grout column (assuming the soil is greater than 4 blow materials). The minimum depth of the grout column in this example is 16 ft. The minimum depth of casing is the deepest depth at which the resultant moment is equal to the cracking moment of the uncased shaft. In this case the minimum cased depth is 14 ft.

GROUP® Results Figure 7 -11

From Figure 7-11, the important thing to look for is that the axial capacity of the pile (from the T-Z curve) is greater than or equal to 2 times the axial reactions (for an axial FS of 2.0). In this case the 85 kips from the T-Z curve is greater than 40*2= 80 kips.

Since the T-Z curve was changed during design, the Helicap® run needs to be run again to get an 85 kip pile. Here are the results. (see Figure 7 -9b)

7-14 | www.hubbell.com/hubbellpowersystems