Transmission Tower Deep Concrete Foundation in Soft Clay

[ClarksonEng]

An outside engineer firm is proposing a 10' diameter by 49' drilled shaft in soft clay soil. Using the geotechnical report, the axial load resistance is 266 kips. The pole supported has an axial load of 58 kips, but the foundation alone weighs 577 kips. This raised some flags for our engineering staff so we asked they provided supporting calculations, they answered with this:

"Note that the mass of concrete is displacing the in situ soil that will be removed and therefore only the net increase in mass need be considered. This net difference plus the axial load imparted by the pole can resisted by skin friction."

Is this an acceptable assumption? Other resources I've been able to find are for piles, and don't specify the type or diameter.

 

[jwkilgore]

Yes, that is a reasonable assumption, assuming that's what the geotech report allows. If 266k were the total capacity the soil couldn't support its own weight.

The weight of soil displaced by a 10' diameter by 49' drilled shaft (assuming 110pcf, no water table, etc., so probably wrong) is 423kips. The difference is 154kips. Add in the 58kips for the pole for a total of 212kips, which is less than 266k. This is assuming that the 266k is allowable capacity, with safety factors already built in. If it's ultimate capacity, then you'll have to apply a dead-load factor to the 154k.

 

[Lomarandil]

jwk is right on -- in nearly all geotechnical reports, the allowable foundation load will be presented not including the weight of the foundation (below grade) -- precisely because the geotech doesn't know the final dimensions of the foundation to be used, and doesn't need to know.

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The name is a long story -- just call me Lo.