Tower Foundation on weak soils 2

[boo1]

An unguided 20 meter tower is to be erected on high water table soils with high organics. The tower is for a privately owned ham radio. The manufacture provided a foundation 100 mph design for dry soils min 2500 psf.

The 3 sec peak in our are is 120 mph.
What is the high water, organic soils foundation design impact? Any links?

 

[MikeDB]

In sandy soil, the capacity can be reduced up to 50% for water table at the ground line. In cohesive soils the impact will be less, but soil unit weight should be reduced. I assume no soil boring info is available. I would ignore the top 2 feet of soil and would hate to use more than 2000 psf without further info on the actual condition.

 

[RDK]

This is a high risk installation.

I would be very concerned about the overturning moment due to wind loading especially with the high water table conditions. Drive a stake into dry ground, push it over. Drive the same stake into the same ground and add water. Push it over. See the difference?

MikeDB's comments refer to the ability of the soil to resist vertical load, resistance to overturning will be less because the soil can move out of the way of the rotating foundation.

I would get the specific advice of a geo-technical engineer familiar with the area. If these soil conditions are common in the local area soil borings might not be needed. If bore logs are needed they can be expensive. The travel time for a rig and personnel to come to site can be a significant portion of the cost. (It sounds like a personal installation. Try trading professional services to keep costs down.)

I’d also check with the tower manufacturer about the ability of this tower to withstand the higher wind loading. No use in having a good foundation with a bent up tower on top of it.

If they gave a typical foundation design, it will most likely have a disclaimer that if special conditions warrant you should get professional advice. This is special conditions. (If it was good soil conditions and the tower fell over, the manufacturer would claim that this proved special conditions and he was not liable.)
Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

http://www.kitsonengineering.com

 

[boo1]

Thanks Mike and RKD, my friend is planning a tower install in his back yard (no permit or $$$ for a geo study). I know the conditions varied from ideal conditions. Looking I'm trying to help him beefup the foundation, with the tips from u guys.

Cheers Boo1