[TowerTalk] Looking for some foundation advice

[jimlux]

On 5/27/20 9:52 PM, Mal Speer wrote:
> About 5yrs ago I put up a TX455. Actually I had it put up by a professional while I was still living up in NY. My QTH is in NC. When he tried to dig the hole they couldn't get down more than 4'. Instead of digging a 6' deep by 4' by4'. They dug a 6' by 6' by 4' deep. I am no engineer, but I don't think that is just as good. I don't think that it is the weight of the concrete that holds the tower up I think that it is the amount below ground compared to the amount above ground. I was pissed when I was told about this. If I was there I would have gone into the hole with a pick and shovel and dug another 2' into the solid clay we have here. I have to say though the tower has been through a few pretty good storms and one hurricane with no ill effects.

The base serves to prevent the tower from tipping over. There's two 
factors that get in here:
One is the weight of the base - the tower is a lever trying to tilt the 
base, so a heavier base requires more force to tip it.
(tipping over a wine bottle vs tipping over a plate laying flat on the 
table)


The other is the bearing area on the soil, which is more a function of 
surface area - as the base tries to tip, the top of the side in the 
direction of the tip is pushing against the soil, the bottom of the side 
away from the tip is pushing. And, if the base has large area, the 
bottom is pushing on the soil.

Narrow deep caissons (and telephone poles) depend on the bearing area 
and soil strength, like a straw standing in a thick milkshake


I suspect your 4 foot deep by 6by 6 foot (140 cubic feet) is probably a 
better base than what you thought you wanted 6 feet deep by 4x4 feet (96 
cubic feet).  Not only is it 45% heavier, but the lever arm is 50% 
longer, so the overturning moment (as if the base were a block sitting 
on the table) is significantly higher.

There are interesting bases that look like a big flat X with two 
crossing long beams.

If you do have a dense hard layer of clay, that might even help, since 
now you have a larger area to push on it (3x6 feet), although bases in 
soils and forces are not something I'm very knowledgeable about.