[TowerTalk] Tower foundations

[Laurent Ferracci]

Dear OMs,

My project is to erect a self supporting tower i bought a few monthes 
ago. The tower is 13 meters (36 ft) high and weights around 250 Kg. I 
planned to dig a square hole of 4x4x4 (based on what i read about 
similar towers), to put a rebar cage and a tower base into it, and to 
pour the concrete.

However, reality differs from my plans, and i have a few questions. 
Please note that the tower has been "home made" (not by myself). I am 
confident in it but i can't simply "ask the tower manufacturer" my 
questions about the foundations. That's why i need your help.

I asked one of my neighbour if he could dig the hole (he has machines 
for it). He accepted, and i explained him what i wanted. Unfortunately, 
he came one day with his machine while i wasn't here and dig (dug?) a 
hole that wasn't deep enough (just 2 ft). When he came back to check if 
that was OK, i told him that i needed twice that depth. He came back 
another day while i wasn't here (really bad luck) and dug again... i 
ended with a hole that is now too big ! It's now 5x6 and of 7 ft depth.

My first question is about the foundations dimensions. What do you think 
about them ? Does 4x4x4 seem sufficient to you, or should i keep 5x6x7 ? 
(of course, i could keep the big one as a "safety margin", but if 4x4x4 
is already safe, i see no interest in paying the additionnal concrete it 
will take to fill the big hole).

If a want a smaller hole, i can simply put back some ground (?) into it 
for less depth, but i can i do for a smaller "area" ? I could build a 
wood frame for the dimensions i want, put it into the hole, and put some 
ground around it, but i wonder if it will be sufficient, because when 
i'll pour concrete in the frame, it will have a great pressure from 
inside to outside.

About the top of the concrete base: Usually, the concrete base is higher 
than ground level. What's the reason for this ? I would like better to 
have it lower than ground level. That way, i could have just the tower 
base coming upside. In my garden, 5x6 of grass would be nicer than 5x6 
of concrete. Of course, rust on the tower base would be a concern, but 
if it's the only reason, i feel i can deal with it.

About the tower base (to be sealed in the concrete). How deep should it 
go into the concrete ?

Thanks for your help ( and sorry for my bad english)

-- 
Laurent F1JKJ