[TowerTalk] new AN-Wireless tower, floundaion question

Jeff Goldman k3dua at erols.com
Fri Jun 30 16:44:13 EDT 2006


I guess the item I would look at is the total cost/effort.  Yes, the 
pad-pier uses less concrete and rebar, a cost savings.  But is the 
difference worth the time, and extra effort of 2 pours and renting the 
tamper and backfilling.  It is much simpler to use there 9 by 9 by 5 
full foundation.    One pour, no back fill, no tamper rental.   Yes it 
takes more rebar and concrete, but its a lot easier in my book.

Jeff, K3DUA

On Jun 30, 2006, at 9:55 AM, ersmar at comcast.net wrote:

> JC:
>      It's nearly impossible to undercut safely the full size of the 
> pad for this type of foundation.  When I was working in the electric 
> utility industry as an engineer, we used these foundations all the 
> time for our substation structures.  The crews dug out the full size 
> hole for the pad (in your case, that would be a hole 9 feet by 9 feet 
> by five feet deep.)  They set the rebars for both the pad and pier, 
> the poured the concrete for the pad.  Next day they set the forms for 
> the pier on top of the pad and poured the pier.  ANd I believe there 
> is a material that is applied to the concrete to allow this second 
> pour to adhere to the first.  Anyone help here?
>      The next step is the trickiest:  backfilling the excavation after 
> removing the pier's form material.  In your case you ought to back 
> fill a foot deep around the pier, then spend a bit of time with a 
> tamper banging the soil in compaction.  You can rent a motorized 
> tamper for this.  Then add another foot and tamp it, etc until the 
> entire excavation is filled and the earth put back in near-undisturbed 
> state.
>      My Trylon tower's foundation is similar in construction except 
> that the undercut is a slanted cut only a foot wider than the main 
> hole.  I was able to use a spade, with proper shoring of course, to 
> loosen the earth and haul it out with a backhoe.  But yours is a bit 
> too wide to safely support the overlying earth.
>      I know - a PITA, but it has to be done that way so you don't 
> injure anyone should the hole collapse.
>      Good luck, and remember - On towers as in driving a car, there 
> are no such things as accidents.
> 73 de
> Gene Smar  AD3F
>  -------------- Original message ----------------------
> From: "JC Smith" <jc-smith at comcast.net>
>> Hello All,
>> As mentioned previously I am stick-building (like a giant erector 
>> set) a new
>> AN-Wireless HD-80.  I will be posting photos at:
>> http://s14.photobucket.com/albums/a317/k0hps/Tower%20Project/
>> if anyone cares to watch it go up and the antenna stack go on it.  We 
>> just
>> started digging the foundation yesterday.  It's a pad and pier 
>> foundation
>> (like an inverted tee) with a 9'x9'x1.5' pad and a 5'x5'x3.5' pier.
>> Has anyone on here ever dug one of these by digging the 5'x5' hole 
>> first and
>> then undercutting the bottom to create the pad?  If so, I'd sure 
>> appreciate
>> hearing how you did the digging for the undercut.  We got the 5'x5' 
>> hole no
>> problem but our experiments (so far) with the undercutting haven't 
>> been very
>> successful.  We are in clay soil, fortunately with no rocks.  If 
>> things
>> don't go better today we will probably dig out the entire 9'x9' hole, 
>> pour
>> the pad, form the pier and backfill around it.
>> Suggestions appreciated.
>> 73 - JC, K0HPS
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