[TowerTalk] Pad and Pier Foundations

[Alan NV8A (ex. AB2OS)]

The ability to dig an "undercut" hole no doubt depends on the soil. I 
gave a local concrete contractor the AN Wireless foundation plan, and he 
said there was no way to dig such a hole. He said the only way to do a 
pad and pier foundation around here would be to dig the hole the size of 
the pad, construct a form the size of the pier, then backfill.

(The photographs on the AN Wireless Web site show both undercut and 
backfilled mathods, IIRC.)

He initially proposed a two-stage pour, but when I told him that my 
understanding was that it was supposed to be a monolithic pour, he was 
quite happy to do it that way.

It had occurred to me that the higher concrete cost of a solid-block 
foundation might be more than offset by the saving of formwork and 
backfilling, but I thought that the smaller exposed concrete area of the 
pad and pier foundation would win on esthetic grounds.

73

Alan NV8A


On 07/05/05 01:09 pm ersmar at comcast.net tossed the following ingredients 
into the ever-growing pot of cybersoup:

>      Digging this kind of excavation is a royal PITA.  First, you dig the hole the size of the bottom pad, e.g., 5X5 feet, to the final depth of the hole.  You place rebar for the pad AND for the pier.  Then you pour only the bottom pad concrete and let it set for a day.  The next day you form up the pier and pour it.  
> 
>      After this entire block cures for another day or two, you can remove the forms.  Now comes the fun part.
> 
>      You'll have to back fill the hole around the pier.  This means you must place some earth into the hole around the sides of the pier, covering the pad in the process.  THEN you must ensure that the replaced earth is as compacted as undisturbed soil by pounding on it with, for example, a 4X4 or 2X4 piece of wood.  Once that layer of soil (abt 6 inches or so) has been compacted, then you add another layer of earth and repeat the pounding (compaction) process until you reach grade.  One might have a tendency to slack off a bit on compacting the upper levels of earth due to fatigue.  This won't give you the mechanical performance from the earth that your tower will need.  
> 
>      I used the Trylon foundation design, that is, a monolithic concrete pour, but with an undercut at the bottom of the hole.  I did this by having the backhoe dig the basic hole (5.5 X 5.5 feet six feet deep) and then shoring up the sides.  I then went INTO the hole (ever look up from the bottom of a narrow hole in the earth?) and hand-dug the undercut.  The backhoe bucket was small enough that it was in the hole with me, but I had very little room to maneuver.  
> 
>      Thinking a bit about this compacting, if you decide to go the way of the two-pour pad and pier design, you might want to really overdig the hole and use a gasoline powered rented compactor to do the job with less human effort (and fatigue.)  It'll sure beat using a two-b'-four!