Jim-
Actually, undisturbed soil is not an urban myth, it's just the simplest way
for the engineer to calculate the foundation design. The vertical walls of
the undisturbed soil create friction against the concrete which counts as
part of the foundation's resistance to the overturning moment.
The key factor in over-excavating is the "high level of compaction". Soils
engineers have a way of calculating that level required, designing the depth
of the engineered lifts, and then measuring the compaction in the field.
But then if the soil is not suitable (ie. too sandy) it won't adequately
compact. In that case you need a mat foundation. The mat design will be
much wider and shallower than the typical pier foundation, but with enough
mass to still resist the overturning moment of the tower.
73,
Steve
N6SJ
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
JVarney
Sent: Friday, February 03, 2017 12:11 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Tower Foundation Pouring
There's an urban myth about "undisturbed soil." Hams believe that pouring
concrete against undisturbed vertical dirt walls is the only way to place a
footing. It is often the easiest way to do it and is perfectly acceptable,
but when soils are soft it can be difficult to do.
The other way to pour a footing, commonly used in commercial construction,
is to over-excavate, build a full form, like this:
https://goo.gl/MKNbLy
After the concrete cures pull the form. Rent a "jumping jack" or "elephant's
foot" compactor. Backfill the dirt around the footing in layers, then
compact it
https://youtu.be/lYndguhIEQc
Done carefully and with a high level of compaction the footing will be as
good as if the undisturbed soil method was used.
73 Jim K6OK
Paul W9AC wrote:
>> Despite efforts to brace the walls from blowing out as concreate was
>> added, we still had slight bowing on the
>> 140 ft. tower base (a 10 ft. x 10 ft. square). That resulted in a
>> less than perfect square on two sides of the top surface.
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