Hi Rudy
Looks like you've had plenty of "advice" from this group. You are spot-on in
your opening sentence, when you say that it's a "rather complex situation".
You have already cast the base, so you do need now to consider carefully the
next steps. Do not hurry.
I can't recommend strongly enough that you should obtain professional
engineering advice. A small soil survey should be procured, in which samples
will be taken for testing. These samples will come from close by your base,
and from the foundation depth and deeper. At the same time, the soil can be
assessed in-situ for its bearing capacity. What's more, your engineer will
be able to see and feel the soil, that's important too. You don't need
expensive deep boreholes, a couple (not just one) of trial pits dug with an
excavator will suffice.
When the soil strength is known, and the wind loads from your mast and
aerial are assessed, you will be told whether your foundation is adequate.
Regarding the concrete base, as others have mentioned, concrete is porous.
Water, during a freeze / thaw cycle can slowly cause deterioration and
cracking to the point where your rebar is exposed. Then it will rust.
Rusting causes expansion and further cracking. Rebar which is completely
encased in concrete is almost entirely protected from this effect. Hopefully
your builder gave sufficient cover to the rebar ? In addition, the concrete
could have had an additive to provide air-entrainment which would have been
helpful.
I wish you luck with your project. It's not all doom and gloom, because the
worst scenario is likely to be that your dream of 90ft may need to be
cropped closer to the ground.
Regards
John G4ZTR
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Rudy Bakalov
Sent: 05 June 2009 20:27
To: towertalk@contesting.com
Subject: [TowerTalk] (Repost) Engineering advice on above ground foundation
Looks like first post came out very difficult to read so here it is again.
I have a rather complex situation that certainly can benefit from the
collective wisdom and experience of this reflector.
My property has a very high water table and as a result my builder and I
decided to build the tower foundation mostly above ground. Specifically, we
dug a hole, filled it with compacted gravel that is above the water table,
covered the gravel with very thick sheets of plastic, and inserted 2'
styrofoam boards around the base. Then we poured the concrete. As a result,
only about 1.5' of the foundation is below ground, 3.5' are above. The
overall dimensions are 10' x 10' x 5'.
Now I wonder what are the additional engineering issues we need to address
to make sure the foundation will last and the 90' AN Wireless tower will
stay up. Currently, we have identified the following next steps:
1) Waterproof the concrete using silica based solutions, such as Penetron,
Xypex, etc. as permanent solution against water getting into the foundation
2) Add extra waterproofing using tar/asphalt-like solutions to make sure we
have extra protection should cracks emerge
3) Add styrofoam boards to prevent against freezing (it gets as cold as -25F
in Ontario)
4) Backfill around the structure to create a mound that takes the rain and
melting snow water away from the concrete. I have also proposed we layer
more plastic sheets about a foot below the mound surface to make sure no
water gets near the foundation
Any further comments and suggestions will be greatly appreciated.
Rudy N2WQ
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