On 3/19/2012 8:36 AM, Larry Libsch wrote:
> Group
Answers interspersed. This is what I did, it is not a recommendation
except to seek professional help. <:-))
> -
>
> Our club has bought a tower for use at our new club shack on
> the edge of the 'glades. The 'glades is fresh water this far north and
> west. The water table at the site is 30 inches below ground level. The
> soil is mixed gravel, sand, and earth fill for about 2 1/2 feet, then
> wet sand below 30 inches. Our tower is an old Rohn with a three legged
> tilt base to which the tower attaches. The Rohn plans for a concrete
> tower base suggest the bottom of the 6 foot, open, galvanized steel legs
> of the tilt base be placed into 9 in of (dry) sand and the concrete base
> poured around the legs above the sand. It seems likely that the the tilt
> base legs below the concrete base will always be in water, and that
> water will stand inside the legs to the level of the water table. Much
> of the concrete base will stand in wet sand below the water table. As
> concrete is water permeable and drys slowly, it is likely that the
> outside of the tower base legs will also be wet from the top of the
> concrete base down. We seek an understanding of the special problems
> this situation presents to the safe construction of a tower base and
> solutions to those problems. So far, we recognize three likely problems:
This tower has a tubular base as does mine. I put mine up before pier
pin bases or tilt bases were really common in this area,
(Unless by tilt base you mean the tower base has "ears" that hold the
bottom tower section)
So the base for mine is a tower section set in concrete. I used the
high strength, high fiber concrete.
>
> 1) Corrosion issues with the steel legs both inside and below
> the concrete base. What can we do to minimize this problem?
This time of year until mid summer (varies considerably from year to
year) the water table is probably within 10 inches of the surface.
Where the legs are *in* water it should not be a corrosion problem, but
the interface between water and air quite likely might. When I dug out
the base for my previous tower which had been in for nearly 20 years
there were no signs of corrosion and remember that on average, 6 months
of the year it likely set in water that came within 10 - 12 inches of
the top of the base. When I dug the base for the 45G in August we hit
water at 5 feet.
The legs can be filled with concrete or an epoxy (ask an engineer). If
filled there should be no voids and no recess at the top to hold water.
That could be worse than no protection. You can use a solid leg base.
Galvanized is preferred but a couple coats of epoxy (or tar) will do a
good job of protecting it. As it's in the ground nothing should disturb
or damage the coatings.
This is what I did for a guyed, 50' 25G that is bracketed to the W. end
of my shop. http://www.rogerhalstead.com/ham_files/Dirt_Base.htm The
effectiveness of dirt bases depends almost completely on the soil.
> 2) It seems likely wet sand is a less firm base than dry
> soil. What changes should we make to the size and /or the shape of the
> concrete base from the Rohn plans?
If this is a guyed tower the base only serves the purpose of supporting
the rest of the tower (keeping it from sinking), or preventing it from
sliding sideways. That's why a properly implemented dirt base works so
well with 25G "In some installations" Yes you can alter the shape of
the concrete, but you need an engineer to calculate the changes. ROHN
might be able to give some recommendations but there is a liability
problem so they may just tell you to hire an engineer. BTW wet sand is,
or can be very firm! It's another, it all depends. There are different
types of sand and sand mixes. If it's clean sand it is usually very
firm. For one, it has no place to go.
> 3) Any hole deeper than 30 inches fills quickly with water.
> Will we be able to dig a hole 7 feet deep and 5 feet by 5 feet wide? If
> we manage to dig an acceptable hole, it will fill with water.
Only by using forms and a pump. Forms are a ROYAL PITA, particularly
when you go to remove them. Do not leave wooden forms in place. Even
with the poor quality wood available now days, they will likely outlast
any of us on the reflector and I'm including teenagers. Depending on
the PH of the soil the wood will become "greasy" as the outer layer
slowly decomposes. In 10 or so years the wood *may* not have any
structural integrity left, but there will still be that greasy layer
between the concrete and soil.
In essence, I would hire that hole dug by an excavating company. You
could build up the rerod cage and put it and the tower base in place
once the hole was dug and forms in place. NOTE working in a deep hole
even with forms is DANGEROUS. To me it's more so than tower climbing by
many times. A 7' deep hole with forms that have water against them up
to30 inches leaves the pressure of 6 1/2 feet of water all the way
around which is a lot of force.
NOTE We dug the hole for the NW tower anchor late in the day. When we
came back the next morning, instead of that big hole we found a very
wide and shallow depression. That portion of the yard is is pretty much
a peat and sand mix while the rest of the yard is a sand and clay mix
down at least 5 feet. We had to build a form for that one.
> Can we pour concrete into a water filled hole and have it set
> up properly?
Yes, but tell the concrete company it will be in still water. IE give
them the details.
You did not give any real particulars on the tower other than it's an
old ROHN with a tilt base. If it's a tilt base I'd assume (assumptions
are never safe) that it's a 25G and guyed. How tall is this thing going
to be and what will be on it?
73
Roger (K8RI)
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