Mike,
I deal with similar situations during my day job, I am a soil
engineer. The great thing about soil engineering is every site is
different, and every loading situation is different. What really
matters here is communication. Your engineer and concrete guy need to
talk, and you need to express your concerns to your engineer.
We can all throw out our ideas on this list, and I can tell you, from
my experience, what you should and should not do. I've never seen the
soil at your house so I don't know, I'm also not familiar with
problems that may arise in your area. Your local engineer is. And by
you paying him for a stamped document telling you what you should do
he is accepting liability.
I would hope you will bring up any concerns you have to the engineer.
Now what your are asking has me a bit confused. I am pretty sure I
have never considered a plywood form backfill. I assume they want to
place the plywood against the cut face of the excavation and pour
against it, leaving the plywood in. I don't see how they could
backfill around that. If they want to widen the excavation, pour the
concrete in a plywood form, then remove the form and backfill around
the foundation I could see your concern. I will say this, saturated
sand has a bad habit of not being very compact, especially if it is a
fine grained sand. In that case properly compacted backfill could
easily be denser and have more lateral resistance than native
material.
You may ask your engineer about a drilled shaft option. Any serious
fence installer will have an auger on a bobcat and be able to drill a
10' hole pretty easily, this would avoid a large excavation, and they
could set a cage and pour immediately so you wouldn't have to let it
stay open long enough to cave in. Just a thought, it may not work
though if the native soils are loose as they will cave in too much to
drill a hole. It really all depends on your individual conditions, of
which I know too little about.
Brian
KF7OVD
On Thu, May 1, 2014 at 5:33 PM, Bryan Swadener via TowerTalk
<towertalk@contesting.com> wrote:
> Hi Mike,
>
> I had a water table issue as well. I solved it with a sump pump.
> I backfilled the hole a bit with some round rock and created a
> small "dimple" in the bottom of the hole for the pump to set in.
> At first, the pump clogged with pebbles between the impeller
> and housing. Some aluminum window screen wrapped around
> the pump solved that issue.
>
> Since the pumped didn't have a float-operated switch, I pumped
> the water out daily (or as-needed). The last pumping came just
> before the concrete arrived. The only wood used was above ground.
>
> Photo story: http://www.tinyurl.com/wa7prc-tower
>
> vy 73,
> Bryan WA7PRC
>
>
> ________________________________
> Date: Wed, 30 Apr 2014 20:20:41 -0400
> From: Mike Reublin NF4L
> To: towertalk reflector
> Subject: [TowerTalk] Crank-up base
>
> I'm negotiating to get the base for my HG70-HD dug and poured. I have a high
> water table, soil tends toward sandy, and an engineer said he would be
> happier if the hole were 10' deep rather than the 7.5' called for in the
> manual.
>
> The concrete guys I'm talking to want to line the hole with plywood to
> prevent collapse. One guy who does a lot of big sign installations says he
> does it regularly, and there would be no stability issues.
>
> I've read several times the pour needs to be into undisturbed soil, no
> backfill, altho that's not in the manual.
>
> What are the facts?
>
> 73, Mike NF4L
> _______________________________________________
>
>
>
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