In a message dated 10/27/99 2:37:20 PM Central Standard Time,
david.b.curtis@intel.com writes:
<< The Rohn catalog talks about "normal" soil.
What is "normal"? What is a simple, cheap test that I can do to see if I
have "normal" soil? Is this something that can be done as a slump test?
Then, of course, that leads to the question: if my soil is NOT normal, what
is the right way to engineer guy anchors and tower bases? (Please, no
"overbuild till it looks good to the eyeball" or "it works for me" answers.
I'm interested in the engineering answer. My mama paid good money for this
engineering degree, and by golly I'm going to use it :-)
73, Dave N6NZ
>>
The 400 PSF/ft of depth is for lateral bearing of guy anchors. The value does
not contain the required factor of safety of 2. If you work in allowable
stresses you end up with 200 psf/ ft of depth with a maximum of 2000 psf.
Now you need to know the depth and thickness of your anchor. For the small
towers with anchors less that 10' deep -- find the depth to the center of the
anchor block -- call it D. Now multiply D x 200 psf/ft and you will get the
allowable lateral bearing pressure for the foundation -- call Q. The lateral
side bearing area x Q must be greater than the horizontal load for the guy
anchor.
Now for your question of "What is normal soil?" Normal soil can be found in
Peoria, IL. It is a cohesive soil with no water (water table below foundation
depth). What is cohesive?
SOILS Mechanics 101
Soils are classified by their grain size. In laymans terms -- there are
bolders, gravels, sands, silts, and clays (large to small). Solid rock has
its own system with RQDs and other properties (another subject). Soils come
in various mixtures and have two major properties -- angle of internal
friction (phi) and cohesion (C). To simplify - pure sands have phi and pure
clays have C. Trust me this is an over simplfication! Now soils are hardly
ever just one type so most soils are classified according to charts rating
their grain size. This test can be done in the laboratory with a grain size
analysis (a series of various size screens) or it can be done by hand and
"feeling" the soil. A sand will feel gritty and a clay will feel smooth. A
silt is in between and can fall either way. Silts are the most difficult to
classify. There are some beach sands that are classified as silts and have
phi angles and some silts are hard as clays. Hard silts will loose strength
when wetted and clays don't.
Now what is Normal Soil? Normal soil is a cohesive soil - normally a clay but
could be a silt. To make a comparison - take Q from above and if it greater
than C (cohesion) it meets or exceed the normal soil parameters. C can be
measured by various means. The laboratory test - unconfied/2, The standard
penetration test (N /8), The pocket penetrometer test /3. There is even a
system of estimating C using your thumb nail. The answer to which test to use
depends on your available equipment and experiance. I once worked with a guy
that went out with contract soils boring crew to oversee the operation. The
crew was using their pocket penetometer and he picked up a piece of the
sample and tasted it and said ".5 tsf". The crew was astouned that it mached
their reading too. What the crew didn't know was that most of soils were caly
.5 tsf soils and the odds were in his favor.
I hope is explaination and quick lesson in soil mechanics gives some insite
to the world of "Normal Soil"
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