PSN-L Email List Message

Subject: floor slab pier
From: sean@...........
Date: Wed, 2 Aug 2000 12:46:07 -0500 (CDT)

Regarding sawing up the slab floor: our experience:

We have an IRIS station just west of Nashville at Waverly, TN (WVT)
at a TN div of geology drilling core storage facility. To make the
original pier, we had a contractor saw a double slit around a 5 ft x
10 ft piece of the floor and remove the concrete between the slits
that were 4" apart. This effectively isolated the "pier" from most
local noise, although we would still get many event triggers when
the geologists were playing with their rocks.

But serious horizontal tilt noise was still a problem for the STS-1
seismometers operating at 360 seconds. So we got $5k from IRIS and
had the slab pier removed, and dug a pit 6 ft deep to a more consolidated
clay layer (bedrock is 30 ft down), and filled the pit with concrete
back up to the floor level. THe gap between the pier and the floor
was filled with tar (a bad idea!: we had to cover it with ethofoam
to keep it off of everything). The new pier reduced the horizontal
noise by greater than a factor of 10.

A note on such piers and the idea of putting pipes thru the floor to
support a pier: don't make an inverted pendulum. The depth of the
arrangement should generally be no more than its smallest horizontal 
dimension.  Even in consolidated clays it could wobble around.

And remember that a horizontal seis is sensitive to tilt noise as
the square of the operating period, whereas a vertical is not.


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Larry Cochrane <cochrane@..............>