Hi Ian,
Dr. Bob Uhrhammer lectured on this point at a PSN meeting at Berkeley a few 
years ago. It has been awhile, however I remember a few of his key points. 
The key point in the paragraph  is "the affect that the pier might have on 
response of the earth or the seismometer." Bob's point was to keep the 
local noise as low as possible while still providing a platform to mount 
the system. He talked about an ideal broadband station in Northern 
California that was at the bottom of an old mine in the mountains. He said 
that it was an extremely quite site and because of it, they were able to 
study the impact of temperature changes and barometric presser changes on 
the enclosure design. The insulation seen in the photos in the Berkeley 
document you referenced are a result of this type of study.
With regard to the 4 inch gap. The 4 inch gap is to reduce noise that could 
be communicated into the pad / device from the outer case or ground around 
the mounting block. Bob took the group down into the basement seismic 
vaults at the Berkeley Lab and showed us several very large blocks of 
cement 5Wx4Hx8L-feet or there about that Berkeley uses to place their 
equipment on. The room were left dark and locked most of the time as you 
might expect. These block are isolated from the connecting floor with an 
air gap and filled with sand. In that way, a person can walk up to the pad 
and adjust the instrument without affecting the performance of the device. 
This type of configuration is not new. For example, some of the very early 
smoke drum seismographs used this same configuration. The Randhall Museum 
in San Francisco still has their pad in the basement of the museum from the 
turn of the century. UCSF used it for one of their instruments and the last 
time I was there the museum used it for an instrument used in their 1906 
exhibit.
For reference, I have pasted my PSN station site photos at 
http://pw2.netcom.com/~shammon1/AptosStn.htm into this note. I built my 
site after hearing Bob's lecture /  discussion and did a few things right 
and a few things wrong.
(1) Cement mixture 50/50 sand and Portland cement. -- this was correct.
(2) insulation -- this was correct
(3) Full solid cover protected from the elements. -- The house and side 
fence shield the box. There is a tin roof over the site. -- this was 
correct.
(3) I tried to incorporate an air gap but had very limited space were the 
seismograph had to be installed and ended up with more problems then I 
expected. I initially built a pier block 1x1x2D-feet with a 3-inch air gap 
around the side walls of the pad which I filled with sand and then poured a 
slab around it. This resulted in the seismometer base being bolted to the 
pad and the magnet / coil located out over and on the slab -- this was 
incorrect. It caused very erratic results. I failed to think this through 
properly. The pads need to include room for all the components of the 
seismograph. Because of the limitation of space, I had to give up the air 
gap and went with a bigger pier with more mass. As you can see in the 
photos of my station, I came back and removed all the sand and clay under 
the slab and poured more cement removing the air gap. The device is 
sensitive to walk-up walk-away noise but I have been having good results 
with the design now that I fixed the two independent slabs to be united.
Regards, Steve Hammond   PSN San Jose - Aptos


-----Original Message-----
From:	ian@........... [SMTP:ian@............
Sent:	Tuesday, November 05, 2002 4:20 AM
To:	psn-l@..............
Subject:	concrete piers


having read the article on installing seismometers,
http://www.seismo.berkeley.edu/seismo/bdsn/instrumentation/guidelines.html 
,
I'm a little unclear on one aspect and would appreciate some advice.

The article states that a 4 inch gap should be left around the base of the
pier.  What about the rest of the pier?

Also, the 4 inch gap seems to be filled with insulation.  Won't this cause 
the
pier to move with the insulation/surrounding ground?

I'll be building on clay soil.  How deep should I go (within reason!) and 
are
there any other tips for clay?

TIA

Ian Smith
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