Kristin,

The plexiglas hypotheis has merit, but only to a certain depth. This material 
is expensive and scratches easily, so can only be used in limited situations. 
Also, moisture builds up to hard surfaces and viewing would soon be obscured. 
Finally, there is the issue of engineered safety beyond a certain depth. This 
is why mining engineers design wood shoring and archaeologists use corrugated 
steel pipes for safety barriers. Federal and state safety laws usually 
require massive shoring at depths greater than five feet. What archaeologists need 
is a grant to fund engineering archaeologists to devise safety systems that can 
breathe moisture molecules and still be visible for strata viewing while 
digging at safe depths. Until that happens, we are back to either wooden shoring 
or creating slopes at a ratio of 2:1 to buttress sidewalls. Oh, and in my 
experience with safety engineers, they want a key-trench cut at least 36-inches 
deep at the toe of the slope to lock it into place. The key-trench could be very 
destructive unless excavated by archaeologists. Finally, achieving a 2:1 slope 
at four sidewalls would either create a test pit the size of a  house 
footprint or result in effectively back-filling the pit. 

Ron May
Legacy 106, Inc.