Liz,
 
I have a suggestion for your drainage problem.  But it may seem naive,
because it is drawn from my former-life experience in landscaping work & not
from management of archaeological excavations.  Anyway, for what it's worth...
 
You noted that you dug a drainage trench DOWNHILL from the excavation. That
sounds like a good first step, but you're still drawing water THROUGH the
site from UPHILL and possibly from the sides.  So I'd suggest a trench
uphill and, if
possible, along the sides, connecting the the uphill to the downhill trench.
These trenches must, of course be deeper than the clay you're trying to drain.
With the site completely "isolated" by surrounding trenches, you won't be
pulling water from outside the site.  However, you must also keep water from
backing up into the site, which would happen if the water level in your
downhill trench gets too high -- you may need to use a pump or may need to
dig a trench perpendicular the downhill trench to carry the water further
away.
 
If this were a landscaping job, I would use drain tiles topped with gravel,
then a layer of soil, rather than open trenches.  And you might consider
doing this too.  All those trenches could be hazardous.  (Perforated plastic
drainage pipe or flexible tubing is relatively inexpensive.)
 
Regarding the cracking clay in dry periods.  Once you have good drainage,
you should be able to mist-spray occasionally to keep the clay moist and
reduce cracking.  Or, if damage to archaeological data is not at issue, you
can try raking powdered clay of the same type over the surface. However,
once the site is "isolated" and not subject to extreme soaking &
desiccation, there should be less cracking, because there will be less
expansion & contraction.
 
Regarding the preservation of the "tavern dump,"  you might need to clarify
your requirements here -- I'm not sure whether you were asking about
preserving the entire cellar wall or, say, just a cross-section of a smaller
dump.  If the latter, I assume that you've already considered 1/4-inch
Plexiglass & other similar materials.
 
In bocco al lupo
BYE -- FJK
 
At 08:19 AM 4/2/97 -0800, you wrote:
>        I am working on a 17th (and 18th,19th...) Century site in
>Edgewater Maryland, about 20 minutes outside of Annapolis. We are doing
>block excavation, sprawling out via 5 X5 ft units that now exceeds a 50
>x 80 foot area. The soil composition (of the subsoil)is compact silty
>clay -which presents us with a drainage nightmare. Conversly, when the
>soil isn't saturated with water (and worms and mesquito larvae) it is
>bone dry and begins to crack.
<snip>
>        Our second challenge is the profile of a 17th century cellar hole
> filled in the first quarter of the 18th with tavern garbage.  We have
>bisected the cellar and we now have a wall profile that extends about
>10feet long and 4 feet deep, with 26 layers of tavern fill and tinglaze,
>bottle, oyster, and pipe sticking out of it. We would like to preserve
>it, perhaps permenatly for the museum, but at least temporarily. Any
>ideas?
>Thanks,
>        Liz West
>        The Lost Towns Project
>
>