[Cob] rubble trench foundation questions

[J Moore]

Hi, here in N central Washington we are building a post and beam straw bale 
house with cob interior walls and floor. Local inspector okayed a 2'X2' 
rubble trench foundation with 4"drain pipe, with the posts on 10"X34"X34" 
footings integral to the inside edge of this trench, but when we started 
digging we found one corner of the house 2' lower (so our "trench" ended 
being at grade!) We plan to berm it with 2' of loam up against the 3' of 2" 
rigid foam on the outside surface (top 1' being a 12" X16" bondbeam), and 
have gravel on the inside (sub floor - insulated only for the first 8' on 
the southside of the living rm. with 2" foam under the floor). My question 
is; should we dig deeper on the low side (hard to do at this point, and just 
increasing the thermal mass more - see concern below)? or insulate more 
(1"X24") with a winged/skirt around the whole perimeter(expensive but 
doable)?

Some factors; the site is dry but on fine clay below 2' of loam, I'm 
conerned about the trench and the post footings being just on this loam. I'm 
also conerned about the increased thermal mass that the extra gravel fill 
has added to the house. Should I insulate under the whole house to reduce 
this mass (with 1.5" foam)? - we have a great passive solar site but it is 
the great NW after all and often snowy/cloudy in the winter with temps down 
to -35 F.... I'm worried about the house becoming a cold heat sink, only a 
good thing in the 100+ degree summers.

Any thoughts, or similar experiences? Thanks for your help - in advance! 
James