Tire wall foundation to cob wall foundation

We are building a half cordwood and half rammed earth tire wall round building. The rammed earth tires will be acting as a retaining wall as that half of the building will be in a large bank. The other half of the building outside the bank will be a 2foot thick cordwood wall. We are at 3200 feet elevation in Canada BC so our cordwood walls will hace a 3.5 ft deep rubble trench foundation but ive read everywhere a tire wall doesnt need a foundation. should we make the rubble trench all the way around the building, or just under the cordwood part that is outside of the bank? It seems that having just half a foundation would be flawed but i cant find any info regarding what to do in this situation. In earthship builidng does the framed walles have a foundation but the tire walls do not? how does this work? Please and thankyou.