With clay or any silt laden soils you definitely should raise the roadbed ( ditch on both sides if at all possible ) such that it will never become saturated with water in the winter months which would promote plastic flow underneath the pavement......
A larger aggregate should (ideally) first be mixed in with the clay soil to some depth above the expected maximum water height in the ditches or at least placed on top and then compacted into the plastic soil first ( this keeps the roadbed dry, and distributes the load into the soil)........a thinner lift of crushed rock or cinders is then placed on top of that, compacted and graded, to distribute the wheel loading directly under the asphalt....and then finally this is capped with the actual bituminous pavement.
To figure your cubic yards of asphalt, a general rule of thumb is roller compaction to 50%.....IE, a 2in lift of hot asphalt mix should be spread to a 4in thickness before rolling in...
Soil and ambient air temperature is very important for obtaining proper asphalt compaction...
Here in the Pacific Northwest, asphalt paving is generally only carried out during the summer months, though often some unscrupulous firms might do slipshod jobs when they can get away with it during unfavorable weather....and all too often the result will be insufficient compaction, which eventually leads to premature failure of the asphalt due to the ingress of water and a general lack of tensile strength because of entrained air voids--at any rate, it is imperative that the asphalt not be allowed to cool too much before final rolling--on the other hand, it can often take several hours if not close to a days time for it to cool sufficiently for rolling if the weather happens to be too hot....