Where it is customary to establish an allowable pressure on the soil as in Spain, the limit states combinations don't contribute much to the (unstated to such purpose) safe or probabilistical establishement of service level loads from which the actual pressure is to be got for comparison, so I understand your question. So on the one hand I understand some approximation would be here possible, since the code itself is undefined, but as well one has always to consider any situation that is likely to occur, in which case drift could become critical sometime somewhere.