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I think that you should deal with nature by using sand located to reach the level bridge proper, no more than1 meter by3-4 layers , with the saturation of sand with water and a very good compaction, and then start bringing A3 or A-4, and should bethe last backfill layers of the kind A1-b or A1-a , with the knowledge that the transportation cost will be more than usual ... but I think the most appropriate solution in this case.