jimyr - You've nailed the problem simply in your statement -
"These roads were not made for the weights that run on them these days"
5" to 6" of gravel base isn't going to carry the weight of heavy trucks for long, if the gravel is sitting on a spongey clay sub-base.
You need a minimum of 9" or 10", and preferably 12". You're going to keep getting soft spots if heavy trucks are going to keep running over that depth of gravel base, particularly on a soft clay sub-base.
The clay needs stiffening up to assist in supporting the gravel base. Remember, that with tandem drive trucks, the load area or stress area under each tire, is basically in the shape of a pyramid, with the "load pyramid" of each rear tyre intersecting at depth.
Where these "load pyramids" intersect, is an overload or high-stress area. In essence, the trucks are generating high pressure loadings at depth, in the road structure. That's why you need good depth of gravel sitting on a supportive sub-base, if you need the road to be able to carry heavy trucks.
The clay needs stiffening up by the removal of moisture, or by the addition of sandy granular material, or by the addition of lime or other clay stabiliser.
Once you have the clay sub-base stiffened up and stabilised, then a thin layer of gravel is capable of supporting higher loads.
