If the subgrade is unsuitable for road construction then replace that subgrade with a suitable type of subgrade that having a good bearing capacity and ability to resist an applied load by vehicles.
there are various techniques to increase the bearing capacity of a suitable subgrade and make it suitable for use.
Refer IRC 002, IRC 064:1990 for various methods.
following are some methods are described briefly for stabilization of the unsuitable subgrade
Removal/replacement of fills. Traditional compaction methods Uses of mechanical stabilization Techniques Use chemical stabilization