My two cents worth: As I understand, you have 3 ft of soft clay overlying 6 ft of very loose (N=3) sand. Presumably, although you didn't say it, underlying the sand would be competent material. You have not indicated the "size" of the construction - i.e., a "house" size area, a industrial bldg sized area or . . . Nor have you indicated if this is for a raft type structure or individual footings or . . .
Assuming you would be developing an area, while three ft of clay seems like a lot, what I would suggest, and again this depends on the nature of the job, is to remove the 3 ft of clay. You have indicated that the groundwater is at the surface of the ground - is this by standpipe or ??? If the water level is at the surface of the sand when the clay is removed, put in some strategically located sump pumps and pump. When the water level is drawn down, put a heavy vibratory steel-drum roller onto the sand which should densify the sand considerably.
Given the soft clay, I don't think that geogrids would be appropriate - they certainly have been used building roads in swamps, etc. but even so, the fills of the embankments above will have flexibility and settlements of the road surface of a few inches would not be detrimental especially with the ability to repave the road from time to time for smoothness.