As a builder in swampy areas we always used a "roadway fabric". Geo-textile fabrics have been used under roadway base aggregates for decades. It creates a stabilized base to build a road off of. We used to buy 1000 foot rolls and literally roll it out and cover it with crushed shale. You guys would be using hardcore which is crushed limestone. I know in wet areas when we used the fabric it would require 75% less material. I am talking about areas where you could sink a metal rod by hand several feet below grade, pretty swampy. We would drive fully loaded tri-axles over them right away (80,000lbs.) The good news is the fabric is dirt cheap here. You could have Hyde ship it down from Miami, if it's not available in the states. We used "Mirafi" brand a lot. If you use these types of products Hollywoods floating road idea will work really well, and you will not have a constantly sinking road.