The costs of four basic approaches to the construction of offshore airports, i.e., fill, dike or polder, pilesupported, and floating structures are compared. The results indicate that the cost effectiveness of the alternates depends significantly on the depth of water at the site considered. The most important conclusion is that in evaluations of offshore airports alternate concepts must be contemplated. Limiting consideration to a single concept makes the evaluations questionable.
There are some compelling reasons for locating airports offshore and this certainly will be a future trend in airport construction. Undoubtedly, the actual cost of constructing an offshore facility will exceed that of placing pavements and structures at an onshore site. It may even exceed onshore construction costs plus land acquisition. The greater costs may well be justified if certain benefits are realized, even though it may be difficult to assign an economic value to some of them. Benefits of an offshore site include; reduction of aircraft noise over populated areas, aircraft pollution will occur over water, land is not removed from present or future tax roles, and airport-residential feuding is eliminated.
Considering factors such as required acreage, gradient, accessibility, and proximity to built-up areas, suitable land sites for new airport construction are becoming increasingly expensive to aquire. Even now, travel time to and from airports often exceeds flying time. Offshore airports appear to be an appealing alternative for many cities. Of the ten busiest airports, eight are adjacent to bodies of water that appear to offer offshore sites. The background for offshore construction modes is plentiful. Though small and lightly trafficked, Chicago's Meigs Field was constructed on an artificial island. Numerous other facilities have also been built on fill. In the Netherlands, a new airport has recently been constructed within a dike. Other American cities (e.g., New Orleans, New York and Los Angeles) have been seriously considered for offshore airport construction.
Though numerous conceptual systems for designing offshore airports may be projects, they seem to fall into four generic types:
Dike or Polder System
Fill or Island System
Clearly the optimum solution for any given site might be some combination of these concepts. Several existing airports in fact use some offshore construction to extend the land-based facility. Consideration of these four generic classes seems to cover the problem in general, and is incorrect only if an alternate generic class is conceived.
The Harza Engineering Company has recommended the polder system for offshore construction in its report "An Appraisal of a Lake Michigan Site for Chicago's Third Major Airport." This approach is based on one already in use for the Netherlands. By this plan a dike would be constructed with a core of silty-sand, sand, then quarryrun stone and gravel; finally massive armor stones would be emplaced to prevent erosion by wave action. A typical section through the proposed dike is shown in Figure 1. After the dike has been dewatered, pavement slabs and buildings may be built on the lake bed.