# Deciding on a design concept

My nephew, a bright boy of 12 or so asked me last year How much would it cost to build a bridge to Hawaii? I replied that it probably could not be done with the resources at our disposal with current technology, but as a good mental exercise for him, and possibly spark an interest in Civil Engineering, I decided to try and answer the question. I laid out basic engineering constraints on fixed structures and discovered early on that the "bridge" would actually be more of an underwater causeway, since the deepest fixed structures possible at this time, the massive offshore oil platforms are anchored in water only as deep as about a thousand feet. A roadbed could be built on a series of bridges linking these platforms set about a mile apart from each other, but the main cost of the project would not be the bridges but the causeway itself.

# Estimating Costs of Construction

#### Estimating Costs of building the Platforms and Bridge Sections

The platforms themselves cost a couple of billion USD apiece, based on the approximate costs of a deep water oil rig, and the bridge sections a couple of billion USD per mile, based on the costs of building an 8 lane suspension bridge. Total cost of the platforms and bridges would be on the order of about 10 trillion USD, or about the annual Gross National Product of the United States

#### Estimating Costs of Construction of the Causeway

To estimate the cost of building the causeway, I had to figure out how much fill would be needed to build a causeway 2,500 miles long and 15,000 feet high, since the seabed is about 3 miles deep between California and Hawaii. I assumed a 45 degree angle of fill, which meant that the base of the causeway would be about 5 miles across, tapering to nearly a point a few hundred feet beneath the waves. This meant each mile of causeway would require about 9 cubic miles of fill to be moved to the ocean floor, not taking into account erosion or sagging of the earth's crust underneath the causeway. 2,500 miles of causeway would need 22,000 cubic miles of fill to be excavated and moved to the seafloor.

Where could we get such a huge volume of Rock? Well, the EPA would probably object to our plan, but leveling the Rocky Mountains would be a start. To keep the calculations simple, I also indicated that we could also dig a mile-deep quarry about 140 miles square out of the high desert of Arizona or Utah, without the severe climatic changes that leveling the Rockies would entail.

Now comes the nitty-gritty, converting cubic miles of rock to tons. I assumed that rock would weigh about 200 pounds per cubic foot. This a specific gravity of about 3, lighter than iron, but about what things like Granite, Basalt, and other common rocks weigh, more or less. I cubic mile is equal to 5,280**3, or about 150 billion cubic feet. Since rock is about 200 pounds per cubic foot, then 10 cubic feet = 1 ton. A cubic mile of rock would weigh about 15 Billion tons. Based on current costs of excavation and transportation, it is reasonable to assume a cost of about \$50 per ton to excavate the rock and move it by rail and barge to the dump site. Each cubic mile of rock would cost about \$750 billion dollars to dump on the seafloor. Multiply 750 billion by 22,000 and the total project cost should come in at about:

### \$17,000,000,000,000,000.00

This is a preliminary estimate, before figuring exact figures for building permits, cost overruns, complying with OSHA regulations, dealing with labor unions, etc.

That is 17 Quadrillion Dollars, as opposed to the few trillion to actually build the platforms and roadway. If the United States actually devoted about a trillion dollars a year to the effort, which is a resource commitment on a par with winning World War 2, the project would progress less than 1,000 feet per year, and would take about 17,000 years to complete.

When all is said and done, it would take about 5 days to drive from California to Hawaii, assuming an average of 500 miles per day. Of course gas stations and motels would have to be provided along the way. Since I only get 2 weeks of vacation every year, I think I will fly instead :o).

### Disclaimer:

The numbers I have provided are rough estimates. A detailed knowledge of Geology, Logistics, as well as detailed surveys would be necessary to come up with more accurate figures, but I would be surprised if my figure are off by more than a factor of 3. In any event, such a project would be Damn Expensive.

To extend the concept, I would like to invite noders to speculate on the geologic and climatic effects of building this bridge. I for one, can see two major effects, even without leveling the Rockies, which I have mentioned previously.

One effect would be to cause the seafloor to sag, as the weight of the stone piled up. The crust is thin on the ocean floor, so there would not only be sagging, but possibly volcanic effects as well.

Another effect would be climatic, since the causeway would act as a dam to ocean currents. I can speculate on some interesting effects, but I am not very knowledgeable concerning the flow of ocean currents.

### An Alternative to the Fixed Bridge

An interesting alternative to building a fixed bridge, which would obviously be unfeasible with current technolgy could be a very long pontoon bridge, built out of supertanker class size barges, with a roadbed running between them. It might be possible to build this for a few trillion dollars but the bridge would have a number of problems, mostly with the cost and challenge of continuing maintenance, and the challenge of keeping the "pontoons" in position against ocean currents and in rough weather. GPS could be used to hold position, but an engine or hull failure anywhere along the chain would result in road closure.

May 24,2003

Another interesting concept is something I saw on a TV show called Extreme Engineering, where the show's authors proposed transoceanic pneumatic tubes which would whisk passengers along in underwater tubes at speeds approaching 5,000 miles per hour. The estimated cost of such an undertaking was estimated in the range of tens of trillions of dollars, based on the cost of constructing floating oil drilling platforms and so on.