This MSNBC article describes the expansion process currently underway at several of the Nation's aiports. 

I fly a LOT on business and most of the time, it's out of SFO:

...San Francisco airport officials, for example, have spent $70 million over the past three years on research, planning and environmental studies just to see if they can start work to separate the facility’s four runways, a delicate process given the airport’s proximity to San Francisco Bay.

SFO is actually directly on the Bay and all plans for runway expansion requires filling in part of the bay.   As you can imagine, this is a HIGHLY politicized issue here stoking the ire of everyone from Environmental groups to wind-surfers.  (the windsurfers actually have one of the better websites w/ overlay picts of what the new runway config might look like:  http://www.sfba.org/sfo/runways.htm)

The SFO airport authority has the following info page describing their rationale for building additional runway capacity.  The most often cited issue:

SFO runways are only 750 feet apart. Federal Aviation Administration (FAA) standards require a separation of 4,300 feet for simultaneous dual landings during poor weather.

...Currently, the three Bay Area airports combined do not have sufficient capacity, on a bad weather day, to meet the region's air traffic demand.

Within 10 years the three Bay Area airports combined will not have sufficient capacity to meet regional air traffic demand even on a good weather day.

In any case, the real reason I started this post was because it had a fascinating discussion of the layers of detailed problem solving construction projects like this require: 

...In Atlanta’s case, the plan is to move the runway dirt-enough to fill the Georgia Dome football stadium six times-without disturbing the environment or neighboring communities. A 5.5-mile-long electric conveyor belt will wind over streams, through woods and across five roads, including an interstate highway. To keep the dust down, the dirt will be sprayed with water and enclosed in covered bins. Two control towers equipped with video monitors will track the dirt’s 30-minute trip to the construction site. Total project cost: $350 million, plus a $10 million bonus for on-time arrival by November 2004.

This is what we call in the software biz, "Real Engineering"   ;-)