Wednesday, December 14, 2011
Making a longer runway at the Carlsbad airport means building the expansion on an unstable old landfill.
SAN DIEGO The McClellan-Palomar Airport in Carlsbad may be the next frontier for San Diego airport expansion. The runway's eastern end lies on undeveloped land, and extending the airstrip just 1,000 feet would allow longer flights and, airport administrators say, less noise impact on homes nearby. But the expansion faces a vexing engineering dilemma.
How do you build the runway extension on an old landfill that won't safely support the tarmac?
The McClellan-Palomar Airport serves both corporate and commercial aviation. But Peter Drinkwater, the director of airports for San Diego County, said the existing 4,900-foot runway has some clear limitations.
"We have airplanes presently based at Palomar that are capable of long-range flight. But they can't depart on the existing runway and take off with a full fuel load and go to the maximum range," he said.
You can change that by making the runway a little bit longer. Drinkwater said the greater takeoff distance would allow the small jets that fly out of Palomar to travel, nonstop, to the Far East. That would help San Diego companies serve foreign markets. It would increase revenue to county airports and take some traffic stress off of Lindbergh Field.
Drinkwater said there are other advantages.
"The runway extension also provides the ability for the airplane to become airborne earlier," he said, "which allows it to gain altitude quicker, which translates to less noise over the community."
Less noise. Longer flights. More airport capacity. It all sounds good. But at what cost?
That's what the county's trying to figure out by commissioning a feasibility study that should be done in about a year and a half. The engineering challenge of this runway extension comes down to the fact that you have to build it on a big heap of garbage.
The abandoned landfill, just beyond the runway's eastern end, is one of three old landfills in the vicinity of McClellan-Palomar airport.
"The problem is that the landfill, itself, is not stable," said Michael Khoury, an engineer with San Diego County public works. "It decomposes and settles over time. So if you put a runway on top of it, the runway will follow the settlement of the landfill."
That would make it unsafe and impossible to use. There are potential ways to solve the problem. One would be to simply dig up all the trash, haul it away and fill the remaining hole with dirt and rocks. But current guesstimates put the cost of doing that at close to $100 million.
Another option is to inject what Khoury calls concrete grout into the garbage heap. Supposedly, that would to fill in the gaps and make it much more stable. Then there's dynamic compaction. That means squashing the pile of garbage with a heavy weight.
"Essentially you have a crane that is holding a heavy weight and you lift the weight and you drop it," said Khoury. "It's constant pounding and you keep doing that. So with that you are trying to achieve all the settlement in a short period of time."
Finally, there is the "bridge" concept. Imagine the landfill is a body of water and the runway extension is a bridge, sitting on cement legs that reach down through the trash to the bedrock below.
"You'd have to build a whole bunch of pilings, like you're building a bridge," said Khoury.
The engineering dilemma is interesting in an academic way. But in the end, if you spend enough money, you'll solve the problem one way or another.
Peter Drinkwater said extending the runway will only be feasible if it's affordable. He said the project must be paid for by airport revenue and federal grants.
The cost of the feasibility study, incidentally, will be $697,000.