FORWARD OPERATING BASE APACHE, Afghanistan -- At FOB Apache, U.S. military engineers are frantically finishing a second chow hall and a new recreational building four times bigger than the old one. The gym will be expanded soon to more than double its current size, and dozens of new tents and rows of gleaming white containerized housing units are sprouting to prepare for an influx of troops who’ll raise the base’s population from several hundred to a few thousand.

Here, amid the shriek of electric saws and sharp scent of fresh plywood, the U.S. and NATO drawdown looks more like a frenzied buildup.

It’s deceptive. The building boom at FOB Apache is a quirk of the planned pullout of more than half the U.S. and NATO forces this year. It’s one of the largest of a handful of construction projects under way across Afghanistan, aimed at fine-tuning where troops and their equipment are based in preparation for their final departure next year.

FOB Apache is taking in troops from about half a dozen smaller, front-line bases elsewhere in Zabul province – which is in the restive southwest corner of the country – so that those bases can be torn down or handed over to the Afghan government. It’s adjacent to a major Afghan army base, Camp Eagle, which makes it easier for U.S. troops at Apache to perform their new main roles as advisers and trainers to the Afghans.

"As the Afghans take the lead, we’ve got to have someplace to move into behind them," said Lt. Col. Andrew S. Ring, who oversees construction for the 555th Engineer Brigade from Joint Base Lewis-McChord near Tacoma, Wash.

A multi-service task force of more than 5,000 active-duty, National Guard and reserve troops led by the 555th is responsible for most of the build-it-so-we-can-leave projects. Those include FOB Apache, a few smaller but similar bases and the massive Camp John Pratt near Mazar-i-Sharif, in the far north, which will be the staging area for some U.S. and NATO equipment to exit via Uzbekistan. From there, shipments can move by rail through Central Asia to ports. The rest of the equipment is expected to leave through Pakistani ports and by air.

The deconstruction of bases is about to accelerate, said Col. Nicholas Katers, the 555th’s commander.

Afghanistan held about 800 U.S. and NATO facilities in late 2011, according to figures supplied by NATO. More than 600 of them had been shut down or handed over to the Afghans by the beginning of this year. Nearly all of those were tiny bases suitable for a few dozen troops or fewer. Some were simply checkpoints.

Now the hard part is beginning, Katers said. His task force is about to begin closing or transferring 15 to 20 bases that once held hundreds or thousands of troops each. That will be significantly trickier than tearing down the smaller bases. The engineers don’t know quite what to expect.

"We do know it’s going to be difficult, because we’ve been in some of these bases a long time; 10 years," Katers said.

They’re in a race with time. The engineers need to finish most of the tear-downs and refits this year, as the drawdown – which is expected to accelerate in the fall as the fighting season ends – will leave little manpower for the mission by next year.

"Summer and fall will be critical,” Katers said, noting that if the drawdown occurs as scheduled, fewer than half the current 66,000 U.S. troops will remain in Afghanistan next year. And they’ll be needed primarily for advising and training Afghan troops, not building and decommissioning bases.