The four-ton hammers have been at it a week now. Pounding the timber pilings through the earth. Bashing the dust from the smooth yellow pines until the wood hits bedrock and the ground starts to shake.

Side-by-side the two rigs work, astride their lumber platforms in the middle of the Lincoln Memorial Reflecting Pool. Over and over, 50-foot timbers are lifted, aimed, driven. One every 12 feet, 9 inches. Almost a hundred a day. Five hundred a week. With 1,600 more to go.

Nearby, the statue of a seated Abraham Lincoln, himself an inventor and admirer of mechanics, looks on.

The 18-month, $30.7 million project to replace the 80-year-old reflecting pool, which began in November, is well underway.

Last week workers started sinking the pilings — 2,133, to be exact — that will support the new pool. The old pool, which dates to the 1920s, was not built on pilings.

The bulk of the old concrete pool has been ripped out, except for remnants of the original asphalt bottom, and construction supervisors explained Tuesday how the pile driving works.

The project is using two mechanical pile drivers, which resemble bulldozers with tall steel masts. Each rig has a long steel cable to lift the pilings, which look like telephone poles.

The top end of the piling is fitted into the square opening of the head of the hammer at the top of the mast. At the same time, the timber is grabbed by a set of metal claws and positioned over the spot where the piling is to be placed.

The hydraulic hammer then pushes the timber into the soft ground, at first only using the hammer’s weight, and then by banging it until it hits bedrock. The procedure must be done with power, precision and caution.

The ground shakes in the process, and vibration monitors have been placed near the Lincoln Memorial.

Sometimes the pilings snap, and two more must be sunk on either side of the broken one.

“You can’t hit it too hard,” said Dennis Brown of Corman Construction, a project engineer. “It’s a very fine line. We have to get it to bedrock . . . but you can’t hit it too much . . . [or] it will break in the ground.”

It’s like hammering “a toothpick through mud and hitting it on concrete,” he said.

Indeed, several shattered pilings were strewn on the ground around the construction site.

“Sometimes we hit it one time too extra,” construction superintendent Kyle Kern said. “Yesterday we only broke one pile,” and only three have snapped since the driving began last week.

“That’s really not bad,” he said.

The ground is essentially a 40-foot deep layer of soft, marshy river clay and some dredged material atop the bedrock, said Dennis M. Quinn, a project specialist with the National Park Service.

Each piling is treated to withstand exposure to the brackish underground water, said Brown, the Corman engineer.

A metal “shoe” is nailed on to the tip of the piling to help it penetrate. The process recalls the mythical steel “driver” John Henry, who with his hammers beat out a steam-powered railroad spike driver in a contest.

Noting that safety always comes first, Brown said that rig operators Matt Deshong and Harry Stavanovich are given a goal every day. “They challenge themselves,” he said. “We do have two machines, and it is competitive for the guys. We want to make sure most of all that they’re safe.”

The reflecting pool is roughly 160 feet wide and 2,100 feet long and is one of the most iconic sites in the nation. It dates from shortly after 1922, when the Lincoln Memorial was dedicated before a crowd that included Lincoln’s only surviving son, Robert, who was then 78. It quickly became a gathering place.

In August 1963, civil rights marchers crowded around it to hear Martin Luther King Jr. deliver his “I Have a Dream” speech. In 1970, war protesters faced off with riot police in it. And in 2009, it was rimmed with people at a pre-inauguration salute to Barack Obama.

Work is to start on the new concrete pool in July. It will require almost 11,000 cubic yards of concrete, hauled in by relays of more than a dozen trucks.

“We build things here for 50 to 100 years,” Quinn said. “Not 20.”