Clang! … Clang! … Clang! … Clang!

That used to be the familiar and incessant sound blaring from construction sites for tall buildings where towering pile drivers rammed concrete pilings deep into the ground. Such hammering was a clear and disturbing indication that crews were laying foundation for another condominium, hotel or office tower that would soon rise.

Nowadays, though, it’s a quieter and largely unnoticed process that is being used to anchor the latest generation of towers in Honolulu — mainly condos in Kakaako.

So, what keeps them from sinking or falling over?

It’s still a network of steel-reinforced concrete piles that act like tree roots to support towers as high as 400 feet where people reside or work.

There can be 200 to 250 piles in a tower’s foundation, or substructure, which also includes beds of concrete inlaid with crisscrossed reinforcing steel rods that lock together clusters of piles. A typical concrete bed, or pile cap, covering four piles can be a cube 10 feet on all sides, weigh 64,000 pounds and shoulder a load of 3 million to 4 million pounds.

All the pile caps are usually 2 to 5 feet below a building’s base floor, so they can’t be seen. But they are connected to, and support, columns that run up into a tower.

What’s changed with the piles in recent decades is how they are formed and installed in the ground. For towers that have come out of the ground in recent years around Honolulu, piles are generally used in fewer quantities, have larger diameters, run to shallower depths, take less time to install and can support greater loads.

“There’s been a lot of change in the last 20 years,” said Robin Lim, president of Honolulu-based geotechnical engineering firm Geolabs Inc.

Lim said the old method of hammering piles into the ground typically involved driving them down until they hit hard earth. That could be 30 to 100 feet deep, though some piles might extend 200 to 300 feet deep. The depth required was determined by a piling’s load need and the resistance of each hammer blow.

The clanging from pile driving rang out around Hawaii into the 1990s as the method for building tower foundations, but that changed due to an experience building a freeway in the 1980s and 1990s.

Lim, who has been a geotechnical engineer in Hawaii for more than 20 years, said that the local construction industry embraced the alternate method of building foundations — drilling shafts into the ground and filling the holes with steel rebar and concrete — after running into numerous difficulties driving piles for the H-3 freeway in Halawa Valley, where cobblestones and boulders inhibited pile driving.

The drilled-shaft technology had been in use on the mainland, but Hawaii’s construction industry was deterred from using it because equipment for a new system would cost a lot and the new system would have to be proven superior in terms of expense and performance. Work on H-3, which finished in 1997, essentially took on and removed those hurdles.

After that, other projects embraced the drilled-shaft method.

By drilling shafts, the sides of a hole are left with rough edges that form a stronger connection with the cured concrete after it is poured into the hole. As a result, the number and depth of drilled shafts are reduced compared with driven piles.

Cedric Ota, vice president of the waterfront and foundations division of Hawaiian Dredging Construction Co., compared drilled-shaft foundations to anchoring something to a piece of wood with a screw, and pile driving to anchoring something to a piece of wood using a nail.

Lim said the last high-rise in Honolulu he remembers using driven piles was the Lanikea at Waikiki condo, completed in 2005.

A few Honolulu tower projects in the 1990s used drilled-shaft foundations, including the One Archer Lane condo. Then a wave followed with the Kalia Tower hotel and time-share at Hilton Hawaiian Village in Waikiki that opened in 2001 and a slew of Kakaako condos including Moana Pacific, Ko‘olani, 909 Kapiolani and Pacifica Honolulu.

Builders also began using an alternate method of foundation construction about 15 years ago that is now used for virtually all new towers in Honolulu. This method is known as augered cast-in-place piles, or ACIP piles.

ACIP differs from drilled shafts in that the hole is created using one long corkscrewlike drill, or auger, that has a cutting tip and a continuous spiraled channel on which earth is moved to the surface. The length of the auger equals the depth of the hole. By comparison, the drilled-shaft method uses a short drill bit repeatedly inserted and removed to dig the hole.

Ota said ACIP equipment, which includes a much more powerful motor at the top of a crane, is more expensive, but the drilling time is quicker. After the industry began using augers 24 inches in diameter, up from 18 inches, ACIP piles became more efficient because they could support more tower weight and resulted in fewer pilings in the ground.

The typical depth for 24-inch ACIP piles anchoring a tower ranges from about 80 to 130 feet.

The maximum depth for ACIP piles is about 130 feet based on available equipment in Hawaii, according to Lim. So in some instances where soft ground runs deep, drilled shafts that reach down farther are necessary. In more solid ground, such as former coral, ACIP piles that go down just 40 feet can anchor a 400-foot tower.

The advantages of ACIP pilings have led to their use in almost all new towers built in Honolulu since 2013, Lim said.

BUILDING A FIRM FOUNDATION