Putting down roots
Rob Carson; The News Tribune
You can't see it when driving on the Tacoma Narrows Bridge, but what used to be Tacoma's War Memorial Park is now a gigantic hole in the ground.
At 116 feet across and 63 feet deep, the hole, just south of the bridge, is big enough to mesmerize. Visiting engineers, officials and journalists gather around its rim and gaze respectfully downward like tourists at the Grand Canyon.
Early Wednesday, crews will begin turning the hole into a gigantic block of concrete, a Goliath designed to hold down 50 million pounds of force exerted on it by the suspension cables of the new Tacoma Narrows Bridge.
Flint Gard, an anchorage superintendent with Tacoma Narrows Constructors, says he doesn't expect the construction process - slated to finish in March 2005 - to be technically difficult.
"Just the sheer size of it makes it challenging," he said, "but it's really pretty much a logical progression of steps.
"It's like what they say about eating an elephant," Gard said with a grin. "You just have to take it one bite at a time."
Building the Tacoma-side anchorage and its twin on the Gig Harbor end of the bridge might not be technically difficult, but the design process that went into them was.
The two identical 90 million-pound structures are the product of thousands of hours of site research and sophisticated calculations involving soil mechanics.
The anchorages not only must provide enough ballast to support nearly the entire weight of the new bridge, but they also must be tough enough to withstand earthquakes and terrorist bombs.
In their innermost cores, they provide the tie-down points for the main bridge cables, artful arrays that resemble the inside of a grand piano.
The science of building suspension bridge anchorages has changed little in the past 150 years. But there are other aspects of the construction process that have changed dramatically.
The anchorages for the existing bridge, completed in 1950 and just a few steps away from the new one, present a dramatic before-and-after study.
"It's a new age now," Gard said. "For one thing, we know more about soil than we used to."
Gard, 42, is a Washington native whose family has a long history in the South Sound. His grandparents settled near Olalla in 1908.
He was born in Enumclaw and studied civil engineering at Washington State University in Pullman.
Gard said he's headed up several big construction projects along the West Coast and in Hawaii, "but nothing this big."
Like most people working on the new bridge, he's thrilled to have a role in such a prominent project.
"This kind of thing only comes around once in a while," he said. "There are not too many people who can say they've done it."
BRIDGING THE GAP
Efficiency is always a goal of construction projects as huge as the new Narrows bridge, but the economics of TNC's design/build contract with the state sort of requires it.
TNC agreed to design and build the bridge for a fixed price of $615 million. The more efficiently it can do the job, the more of that check will be profit.
What that meant in terms of designing the anchorages, Gard said, was that the less mass they required, the less they would cost.
The new anchorages are smarter than the ones on the existing bridge, Gard said. Rather than sheer bulk, they rely on shape to do their jobs with finesse and technique.
Designers searched for a shape that would maximize the anchorages' holding power and put their bulk precisely where it is needed to resist forces put on them by the mile-long bridge cables. That required an exacting analysis of soil conditions and of the forces the new bridge will exert.
The new anchorages will be slightly beefier than those holding up the existing bridge, but only because the new bridge is being built to accommodate a second deck in case such an addition is ever deemed necessary.
On a strength-per-pound basis, the new anchorages are much more efficient. According to bridge engineers, they have a built-in safety factor enabling them to hold down nearly twice as much force as the 25 million pounds each cable will exert.
In designing the new anchorages, designers began with some nonnegotiable givens, according to Tim Moore, the state Department of Transportation's authority on bridges and the man in charge of reviewing TNC's designs.
The center of the new bridge will be just 200 feet from the center of the old bridge, which restricted the anchorages' width.
And the requirement for a potential second roadway, beneath the first one, meant the anchorages had to have an opening through them to accommodate possible future traffic.
The depth of the water table also limited the design. Water can act as a glue in soil, but also as a lubricant. The anchorages had to built above the water table to avoid landslides.
On the Gig Harbor side, the water table is low enough so that it did not present a problem. But on the Tacoma side, the water table is only about 60 feet below the surface. That set a depth limit for the anchorages.
AN EXACT SCIENCE
To make sure the force of the bridge wouldn't pull the anchorages out of the ground, designers had to calculate the size and direction of pull and again use detailed analysis of soil properties.
To demonstrate, Moore placed a book flat on the top of his desk and nudged it with his index finger.
"How do you figure how much force it's going to take to move this book?" he asked. "It's the weight of the book and the amount of friction between it and the table. The same is true with the anchorages."
To determine how much friction the massive concrete blocks will create against the soils surrounding them, engineers mathematically subdivided the surrounding earth into thousands of individual cubes, each with its own individual physical properties and susceptibility to stress.
Using computer simulations, they combined thousands of individual predictions, arriving at a composite that showed how the structures would react as a whole.
The direction of force was also critical in designing the anchorages, Moore said.
The bridge cables will pull forward and upward on the anchorages, tending to push their forward edges downward and making the rear ends want to rise, rotating on their center of gravity.
"You have to have enough mass in there to keep it from being unstable," Moore explained.
For that reason, the rear ends of the anchorages are deeper than the front. In profile, the buried anchorages resemble a map of Texas, with a huge hump in the rear.
The irregular shape fits into the earth like a key in a lock, making it less likely to slide.
"The question was not only, 'Is the bridge going to move the anchor?'" Moore said. "It was, 'Is it going to try to pull the entire hillside apart?'"
According to geologists, that is a substantial concern.
The standard geologic theory used to be that the water rushing through the Narrows was steadily increasing the channel's depth. Now, geologists know the banks are steadily crumbling off in landslides, gradually filling up the channel.
To figure out how the soils on the uplands are likely to react under forces exerted on them by the bridge, engineers first had to determine what the hillsides were made of.
To do that, they bored 3-inch holes 150 feet deep at the anchorage sites and extracted core samples.
They found bands of silt, sands and clays, some firmly compacted by the mile-high glacier that covered the South Sound during the last ice age. Other samples had loose deposits with low density and strength.
They then used the borings to construct a detailed, three-dimensional soils map, enabling them to come up with potential "slip planes," where landslides are most likely to occur.
The anchorages had to be designed to put the least stress on those areas.
Another of TNC's efficiency moves was more controversial. Rather than make the anchorages out of solid concrete, the builder will provide about 7 percent of the anchorages' total mass using compacted sand in a 20-foot-tall sandbox set in the anchorages' rear ends.
That technique has never been used in a suspension bridge anchorage, Moore said, and it offends some purists' sense of construction aesthetics. Moore said he was not particularly proud of the cost-saving measure, though he agrees it will work.
Gart agreed the sandbox is simply sensible: "Making it solid concrete would have been a waste."
Taken as a whole, the scientific innovations give engineers a sense of control and confidence previously impossible in such a large anchorage project.
Moore said they can predict to the mere fraction of an inch how much the anchorages will move when the cables are attached to them and the full load of the bridge is applied.
When fully loaded, the anchorage at the Gig Harbor end of the bridge will move three-fifths of an inch horizontally and three-quarters of an inch downward, Moore said.
The Tacoma anchorage, slightly stiffer, will budge even less - just two-fifths of an inch in each direction - about the width of a ballpoint pen.
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Rob Carson: 253-597-8693
rob.carson@thenewstribune.com
