Freeway tunnel’s long road back
Huddled in the soot-blackened tunnel that burned in the aftermath of last Saturday’s freeway crash in Elysian Valley, a team of fluorescently dressed, hard-hatted engineers peered at the mottled ceiling. One shined a penlight into a far-away corner. Another took pictures.
A drilling machine nearly drowned out their voices as they discussed the fire-eroded surface, a roughened texture of stones and concrete that looked like a neo-Expressionist art installation.
They had just come from a conference room where they spoke in the easy shorthand of the trade, words passing among them like “carbonation,” “incipient spalls” and “surface delamination.”
Five days earlier, a tanker truck overturned in this cavernous hollow connecting the northbound 2 Freeway with the northbound 5 Freeway. Fed by 8,500 gallons of gasoline, the fire peaked at more than 1,000 degrees and burned for more than an hour with the intensity of a blast furnace. Its flames licked the upper deck of the 2 Freeway, 40 feet above the tunnel.
As impervious as concrete might seem to both fire and heat, engineers know the contrary.
Tom Brake, a senior bridge engineer for Caltrans, stepped away from the huddle -- and talked about the worst-case scenario. When asked, he admitted that the tunnel could have collapsed, carrying with it a stretch of the 5 Freeway 20 feet overhead.
After the fire, the steady concussion of traffic on the roadbed above would have put the fire-damaged concrete at risk of failure, and Los Angeles hasn’t seen a freeway collapse since the Northridge earthquake in 1994.
Now with the shoring safely in place -- long girders and thick posts supporting the ceiling -- Caltrans was assessing the damage. Brake and a dozen other engineers needed to know if the fire had compromised the strength of the concrete. Until that is known, the tunnel will be closed and the bulwark-like shoring will stay in place.
Standing near the southern entrance of the tunnel, Brake unfolded a set of blueprints for Bridge No. 53-0577.
The southern stretch of the 2 Freeway is more of a stub than a thoroughfare. Designers had intended for the route, conceived in the 1950s, to connect to the Hollywood Freeway less than a mile away, but this intention was never realized.
Once known as the Alessandro Freeway, named for a character from the novel “Ramona,” the 2 Freeway has little romance as it descends the eastern flanks of the Hollywood Hills toward the industrial surrounds of the Los Angeles River. Its intersection with the 5 Freeway is a skein of on- and off-ramps that on a map look like a game of cat’s cradle.
Brake had been reading the blueprints since Monday and was encouraged to learn that the walls and ceiling were at a minimum 2 feet thick.
“Fifty-three years later,” he said, referring to the date on the blueprints, Jan. 25, 1960, “and the tunnel is still functional.”
Over the weekend, he might not have been so optimistic. He had just returned from an out-of-town trip when his phone rang. He had seen the pictures on television, and the thick smoke and the purling flames didn’t bode well. His supervisor told him to be prepared for a busy week.
Years ago as a student in Chicago, Brake had wanted to design high-rises, but by the early 1990s, no one was hiring, so he turned to bridges and forgot the tall buildings.
“These are massively huge structures that take incredible loads and can be very beautiful as well, and the entire economy relies upon them,” said Brake, 50.
His work took him from Washington to California, and for a time he lived in the Bay Area, where his job had him rappelling down the sides of the Bay Bridge to examine its girders, joints and tower legs.
Initial assessments of the damage in the tunnel suggested that the repairs would be extensive. When concrete is exposed to heat and fire, its chemical composition changes. Water molecules in the cement begin to evaporate, creating cracks that weaken the structure.
The cracks also make the concrete brittle, a phenomena known as spalling. It flakes and crumbles, and the floor of the tunnel was littered with pieces of the walls and ceiling. The metal reinforcing rods that support the concrete were exposed.
Only by taking samples of the concrete would the engineers know the extent of the damage.
On Tuesday, they identified 32 locations to be tested, and the next day, crews began cutting holes into the walls and the ceiling.
Standing in a dusting of concrete, a man on a boom lift 12 feet above the roadbed drilled into the west wall. A light tower gave the scene a cinematic drama. He extracted a small column of concrete, 4 inches in diameter and 10 inches long.
On the east wall, pigeons tried to find a place atop a tunnel light that hadn’t melted. A tagging crew’s incinerated cursive glazed the wall above the shoulder.
Some samples would be placed in a vise-like machine that would slowly compress them, measuring pounds per square inch, until they shattered. Other columns would be sliced like bread and impregnated with a dye that would filigree the microscopic cracks.
Other tests are not as sophisticated. A hammer swung onto a solid piece of concrete delivers a high, resonant ping. A hammer swung onto a compromised piece of concrete delivers a hollow thud.
From what Brake had seen and heard so far, he believed that the repairs would be modest. He charted out a possible scenario.
Crews would demolish the burned walls and ceiling with a blast of water that could take the concrete down to the metal rods. Then a new surface of concrete would be shot onto the walls with a pressurized cannon.
Two men push a broom through the tunnel, shoveling fallen debris into a skip loader.
More tests will be conducted, including an impact-echo test that drives sound waves through the walls, which are then measured with a sonar-like device.
The road, which is also marked with pocks and divots, needs to be studied. One of Brake’s colleagues was concerned that the gasoline had seeped beneath it and burned like an underground mine fire.
Any predictions, Brake says, are premature. Test results will be available next week, but he has been told that a few of the core samples shattered at 3,800 to 5,000 pounds per square inch.
“Well above the specified design strength,” he said, of 3,250 pounds per square inch, the standard established by his agency decades ago.