While the recent shutdown of an 11-mile stretch of the Interstate 405 in Los Angeles proved to be more “Carma-good” than “Carmageddon,” Southern California transportation officials are quick to point out the operation didn’t go smoothly by accident.

“This is a situation where all parties involved were 100% focused on getting the job done safely and effectively – and exceeding expectations,” said Ron Rattai, project manager for Kiewit Infrastructure Group, Los Angeles, in a statement issued July 18. Kiewit is general contractor for the effort, dubbed the I-405 Sepulveda Pass Widening Project.

Kiewit earned a $300,000 bonus on the quick completion. The contractor had faced penalties of $72,000 an hour for any delays past the agreed shutdown time.

For two days beginning the evening of July 15, the 405 from the I-10 to the U.S. 101 was shut down in both directions to allow workers to partially demolish the Mulholland Drive Bridge. While many observers worried the closure would wreak traffic havoc across L.A., the event turned out to be decidedly uneventful:  Forewarned motorists stayed well away from the 405 that weekend, and road crews were able to complete the needed work by July 17 – a full 18 hours ahead of schedule with minimal disruptions to traffic.

But, despite all the exceeded expectations, what was it about the Sepulveda Pass improvement project that required transportation officials to completely shut down the fearsome 405 in the first place? According to Metropolitan Transportation Authority spokesman Dave Sotero and others, the answer resided in a simple engineering decision made more than half a century ago.

The 405 closure was part of an ongoing program to improve the freeway between the I-10 and U.S. 101 freeways. Whether all these improvements actually happen is anything but certain: Only $614 million in state bond money has been designated for the $1 billion project. Even that revenue is up in the air since the state canceled its early 2011 bond sales, though prospects improved with Sacramento’s recent budget agreement and improved bond rating.  But, assuming the joint effort by the MTA and Caltrans is fully funded, it would standardize lane sizes, add 10 miles of high-occupancy-vehicle lanes and 18 miles of sound- and retaining walls, install a flyover ramp on Wilshire Boulevard, widen 13 underpasses and re-align 27 on- and off-ramps.

The project also includes the replacement of three overpasses, and therein laid the seeds of Carmageddon. Two of the overpasses – the Sunset Boulevard and Skirball Center Drive bridges – were designed with center support columns. The load-bearing pylons enabled workers to demolish the bridges in stages while keeping the freeway underneath open. The Sunset overpass was demolished in January 2010, the Skirball Center Drive overpass 10 months later.

But the 579-foot-long Mulholland Drive Bridge was different. Built in 1960 (coincidentally enough, by Kiewit), when no one realized just how much Angelenos would come to rely on the highway that ran beneath it, the bridge had no center columns. “There was simply no way we could safely demolish that bridge with the freeway open,” Sotero said.All that could be done, he said, was for the 405 to be shuttered in both directions, while workers destroyed the southern half of the bridge, rebuild it over an 11-month period, then destroy and rebuild the northern half. When finished, expected sometime in 2013, the new bridge would be 680 feet long.

And so, at 7 p.m. Friday, July 15, workers began closing lanes on the Sepulveda Pass one at a time until the entire freeway was offline. Right around midnight, 85 trucks hauled in a veritable mountain of dirt, which an army of workers used to build a 4-foot high dirt stack under the Mulholland Drive Bridge to cushion falling debris. Sometime around 2 a.m. July 16, crews began cutting away at the southern half of the bridge, starting at the center and moving out. By the middle of that day, massive hoe rams with 80-foot arms began slowly chipping away at the southern portion from underneath the bridge.

“The hoe rams have to cut the concrete pieces down to sizes that can be easily removed from the demolition site,” said Dan Kulka, community relations manager for Kiewit, in June. “We’ll be recycling the concrete and rebar, which will be stored at yards scattered across the project area.”