Blasts test foundations for high-speed rail bridge over Highway 99

Engineers and contractors for California’s high-speed rail authority had quite a blast Thursday — four of them, actually.

Using increasing amounts of an explosive called nitrocellulose, they conducted four stress tests on a concrete piling festooned with sensors and buried 80 feet deep in the earth near Highway 99 at the south end of Fresno. The testing was part of engineering work for a 3,700-foot-long elevated bridge to carry high-speed trains over Cedar Avenue and the freeway.

Each blast propelled a piston ram into the side of the piling to simulate the lateral stresses that an earthquake might inflict on a bridge foundation.

The goal of the test was to try to batter the concrete to its breaking point. But Bobby Pentorali, a construction project engineer for the rail authority, said the tests are intended more to measure the properties of the soil rather than the piling itself.

“Those properties will guide us when we design the foundations for the larger viaduct structure that will cross right here over Cedar Avenue and above Highway 99,” he said. “The soil is what holds the piling, and when we put these large loads on it, how much it moves and strains depends on the properties of the soil.”

More than 40 pilings will eventually be needed to support the elevated bridge, Pentorali added.

“These expensive tests actually pencil out when we have so many of these piles to build,” he said.

The testing happened in a vacant field at the intersection of Cedar and North avenues, just east of Highway 99. The fourth, and most powerful, of the blasts happened shortly before 1:30 p.m. Crews with subcontractor Becho Inc. said the final test generated about 1,400 tons of force against the exposed top of the concrete pillar. (Correction: The original version of this story incorrectly reported the force as 280,000 pounds.)

The thrust of the day’s final blast forced a massive set of steel weights — which formed the cylinder that held the ram — backwards about 20 feet. Think of it like the recoil of a gun, but on a much-magnified scale and a heck of a lot louder.

Similar tests were carried out last summer in Madera near the Fresno River, where construction got underway this week on a viaduct that will span the river, Highway 145 and Raymond Road near the BNSF Railway freight tracks. Such testing also occurred several months ago on the south side of the San Joaquin River near Highway 99 for a bridge over the Union Pacific Railroad tracks and Golden State Boulevard near the Fresno-Madera county line.

All three bridges are part of the authority’s Construction Package 1, a 29-mile stretch of the rail line from the northeast edge of Madera to the south end of Fresno.

This story was originally published June 18, 2015 at 4:49 PM with the headline "Blasts test foundations for high-speed rail bridge over Highway 99."