Bottoms up

Eighty years after piers were constructed in San Francisco Bay to hold up the eastern span of the San Francisco-Oakland Bay Bridge, a specialized contractor was preparing to blast its sturdy footings. Demolishing the reinforced concrete piers was the final step in the removal of the nearly 2-mile-long span. To accomplish this complex task in an environmentally friendly way, a Ranger DX800 drill rig from Sandvik Construction has been working on-site.

The San Francisco-Oakland Bay Bridge already has been replaced by an adjacent $6.4 billion structure that claims the title of world’s widest bridge. While the new bridge is impressive, so is the demolition job being undertaken by Florida-based Contract Drilling & Blasting LLC. The company was tasked with making a pier disappear cleanly beneath the bay bottom without disturbing avian and marine life.

The contractor was tasked with drilling 159 holes in Pier E3, chosen to be the demonstration pier for explosives demolition.

Cartridge charges were placed in the holes at several levels, and the pier imploded without negative impacts to marine life or environment.

Environmental concerns were magnified because this is California’s first major blasting demolition in water. This means that the dust and slurry created by concrete dust emanating from the demolition could not be allowed to degrade water quality.

To deal with this potential problem, a system of garbage cans was put around the drill, and all drilling residue and cuttings, exclusive of aggregates, were collected in a covered container for hauling to shore. It was essential that no leaks or blown hoses occur as this could lead to contamination of the water. Clearly, Contract Drilling & Blasting had a big job on its hands.

HAVING A BLAST

The job was, however, approached with confidence as the contractor had done similar work before, though perhaps not under such tight environmental restraints. The company has developed a first-class reputation in its field and now travels the world undertaking specialized blasting and demolition projects. This includes the demolition of marine structures and bridge superstructures, as well as other underwater blasting projects.

Ken Tully, blasting specialist for the company, recently successfully demolished bridge piers in British Columbia that were diligently monitored for any negative environmental impact and was the controlled blasting specialist for the E3 demolition project. Tully was responsible for designing the blast, overseeing the drill operations and conducting the blasting operations.

This emphasis on employee expertise on the project led Danny Deskins to drill the holes in the demonstration pier. Though Pier E3 was the first pier he worked on, Deskins is a 26-year veteran of precision drilling and was able to meet the challenges head-on. His expertise proved vital as the structure’s vertical rebar and crossties tested both machine and operator but were drilled through successfully. Other surmounted challenges included jagged surfaces on the tops of walls and soft spots in the concrete, both of which can send a drill bit skittering one way or the other.

The drilled pier is 80 feet wide by 130 feet long and stretches downward 289 feet from its cap, with the last 180 feet being in the muddy bottom of the bay. Beneath the pier’s cap is a supporting grid honeycombed with voids. Horizontal cross-sections periodically intersect with 3-foot-thick reinforced concrete walls. Holes were drilled into each of the 3-foot interior walls as well as in 4-foot-thick exterior walls. The holes were drilled in two depths—64 feet and 86 feet—in a pattern designed to neatly capture the exploded and inward-collapsing material at the bottom of the pier’s footprint. More than 558 individual electronic detonations would be separately initiated on the multiple decks, with a total detonation time of 4.6 seconds.

PRECISION DRILLING

To accomplish the demolition, a 37,000-pound drilling machine was crane-loaded onto a barge, floated to the pier and offloaded atop it. When in place, Deskins drilled 10 hours per day, five days per week. What made the drilling especially tricky, aside from environmental considerations, was the relatively thin walls. Drilling a hole 2.75 inches in diameter for up to 86 feet with little to no deviation is not a simple task. “The trick was getting to the bottom of the hole without going out one side of the wall,” Deskins says.

Tully, the blasting specialist, specifically chose a Sandvik Ranger DX800 drill rig for the challenging job. “I wanted this machine,” he says. “I have used similar Sandvik equipment on other projects and was very satisfied. The accuracy and trueness of the holes was fantastic.”

Aggregate Crusher Specialists (ACS), the Sandvik drilling and stationary crushing and screening equipment dealer for Nevada and California, supplied the machine for rental when Contract Drilling & Blasting needed it. ACS President Mike Murphy says, “Time is of the essence in this industry; we strive to be able to supply the right equipment and the best service at the right time. Barry Wells, our drill specialist, was present on-site the first few days of the drill arriving to the pier and supplied the technical support during the project.”

Tully has worked around Sandvik drilling rigs for 20 years—including with predecessor Tamrock units—and says he always has been impressed with their productivity. He says he believes three features on the Ranger DX800 were critical on drilling Pier E3. One was the rig’s reach, which in the limited pier area can be difficult and sometimes up to 2 to 3 feet. Another was its ability to revolve its superstructure up to 180 degrees and drill multiple holes from the same location. Those features minimize setup time and, consequently, increase time spent drilling.

The other critical feature was Sandvik’s TIM5300 system, which measures depth and inclination. The drilling accuracy of the TIM5300 was needed because the pier’s 3-foot-thick walls were poured in place, sometimes in multiple pours, and were not expected to run true.

Guided by the system, combined with Tully’s selected drill string and operator, the Ranger DX800 proved productive. “There were lots of surprises and adjustments to make, but we made them successfully,” Tully says.

“This project was undertaken in a controlled drilling atmosphere rather than a production drilling atmosphere,” he continues. “I would rather take an hour on a single hole than quickly drill a bad one.”

Deskins says he couldn’t imagine tackling the task with any other drill. He calls the Ranger DX800 operator friendly. The Rock Pilot+ control system measures the hardness of the material and adjusts accordingly to produce a straight hole.

It took less than six seconds to destroy the pier after it was drilled and laced with electronically sequenced explosives. Contract Drilling & Blasting set up the blast in Nov. 14, 2015. This date was selected because demolition during the fall poses the least risk to San Francisco Bay’s fish and wildlife populations, including porpoises, sea lions and seals.

Delicate tasks like this one are doable because drilling specialists and Sandvik continue to refine the machines that can do the job.

Giles Lambertson is a freelance writer who has been writing about the construction industry for nearly two decades. A version of this article, which was submitted by Sweden-based Sandvik Construction, first ran in the March/April issue of Construction & Demolition Recycling, a sister publication to Recycling Today.