New Construction/ Retrofit Helical Piles at 1623 Farnam (Whiting-Turner)

Thursday, June 25th, 2020 by Kendra Dunnigan

Challenge

A large corporation was expanding its footprint in Omaha, Nebraska. This expansion involved moving into an existing downtown building occupied by other businesses. Logistically, the build-out was tricky because the other businesses would remain in operation for the duration of the project that involved large amounts of structural steel being added to the interior of the building to support new, heavy equipment that was to be moved in. A deep foundation system and helical piles were only part of the challenge.

The extra equipment and steel would cause an excess amount of load on the existing foundation, beyond its original design. Also, the project designers engineered the deflection criteria to a critical standard that necessitated little movement. The structure would have to be supported on a deep foundation system to better meet the engineering requirement. Large pile installation equipment couldn't be used due to the limited site access and low overhead clearance inside the building.

The location of the planned deep foundation system in the building was challenging, as half of the project was located in the lower level, 15 feet below street level. The second half of the project was in the sub-basement, 30 feet below street level.

Solution

“Thrasher created workarounds that had the General Contractors shooting video of us in amazement,” said Todd Royal, Solutions Specialist & Project Lead. “Often, we come up with an option B or C for projects. We were working on Option Z, this time.”

“It's not always this challenging. Nowhere on our proposals do we have a line item for renting a 100-ton crane to fly in our materials and machine. Often, we run into the differences between the conceptual needs that come to us “on paper” and what needs to happen. We have to be creative in delivering the means and methods – construction-speak for “figure it out,” Royal added.

The engineer of record for the project determined that helical piles were the best solution to ensure the long-term stability of the newly added infrastructure. Helical piles were selected as the preferred deep foundation due to limited access in the working area and the ability to install helical piles quickly with minimal ground disturbance and no spoils to maintain the tight construction schedule. The helical design included (61) HP350 (3.50-inch OD by 0.340-inch wall) hollow round shaft helical piles with a 10”-12”-14”-14”-14” helix blade configuration to support design working loads of 35 kips at each pile location. The helical piles were advanced to depths from 25-30 feet to achieve torque-correlated ultimate capacities of at least three times the design working loads (FOS ≥ 3.0).

Product installation was challenging due to limited building access. After “flying in” a 13,500-pound machine that would drive the piles, we had to get it into the right place in the building. To enter the sub-basement, a 7 ft x 12 ft hole was cut into the concrete floor at both ground level and the lower level at the south end of the building. A 7.5-ton gantry crane was used to lower both material and machine down 30 feet into the sub-basement. The lower level required 15 of the new construction helical piles and 8 of the retrofit piles. The sub-basement required 33 of the new construction piles and 5 of the retrofits.

Even after all of this, the project almost ground to a halt. The only way to access the location of several of the piles was a small room. The only skid-steer able to fit in the room wouldn't be able to handle the load necessary to drive the piles to the proper depth. Brent Foreman, Production Manager came up with the idea of “daisy-chaining” a second machine to the first one. With a series of hydraulics and hand-signals, they were able to drive the piles to their required depth.

In all, the project was split into three smaller projects. It took a combined 20 working days to complete with a 3-man crew.

Results:

A full-scale load test was performed in the lower level of the building before the installation of the helical piles. The results determined that the installed deep foundation system would deliver a capacity of at least three times the designed working loads.

"We have to be creative in delivering the 'means and methods' – construction-speak for 'figure it out' ” “Thrasher's work is always “out-of-the-gate” on a project. People are waiting, so we can't be late,” said Matt Cabrera, General Manager. “What we did on paper would be nearly impossible. I'm so proud of our crew for how they kept on this one. Obstacles and challenges kept threatening and they kept answering the bell. I'd put them up against anyone. They deliver!”