Geotechnology Digs Deep to Evaluate an 80-year-old Structure

The U.S. General Services Administration (GSA) hired McCarthy Building Companies, Inc., to design and build a seismic retrofit of the historic structure along with seismic bracing of non-structural components such as ceilings, partitions and mechanical, electrical, plumbing and fire protection systems.

Although the GSA gave McCarthy initial geological and seismic information, including design documents from the 1920s, McCarthy needed more information to develop proper design parameters for an effective renovation. McCarthy’s team needed more information about the soil, bedrock, and ground water conditions around the building, and they also needed to know if the building was built in accordance with the design.

The Robert A. Young Federal (RAY) Building was completed in 1933 for the Terminal Railroad Association. The federal government bought the 1,050,000-square-foot, L-shaped building during World War II. It houses dozens of offices for various federal agencies, including the Internal Revenue Service, State Department, U.S. Army Corps of Engineers, and the U.S. Citizenship and Immigration Services.

The RAY building stands at the corner of Spruce Street and South Tucker Boulevard in downtown St. Louis, near the former course of Mill Creek along what was once Chouteau’s Pond. Other buildings to the east, near the edges of the former pond, had had foundation issues stemming from poor soils, which had to be repaired before the buildings could be renovated.

McCarthy hired Geotechnology, Inc., to explore soil, bedrock, and groundwater conditions, and assess the condition of the existing structure.

Geotechnology conducted a range of tests with test borings, seismographs, and scanning technology. Perhaps the most unusual was the effort Geotechnology made to determine if the foundations supporting the 83-year-old building matched the foundations drawn in the design documents.

To determine if the building really was supported by driven piles founded on bedrock, Geotechnology exhumed some of the piles beneath the pile caps. According to Geotechnology Project Manager Dan Greenwood, that meant going into the basement and excavating 10 feet below the basement slab: six feet to get to the pile caps, and then another three to four feet to get below the bottom of the pile cap. McCarthy/Castle excavated to the bottom of the pile cap and installed shoring, he said, them Geotechnology technicians excavated by hand at the bottom of the hole to get underneath the cap in order to test the piles.

“The drawings showed the type, number, spacing, and capacity of the piles, but as part of McCarthy’s due diligence, we had to verify that they were installed as per the plan,” Greenwood said.

Excavations exposed fluted columns, as shown in the drawings, but the structural engineers wanted to know if they could be relied on. Geotechnology suggested load-testing them.

In selected locations, Geotechnology cut 18-inch sections from four piles and tested them with a Pile Integrity Tester, which is essentially a hammer wired to some instruments. The reflected signals from the hammer taps showed piles that were in good condition and rested on bedrock 22-feet below the pile cap.

“We got a good, strong signal, which was reassuring to everyone,” said Greenwood.

That meant the piles were in good shape, but could they bear the 35-ton design load?

Geotechnology used a hydraulic jack to find out.  They inserted a calibrated jack in the space where they had cut out a length of pile and tested to 70 tons.

“There was no safe way to be in the hole while running a jack, so we used a down-hole camera and ran the test from a safe spot on the basement slab, remotely controlling the settings and watching it through a monitor,” Greenwood said.

The piles passed the 70-ton test, which provides a safety factor of twice the design rating. That was a big milestone.

“It has a huge impact on the design,” Greenwood said. If either pile or pile cap hadn’t withstood a 70-ton load, the amount of additional structural shoring required on the project to meet seismic guidelines “would have been a big deal,” Greenwood said, maybe even enough to kill the project.

After completing the tests, Geotechnology refastened the piles to the pile caps.

“It was truly an amazing blend of state-of-the-practice seismic evaluation with structural engineering and a nearly century old structure,” Greenwood said.

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