New Mississippi River Bridge, St. Louis, MO

Project : I-70 Bridge over the Mississippi River
Location : St. Louis, Missouri
Client: Massman, Traylor Bros. and Alberici, JV

St Louis Mississippi River Bridge web.jpg

Project Description
With a main span of 1,500 feet, the new Mississippi River Bridge in St. Louis, Missouri will be the third longest cable-stayed bridge in the United States, yet has already set a new world record for the highest static load test ever recorded – 36,067 tons (321MN). The previous world record 31,300 tons (278MN), set in Korea by Loadtest at the site of the new Incheon Bridge, was surpassed by over 4,700 tons in St. Louis, with a total load of 36,067 tons (321 MN).

The Mississippi River crossings in downtown St. Louis and southwestern Illinois are some of the busiest in the U.S. where several interstates carry traffic across the river. The new bridge is expected to transport up to 55,000 vehicles daily, re-routing Interstate 70 from an overly-congested nearby bridge.

When the massive eight-lane original bridge design was deemed uneconomical, co-owners Missouri DOT and Illinois DOT opted for a sleek 1,500ft main deck carrying four lanes across the Mississippi, although the design calls for room to restripe to six lanes as needed.

The $640 million project includes the 1.22-mile bridge, supported by symmetrical cable-stayed delta towers, over 400 feet tall. Two large concrete footings, one near each bank, anchor the bridge below. Each pier rests on a series of concrete-filled drilled shafts, extending over 100 feet socketed into the limestone bedrock

Massman, Traylor Bros. and Alberici (MTA) presented an Alternative Technical Concept that included plans for six 11.5ft diameter drilled shafts, as opposed to the fourteen 10ft diameter shafts initially proposed. This new design, using fewer but wider supporting columns, is significantly more cost-effective since it halves construction time for the bridge foundation. The design has the additional advantage of a reduced environmental footprint with fewer drilled shafts.

Bi-directional load test arrangement
To perform the world record test, a 119ft deep test shaft was drilled into the sub-surface, which consisted primarily of sand and gravel underlain by solid rock. A core barrel roller bit and core extractor were used to excavate the rock socket, which was then airlifted after drilling was completed. A SONICALIPER was then used to profile the shaft excavation sidewalls.
Four 34in O-cells attached to a steel reinforcing frame were installed at the base of the 11.5ft diameter drilled shaft and reinforced concrete poured in to fill the socket and encase the cell. After concrete curing, the O-cell assembly was pressurized, loading the shaft in 19 loading incrementswith each successive load increment held constant for eight minutes.

The shaft was loaded to a maximum bi-directional load of over 18,000 tons, mobilizing a combined end bearing and side shear resistance of 36,067 tons (321 MN).

Summary
As is the case on many projects, the results of the O-cell test confirmed the use of an optimized engineering design, by verifying MTA’s alternative technical concept and allowing them to utilize a much more economical alternative than the original conventional design. Additionally, the testing footprint was minimal, despite the record-breaking loads applied. Loadtest’s company objective, which is to provide an accurate, high-quality tool for value-engineering, proved to be an invaluable benefit to this project.


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