Challenges and Lessons Learned from Shoring Design of 2nd/Broadway Metro Station-Paper

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The 2nd/Broadway Metro station required a 500-foot-long excavation nearly 100 feet deep beneath an operating downtown Los Angeles street. Its support-of-excavation system used drilled soldier beams with tiebacks and internal struts. Design challenges included deep excavation, nearby utilities and buildings, traffic, Metro requirements for tieback de-tensioning, and underpinning part of the Los Angeles Times building that extended into the station.<br /><br />The most complex element was underpinning the building’s basement wall and supporting adjacent foundation loads while limiting settlement. The system combined ground anchors with concrete panels and diaphragms. Panels were installed one at a time beneath the wall to reduce movement; the first row used inverted-L panels to support the footing, and connected panels helped distribute loads across the system.<br /><br />Design followed Metro criteria. Limit-equilibrium analyses using DeepEX assessed loads and structural demands at every construction stage, while PLAXIS 2D finite-element analyses evaluated cumulative deflection. Modeling intermediate stages proved essential: removing a level of bracing increased the load on another level by 26 percent. Calculated bracing loads also served as construction benchmarks and informed contingency planning.<br /><br />The underpinning’s predicted maximum deflection was 0.41 inches, below Metro’s one-inch limit. Inclinometer, settlement, and tilt-monitoring data showed minimal movement, consistent with the model results.<br /><br />The key lessons were to analyze all construction stages—not only final excavation—and to combine conventional load-based design with finite-element deformation checks. Monitoring results helped validate the design assumptions and demonstrate that the underpinning system performed as intended.

Keywords

2nd/Broadway Metro station

deep excavation

support-of-excavation

drilled soldier beams

tiebacks

internal struts

underpinning

finite-element analysis

settlement monitoring

Los Angeles Times building