The Use of Rigid Inclusions to Support Large Spans-Paper

Paper

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The South Capitol Street Corridor project used rigid inclusions (RIs), specifically Controlled Modulus Columns, to reduce settlement beneath roadway fills and MSE walls while protecting aging sewer utilities. This paper focuses on a support system spanning twin utilities beneath the East Oval, where up to 14 feet of fill would impose about 2,000 psf. The utilities could not tolerate construction-related stress or settlement.<br /><br />The team selected adjacent, prestressed 48-by-48-inch concrete box beams spanning approximately 62 feet across the utilities. The beams rest on continuous concrete grade beams supported by 77 RIs. The RIs are not structurally connected to the grade beams, limiting the forces they receive to mainly axial loads. A void beneath the superstructure prevents vertical load transfer to the existing utilities.<br /><br />Three-dimensional Plaxis modeling informed RI layout and predicted post-construction settlement of 0.15 to 0.8 inch, within the project’s one-inch limit. RI spacing was adjusted outward from the utility to reduce differential settlement. Because modeled RI loads were unusually high, the design team verified capacity through a static load test on an instrumented, 18-inch-diameter, 58-foot RI. The test reached 580 kips—twice the 290-kip design load—with about 0.36 inch of deflection. Movement at design load was approximately 0.15 inch, less than predicted. Strain-gauge results also showed adequate soil resistance, though production columns still needed to extend into the underlying Potomac clay.<br /><br />The project demonstrates how RIs can provide a cost- and schedule-efficient alternative to conventional pile-supported systems. Coordinated structural and geotechnical design, settlement modeling, and load testing confirmed that the unconnected RI and grade-beam system could protect the utilities and meet long-term settlement requirements.

Keywords

controlled modulus columns

rigid inclusions

settlement reduction

MSE walls

utility protection

Plaxis modeling

grade beams

static load test

post-construction settlement

South Capitol Street Corridor