Abstract:
The Minnesota Department of Transportation undertook a large-scale reconstruction of the I-35/I-535/Hwy 53 interchange along the Saint Louis Bay of Lake Superior in Duluth, MN. The Twin Ports Interchange Project consists of replacing the outdated system of bridges that connected the highways, long referred to as the “Can-of-Worms.†This system comprised 33 bridge structures supported by various piling types due to the highly variable soils across the project. The soil profile consisted generally of urban fills underlain with varying glacial deposits of sands, silts, and clays, overlying till above the bedrock. Reconstruction involved replacing many of these bridge structures with new ramps and approach embankments built on improved ground. The ground improvement system included over 6,600 unreinforced grouted rigid inclusions totaling over 330,000 LF. The rigid inclusions were 16-in diameter and extended as deep as 105-ft to their targeted bearing layers. While an extensive soil investigation program was done before construction, assumptions were necessary between borings to assign target depths to each rigid inclusion element. During the construction of the rigid inclusion system, bearing criteria were established by the project team using the target depths as a guide and then validating the actual bearing layers through real-time monitoring of the drill rig response/pressures. This allowed the project team to ensure during installation that each element was bearing on the appropriate strata while also providing the means to shorten the piles where possible. Following the installation of each element, the data acquisition on the drill rig was reviewed by the project team for acceptance. Finally, load test data confirmed the design assumptions and installation data monitoring.