Design-Build of Watertight Underground Structures for Tunneling in Miami, Florida - The Norris Cut Utility Relocation Project-Paper

Paper

Pdf Summary

The Norris Cut Utility Relocation Project replaced a 54-inch wastewater force main beneath Biscayne Bay, Miami, with a 60-inch pipe installed through a tunnel more than a mile long. The tunnel boring machine (TBM) was launched from Virginia Key and retrieved on Fisher Island. Both shafts had to be built through highly permeable limestone with sand- and silt-filled cavities.<br /><br />The 89-foot-deep, 42-foot-diameter launch shaft used secant piles, wet excavation and a reinforced tremie base slab. Its design addressed uneven loads from TBM launch, ground freezing and cranes. Precise pile alignment and reliable interlocks were essential to limit water ingress.<br /><br />The 72-foot-deep, 28-foot-diameter retrieval shaft was initially planned with secant piles but changed to deep soil mixing (DSM), partly because Fisher Island’s ferry access made concrete deliveries difficult. A full-scale trial found lower-than-expected soil-mix strength, prompting adjustments to curing time and grout strength. The DSM shaft was excavated dry, allowing leaks to be identified and repaired during excavation.<br /><br />The two methods involved trade-offs. DSM construction and excavation took about half the time per square foot of wall, provided safer working conditions and suited the island’s logistical constraints. However, it cost about 10% more per square foot than the secant-pile shaft. The secant-pile method was more demanding to excavate and required divers to build the tremie slab, but proved economical and performed well.<br /><br />The project demonstrated that DSM can be used at depths approaching 75 feet in hard limestone, provided field trials confirm achievable strength and construction quality. The design-build approach enabled construction to proceed alongside design development and allowed the retrieval-shaft method to change without delaying the overall project.

Keywords

Norris Cut Utility Relocation Project

Biscayne Bay

Miami

wastewater force main

tunnel boring machine

Virginia Key

Fisher Island

deep soil mixing

secant piles

hard limestone