DESIGN-BUILD OF WATERTIGHT UNDERGROUND STRUCTURES FOR TUNNELING IN MIAMI, FLORIDA - THE NORRIS CUT UTILITY RELOCATION PROJECT

摘要

In 2017, Nicholson Construction Company completed a $72.1 million design-build contract for MiamiDade Water and Sewer Department to replace an existing 54-inch diameter pre-stressed concrete force mainthat runs from the Virginia Key Central District Wastewater Treatment Plant, under the Biscayne Bay atNorris Cut, to Fisher Island. In order to install the new sewer pipe below the bottom of Biscayne Bay, atunnel boring machine (TBM) was launched from a 42ft diameter and 89ft deep shaft on Virginia Key anda precast segmentally lined tunnel was constructed towards a 28ft diameter and 72ft deep retrieval shaft onFisher Island.Both shafts were constructed through sand and highly permeable limestone, primarily the Fort ThompsonFormation which is part of one of the most permeable aquifers in the United States and two differenttechniques were used to ensure a water-tight shaft at each end of the tunnel drive. The launch shaft structurewas built using the secant pile technique, wet excavation and a tremie plug. For the tunnel break-in a soilfreeze block was installed after the ground was treated with low mobility grout. The retrieval shaft was builtusing the Deep Soil Mixing (DSM) technique and excavated in the dry. Precedence of any other structurebuilt this deep through hard layers (UCS over 3,000 psi) using this technique was not found.This case study addresses the challenges of constructing watertight underground structures in highly porousground for launching and retrieving a TBM. Design challenges from complex asymmetric loading, selectionand comparison of the construction techniques, the installation and excavation of both shafts, and theirperformance are also discussed herein.