Retention Systems in Port of Miami’s Tunnel Project
The Florida Department of Transportation (FDOT) contracted with the Miami Access Tunnel (MAT) concessionaire group to construct two tunnels, each 42 feet in diameter and 4,000 feet long, with two lanes of traffic each way between Watson and Dodge Islands to alleviate commercial traffic congestion in downtown Miami. This Port of Miami Tunnel (POMT) project is a public-private partnership (PPP).
Due to the high permeability of the subsurface materials and the high static groundwater levels, the excavation support system consisted of a cutter soil mix (CSM) wall with embedded soldier piles designed to serve as lateral structural support of excavation (SOE) and groundwater cut-off. Additional lateral support at the top of the CSM wall was provided via pre-stressed, 5 to 9-strand, 6-inch diameter tie-backs connected to a system of double-channel whalers. The bottom tremie concrete seal was anchored with a combination of H-pile reinforced 36-inch diameter cast-in-drilled-hole (CIDH) elements and 8.5-inch diameter reinforced minipiles. At the break-in/break-out location for the Tunnel Boring Machine (TBM), a plug was constructed immediately adjacent to the SOE system incorporating CSM panels and 12-foot diameter unreinforced secant piles configured in an overlapping and integral pattern as an alternative to more traditional soil improvement techniques. The purpose of the plug is to provide the TBM a watertight entry point for the start of the tunneling process. Lateral resistance was achieved from side and base shear, as well as shear keys at the base, as required, providing an adequate factor of safety against sliding and overturning.