Excavation Support for Jacking and Receiving Shafts on the East Boston Sewer Relief Project

Abstract

Construction of new 914 mm, 1,219 mm, and 1,676 mm diameter sewer pipes by microtunneling on the East Boston Branch Sewer Relief Project for the Massachusetts Water Resources Authority (MWRA) required installation of 19 excavation support structures at jacking and receiving shaft locations. Three methods of excavation support were used: drilled soldier piles and lagging, drilled micropiles and contact lagging, and drilled secant piles. Jet grout bottom seals were installed at two excavation support structures. Jet grouting was also utilized to seal around existing utilities at one excavation support structure. The typical depth of excavation varied from 5.8 to 13.4 m. The excavation support method used at each jacking and receiving shaft was determined after evaluation of the existing soil conditions and logistical site constraints. Since portions of the tunnel alignment intercepted made-ground (fill), the soil conditions varied considerably along the tunnel alignment. Each excavation support structure was constructed in a congested urban environment. Presence of overhead and underground utilities and proximity to existing buildings impacted the excavation support method chosen for each shaft location. This paper presents an overview of the rationale used to determine the excavation support method utilized at each drop shaft, a review of the construction techniques used to build the excavation support structures, and the major lessons learned.