GEOSTRATA Magazine

Cities are becoming very dense, and the use of surface space is usually at a premium. Efficient infrastructure and transportation solutions are becoming an absolute necessity, and sustainable, planned use of both surface and underground spaces is now critical. As cities develop infrastructure and mobility solutions to keep up with increased population and economic growth, the issues of livability, quality of life, community preservation, and effectiveness of infrastructure become of special concern.

The Midtown Tunnel Project team had to design safe, yet economical slope geometries for the subaqueous dredge slopes for a new roadway tunnel using immersed tube construction techniques. The design of safe slopes for this project was of paramount importance, because a dredge slope failure could cause complete failure of the existing adjacent immersed tube, sloughing slopes would create a costly maintenance issue during construction, and divers and equipment would be working within the dredge prism.

Geotechnical uncertainty is the governing source of risk on urban tunnel projects. A typical preconstruction geotechnical site investigation for a tunnel project involves vertical and/or inclined boreholes spaced at 50-m to 75-m intervals along alignments often extending for kilometers. This leads to a sampling rate of less than 0.1 percent of the ground. The result is a very high level of geotechnical and geological uncertainty, with reliance on desktop study and understanding of geological processes to help fill in very large information gaps.

The tunneling industry has many techniques for creating space underground, which vary from excavation with conventional construction equipment, to drill and blast techniques, to highly specialized tunnel boring machines. The most traditional form of tunneling, which was first used thousands of years ago to create ancient Roman and Persian aqueducts, has evolved into what is now known as the Sequential Excavation Method, or SEM tunneling. SEM tunneling is an observational method that involves evaluating the type and behavior of the ground during excavation and determining the best combination of ground support components.

The use of “stacked drifts” in the sequential excavation method of tunnelling has been employed for over four decades and has allowed for the construction of tunnels and underground caverns approaching nearly 30-m width under clear cover thickness of 5 m or less, more typically in soft ground geology. The method essentially involves constructing a series of smaller-diameter tunnels, configured side-by-side, arched, and/or vertically “stacked.” Typically, each small tunnel is backfilled with concrete and linked to adjacent tunnels with mechanical or shear interlocks to either directly form or facilitate the construction of a single, larger-diameter tunnel or underground cavity.