Immersed Tube Tunnels: Building Beneath Harbors with Fort McHenry Case Study

Overview

This video explains immersed tube tunnel construction as a harbor crossing alternative to tall bridges. Using the Fort McHenry area in Baltimore as a focal point, Grady outlines how large tunnel segments are built in dry docks, floated into place, and joined underwater with careful alignment, gaskets, and backfill that locks the segments together. The discussion covers the practical advantages of reduced surface footprint, the challenges of underwater construction, and examples from around the world that illustrate how this method is used in diverse settings.

Introduction

The video introduces immersed tube tunneling as a clever solution for spanning navigable waterways without building extremely tall bridges. By focusing on Baltimore's Fort McHenry area, the host explains why a high bridge would obscure a cherished landmark and how an underwater tunnel preserves the landscape while still enabling highway and rail traffic.

Why Bridges Are Problematic in Dense Waterways

High bridges require long approaches and steep grades, which increases land use, cost, and disruption in dense urban environments. A submerged option can dramatically reduce surface land needs and streamline integration with city infrastructure.

Core Concept of Immersed Tube Construction

Immersed tubes are manufactured in dry docks, then towed into position. Segments can be very long, often up to 650 ft, and are largely air filled so they behave like boats during transit. Once aligned, the segments are lowered into a shallow trench on the water side, avoiding the deep ground conditions typical of boring projects.

From Fabrication to Placement

Before installation, sections are built in shipyards or dry docks under controlled conditions. They must be designed to function as watercraft first, with buoyancy and balance carefully accounted for. A lay barge, acting as a giant floating gantry crane, places each segment with precision as it sinks to the trench floor. Subsea alignment relies on rubber seals, gaskets, and sometimes surveying towers that rise above the water for final adjustments.

Joining, Sealing, and Stability

Joining the segments is the most delicate step. Jacks or divers connect the fresh segment to the one already in place, compressing gaskets to seal the joint. After positioning, valves are opened to drain water between bulkheads, creating hydrostatic pressure that further tightens the seal. The joint is designed to allow small movements from temperature changes or settlement, while still transferring shear forces through dowels and continuity of the backfill to lock the structure in place.

Backfill and Finishing Touches

The first backfill layer is locking fill, typically angular rock that self compacts, followed by conventional backfill and a protective layer on top to resist currents and ship anchors. Once a segment is secured, access can be opened from the inside through bulkheads under strict safety rules. After backfill, the tunnel is equipped with the usual road or rail cross sections, ventilation, drainage, and lighting.

Environmental Considerations

While immersed tube tunnels minimize surface disruption, dredging and sediment disturbance are substantial concerns. Turbidity can affect aquatic habitats and buried contaminants may be mobilized. Mitigation strategies include using clamshell dredging to reduce turbidity, scheduling construction to avoid sensitive wildlife periods, and employing careful sequencing to limit ecological impact.

Global Examples and Future Projects

World examples include the Orrison Link between Denmark and Sweden, the Busan Goje Tunnel in South Korea, the Marmara Tunnel crossing the Bosphorus in Turkey, the Fort McHenry Tunnel in Baltimore, and the BART Transbay tunnel in California. Ongoing projects like the Marn Belt between Denmark and Germany aim to become the world’s longest immersed tunnel. The video notes how these projects demonstrate the adaptability of immersed tube designs to various shapes and traffic mixes, including combined road and rail corridors.

Conclusion

The host reflects on the remarkable challenge of turning giant underwater segments into a seamless, open path for drivers and train riders, highlighting how immersion tunnel construction blends math, physics, and hands on engineering. The video invites viewers to appreciate the complexity behind a seemingly simple idea and to explore the future of factual content about engineering.