Tunnel Construction: Engineering the Emerald Mole Project
The creation of a modern transit tunnel requires a sophisticated blend of heavy machinery and precise engineering techniques. Between 2005 and 2009, a significant tunneling project was undertaken to establish a new transit link, utilizing both automated boring technology and manual excavation methods to navigate the subterranean landscape.
Project Timeline and Management
Construction officially commenced in March 2005. The project was managed by the Obayashi Corporation, who served as the general contractor. After over four years of intensive work, the project reached completion in July 2009.
Engineering Methods and Technology
To ensure efficiency and structural integrity, the project employed two distinct tunneling methodologies based on the specific requirements of the tunnel sections.
The Emerald Mole TBM
The twin running tunnels were excavated using a specialized Tunnel Boring Machine (TBM)—a massive piece of equipment that excavates a circular tunnel full-face. This specific machine, manufactured by Mitsubishi, was named the Emerald Mole.
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Sequential Excavation Method (SEM)
While the main tunnels relied on the TBM, the station and crossover tunnels required a more flexible approach. These sections were constructed using the Sequential Excavation Method (SEM), which is also widely known as the New Austrian Tunneling Method (NATM). This technique involves excavating the tunnel in small stages and applying a shotcrete lining to stabilize the rock or soil immediately.
Project Costs and Safety
The financial scope of the project saw an increase from the initial estimates. While Obayashi Corporation originally submitted a bid of $280 million, the final cost of the completed tunnel rose to $309 million.
The construction process was not without its challenges. Safety incidents occurred during the build, resulting in several worker injuries and the tragic death of one worker.
Key Facts
- General Contractor: Obayashi Corporation
- Construction Period: March 2005 – July 2009
- TBM Name: Emerald Mole (built by Mitsubishi)
- Excavation Methods: TBM for running tunnels; SEM/NATM for stations and crossovers
- Final Cost: $309 million
- Initial Bid: $280 million
| Category | Details |
|---|---|
| Contractor | Obayashi Corporation |
| Duration | Approx. 4 years and 4 months |
| Primary Machinery | Mitsubishi "Emerald Mole" TBM |
| Budget Variance | +$29 million over bid |
Frequently Asked Questions
Who was the general contractor for the tunnel?
The Obayashi Corporation served as the general contractor for the project.
What was the "Emerald Mole"?
The Emerald Mole was a Tunnel Boring Machine (TBM) built by Mitsubishi used to excavate the twin running tunnels.
What is the difference between the methods used for the running tunnels and the stations?
The running tunnels were bored using a TBM, whereas the stations and crossover tunnels were built using the Sequential Excavation Method (SEM), also known as the New Austrian Tunneling Method (NATM).
How much did the project cost in total?
The final cost of the tunnel was $309 million, exceeding the original bid of $280 million.
When was the construction completed?
Construction began in March 2005 and was completed in July 2009.