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Tunnel Jacking

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Tunnel jacking or jacked-box tunneling is a non-intrusive method of constructing railway or subway tunnels, large oversized culverts, or an under-bridge by pushing a huge concrete box through the soil using specialized jacking equipment and hydraulic jacks.[1] The obvious benefit of the tunnel jacking method is that construction of shallow tunnels can resume under existing infrastructure or structures above with minimal disturbance.

Contents

[edit] History

The concept of tunnel jacking originated from pipe jacking in the 1960s whereby small pre-cast concrete boxes were jacked under existing infrastructures to form pedestrian subways, and portal bridges, and in underground pipeline construction. Eventually, the technique was adapted in the building of bridge abutments. Small boxes were jacked into place on top of each other and then filled with concrete. The technique eventually evolved into jacking concrete tunnel sections of monolithic proportions that were big enough to accommodate the construction of three-lane highways running underneath railways, highways, rivers, and airports.[2]

[edit] Tunnel Jacking on a Grand Scale

One notable example in tunnel jacking advancement in the last decade was the construction of three tunnel sections collectively referred to as the “Boston Jacked Tunnels” in the Central Artery Tunnel Project, also known as the "Big Dig," in Boston.[3] The construction of the three tunnels included an extension of the city’s I-90 route that passed under a railway system and lead out to Boston’s Station Railway Terminal.

A team of engineers from Mott MacDonald, the company responsible for overseeing this phase of the Central Artery Tunnel Project, was presented with a huge challenge in trying to advance a tunnel through a 2,000-foot by 1,000-foot (610-m by 305-m) plot of harbor-side land comprised of very soft ground that contained over 200 years of uncontrolled landfill and  full of archeological artifacts that needed to be preserved.[4] On top of all that, the tunneling site crossed a waterway and several railway lines.

Mott MacDonald had to figure out a way to dig the three tunnels through such fragile terrain without bringing train operations above to a complete standstill. No conventional tunneling methods, such as the cut-and-cover method, seemed applicable for the conditions Mott MacDonald was facing or addressed the requirements set out by both Amtrak and the Massachusetts Bay Transportation Authority. That was when the company came up with the idea to use tunnel jacking in order to construct the three interstate highway tunnels. The deployment of this technique in the project would set a new precedent in the world becoming the largest and most complex set of tunnels ever installed using the tunnel jacking method. In fact the actual size of the jacked tunnels was 10 times the size of any jacked tunnels ever attempted within the U.S before.[5] Site constraints and available working space were also considerably less than what is typically required for tunnel jacking operations but was addressed with the use of jacking pits and ground freezing.[6]

The first step in tunneling was to transform the entire tunneling zone from soft ground into solid ground making it easier to excavate. This was accomplished with the installation of 2,000 vertical steel pumps drilled into the ground at a depth of 70 feet (21 m) through which a cold brine solution was injected that literally froze it thereby creating the world’s largest man-made iceberg.[7]

Once the soil was hardened for excavation, three concrete jacking pits were dug out and inside each pit, tunnel boxes measuring 80 feet (24 m) wide and 40 feet (12 m) high were constructed.[8] Crews then broke the head end of the concrete pit and began excavating soil in three-foot increments using a road header. The soil was removed from outside the back of the tunnel box using large buckets that were then lifted to the surface using a crane. Hydraulic jacks, as many as up to 50 at one point during construction, were used to push the tunnel boxes forward inside the concrete jacking pits. This process was repeated over and over again. Each jack had a maximum pushing capacity of 10,000 pounds per square inch (68,948 kPa) and on average achieved moving the tunnel sections through the ground about three to six feet (0.9 to 1.8 m) per day with the trains rumbling just 20 feet (6.1 m) above. Once the jacked sections were permanently placed, the jacking pits were incorporated as part of the highway’s path.[9]

The application of this technique in constructing the tunnels cost approximately $150 million but was nothing compared to the $300 million saved in construction costs.[10] The successful execution and completion of the “Boston Jacked Tunnels” was a true testament to the marvel of the tunnel jacking method. The project was awarded the prestigious NOVA Award from the Construction Innovation Forum in 2004. The award “recognizes innovations that have proven to be significant advances that have positive important effects on construction to improve quality and reduce cost.” [11] The project also received numerous accolades and industry awards including the 2003 Quality in Construction Awards, the International Achievement prize in the 2003 Building Awards, the 2002 British Construction Industry International Award and the American Society of Civil Engineers’ top recognition award for innovation, the Charles Pankow Award.[12]

[edit] Process

Hydraulic jacks force the concrete box resting on top of a jacking slab forward in small increments. The soil is then excavated from inside the tunnel box with the use of a specialized tunneling shield that advances through the ground as a prefabricated section of tunnel lining.[13] The top and bottom of the concrete tunnel box support the surrounding soil outside of the box and additional measures are taken to ensure ground stability at the face of the tunnel and the prevention of the ground underneath the tunnel from being dragged forward as the concrete box advances.[14] The process of jacking and excavation alternates and this is repeated over again until the concrete box is in its final position. Heavy-pipe like spacer tubes are also installed to fill in any gaps created by the process.

[edit] Equipment Used

[edit] References

  1. Jacked box tunneling, our speciality. Ropkins, 2008-09-29.
  2. Tunnel Jacking. Geotechnics, 2008-09-29.
  3. Concrete Tunnel Jacking on Central Artery. Construction Innovation Forum, 2008-09-29.
  4. Concrete Tunnel Jacking on Central Artery. Construction Innovation Forum, 2008-09-29.
  5. Projects. Mott MacDonald, 2008-09-29.
  6. World-class tunnels portfolio. Mott MacDonald, 2008-09-29.
  7. Projects. Mott MacDonald, 2008-09-29.
  8. Tunnel Jacking. Massachusetts Turnpike Authority, 2008-09-29.
  9. Tunnel Jacking. Massachusetts Turnpike Authority, 2008-09-29.
  10. World-class tunnels portfolio. Mott MacDonald, 2008-09-29.
  11. The Nova Award. Construction Innovation Forum, 2008-09-29.
  12. Projects. Mott MacDonald, 2008-09-29.
  13. Tunnel. Answers.com, 2008-09-29.
  14. Jacked box tunneling, our speciality. Ropkins, 2008-09-29.