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Pipe Jacking

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Pipe jacking is a technique used for installing pipes under the ground using a tunnel boring machine and hydraulic jacks located inside a jacking pit. A shield is inserted into the ground through a drive shaft or reception point and cuts a bore. As the shield advances forward, excavating the earth in its path, sections of pipe are jacked into place directly behind it using powerful hydraulic jacks.

The technique is a widely employed alternative to open-cut excavation and other tunneling methods and is also sometimes referred to in smaller diameters less than 35 inches (900 mm)[1] as microtunneling, a trenchless construction method used for installing pipelines in a predetermined line by remote control.[2] In the U.S., however, no clear definition of microtunneling exists, as the method has applied to the laying of pipeline ranging from 0.5 inches (12 mm) to 12 feet (3.7 m) in diameter.[3] That said, pipe jacking today is a process used in combination with microtunneling and differs in that the pipe jacking has developed over the last several decades to encompass the jacking of larger sized sections of pipe and even the installation of whole concrete tunnel sections or boxes used to build railway or highway tunnels underground. This is evidenced in the Central Artery Tunnel Project in Boston.

In general, the technique is still widely used primarily for the installation of utility purpose pipelines such as sewage, oil, gas, electricity, cable, and water mains. The method may also be applied in the construction of small-scale infrastructure projects such as rectangular or circular pedestrian subways, culverts, road underpasses and bridge abutments. In addition, it is used in negotiating obstacles such as motorways, railways, rivers, canals, buildings, and airfields that are located directly in the path of pipelaying projects.[4]

Contents

[edit] History

Pipe jacking technology originated in the United States some 50 years ago.[5] When first developed, the method was used as an alternative to open trenching and existed as a means for crossing under roads, railways, rivers, and other projects with the goal of minimizing or attempting to avoid disruption of traffic altogether. The benefits of the application soon were realized all over the world.[6] By the 1980's microtunneling was well established in most of the industrialized world.[7]
When circular pipe sections became too large to maneuver inside thrust pits, pipe jacking evolved into the actual jacking of large concrete tunnel sections or boxes using jacking equipment and hydraulic jacks. First used in the U.K. in the 1960s, tunnel jacking proved to be a practical solution to achieve construction of a shallow tunnel beneath an existing facility or operational structure.[8]
Another development in the technique was the introduction of slurry pipe jacking established in Japan as a non-disruptive construction method for the installation of underground pipelines.[9]

[edit] Process

Pipe jacking is an effective method for constructing smaller tunnels and provides a very flexible, structural, watertight pipeline while the tunnel is actually being excavated. Pipes can be driven in a range of directions from a straight line, to a radius, or a series or radii via a number of excavation systems that include manual, mechanical, or remote control.[10] Pipes installed range in size from six to 118 inches (150 to 3,000 mm)[11] and can be fabricated as a reinforced concrete pipe (RCP) or a reinforced concrete box (RCB) in addition to arch and fiberglass piping, and steel casing.[12] Other options include polymer concrete, clay, and ductile iron. Any of these items can be installed as a primary liner or carrier piping and are most commonly circular or rectangular in shape.[13]

Though the technique is best deployed in cohesive soil, the mechanical excavation methods used in pipe jacking are very similar to other tunneling methods with a wide range of shields, excavation methods, and face supports available to make pipe jacking possible in a variety of ground conditions. For example, there are now construction methods available to effectively deal with pipe jacking either in cohesive or non-cohesive soils in dry or watery conditions to jacking through very hard rock and ground conditions comprised of mixed composites such as cobbles and big boulders[14].

The pipe jacking process begins first by constructing a drive or launch pit or shaft at the face and a thrust or reception pit or shaft at the corresponding end of the pipeline. These pits will vary in size based on the excavation method deployed in the pipe jacking process.[15]

The tunnel or jacking shield is then launched inside the launch shaft. A thrust wall is constructed to provide force against which to jack pipes into place behind the jacking shield. To ensure the jacking force is distributed equally through the entire diameter of the jacked pipe, a thrust ring is also used to transfer loads. Jacks are interconnected hydraulically to ensure that the thrust from each pipe is consistent. The amount of jacks needed to install the pipes is contingent upon the size and length of the pipe being installed, the strength of the jacking pipes, and how much frictional resistance is required.[16]  

As the pipes are jacked into place, the correct alignment of the pipeline is routinely checked. Accurate pipe alignment can be checked and carried out with the use of a steerable shield and by placing guide rails inside the thrust pit upon which the pipes are laid. For shorter pipelines, checks are conducted by means of traditional surveying equipment.[17] The shield is then removed from the reception pit once jacking is completed and the pipeline is installed.

To minimize the likelihood of the occurrence of ground settlement, various ground treatment works can be employed, including jet grouting, silica/cement grouting, grouting using tube-a-manchettes, dewatering, and ground freezing.[18]

[edit] Microtunneling

In some circles, microtunneling is considered remote control pipe jacking.[19] Shield boring and pipe jacking are conducted entirely by remote control, managed by an operator above ground. No personal entry is required inside the shaft or tunnel and spoil or muck is removed from a cutting head positioned in the new pipeline advanced by pipe jacking. The primary difference in microtunneling is that excavation is always closed trench or trenchless and pipe jacking, excavation, and transport all take place simultaneously. Microtunneling eliminates the need for long, extended stretches of open trench for pipelaying and is therefore deemed to provide a much safer work environment.

Special boring machines or shields used in microtunneling are called moles or microtunneling tunnel boring machines (MTBM), and are basically miniature replicas of full scale, commercial tunnel boring machines except they are electrically powered and attached to the head of the pipe that follows the path of the tunnel as it is being bored.

Major strides have been made in the advancement of MTBM in recent years. There are machines that drive 328 feet (100 m) more in soft ground for sizes of four inches (100 mm) in diameter upwards and from drive shafts no bigger than 10 feet (3 m) in diameter.[20] More significantly, machines now feature laser guidance control systems and computerized monitors that steer the machine with a high degree of accuracy.[21] For example, it is noted that sewer pipelines laid in this way are more accurate and evenly stressed than those laid using the  conventional open trench method. 

[edit] Slurry Pipe Jacking

Recent technological developments have led to the ability of stabilizing unstable ground strata by eliminating water from excavation by means of placing mud slurry around the pipes. During the jacking process, the slurry is injected into the face and what is known as the over-cutting area, the area between the pipe and soil. The slurry acts as a solidifier—it fills up the void space, exerts pressure and stabilizes the loose ground soil. Grouting is then injected into the over-cutting area to provide permanent ground stability.[22]

[edit] Parallel Drift Method

The parallel drift method entails boring a series of horizontal holes or drifts and then using microtunneling machinery such as augers, MTBMs, and moles to drive pipes into them, packing them with grout and then filling them up again. The filled drifts naturally form a protective archway around the shell of the tunnel.[23]

[edit] Pros of Pipe Jacking

There are many advantages to pipe jacking and microtunneling. The most obvious is minimal surface disruption. The two are especially suited for any urban environment where a disruption to business, traffic, and existing utilities is likely. As well, because the method relies completely on trenchless excavation, there is less of a direct impact on the environment than in an open-cut or trenched excavation operation.[24] Microtunneling is therefore more economically feasible because ground reinstatement and restoration costs are practically nil. With microtunneling and pipe jacking, longer drives can also be achieved yielding a faster aster rate of progress than in conventional tunneling.[25]

Some of the technical benefits to pipe jacking include:

  • Inherent strength of lining
  • Smooth internal finish provides good flow characteristics
  • No secondary lining is required
  • Fewer joints than in a segmental tunnel
  • Sealed pipes with flexible joints prevent ground water ingress
  • Reduced risk of ground settlement
  • Reduced need to divert or relocate utilities in urban areas[26]

[edit] Equipment Used

[edit] References

  1. Singh, Poonam. Micro-tunneling: Technique of proven ability. Project Monitor, 2008-09-29.
  2. Glossary: Pipe Jacking. Sacramento Regional Count Sanitation District, 2008-09-29.
  3. Microtunneling Solutions. Huxted Tunneling, 2008-09-29.
  4. Applications & Benefits. The Pipe Jacking Association, 2008-09-29.
  5. Shimada, Dr. Hideki. Theoretical and practical researches for tunnel excavation using slurry pipe jacking in Japan. Kyushu University, 2008-09-29.
  6. Shimada, Dr. Hideki. Theoretical and practical researches for tunnel excavation using slurry pipe jacking in Japan. Kyushu University, 2008-09-29.
  7. Hapgood, Fred. Moles Under Houston, 2008-09-29.
  8. Tunnel Jacking. Geotechnics, 2008-09-29.
  9. Shimada, Dr. Hideki. Theoretical and practical researches for tunnel excavation using slurry pipe jacking in Japan. Kyushu University, 2008-09-29.
  10. About the Technique. The Pipe Jacking Association, 2008-09-29.
  11. About the Technique. The Pipe Jacking Association, 2008-09-29.
  12. Microtunneling NASTT, 2005. (accessed: 2008-09-29)
  13. Pipe Jacking. Boring and Tunneling Company of America, Inc., 2008-09-29.
  14. About the Technique. The Pipe Jacking Association, 2008-09-29.
  15. About the Technique. The Pipe Jacking Association, 2008-09-29.
  16. About the Technique. The Pipe Jacking Association, 2008-09-29.
  17. About the Technique. The Pipe Jacking Association, 2008-09-29.
  18. Ground forum on micro-tunneling in the urban environment. Maunsell Geotechnical Services Ltd., 2004. (accessed: 2008-09-29)
  19. Product Selection. Akkerman, 2008-09-29.
  20. Pipe Jacking/ Micro Tunneling. United Kingdom Society for Trenchless Technology, 2008-09-29.
  21. Pipe Jacking/ Micro Tunneling. United Kingdom Society for Trenchless Technology, 2008-09-29.
  22. Shimada, Dr. Hideki. Theoretical and practical researches for tunnel excavation using slurry pipe jacking in Japan. Kyushu University, 2008-09-29.
  23. Tunnel. Answers.com, 2008-09-29.
  24. Applications & Benefits. The Pipe Jacking Association, 2008-09-29.
  25. Microtunneling Solutions. Huxted Tunneling, 2008-09-29.
  26. Applications & Benefits. The Pipe Jacking Association, 2008-09-29.