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ndrc crossing sections details cad drawings sample

 The meaning of NDRC is Non Disruptive Road Crossing and other meanings are located at the bottom which take place within Drilling terminology.

Micro tunnelling is a name generally associated to unmanned pipe jacking methods for pipes smaller than 1000 mm diameter. However larger diameter machines and equipment are readily used and it would be more appropriate to use the term micro tunnelling for any unmanned pipe jacking using a steerable tunnelling machine.

In micro tunnelling the pipeline is installed by pushing (jacking) the pipes forward from the starting shaft as the tunnelling machine excavates the soil at the front of the pipeline. The excavated soil can be removed through the already laid pipes by various methods. Examples of this are auger soil removal or slurry shield micro tunnelling as shown in Figures 7 and 8.

 

In the auger method the excavated soil is removed mechanically with the continuous line of augers. In the slurry shield method the excavated soil is mixed with a bentonite slurry suspension and pumped out of the pipeline. After being pumped out the soil is settled or separated from the slurry in a tank or separation unit and the slurry is reused.

 

When working below ground water levels the auger soil removal may pose the same problems, where the auger may flood and give way for excessive soil loss. The slurry shield micro tunnelling machine is typically designed with a pressurized bulkhead, where slurry is pumped at a sufficient pressure to stabilizes any loose soil and balance ground water pressure.

 

The tunnelling machine can be controlled by an operator outside the pipeline. The alignment is usually controlled by laser or by gyroscope and water level. The line is typically straight, but using gyroscope or specialized surveying equipment and short pipe lengths, curved tunnels can be constructed.

 

Piping materials must be designed to withstand the jacking forces acting on them under installation. Concrete is often used as piping material along with fibreglass or composites with concrete and fibreglass. Polymer materials especially polymer concretes are  becoming more common due to their strength and corrosion resistance.


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                Figure 8: Slurry shield micro tunneling

Advantages

  • Micro tunnelling works in almost all soil conditions and cutting heads can be modified to deal with weak rocks.
  • Pipes of up to 2000mm diameter can be installed and can be constructed to a high degree of accuracy which makes the technique suitable for pipelines that require precision in alignment or gradient.

Disadvantages

  • Obstacles (large rocks/boulders or other materials) may stop machines not designed for cutting through these materials. In such cases there may be no other solution, than to excavate from the surface to remove the obstacle. If this is not possible, the tunnel and machine may have to be abandoned.

  • Working in mixed face conditions can be problematic, particularly where slurry support is used below groundwater as the slurry pressure may be difficult to control. Loss of slurry/face support can lead to instability and ravelling leading to large surface settlements and, in the extreme case, abandonment of the pipe and machine.
  • Micro tunnelling is generally more expensive than many of the other NDRC methods, requiring relatively large entry and exit shafts and more advanced equipment and materials.


 

   

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