Hydraulic transportation.

Slurry Circuit for Consistent Removal of Excavated Material.

When tunnelling through sandy and gravely (non-cohesive) mixed soil with high water permeability, the excavated material is usually handled by a hydraulic transportation system. For this, either water or a bentonite suspension is used as the transportation medium. The transportation medium is pumped to the tunnel face from the start shaft by one or several feed pumps using the tunnel boring machine's feed line. There it stabilizes and supports the tunnel face and is mixed with the excavated material.

 

The mixture of soil and transportation medium is exhausted out of the excavation chamber through the slurry line and conveyed to the separation plant. To prevent the slurry line from getting blocked, any major lumps of rock or erratics are pulverized by the cone crusher in the working chamber before passing into the slurry line. A sieve located in front of the suction pipe prevents unsuitably large pieces from being drawn in.

 

In the separation plant, the charged transportation medium is separated from the loose soil using screens, cyclones and, if necessary, centrifuges as well. Efficient separation means that a large proportion of the medium can be treated and sent back into the transportation circuit. If the soil has a high proportion of cohesive particles, water is often an adequate transportation medium instead. However, if the soil is sandy and gravely and has few or no cohesive particles and is relatively compacted, larger individual pieces could still deposit on the bottom of the line and block it. Instead, with this sort of soil, it is often necessary to reduce the internal compacting of the soil particles by immersing them in a suspension, which creates a cohesive flow.

 

This is why bentonite, a mixture of absorbent clay minerals and water, is used for such soils. It has tixothropic properties and stabilizes the tunnel face during tunnelling. The other effect is that it creates a perfect compound with the excavated material as it flows along, which means that it can be transported and removed through the slurry line without damage to the equipment in use. The choice of transportation medium depends in the main on a variety of individual factors and cannot be generalized, because such choices very much depend on the nature of the project.

 

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