Properties of slurry for TBM tunnelling

What is slurry used in TBM tunnelling?

The slurry used in TBM tunnelling is basically a water-based suspension fluid with added solids that can be either bentonite or polymers. This is similar to other uses of bentonite suspension fluids used in piling or vertical drilling for example.

Slurry is made up of:

• Water, but not just any water. The chemical content and temperature of the water has a significant bearing on the properties of the mixed slurry

• Bentonite type: mainly clays or polymers. Clay based bentonite in comparison to polymer based bentonite generally has lower cost, is environmentally benign and can cope with a wide variety of ground conditions. However, polymer based bentonite can be modified to deal with specific challenging conditions within a wider range of varieties.

• Additives, which improves the properties further to meet requirements for tunnelling

What is slurry used for?

The following are the commonly expected functions of slurry used for TBM tunnelling:

• The ability to stabilise the face and control groundwater. The slurry needs to create an impermeable membrane (filter cake) at the face and around the shield, and be able to maintain the pressure to ensure face stability. The formed filter cake ideally will be able to undergo significant deformation without breaking or losing water tightness.

• Medium for muck removal. The slurry carries the muck from the tunnelling face back to the separation plant. The slurry should be able to encapsulate the muck reducing its hydration to reduce the likelihood of muck sticking to something causing blockage in the removal pipe.

• Lubrication. The slurry can limit the wear of plants by reducing the friction. When used around the TBM shield it can also reduce the TBM thrust force against the segments due to reduction of friction.

What are the key properties of slurry?

The apparent viscosity (what you see) of the slurry has some interesting properties:

• High when the flow velocity of the slurry is low, such as penetration into the pore space of the ground. This is why filter cake can form.

• Low when the flow velocity is high, such as being pumped in pipeline. This is why slurry can be used as a muck removal medium.

Some key performance measurements of the slurry include:

• Plastic viscosity. It depends on the solid content within the slurry mix as well as the properties of the solid.

• Yield point. The yield point of slurry can be understood as the shear stress that the slurry has to overcome in order for it to flow (become a fluid). The higher the yield point is, the harder it is for the slurry to flow.

• Density. Density has correlation with the ability to form filter cakes. The higher the density is, the easier it is to form filter cakes.