What Does Slurry Tunnel Boring Machine (Slurry TBM) Mean?
A slurry tunnel boring machine (slurry TBM) is a type of underground excavator that removes excavated material by hydraulic means. It is considered a “soft ground” tunnel boring machine (TBM) along with the earth pressure balance tunnel boring machine (EPB-TBM).
The excavated soil is mixed with slurry to withstand the excavation by creating a positive face pressure. The soil to be removed is pumped out along with the slurry to a separation plant situated outside the tunnel where the slurry is separated from the muck for recirculation. The slurry TBM is one type of large tunneling machines used in trenchless construction.
A slurry tunnel boring machine (slurry TBM), is also known as a slurry shield machine or a closed-face machine.
Trenchlesspedia Explains Slurry Tunnel Boring Machine (Slurry TBM)
The slurry shield method is safe, fast, and has a minimum impact on the environment. The method is mainly used in areas where ground movement control is essential although it does cause some ground movement.
The selection of a tunnel boring machine (TBM) for a particular job is dependent on the type of soil being bored. A slurry tunnel boring machine can be used where there is a significant amount of groundwater, and the presence of high water pressure is a deciding factor for selecting the slurry TBM method for excavation.
History of Tunneling Shields
Tunneling shields are nothing new. The first use of the technology was by Marc Isambard Brunel in 1818. He used the method to create a tunnel under the River Thames. The world’s largest TBM is the Tuen Mun - Chek Lap Kok TBM, a slurry TBM. It is part of a Hong Kong project to create a tunnel 17.6-meters (58-feet) in diameter beneath the sea.
The Slurry Shield TBM
The slurry shield TBM is widely used in non-cohesive soils ranging from fine-grained sands to coarse-grained gravel. The International Tunnel and Underground Space Association explains how the bentonite suspension is pumped into the excavation chamber where it reaches the face of the excavation and penetrates into the earth, forming a filter cake. The pressurized bentonite slurry counteracts the earth and groundwater pressure while also helping to suspend the cuttings and convey them out.
While the slurry TBM is preferred for soft ground conditions it isalso used in mixed ground conditions including igneous rocks, such as granites, quartz, clays, and sands. However; this can pose significant challenges during the construction of segmentally lined tunnels.
The method is used in these mixed ground conditions to maintain positive face pressure while supporting the ground by applying pressure through a bentonite slurry suspension.
The main parts of a slurry shield TBM are:
- Cutter head fitted with teeth, discs or blades
- A protective shield
- Thrust jacks
- Slurry system for debris separation