Tunnel boring machines, or TBMs, excavate tunnels while supporting the ground at the tunnel face. Slurry TBMs and earth pressure balance TBMs both solve the same problem: they keep soil and groundwater from collapsing into the excavation. This matters in cities because tunnels often pass below buildings, roads, rivers, and utilities where even small ground movement can cause damage.
The main difference is the material used to balance ground pressure at the cutterhead.
Key Facts
- Face support condition: support pressure at the cutterhead should approximately balance soil pressure plus water pressure.
- Slurry TBM: bentonite slurry supports the face and carries excavated soil through pipes to a separation plant.
- EPB TBM: excavated soil is conditioned into a plastic paste that supports the face inside the excavation chamber.
- Water pressure increases with depth: p = rho g h.
- Total vertical stress in soil can be estimated as sigma = gamma z, where gamma is soil unit weight and z is depth.
- Slurry TBMs are often best for very wet, permeable ground, while EPB TBMs are often best for clay, silt, and mixed soils that can form a stable paste.
Vocabulary
- Tunnel boring machine
- A tunnel boring machine is a large machine that excavates soil or rock while installing tunnel support behind the cutterhead.
- Slurry TBM
- A slurry TBM uses pressurized bentonite slurry at the face to support the ground and transport excavated material away through pipelines.
- EPB TBM
- An earth pressure balance TBM uses the excavated soil itself, often mixed with foam or additives, to create pressure that supports the tunnel face.
- Cutterhead
- The cutterhead is the rotating front disk of a TBM that cuts soil or rock and feeds material into the machine.
- Face pressure
- Face pressure is the pressure applied at the tunnel face to prevent soil collapse and control groundwater inflow.
Common Mistakes to Avoid
- Assuming slurry and EPB TBMs differ only in size is wrong because their main difference is the face support medium and spoil transport system.
- Choosing an EPB TBM for highly permeable water-bearing sand without soil conditioning is wrong because the excavated material may not form a stable plug or paste.
- Ignoring groundwater pressure is wrong because water pressure can be a major part of the load acting on the tunnel face.
- Thinking higher face pressure is always safer is wrong because excessive pressure can heave the ground, cause slurry loss, or damage nearby structures.
Practice Questions
- 1 A tunnel face is 20 m below the groundwater level. Using rho = 1000 kg/m^3 and g = 9.8 m/s^2, estimate the water pressure at the face in kPa.
- 2 Soil above a tunnel has unit weight gamma = 18 kN/m^3 and the tunnel axis is 15 m below ground surface. Estimate the vertical total stress sigma = gamma z at that depth in kPa.
- 3 A tunnel must pass through loose, highly permeable sand below a river. Explain whether a slurry TBM or an EPB TBM is more suitable and justify your choice using face support and groundwater control.