What is an Earth Pressure Balance (EPB) Tunnel Boring Machine?

Posted by Mike Sposetti , - 26 April, 2022

Tunnel boring machines (TBMs) are highly specialized machines designed to bore through the earth to construct a variety of infrastructure, from super-highways to water-overflow systems. Different types of TBMs can be used, depending on the geological conditions. So what is an earth pressure balance (EBP) tunnel boring machine, and what advantages does it bring?

TBM-image

Simply put, earth pressure balance machines (EPBs) are shield TBMs specially designed for operation in soft ground conditions containing:

  • Water under pressure
  • Loose sedimentary deposits
  • Sands, gravels, silts, clays
  • Formations with large boulders
  • High water table

These machines are usually referred to as ‘Single Shield EPB TBMs’. First, let’s look at exactly how EPB TBMs work, before explaining how our products support efficient excavation in this tunneling technique.

How do EPB TBMs work?

The standout feature of EPB TBMs is that they use the excavated soil as a support medium. The rotating cutterhead, equipped with cutting tools, pushes into the tunnel face and excavates the soil, which enters the excavation chamber through openings. Here, it mixes with the soil paste already present.

EPB TBMs-work-process

  • Mixing arms on the cutting wheel and bulkhead mix the paste until it has the required texture.
  • The bulkhead transfers the pressure of the thrust cylinders to the pliable soil paste.
  • When the pressure of the soil paste in the excavation chamber equals the pressure of the surrounding soil and groundwater, the necessary balance has been achieved.

 

Operating modes

EPB TBMs control the stability of the tunnel face and subsidence of the ground surface by monitoring and adjusting the pressure inside the cutterhead chamber to achieve a balance with the water and ground pressure in front of the cutterhead, hence the name ‘pressure balance’. The way these machines operate is called ‘close mode’ – here, the whole excavation chamber is under pressure.

The EPB TBM turns the excavated material into a soil paste that is used as a pliable, plastic support medium. This makes it possible to balance the pressure conditions at the tunnel face, avoid uncontrolled soil inflow into the machine and create the right conditions for rapid tunnelling with minimum settlement.

During excavation, the soil within the excavation chamber is treated with foams and polymers. The conditioned soil achieves a plastic rheology so it can be extracted from the TBM and the tunnel. When the soil is mixed with the foam, its viscosity or thickness changes, transforming it into a flowing material (muck). This muck is then used to provide support and balance pressure at the tunnel face during the excavation process.

As well as being ideal for soft ground conditions, EPB TBMs can also be used in stable ground conditions containing:

  • Water under pressure
  • Karstic environments
  • Limestones, dolomite formations
  • High water table
In these situations, the machines operate in ‘semi-close mode’ or ‘open mode’ – the excavation chamber is either semi-pressurized or not under pressure at all. When the working chamber is pressure-free, there’s a potentially high risk of settlements and water inflows, depending on geological conditions.

So what’s the difference between an EPB TBM and a Hard Rock TBM? Generally, they have a different cutterhead configuration, with EPB TBMs having a bigger opening ratio in the cutterhead.

Parts of the EPB TBM

  • Mixing arms: They mix the soil to obtain the required rheology
  • Cutterhead: It removes the soil from the tunnel face
  • Bulkhead: Transfers the thrust force to the soil paste in the excavation chamber where it is controlled using pressure sensors
  • Excavation chamber: The plastic soil in the excavation chamber transfers the support pressure at the tunnel face
  • Screw conveyor: The rotation speed determines how much material is removed from the excavation chamber, thus regulating the required support pressure
  • Erector: To lift and position the segments during ring building
  • Tail shield: Wire brushes seal the gap between the inside of the tail shield and the outside of the segmental lining
  • Backfilling: The annular gap between the excavated surface of the ground and the outside of the tunnel lining is filled with grout
  • Tunnel lining: The precast concrete segmental lining is installed with precision

 

How do our products help?

Optimal TBM performance demands a combination of mechanical and chemical engineering. Master Builders Solutions supports underground construction with hi-tech foams, polymers and anti-clay agents that enable smooth operations in every geological situation, backed up by an expert support team and local lab facilities.

In EPB tunneling, adapting to the geology of the environment through the correct use of soil conditioners reduces cutterhead torque and tool abrasion and increases advance rates. It also helps to facilitate the cost-effective soil disposal – and even the re-use – of excavated material. 

  1. Foams – MasterRoc SLF 30, 32, 34, 41, 47, 50, and recent introductions 325, 419, and 543
    Torque and wear reduction are key to safe and economical TBM tunneling. MasterRoc SLF foams ensure maximum performance and can be adapted to the local geology.

  2. Anti-Clay Additives – MasterRoc ACP 127, 147, 143, 211, and 214
    MasterRoc ACP anti-clay additives are the world-leading tech for powerful and cost-efficient EPB tunneling in clayey soils, with continuous development underpinning the range. The portfolio includes pure anti-clay additives and formulated products based on foams and polymers.

  3. Polymers – MasterRoc SLP 1, SWA 710, and 750
    Stabilizing polymers combat high water content or pressure, unstable ground conditions and sharply graded soils. These unique polymers have a low impact on the environment and can be used via the foaming system or injected by separate lines in the chamber or screw.

  4. Laboratory testing & expert support
    Safe and efficient tunneling starts at the planning stage, so detailed laboratory testing of soil from the project location before excavation is essential. Based on the test results, the contractor can select the most suitable product and receive the necessary information to set up the foam parameters on the TBM.

Are you ready to find out more about our solutions for EBP tunneling? Please get in touch or visit https://ugc.master-builders-solutions.com/en!

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Topics: Tunnel Construction


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