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?
Simply put, earth pressure balance machines (EPBs) are shield TBMs specially designed for operation in soft ground conditions containing:
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
The standout feature of EPB TBMs is that they use the excavated soil as a support medium. The rotating cutter head, 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.
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.
EPB TBMs control the stability of the tunnel face and subsidence of the ground surface by monitoring and adjusting the pressure inside the cutter head chamber to achieve a balance with the water and ground pressure in front of the cutter head, 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 the minimum settlement.
During excavation, the soil within the excavation chamber is treated with foams and polymers. The conditioned soil achieves 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 cutter head configuration, with EPB TBMs having a bigger opening ratio in the cutter head.
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 cutter head 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.
If you're interested in learning more about our solutions for EBP tunneling, we encourage you to reach out to Wolfgang or visit our website. We would be delighted to provide you with further information and support.