Understanding the Role of Bentonite Slurry in Tunnel Boring Machines

Photo: Arne Bezuijen (large view)

Photo: Arne Bezuijen

(30-01-2025) Chenghao Zhang's research reveals how bentonite slurry's unique infiltration and sealing properties are critical for tunnel stability.

Chenghao Zhang's research reveals how bentonite slurry's unique infiltration and sealing properties are critical for tunnel stability, with insights on optimizing its use under varying soil and water conditions to enhance tunneling safety and efficiency.

When it comes to large-scale tunneling projects, particularly those beneath cities, rivers or estuaries, stability and safety are paramount. A critical player in this field is bentonite slurry, a mud-like substance with exceptional properties that supports of the soil in front of tunnel boring machines (TBMs). In his recent doctoral research, Chenghao Zhang from Ghent University explored how bentonite slurry infiltrates into soil and why understanding this process is crucial for safe tunneling operations.

What Is Bentonite Slurry and Why Does It Matter?

Bentonite slurry is a mix of water and bentonite clay. This slurry is injected under pressure to stabilize the soil around a TBM. "Bentonite may form a protective layer—a filter cake—that seals off the tunnel walls," Zhang explains. The infiltration of bentonite and/or filter cake formation prevents soil and water from entering the tunnel and ensures the stability of the tunnel face, particularly in loose or permeable soils.

However, not all bentonite slurries behave the same way. According to Zhang’s research, factors such as the type of bentonite, soil saturation, and the presence of salty water can drastically affect how the slurry infiltrates the soil and forms a filter cake.

Key Findings from the Research

  1. The Mud Spurt Phenomenon:

Bentonite slurry initially penetrates the soil quickly in a phase called the "mud spurt."

"We discovered that the speed and distance of this mud spurt depend more on the slurry’s yield stress than its plastic viscosity," Zhang notes. Yield stress is the force needed to initiate flow in the slurry, a critical parameter in its performance.

  1. Impact of Salty Conditions:

Salty water, common in undersea tunneling, weakens bentonite’s ability to swell and seal. Zhang’s experiments showed that pre-hydrating bentonite with fresh water before adding salty water can mitigate this effect. “This preparation step helps to maintain the mud spurt behavior and the integrity of the filter cake,” he says.

  1. Soil Saturation Matters:

In partially saturated soils, filter cakes form more quickly and are just as effective as those in fully saturated conditions. "This finding challenges traditional assumptions that fully saturated soils are always ideal," Zhang explains.

  1. Airtightness of the Filter Cake:

The filter cake must remain intact during TBM maintenance, such as cutterhead replacement. Zhang identified two failure mechanisms: holes and cracks. "Understanding these mechanisms allows us to design better protocols for maintaining air pressure during these critical operations," he emphasizes.

Practical Implications

Zhang’s work provides actionable insights for tunneling projects. For example:

  • Lowering the TBM advance rate or increasing the inlet slurry flow can reduce the amount of seawater mixing with the bentonite slurry.
  • Pre-hydrating bentonite slurry with fresh water is a simple yet effective strategy for undersea projects.
  • Designing better maintenance protocols based on the identified failure mechanisms can prevent costly and dangerous accidents.

A Path Forward

This research highlights the complexity of using bentonite slurry in real-world scenarios. "Every tunneling project is unique," Zhang concludes. "By understanding how bentonite interacts with different soils and conditions, we can make these projects safer and more efficient."

As cities expand and infrastructure grows, Zhang’s findings will undoubtedly play a role in shaping the future of tunneling technology.

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PhD Title: Pressure Infiltration of Bentonite Slurry into Sand: Experiments, Mechanisms, and Significance for Practical Implications

Promotors: Wim Haegeman and Adam Bezuijen