Ensuring safety while excavating tunnels

Tunnel excavation under high water pressure, such as for creating underwater river crossings, uses rotating cutterheads to cut through wet, muddy slurry. Despite its dynamic interactions with the soil and the slurry, most excavation studies neglect the cutterhead’s rotation. Bai et al. created a physical model to experimentally measure these effects and increase excavation safety.

“The common practice of using static methods to demonstrate support pressures can significantly underestimate actual requirements, creating unnecessary safety risks in high-risk tunneling environments,” said author Yang Bai.

In studying the effects of water pressure, the number of cutters and spokes on the cutterhead, and the cutterhead’s rotational speed, the researchers found rotational speed to be the most important factor to consider in terms of excavation safety. In particular, the speed changes the way the so-called “filter cake” forms — a barrier of slurry at the excavation face that balances the pressure between the tunnel and its surrounding soil. At too high rotation speeds, the filter cake is not continuous, and the tunnel is susceptible to infiltration of slurry if its pressure drops; even a small increase in cutterhead speed therefore requires a significant increase in support pressure to maintain stability.

“The failure to form an effective filter cake will expose tunnel construction to a substantial risk of excavation face instability, which severely compromises the safety of the entire tunneling process,” Bai said. “Its importance cannot be overstated: The filter cake is the sole medium that enables efficient transfer of slurry pressure to the soil mechanism.”

Crucially, the tunneling failure mode the researchers observed is fundamentally different from what has been widely reported in the past.

“As the cutterhead rotational speed increases, the instability zone not only expands forward ahead of the excavation face but also extends significantly behind it — a critical phenomenon that has been completely overlooked,” Bai said.

Together, these findings show that strict control of cutterhead speed is necessary for the safety of high-water-pressure tunnelling.

Source: “Experimental study on dynamic effects of face stability in sandy-strata slurry shield tunneling,” by Yang Bai, Yapeng Li, and Yingji Bao, AIP Advances (2026). The article can be accessed at https://doi.org/10.1063/5.0331835 .