Tunnel Boring Machines (TBM): Geotechnical Challenges in Rock and Soft Ground

Tunnel Boring Machines (TBMs) have revolutionized underground excavation, offering a safer and more efficient alternative to traditional drill-and-blast methods. However, tunnel construction through variable geological formations presents severe geotechnical risks, including face instability, excessive ground settlement, and rockbursts.

Geotechnical Characterization of Rock Masses

In rock tunneling, TBM performance is heavily dependent on the mechanical properties and jointing of the rock mass. Geotechnical engineers use empirical classification systems to predict support requirements and boring rates. The two primary systems are the Rock Mass Rating (RMR) and the Barton Q-system.

The Q-system calculates rock mass quality using six parameters:

$$Q = \frac{RQD}{J_n} \times \frac{J_r}{J_a} \times \frac{J_w}{SRF}$$

Where:

• RQD is the Rock Quality Designation $percentage of intact core \pieces >10 cm$.
• $J_n$ is the joint set number (representing degree of jointing).
• $J_r$ is the joint roughness number.
• $J_a$ is the joint alteration number $representing weathering/filling$.
• $J_w$ is the joint water reduction factor.
• SRF is the Stress Reduction Factor (accounting for active stress states).

Soft Ground Tunneling and Face Stability

In soft ground (clay, sand, silt), slurry shield and Earth Pressure Balance (EPB) TBMs are deployed to control water inflow and maintain stability. Maintaining face pressure is critical to preventing tunnel collapse. The required face support pressure $\sigma_{T}$ can be estimated using the horizontal earth pressure and hydrostatic pressure:

$$\sigma_{T} = K_0 \sigma’_{v} + p_w$$

Where $\sigma’_{v}$ is the effective vertical overburden pressure, $K_0$ is the coefficient of earth pressure at rest, and $p_w$ is the pore water pressure.

Ground Settlement and PECK Trough

Excavation inevitably causes volume loss, manifesting as a settlement trough at the ground surface. Under greenfield conditions, Peck’s empirical formula modeling the settlement profile $S$x$$as a Gaussian distribution curve is applied:$$S$x$ = S_{max} expleft$-\frac{x^2}{2i^2}\right$$$

Where $S_{max}$ is the maximum centerline settlement, $x$ is the lateral distance from the tunnel centerline, and $i$ is the trough width parameter $ty\pically$i = K cdot z_0$, where$z_0$is the tunnel depth and$K$is the settlement trough width parameter based on soil type$.