The Main Geological Factors Affecting The Excavation Efficiency Of TBM

The uniaxial compressive strength (UCS) of rock is a key indicator for evaluating the applicability of Tunnel Boring Machines (TBMs) and predicting driving efficiency, although it is not the sole determining factor.

In TBM tunneling projects, comprehensive geological investigations are essential. These form the basis for scientific equipment selection and design. Furthermore, the construction phase must be supported by robust advance probing systems and flexible response strategies to ultimately ensure the safe and efficient advancement of the TBM.

The uniaxial compressive strength of rock is one of the critical geological factors determining TBM driving efficiency. Generally, TBMs achieve optimal performance in hard rock with a UCS ranging from 30 to 150 MPa. When the rock strength exceeds 150 MPa and the rock mass is intact with poorly developed joints, a significant decline in penetration rate occurs, accompanied by a series of issues: exacerbated cutter wear, abnormal cutterhead vibration, accelerated wear, and even weld seam cracking. These conditions substantially increase downtime for cutter replacement and equipment maintenance. Consequently, the impact of this factor must be fully considered during project scheduling.

The degree of development of discontinuities within the rock mass (including joints, bedding, foliation, minor faults, etc.), i.e., the degree of fracturing or integrity of the rock mass, is another crucial geological factor influencing TBM driving efficiency. Even if rocks have similar UCS, hardness, and abrasiveness, the penetration rate (depth of cut per revolution) and net driving rate of a TBM can vary significantly if the degree of discontinuity development differs.

Parameters such as the Rock Mass Integrity Coefficient (Kv), Volumetric Joint Count (Jv), or Rock Quality Designation (RQD) are commonly used to quantify the development of discontinuities. Generally, a higher net driving rate is achieved in rock masses with good integrity and relatively closely spaced discontinuities, as moderate discontinuities facilitate rock breakage and reduce cutting resistance.

However, the situation reverses when the rock mass is intensely fractured (e.g., extremely high joint density resulting in a Kv value below 0.25). In such cases, the rock mass becomes broken or even loose, its overall strength decreases drastically, and it lacks self-supporting capacity. Although TBM cutters may achieve greater penetration (i.e., easier cutting) in such fractured rock, the highly unstable surrounding ground is prone to collapse or convergence deformation. This necessitates significant time investment for temporary support measures (e.g., installing steel arches, shotcreting, rock bolting) to ensure construction safety and tunnel wall stability. These additional support operations significantly consume advance time, ultimately leading to a decrease in the net driving rate of the TBM in these sections. Therefore, excessively developed discontinuities, while reducing the intact strength of the rock, become a constraint on efficient TBM advancement due to the dramatically increased demand for support.

Rock hardness is a critical factor determining its abrasiveness. Generally, the higher the rock hardness, the greater its abrasiveness, which directly accelerates the wear of TBM (Tunnel Boring Machine) cutters. This results in faster cutter consumption, not only increasing construction costs but also leading to more frequent downtime for cutter replacement, thereby reducing the overall advance rate.

The abrasiveness of rock is significantly influenced by its mineral composition, particularly the content and particle size of hard minerals such as quartz. Higher content and larger particle size of these abrasive materials lead to increased cutter wear.

To objectively evaluate the wear potential of rock, the CERCHAR Abrasivity Test is internationally adopted to determine the Cerchar Abrasivity Index (CAI). The CAI value has become an important indicator for quantifying rock abrasiveness and predicting its impact on cutter wear and TBM driving efficiency.

When the angle between the strike of major discontinuities in the rock mass (e.g., joints, bedding) and the tunnel axis is less than 45°, combined with a gentle dip angle (below 30°), unstable wedge-shaped blocks tend to form in the surrounding rock above the tunnel haunches and in the crown area. This unfavorable structural combination makes these wedges highly prone to loosening, falling, or even large-scale collapse, which not only severely disrupts normal TBM operations and reduces efficiency but also poses direct threats to equipment and personnel safety.

It should be noted that an angle between 50-70 degrees between the major discontinuities and the tunnel axis is generally most conducive to achieving higher advance rates.

Therefore, accurate pre-construction survey of rock mass discontinuity orientations is crucial for predicting and preventing such risks, ensuring safe and efficient TBM construction.