Quantitative evaluation method of rockburst prevention effect foranchoring rock masses around deep-buried tunnels

Rockburst has long been one of the most unsettling hazards in underground tunneling, especially when projects extend to depths where stress conditions in the rock approach their limits. Rockburst erupts suddenly not like gradual deformation, which gives engineers at least some time to react and the elastic energy stored in the surrounding rock mass is released in an instant, often with explosive violence. Workers describe it less as a structural failure than as the earth itself striking back—walls shattering, fragments ejected at high speed, and noise so sharp it resembles an underground blast. Beyond the immediate danger to crews, such events halt excavation, destroy equipment worth millions, and compromise support systems that were already installed at significant cost. Despite steady advances in tunneling technology, rockburst remains unpredictable, and its occurrence is a constant reminder of the limits of engineering control in deep geological environments.

Rockbolts are among the most common attempts to tame this hazard and by anchoring fractured rock and binding it into a stronger composite body, bolts are expected to redistribute stress and suppress crack propagation. In theory, this reinforcement should reduce the chance of violent failure, however, the record from the field is mixed. Engineers have seen bolts snapped cleanly in half or even expelled from tunnel walls during bursts, raising uncomfortable questions about whether bolting is as protective as textbooks suggest. The persistence of rockburst in heavily bolted tunnels makes it clear that the interaction between bolts and stressed rock is more complex than often assumed. Part of the difficulty lies in how effectiveness is measured. Much of the evaluation to date has been qualitative—engineers noting whether bursts seem to occur less frequently after installation and drawing conclusions accordingly. While such judgments are practical, they rarely provide the quantitative basis needed to refine support design. Compounding this is the fact that the most widely cited rockburst prediction criteria—those of Barton, Hoek, and Kaiser—tend to emphasize rock strength and in situ stress. These are useful parameters, but they leave out something crucial: the destabilizing role of excavation itself. The excavation disturbance is now recognized as a key trigger of bursts, yet it is not consistently integrated into existing models. This gap explains why predictions may appear reasonable at the design stage but prove unreliable once excavation is underway. To this account, new research paper published in Tunnelling and Underground Space Technology and conducted by Dr. Jinhao Dai and Professor Fengqiang Gong from the Southeast University alongside Professor Da Huang from the Chang’an University and Professor Qinghe Zhang from the Anhui University of Science and Technology, the researchers developed a quantitative evaluation method to measure how effectively anchoring rock masses prevents rockbursts in deep tunnels. By introducing an enhancement coefficient that accounts for the strengthening effect of rockbolts, they built a model to calculate potential rockburst pit depths before and after anchoring. This approach allowed them to quantify prevention efficiency through a decrease ratio, linking excavation damage, rock properties, stress conditions, and bolt parameters. Their framework provides engineers with a predictive tool to design safer and more efficient reinforcement strategies in high-stress tunnelling environments.

The researchers turned to three large-scale tunnel projects in China where rockburst had posed severe risks despite reinforcement. Each tunnel presented a unique geological environment, excavation method, and stress regime, offering a natural laboratory to test whether anchoring could be systematically evaluated. In the Caoguoshan tunnel, located in Yunnan Province at depths surpassing 900 meters, excavation through monzonite and granite frequently triggered bursts. Observers noted violent cracking and the sudden formation of shallow pits along tunnel walls, some extending two meters in length. After installing Φ22 mm grouted bolts with a 2.5 m length at a spacing of 1.0 m by 1.0 m, the visible signs of bursting diminished noticeably. Workers reported fewer rockfalls and less cracking near the excavation face, suggesting that bolting transformed vulnerable zones into more stable rock masses. The finding was important: shallow potential bursts, which engineers feared would expand during excavation, often disappeared completely after anchoring. The Daxiagu tunnel in Sichuan Province told a slightly different story. With a depth nearing two kilometers and host rocks of dolomite and rhyolite reaching compressive strengths above 245 MPa, the stresses were severe. More than two hundred rockbursts were recorded in a 4.4 km stretch, with ejected rock blocks traveling up to ten meters. In response, engineers applied different bolt designs tailored to predicted burst intensities. Resin cartridge bolts were installed in zones of weak or moderate bursts, while prestressed hollow grouting bolts—denser and stronger—were deployed in high-risk sections. The contrast was striking. Where ordinary bolts were used, bursts continued but with reduced severity, whereas in zones supported with the high-performance bolts, the frequency of incidents fell sharply, allowing excavation to proceed faster with fewer interruptions. These observations reinforced the idea that bolt type and configuration are not secondary details but fundamental to prevention outcomes.

Qinling tunnel which is part of a massive water diversion project is considered a truly challenging environment because horizontal tectonic stress pushed the maximum principal stress above 100 MPa, and nearly one thousand bursts were recorded over two years of excavation. Standard mortar bolts initially offered some relief, but bursts of over a meter deep still formed. The researchers then introduced Negative Poisson’s Ratio (NPR) bolts in a test section, tightening the spacing compared with the earlier scheme. The outcome was measurable: maximum pit depths fell from 1.55 m to 1.15 m, and average depths were cut by more than half. This experiment confirmed quantitatively what many engineers had intuited—the right kind of anchoring could alter burst behavior in a predictable, beneficial way.

In conclusion, the true significance of the research work of Fengqiang Gong and colleagues is in successful ability to transform rockburst prevention from a largely qualitative practice into one guided by measurable and reproducible indicators. Indeed, the authors have given practitioners an important tool that not only interprets past outcomes but also forecasts the effectiveness of reinforcement strategies before conditions escalate into dangerous territory by establishing a quantitative framework centered on the depth of potential rockburst pits. This shift from descriptive observation to predictive analysis marks a turning point in tunnel safety management. The implications extend well beyond theoretical modeling. In practice, the method provides engineers with the capacity to distinguish between situations where rockbolts alone can neutralize rockburst risks and additional reinforcement must be incorporated. This clarity is invaluable because it allows scarce resources—time, material, and labor—to be allocated with precision. For example, when the model indicates that minor excavation damage will be fully compensated by anchoring, crews can proceed confidently without costly over-design. Conversely, when calculations show that deep damage zones will only see marginal improvement, managers can immediately plan for supplementary supports such as cables or steel arches, avoiding a trial-and-error approach that often delays construction and endangers workers. Another equally important implication is in excavation strategy itself and the results clearly demonstrated that the extent of excavation damage before bolting largely dictates prevention outcomes. These findings places renewed emphasis on cautious excavation techniques aimed at minimizing disturbance in the first place and that prevention is not just a matter of installing more or stronger bolts; it begins with how the rock is excavated. By connecting excavation practice directly with the later effectiveness of anchoring, the study proposes a more integrated approach to tunnel design and construction.