Study on the Influence of in-situ Stress Levels on the Damage Characteristics of Surrounding Rock during Drill-and-Blast Expansion of TBM Pilot Tunnels

The “Small-Diameter TBM Pilot Tunnel + Drill-and-Blast Expansion” method is a key excavation approach for long tunnels, combining the advantages of TBM and drill-and-blast methods to enable rapid and efficient construction. However, the understanding of surrounding rock damage caused by this method remains limited. This study combines field experiments and numerical simulations to analyze surrounding rock damage during drill-and-blast expansion and investigates the effects of in-situ stress factors, such as tunnel burial depth and lateral pressure coefficient. Results show that excavation advancement is closely linked to the ratio of advancement between full-face drill-and-blast and TBM methods. When both methods achieve double the advancement of full-face drill-and-blast, significant time-saving advantages are evident. Surrounding rock damage is positively correlated with burial depth, with deeper tunnels shifting failure modes from compressive-shear to tensile-shear composite failure, increasing tensile cracks. In shallow tunnels (burial depth < 450 m), drill-and-blast expansion reduces rock damage by approximately 10% compared to full-face drill-and-blast, but this effect weakens with depth. Lateral pressure coefficient impacts damage nonlinearly, with high values leading to extensive tensile cracks near sidewalls. Regression analysis reveals normalized contribution rates of 61.2% for burial depth and 38.8% for lateral pressure coefficient. These findings offer theoretical support for applying this method in long tunnel construction.