Generation of axial chain rockbursts in deep tunnels with drilling and blasting methodology in locked-in stress zone

Abstract

Axial chain rockbursts (ACRs) repeatedly occur in deep tunnels during drilling and blasting methodology (D&B) within locked-in stress zones, severely hindering construction progress. In extremely cases, ACRs can persist for 7−10 d and affect areas exceeding 20 m along tunnel axis. Through integrated geological investigations and microseismic (MS) monitoring, the geological characteristics, MS activity patterns, and formation mechanisms of ACRs were analyzed. In tectonically active regions, locked-in stress zones arise from interactions between multiple structural planes. Blasting dynamic disturbances during tunnel excavation in these zones trigger early slippage along structural planes and fractures in the surrounding rock, with MS events developing ahead of the working face. High-energy MS events dominate during the development and occurrence stages of ACRs, extending 20−30 m (3−4 tunnel diameters) ahead of the working face. Following the ACRs, low-energy MS events primarily occur behind the working face. Tensile fracturing is the predominant failure mode during ACRs. Shear and mixed fractures primarily occur within the ACRs zone during the intra-ACR phase. Monitoring MS event locations ahead of the working face provides a reliable approach for prewarning potential ACR-prone zones.

1. Introduction

Rockburst issues in deeply buried hard rock tunnels have been becoming increasingly prominent [1]. Rockburst generation is influenced by various factors, including in-situ stresses, geological conditions, and excavation methods, resulting in diverse characteristics [2-4]. Based on the mechanisms, timing, and spatial location of rockburst, rockbursts can be classified into strain rockburst, strain-structure slip rockburst, fault-slip rockburst, immediate rockburst, and time-delayed rockburst [5]. In the active tectonic regions of Southwest China, multiple rockbursts have been observed in the same area during the excavation of deeply buried tunnel. These repeated rockbursts severely impact safety and progress during tunnel excavation. This occurrence of repeated rockbursts in the same area has been classified as a new type of rockburst, referred to as “chain rockburst” [6]. Based on the spatial locations of multiple rockbursts, chain rockbursts are categorized further into axial chain rockbursts (ACRs) and radial chain rockbursts (RCRs). During the tunnel excavation process, a rockburst occurs with each blast, and the rockburst-induced crater boundaries of two adjacent rockbursts are connected. This series of rockbursts is defined as ACRs [7].

The geological environment of deep engineering projects is primarily influenced by the coupling of tectonic and sedimentary processes, resulting in exceptionally complex primary structures of the surrounding rock (as shown in Figure 1). During deep rock mass excavation, the mechanisms underlying numerous phenomena remain unclear, such as chain rockbursts, extremely intense rockbursts, and floor heave [8]. MÜLLER [9] first noted the overlooked issue of locked-in stress in rock mechanics research. During geological tectonic movements, the presence of multiple structural planes induces non-coordinated deformation in the rock, forming an uneven in-situ stress field. Within this field, some stresses remain constrained, forming a locked-in stress field [10, 11]. Tunneling through locked-in stress zones sharply increases rockburst probability.

Both laboratory physical simulations and documented field cases have revealed the characteristic phenomena of ACRs. For instance, large-scale physical model tests demonstrate that joints can induce significant compressive stress concentrations, leading to the occurrence of intensive rockbursts [12]. Similarly, tunnel excavations in discontinuous bedrock within the Himalayan tectonic active zone, such as at the Neelum-Jhelum Hydroelectric Project, freque