Pull-out capacity and energy absorption of cable bolts under impact loading

Abstract

This study investigates the performance of high-strength cable bolts under impact loading conditions representative of rock bursts in underground environments. Although widely used, the dynamic behaviour of these cable bolts has received limited experimental attention, and their effectiveness in seismically active zones remains a subject of ongoing debate. To address this gap, a reverse pull-out test machine integrated with a drop hammer rig was employed. Tests were conducted on 70-t SUMO bulbed and non-bulbed cable bolts with encapsulation lengths of 300 and 450 mm, subjected to an impact energy of 14.52 kJ. Results indicate that non-bulbed cables, despite showing lower initial peak loads (average 218 vs. 328 kN for bulbed cables at 300 mm encapsulation), demonstrated superior energy absorption (average 11.26 vs. 8.75 kJ) and displacement capacity (average 48.40 vs. 36.25 mm). Increasing the encapsulation length for bulbed cables led to a reduction in initial peak load but improved displacement and energy absorption. The dominant failure mechanism was debonding at the cable-grout interface, characterised by frictional sliding and cable rotation. These findings provide new insights into the energy dissipation mechanisms of cables and support the development of more resilient ground support systems for dynamically active conditions.

1. Introduction

In underground mining and tunnelling projects, the safety of working spaces is always a paramount concern. To address this concern, stranded steel tendons, known as cable bolts, have been effectively employed as a rock reinforcement tool [1]. A cable bolt is a flexible tendon consisting of several steel wires, wound into a strand, which is grouted into a borehole [2]. The length of cables typically ranges between 3 and 15 m, while the rock bolt's length would generally be less than 3 m [3]. According to Fuller [4], the first application of cable bolt for rock reinforcement dates back to 1964 in South Africa, closely followed in Australia in the late 1960s at the North Broken Hill, where 15 m long and 12.5 mm diameter cables were used. The principal function of tendons like cable bolts is to maintain and improve the strength of the rock mass, to prevent rock layer separation, and to control the post-failure deformation [5]. In this regard, cable bolts develop forces as the ground starts to move. The weight of the unstable rock block close to the excavation boundary is conveyed to the firm rock strata beyond the unstable region in the roof and sidewalls of the underground opening [6].

Since cable bolts play a pivotal role in resisting both axial load due to the dead weight of the rock mass and shear load due to slippage along discontinuities [2,7], it is necessary to assess their performance by subjecting them to both axial and shear loading modes. Based on the available literature, various studies have been carried out worldwide to investigate axial and shear capacities of cable bolts subjected to static loading mode, demonstrating the long-term ground settlements, during which no impact load is applied [1]. But tendons are also susceptible to impact loads, especially in dynamically active zones [8,9]. In the presence of seismic events (e.g., coal burst, rock burst, and large-scale blasting practices), high stress loading due to impact loading can also result in steel yielding, anchorage loosening, and maybe tendon complete failure [10]. Depending on the seismic event scale, the velocity of ejected rock pieces may vary between 3 and 10 m/s, and the corresponding energy levels may vary from 10 to 50 kJ/m2 [11,12], resulting in severe damage to underground openings as shown in Fig. 1. Thus, from a safety point of view, it is also necessary to examine the performance of tendons under seismic loading.