Fragmentation characteristics and mechanical response of hard rock indented by cutting picks: Effects of confinement, spacing, and pre-grooving

Abstract

Efficient hard-rock fragmentation remains a critical challenge in mechanized mining. This study designed an adjustable-spacing mold and conducted double cutting pick indentation tests on granite. Mechanical responses and fragmentation characteristics under varying horizontal stresses, pick spacings, and groove depths were systematically analyzed. Unidirectional stress concentration altered the rock fragmentation modes, exhibiting a dual effect on the fragmentation process. The maximum indentation force (Fmax), indentation hardness index (IHI), indentation modulus (IM), and indentation energy (W) initially increased and then decreased with rising horizontal stress. Appropriate spacing promoted radial crack coalescence, whereas too small a spacing (20 mm) caused repetitive re-fragmentation of rock chips, and too large a spacing (50 mm) resulted in unbroken ridges. Pre-cut grooves weakened the rock, reducing Fmax and specific energy (SE), thus improving fragmentation efficiency, although the improvement slowed beyond a 10-mm groove depth. Based on the results and rock-mass conditioning assisted fragmentation mechanism, a "stress-structure dual control" assisted fragmentation mechanism was proposed, and a "pre-drilling unloading −alternate stopping" mining scheme was exploratorily designed. This approach creates favorable conditions for rock fragmentation by reducing stress levels and rock mass integrity in target zones, providing theoretical support and an engineering paradigm for mechanized mining of deep resources.

1. Introduction

Mineral resources are recognized as fundamental material resources for human survival and development, and serve as critical guarantees for national strategic security and economic construction [1]. With continuous global economic growth and escalating demands for human living standards, the construction of underground engineering projects, as well as mineral resource exploration and development, have witnessed significant expansion [2,3]. However, as shallow metal mineral resources have been progressively depleted, the global mining industry has accelerated its advancement into deeper regions, where kilometer-depth resource extraction has emerged as a new norm internationally [4]. Over 100 mines worldwide have exceeded 1000 m in mining depth [5]. Among the representative deep mines illustrated in Fig. 1a, the Western Deep Levels Gold Mine in South Africa had reached 4800 m, while Pb-Zn-Au-Ag mines in the United States, Canada, and India have surpassed 2900 m [6,7]. As shown in Fig. 1b, Chinese underground mines have reached depths of 1600 m, with metal mining poised to fully enter the second-depth exploration space of 1000–2000 m [8]. According to projections, more than one-third of China’s metal mines are expected to extend beyond 1000 m within the next decade [1], with maximum depths anticipated to reach 2000–3000 m [2]. Contemporary deep metal mines universally exhibit characteristics including dramatically increased burial depth, complex high-stress environments, pronounced nonlinear mechanical rock behaviors, and heterogeneous principal stress distributions.

As illustrated in Fig. 2, geological conditions in deep mines have become increasingly complex with greater mining depths, where challenges such as high ground stress [9], elevated geothermal gradients, and strong mining-induced disturbances are prevalent. These conditions significantly affect the fragmentation efficiency of mechanical cutting tools, necessitating innovative approaches to rock breakage. The present study focuses on the effects of confinement, pick spacing, and pre-grooving on the fragmentation characteristics and mechanical response of hard rock, aiming to provide insights for optimizing mechanized mining schemes in deep resource extraction.