Tensile-shear collaborative fracturing in hard rock induced by a controllable free surface: Mechanism and application

Abstract

In deep hard rock mining, high confining pressure inhibits tensile failure, leading to low efficiency and severe tool wear in conventional mechanical rock breaking methods. To solve this problem, we propose a Controllable Free Surface Induced Tensile-Shear Collaborative Fracturing (CFS-TSCF) method. The method pre-forms an engineered controllable free surface (CFS) to reconfigure the local stress field, enabling a specialized device (FIPFD) to apply directional tensile-shear loads for low-energy breaking. A multi-scale approach integrating lab AE tests, DEM simulations, and field verification investigated the fracture mechanism and performance. Results revealed a predominantly tensile-driven (>50%) process. The CFS transforms the rock's triaxial compression into a specific stress path. This path, dominated by directional tension and constrained by lateral compression, guides the fracture along a low-energy channel. This also dictates the micro-mechanism's evolution from central quasi-tensile to peripheral tensile-shear failure. Field trials in hard rock (>200 MPa UCS) validated the method, demonstrating controllable, blocky spalling and achieving an average mining efficiency of 52.03 t/h. This research validates the CFS-TSCF method, offering a new technical paradigm for safe, efficient, continuous hard rock mining.

1. Introduction

As the demand for mineral resources in China continues to grow, decades of large-scale extraction have led to the depletion of shallow deposits. Consequently, the future of China's mineral development is shifting towards deep ore bodies, entering what is known as the 'second deep space' (1000–2000 m), where deep mining for metallic minerals will become standard practice [1–3]. However, as operations advance to greater depths, the rock mass becomes harder and more compact, in-situ stresses escalate, and complex ore body geometries are encountered. These conditions present significant operational challenges, such as difficulties in hard rock cutting and the inability to achieve continuous stoping with the drilling and blasting method, posing a severe test for safe mining technologies [4,5].

Currently, the drilling and blasting method remains the dominant technology for deep hard rock mining in China and globally, owing to its versatility and cost-effectiveness [6]. Extensive research has been conducted on the mechanisms of rock mass damage and crack propagation under blasting loads, aiming to elucidate the complex interactions between the blasting load and the rock mass. This has led to the development of techniques for directional crack control and sophisticated methods for multi-scale damage assessment. Regarding fracture mechanisms, Liu et al. [7], through theoretical and experimental analysis, revealed that confined blasting results from a combination of pre-cracking by the shockwave and subsequent wedging expansion by the gas products. To quantify the fracture resistance of rock under blasting loads, Zhu et al. [8] proposed a novel method for determining the Mode I dynamic fracture toughness. For crack propagation control, researchers have sought to achieve directional fracturing by optimizing the layout of boreholes and the charge structure. For instance, Pu et al. [9] employed numerical simulations to investigate the effects of hole spacing and delay timing on crack propagation and coalescence in dual-hole blasting. Similarly, Cai et al. [10] assessed shaped-charge directional fracturing in jointed rock masses using physical and numerical models. Xiao et al. [11] innovatively applied fractal theory to propose a multi-fractal method for comprehensively assessing rock damage under blasting.