Research on fracture characteristics and support mechanism of shallow buried double-soft composite roof

Abstract

Affected by the geological characteristics of coal bearing strata in western mining areas of China, the double-soft composite roof has low strength and poor integrity, which is prone to induce disasters such as large deformation and roof collapse. Four-point bending tests were conducted on anchored double-layer rock beams with different pre-tightening force and upper/lower rock strength ratios (I/II) based on the digital speckle correlation method (DSCM). The research results indicate that the instability process of anchored roof can be divided into stages of elastic deformation, crack propagation, alternating fracture, and failure collapse. The proportion of crack propagation and alternating fracture processes increased with the increase of pre-tightening force and I/II. The pre-tightening force can suppress the sliding of the upper/lower rock interface, and delay the initiation and propagation of cracks. As I/II increases, the failure mode changes from tensile failure steel strip to shear failure anchor rod. Steel strip can improve the continued bearing effect of anchored roof during crack propagation and alternating fracture processes.

1. Introduction

The excavation of roadways has disrupted the balance of initial geostress, resulting in uneven distribution of mine pressure and directly leading to difficulties in roadways support [1−3]. Maintaining the stability of the roadway, especially the roof, is the key to ensuring safe mining [4−6]. With the westward movement of mining areas in China, the low strength and poorly cemented rock layers have increased the complexity of roof support [7−9].

Affected by the sedimentary environment and geological structure, the strata in the western region are more likely to form a double soft composite roof structure [10−12]. The anti-deformation ability of the double soft composite roof is poor, and it is highly sensitive to engineering disturbances. Excavating roadways can easily lead to roof collapse, which has a significant impact on safety production of coal mine safety production [13−15]. In order to study the mechanism of roadway instability caused by differences in rock types of the roof, the convergence deformation and evolution process of the surrounding rock in composite strata were studied by ZHANG et al [16], and the failure types and dominant factors were proposed. FAN et al [17] analyzed the deformation characteristics caused by roadway construction in the composite strata, and explored the influence characteristics of roadway spacing and excavation sequence on surface subsidence. The structural characteristics of the composite roof were studied through similar simulation experiments by WU et al [18].

The occurrence environment of surrounding rocks in the western region has deteriorated, which has led to the deepening of the problem of anchor rod failure [19−23]. The failure of anchor rods in the double-soft composite roof will inevitably lead to a significant decrease in the strength of the roof support and become a key factor causing roof deformation [24−28]. AGHESHLUI et al [29] used numerical simulation and theoretical analysis to study the failure mechanism of anchor rods in composite roof roadways. AFRASIABIAN et al [30] proposed the theory of strong support and developed high pre-tightening force support system. TAO et al [31] proposed to use a combination of grouting and anchor rod support, and fully utilize the bearing capacity of the surrounding rock itself. The above research indicates that in order to ensure the stability control of the roadway, it is necessary to study the rock deformation and instability laws of the double soft composite roof [32−35]. Through bearing tests of composite beams with different roof combinations and pre-tightening force, the crack propagation characteristics during the fracture process are analyzed. And the mechanism of anchor rod action in the fracture process of double-soft composite roof is explored.