Dynamic tests and mechanical model for water-saturated soft coal with various particle gradations

Abstract

Particle gradations significantly affects the dynamic response of water-saturated soft coal. To obtain the dynamic response of soft coal to various particle gradations, static and dynamic mechanical properties of water-saturated soft coal with different particle gradations were tested and compared. During these processes, the enhancement coefficient of the dynamic mechanical parameters to the corresponding static mechanical parameters was set as an index to describe the effect of non-uniformity coefficient Cu and median diameter d₅₀. The results demonstrate that both the static and dynamic mechanical parameters of the water-saturated soft coal were affected by particle gradations. The enhancement coefficient was affected by both the d₅₀ and Cu, which was microscopically attributed to effect of the d₅₀ and Cu on the average dynamic liquid bridge force and scale dynamic rupture energy among the particles based on the microstructure and dynamic fracture evolution processes. In addition, the dimensionless scale rupture energy model among the water-bearing particles varied with the d₅₀ and Cu were further established and applied to discuss the effect mechanism of particle gradations on the dynamic response of water-saturated soft coal. On this basis, the support measures of roadways in coal seam with different particle gradations were further discussed.

Introduction

The developed geological structure in many coalfields in China makes many coal seams present as a soft state. The soft coal generally refers to the loose, soft and weak coal with low cohesion, weak bearing capacity, and extremely poor plasticity.¹ Its mechanical characteristics are primarily dependent on particle geometry and physical characteristics, of which particle gradation is an important factor when evaluating soft coal.² In addition, coal mining is usually performed in a water environment, and the water content in the soft coals is regulated by the particle gradation for the water-saturated soft coal.³ In previous studies, the physical and static mechanical properties of water-saturated coals with different particle gradations have been extensively investigated.^4,5 However, the variation in the dynamic mechanical properties of water-saturated soft coals with different particle gradations have rarely been studied. Generally, the loading rate has a significant effect on the rock fragmentation processes, including drilling, blasting, hydraulic fracturing, crushing, and grinding, and on the failure modes, including rock bursting, impact failure, and others.^6,7 Thus, the investigation of the dynamic properties affected by the particle gradations and the correlated mechanism are greatly meaningful for designing the impact-resistant engineered infrastructures and constructions.

Soft coal seams are widely distributed in China. Due to the interaction of geological conditions, mine pressure, and other factors, the particle compositions of soft coals can be quite different. To distinguish these differences, the particle gradation has been developed to characterize different soft coals, which are characterized by multiple metrics. Among these metrics, the non-uniformity coefficient C_u and the median particle diameter d₅₀ are most widely used to describe the non-uniformity degree and particle size, respectively.⁸ Changes in these parameters may lead to a variation in the micro-properties of soft coals, such as porosity and density. During the coal mining process, a coal seam is inevitably immersed in a watery environment that is mainly caused by the goaf water, aquifer leakage, coal seam water injection, and inherent water content of the soft coal.⁹ This causes the soft coal to stay in a liquid-solid coupling state, and microscopically changes the interaction between the particles.^10,11 When ignoring the effects of possible trapped air bubbles in the water-saturated soft coal, the pores can be assumed filled with water. In this condition, the porosity determines the water content of the water-saturated specimen. Its negative correlate with the density of soft coal, which directly related with the particle gradation. Thus, it can be concluded that the water content of the water-saturated soft coal is closely correlate with the particle gradation. The effect of the water content on the mechanical properties of soft coal can be translated into the effect of the particle gradation on the mechanical properties of soft coals to some extent.⁸ To study the effect of particles gradation on the mechanical properties of water-saturated soft coal, many studies have performed extensive research on the mechanical properties of graded particle materials. It obtained that due to the considerable variations of particles gradation, the extent of the water content effects is highly varied.¹² However, the studies mentioned above were mainly based on static tests and a limited number of studies can be found on the dynamic properties of water-saturated soft coal. In fact, the soft coal usually fail dynamically in engineering applications, such as impact, explosion, and seismic events.¹³ Thus, the investigation of the dynamic response of soft coals to particle gradation is greatly meaningful to assess the dynamic stability of soft coal structures and control the engineering hazard.

The dynamic failure process for coal is macroscopically characterized by fracture initiation, propagation, and coalescence with associated damage and evolution.¹⁴ Various scales are involved in the failure process, from the nano- or micro-level to the macro-level.¹⁵ The fundamental cause lies in the variation of the interaction and the micro-mechanics between the particles in soft coals under external loads.¹⁶ The main forces between the particles include van der Waals, electrostatic, and liquid bridge forces. To calculate the stress between particles with different sizes, the particles are usually simplified as spheres and the roughness of the spheres is characterized by the specific space.¹⁷ Moreover, to simplify the process, the force between unequal spheres is usually converted into the liquid bridge between the equal particles though a reasonable transformation. The liquid bridge force among soft coal particles consists of two parts: static and dynamic.¹⁸ Under a dynamic load, the dynamic liquid bridge force cannot be ignored due to the rapid relative motion between the particles. There are several methods used to calculate the solution-liquid bridge force. They can be classified as the Laplace-Young’s exact numerical solution method and the Fisher’s approximate solution theory method.^19,20 However, the both the methods are not adequate for modelling the particle gradation effects because the calculation parameters do not consider the particle size distribution. Previous research mainly focused on the experimental analysis, and the fundamental mechanisms have not been elucidated in these experimental studies.²¹ An alternative approach is to consider the particles material as an assemblage of particles, where the physical processes that govern the constitutive behaviour can be rigorously understood starting at the particles scale level.²² This approach can in principle characterize the effect of particles gradation on the mechanical properties of soft coals based on the inter-particles mechanical principle. However, the previous theoretical analysis does not involve these studies.

Based on the above analysis, this study attempts to reveal the effect and its mechanism of the particle gradations on the dynamic mechanical properties of the water-saturated soft coals. Static and dynamic tests for a series of water-saturated coal specimens with different particles gradation were performed using an MTS-322 rock mechanics testing machine and the Split-Hopkinson pressure bar (SHPB). This study was carried out in two parts. The first part was the tests and analyses of the strength and elastic modulus properties of water-saturated soft coals with different particle gradations under dynamic and static loads, respectively. The second part was the theoretical analysis of the effect mechanism of particle gradations on the dynamic and static mechanical properties of water-saturated soft coals based on the established scale rupture energy.