# Experimental Research into the Ultrasonic P-Wave Velocity of Coal Slime Based Backfill Material
## 1 Introduction  
At present, the finite nature of coal resources has become an issue of note and the demand for mining engineering to use eco-friendly mining technologies has increased. In such a context, the application of coal slime based backfill materials, as environmental-friendly cemented backfill materials, into coal mines has received wide attention [1]. Coal slime based cemented backfill materials are formed using cementing materials to solidify coal slime, which can be used as the main raw material. Coal slime based cemented backfill materials are generally applied to mine filling engineering during coal mining, to fill mined-out areas and abandoned roadways, stabilize the surrounding rock, and improve the safety factor of mining and the rate of resource utilization. They lay a solid foundation for safe, efficient coal mining [2; 3; 4; 5; 6].  
It is always a difficulty in project implementation to monitor and evaluate the strength characteristics of coal-tailings based backfill materials in real-time, practical, engineering applications. Conventional mechanical test methods commonly take a long time and call for many resources, and it is challenging to achieve real-time monitoring in the mining environment. Therefore, there is an urgent need to develop a novel method that can rapidlyand accurately assess the strength characteristics of coal-tailings based backfill materials [7; 8; 9]. Ultrasonic detection has been widely applied to the evaluation and monitoring of the mechanical properties of materials because it is non-intrusive, highly accurate, and works in real-time. By measuring the propagation velocity of ultrasonic waves in materials, ultrasonic detection can determine mechanical parameters including the elastic modulus and Poisson's ratio of materials, thus providing an important reference for the strength characteristics of materials. However, for materials with complex porous structures, such as coal-tailings based backfill materials, the relationships of the propagation characteristics of ultrasonic waves with the internal structures and mechanical properties remain to be investigated [10; 11; 12; 13].  
The proportions and properties of cementing materials may affect the density and strength of backfill materials, thus affecting the measurement of wave velocity. Generally, a high proportion of cementing materials will induce a high wave velocity, because a large amount of cementing materials can improve the strength and density of the backfill materials. Different proportions of cementing materials cause the ultrasonic wave velocity to change in different manners. Moreover, the influence of the mix proportion of cementing materials is associated with those of other factors on the measurement of the ultrasonic wave velocity [14; 15; 16; 17; 18]. Variation of one factor may change other factors, thus influencing measurement results of the ultrasonic wave velocity. Through experiments and theoretical analysis, this research aimed to expound the influencing mechanism of mix proportions of a coal slime based backfill material on the ultrasonic wave velocity [19; 20; 21]. In addition, the association of mix proportions with the material strength was revealed and the corresponding prediction method was proposed. This research provides a novel and efficient strength assessment method for coal mines and solid technological support for the safe and stable operation of coal mines [22; 23].