# Effect Mechanism of Material Ratio on Ultrasonic P-wave Velocity in Coal Based Paste Fill Materials
## 1 Introduction  
As modern mining engineering continues developing, the use of mine fill as an important support technology has received increasing attention [1]. This scrutiny pertains to the selection and evaluation of the materials used for fill, which are pivotal in dictating the fill's quality and the overall mining safety [2; 3; 4; 5; 6; 7]. Within China's coal mining industry, which is a significant contributor to the nation's ore resource pool, the choice and application of fill materials are integral to both the mining operations and the fill processes [8; 9; 10; 11].  
In closed/closing coal mines, the main remaining recoverable resources are usually pillars, such as main roadway protection pillars and rise-and-dip protection pillars. These pillars are surrounded by networks of vacant roadways and disused spaces. Recovering these pillars via the caving method may undermine their stability and encourage the accumulation of water and gas, hence jeopardizing safety. To counteract this, the implementation of sophisticated fill methods is proposed to restore integrity to the vacated areas,thus enabling the safe extraction of the pillar resources. This approach has been shown to enhance the safety quotient of mining procedures significantly [12; 13]. However, the use of traditional gangue-based pastes as fill materials has been linked to complications in subsequent coal processing stages, potentially degrading the extracted coal's quality. In contrast, paste fillers derived from coal bases circumvent such issues [14], suggesting their broader applicability and value in the industry.  
Ultrasonic inspection is a non-destructive examination method that has been extensively utilized for the evaluation of material characteristics and the monitoring of quality, being favored for its efficiency, rapid execution, and preservation of the test subject's integrity. Attention has increasingly been given to this method in the study of fill materials in recent times. By means of ultrasonic inspection, critical parameters such as the velocity of sound, density, and materials' elastic modulus have been acquired, enabling the assessment of their physico-mechanical attributes [15; 16; 17; 18; 19; 20]. Consequently, a scientific foundation for the selection and enhancement of fill materials has been provided. Additionally, the capability of ultrasonic inspection to track variations and defects in materials has been confirmed, delivering an immediate insight into the operational condition of fill structures and supporting the safeguarding and dependability of fill procedures [21; 22; 23; 24; 25; 26; 27].  
The intent of the current investigation is to determine the interplay between the velocity of ultrasonic p-wave and the strength of fill materials composed of coal aggregates, with the materials' performance being appraised through ultrasonic methods. Critical to this inquiry was the examination of ultrasonic p-wave velocity features within the materials, through which an understanding of the connection between internal structures and overall performance was unveiled, offering a scientific foundation for the materials' selection. Additionally, the research has aided in the enhancement of fill engineering's quality and dependability, as well as the promotion of advancements and innovations in mining engineering and technology through the exploration of ultrasonic inspection's application to coal based materials [28; 29; 30; 31; 32; 33; 34; 35; 36].