# Experimental Study on Permeability Characteristics of
## 4 Conclusions  
In this study, a multi-field coupled seepage test system for backfill materials was developed, and the pressure bleeding characteristics, permeability changes, and the determination method for the permeability level of the CBB and its optimization methods were investigated. The main conclusions are as follows:  
1. The developed multi-field coupled seepage test system for backfill materials mainly consists of five parts: material loading module, axial stress loading module, liquid seepage module, seepage outlet measurement module, and data acquisition and processing module. With a maximum axial pressure of 30 MPa and a maximum seepage pressure of 15 MPa, this system can monitor and collect key parameters, such as stress, displacement seepage flow rate, and water pressure, in real time during the experiment.
2. After being injected into the mining space, the gangue slurry is subjected to pressure and local stress from the overburden, resulting in a pressure bleeding phenomenon. After the free water is completely drained, the CBB is formed. A laboratory preparation method for the CBB is proposed, and the waterproof performance of the gangue grouting backfill materials is evaluated by testing the permeability of the CBB.
3. The pressure bleeding of the gangue slurry exhibits obvious phased characteristics. In the first stage, it shows a linear growth trend with axial loading time. In the second stage, the increasing amplitude of pressure bleeding gradually weakens. In the third stage, the pressure bleeding is reduced to 15 MPa. The pressure bleeding is reduced to 15 MPa.  
stage, pressure bleeding gradually stabilizes and finally stops. Moreover, the second stage has a significantly longer duration compared to the first and third stages.
4. The permeability of the CBB decreases exponentially with the increasing axial pressure. After adding cement to the gangue slurry, the permeability of the CBB exhibits a higher sensitivity to axial stress. As the cement dosage increases, the permeability change in the CBB transfers from linear to nonlinear, and the cement dosage of 2% is the turning point of the linear to nonlinear transition. With the same axial stress, the permeability of the CBB with a particle size range of 0-5 mm is greater than that of 0-2 mm. This is because there is a higher porosity in the CBB with a particle size range of 0-5 mm. As the cement dosage increases, the permeability of the CBB with a particle size range of 0-5 mm gradually decreases, and the addition of cement reduces the permeability of the CBB by 1-2 orders of magnitude.
5. The permeability level of the CBB of gangue slurry is within the range of _poor permeability_ and _extremely poor permeability_. After entering the mining space, the gangue slurry has a good water-blocking effect. Increasing the axial stress on the gangue slurry and adding cement can both improve the permeability level of the CBB and improve its waterproof performance.  
Conceptualization, writing--original draft preparation, writing--review and editing, J.X.; investigation, conceptualization, writing--original draft preparation, S.L.; methodology, writing--review and editing, H.W.; methodology, funding acquisition, N.Z.; data curation, Y.Z. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Key R & D Program of China (2023 YFC3904304), the Key R&D Program of Xinjiang Uygur Autonomous Region (2023B01009), the Graduate Innovation Program of China University of Mining and Technology (2024 WLKXJ017), the Fundamental Research Funds for the Central Universities (2024-10957), the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX24-2851).  
Not applicable.  
The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.  
The authors declare no conflicts of interest.