## 5 Conclusions  
In this work, by applying a seepage test system for damaged coal-rock masses, seepage tests were carried out on the broken coal mass under cyclic loading and unloading conditions. First of all, the permeability evolution law of broken coal samples under cyclic loading and unloading conditions was obtained, and the sensitivity of permeability of broken coal samples was also analysed. Afterwards, the particle breakage and distribution law after each loading and unloading seepage test was analysed. Finally, the reasons for the changes in permeability of broken coal samples under cyclic loading and unloading conditions was revealed, and the relationship between permeability and strain, and porosity under cyclic loading and unloading conditions was discussed. The specific conclusions are shown as follows:  
(1) The relationship between the permeability of broken coal samples and effective stress during cyclic loading and unloading conformed to a logarithmic function model. Moreover, the fitting equations for the permeability of coal samples and effective stress under various loading and unloading conditions were attained.  
(2) The loss (\(D_{1}\)) of permeability of broken coal samples in the loading stage, the recovery (\(D_{2}\)) of permeability in the unloading stage and irreversible loss (\(D_{3}\)) of permeability after one loading and unloading cycle all gradually decreased with increased numbers of cyclic loading and unloading events. During the same loading and unloading cycle, the loss (\(D_{1}\)) of permeability in the loading stage was much greater than the recovery (\(D_{2}\)) of permeability in the unloading stage.  
(3) The stress sensitivity of permeability of broken coal samples gradually decreased with increased numbers of loading and unloading cycles. During the same loading and unloading cycle, the stress sensitivity of permeability of coal samples in the loading stage was much greater than that in the unloading stage.  
(4) The larger the particle size of the coal sample is, the greater the breakage probability and crushing amount of the particle will be after each loading and unloading seepage test. With the increase of cyclic loading and unloading times, the crushing amount of the broken coal sample decreased gradually, but the particle size gradation of the broken coal sample gradually became better.  
(5) Rearrangement, breakage and compressive deformation of coal particles in the loading stage can cause a significant decrease in the porosity of coal samples, thus resulting in a decreased permeability thereof. In the unloading stage, only the loss of permeability resulting from particle deformation could be recovered.  
(6) The permeability of broken coal samples decreased with increasing strain and increased with the increasing of porosity of the broken coal samples. The fitting curves between permeability and strain, and porosity are a cubic function and power function, respectively. Besides, the permeability of broken coal samples under cyclic loading and unloading conditions was determined by its porosity, which conforms to the cubic law.  
Data curation, B.L.; Funding acquisition, Y.L.; Methodology, Y.L.; Validation, Q.Z.; Writing--original draft, B.L.; Writing--review & editing, Q.Z.  
This work is financially supported by the National Science and Technology Major Project of China (Grant No. 2016 ZX05043005), the National Natural Science Foundation of China (51674050 and 51704046), the State Key Research Development Program of China (Grant No. 2016 YFC0801404 and 2016 YFC0801402), and the Fundamental Research Funds for the Central Universities (2018 CDQYZH0001) which are gratefully acknowledged.  
The authors declare no conflict of interest.