Influencing factors of accumulative effect of overburden strata expansion induced by stress relief

The overburden strata undergo a dynamic process of stress relief-induced expansion and re-compaction. The phenomenon that the total amount of overburden unloading expansion changes dynamically with the continuous accumulation of unloading height and bearing load is defined as the cumulative effect of overburden strata expansion induced by stress relief. This cumulative effect caused by mining is a natural phenomenon in the process of strata movement and has an important impact on the movement law of strata. Based on the revised mechanical model of the cumulative effect, and the measured results of internal strata movement at the 1311 working face of Zhaozhuang Coal Mine, China, the impact of mining height, mining depth and lithology on the cumulative effect was studied theoretically. The results show that the total amount of overburden unloading expansion is mainly composed of the plastic expansion of the caving zones and the fracture zones and the elastic expansion of the continuous bending zones at the lower part of the primary key stratum. The plastic expansion of the caving zones accounts for the largest proportion, and the elastic expansion of the continuous bending zones accounts for the smallest proportion. With the gradual increase of the unloading height of the overburden strata, the total amount of overburden unloading expansion will first increase and then gradually decrease. When the primary key stratum of the overburden strata breaks, the elastic expansion of the continuous bending zones drops to 0, at this time the total amount of overburden unloading expansion reaches the minimum value. The influence of mining height on the cumulative effect is mainly reflected in the different heights of caving zones and fracture zones under different mining height conditions, which affects the total amount of overburden unloading expansion. The total amount of overburden unloading expansion increases with the increase of the mining height. When the mining height increases from 2.5 m to 10.5 m, the peak value of the total amount of overburden unloading expansion and the residual crushing expansion both increase by more than 3 times. Compared with the mining height, the mining depth variation generally does not change the height of the caving zones and the fracture zones, and the mining depth has less influence on the peak value of the total amount of overburden unloading expansion. The influence of mining depth on the cumulative effect is mainly reflected in the difference in the residual expansion of the overburden under different mining depth conditions. The residual expansion of the overburden decreases with the increase of mining depth. When the mining depth increases from 496.6 m to 896.5 m, the residual expansion of the overburden is reduced by 32.5%. The influence of lithology on the cumulative effect is mainly reflected in the different degree of compaction of the expansion coal and rock with different lithology. The total amount of overburden unloading expansion increases with the hardening of lithology. When the initial tangent modulus and elastic modulus vary from 50% decrease to 50% increase, the total peak amount of overburden unloading expansion increases by 4.3%, but the residual expansion of the overburden increases by 110%. The residual expansion of the overburden determines the surface subsidence factor. Therefore, the cumulative effect can affect the variation law of the surface subsidence factor. The influence of mining height on the surface subsidence factor is not monotonous linear. Whether the subsidence factor decreases or increases slightly with the increase of mining height depends on the influence of the position of the key stratum on the height of the fracture zones. Under the condition of critical mining, since the residual expansion of the plastic expansion zones decreases with the increase of mining depth, the subsidence factor increases with the increase of mining depth, and the increase rate keeps decreasing. When the lithology gradually changes from soft rock to hard rock, the plastic expansion zones is less likely to be compressed, and the surface subsidence factor decreases accordingly.