Technology of raising upper limit of mining by grouting reconstruction in thick water-bearing sand layer and its application

In order to set anti-collapse coal (rock) pillars in the thick loose water-bearing sand layer to raise the upper limit of mining, the grouting transformation technology of thick water-bearing sand layer was studied. Aiming at raising the upper limit of mining in the fourth aquifer in working face 1010-1 of Wugou Coal Mine in Huaibei, Anhui. The analysis is conducted through theoretical calculation, experiments on of grouting materials ratio, and in-situ grouting test, with a focus on the scope and horizon of transformation, the applicability of grouting materials, the mix ratio of grout at different depths, the arrangement of drilling holes and the grouting methods. Meanwhile, the effect of grouting was verified by core test, hydrogeological test and underground drilling. The results indicate that: The loose sand layer and the weathered bedrock were modified by grouting while the upper limit mining was raised under the conditions without clay but with water-bearing weathered bedrock on the bottom of loose water-bearing sand layer in the “quaternary aquifer” modification area of Wugou Coal Mine. Besides, the fly ash-cement slurry has better volume stability than the gypsum-cement slurry. Definitely, the fly ash-cement slurry with 20%‒50% fly ash has the hardening rate greater than 98%, and has a relatively stable condensation strength, which could meet the requirements of splitting grouting in the sand layer. In terms of the weathered bedrock, the pure cement slurry or the fly ash-cement slurry with less than 10% fly ash were adopted. Specifically, the sand layer twice the thickness of mining above the bedrock was grouted with fly ash-cement slurry at the fly ash content of 10%‒20% to act as a protective layer. The upper layer was filled and compacted with the fly ash-cement slurry at the fly ash content of 20%‒50%. Generally, the fly ash content decreases with the grouting depth. The final grouting pressure can be increased by the formation pressure and pore water pressure gradient +0.3 MPa/10 m on the basis of the grouting splitting pressure in the shallowest sand layer. The grouting induced by pressure relief can effectively control the splitting diffusion direction of grout, and the maximum diffusion distance of grout reaches 89.5 m. After grouting, the integrity of the sand layer and the weathered bedrock is improved significantly, the strength of rock (soil) test block is increased 2‒5 times, and the sand layer is transformed into water barrier layer with poor water richness and extremely low to low water permeability, capable of raising the upper limit of mining by setting anti-collapse coal (rock) pillars.