Analysis on spatio-temporal evolution law of freezing temperature field in eastern shaft of Qinan Coal Mine

Three numerical calculation models of the freezing temperature fields with different soil nature and different burial depths, i.e. the calcareous clay horizon with the burial depth of 218 m, the fine sand horizon with the burial depth of 225 m and the sandy clay horizon with the burial depth of 259 m, were established respectively using the finite element software COMSOL Multiphysics based on the actual forming locations of freezing holes, with the engineering background of using the freezing method in shaft sinking for the eastern air shaft of Qinan Coal Mine Anhui Province and the study object of the surface soil with different soil nature that was frozen by multi-circle-pipe method. In combination with the field measured data, the space-time evolution regularity of the freezing wall temperature field was calculated and analyzed. As indicated by the results, under the same freezing conditions, the effective average temperature of the fine sand horizon with the burial depth of 225 m was 0.09-0.72℃ and 0.44-1.95℃ lower than those of the sandy clay horizon with the burial depth of 259 m and the calcareous clay horizon with the burial depth of 218 m, respectively; the effective average thickness of the fine sand horizon with the burial depth of 225 m was 0.17-0.38 m and 0.29-0.47 m larger than those of the sandy clay horizon with the burial depth of 259 m and the calcareous clay horizon with the burial depth of 218 m, respectively. According to both the field measurement and numerical calculation, when the freezing wall of each horizon was excavated, the average temperature was lower than −15℃ for each, the effective thickness was larger than 6.2 m for each, and the flank temperature at the deep surface soil below 200 m was lower than −4℃, which met the construction requirements; both the freezing wall strength and the stability were in safe status; the freezing temperature field was divided into 3 areas radially by the freezing hole. The temperature of area B dropped fastest due to the effect of the cooling capacity superposition of freezing holes; the temperature drop in area A was moderate; the temperature drop of area C was the slowest, as the area was far away from the freezing pipe, and the external soil continued to transfer heat to it. The study can provide certain theoretical references to the construction of the freezing wall in the shaft sinking using the freezing method.