Experimental study on the expansion law of local horizontal frozen body under seepage

At present, when the freezing temperature field under seepage is studied by physical model test, the homogeneous sand layer is mostly used as the heat transfer medium in the stratum, and most of them are single-row pipes frozen along the length direction.There are few model tests in which sand and pebble are used as heat transfer medium in formation, and there are few local freezing model tests with double-row pipes.In order to study the expansion law of local horizontal frozen body of double-row pipes under seepage, according to the theory of similarity, the local freezing model test of double row pipes in sandy gravel stratum under seepage is carried out.Research shows that the forming process of local horizontal frozen body of double-row pipes under seepage can be divided into three stages:the second row of frozen soil columns intersecting, the formation of complete horizontal frozen body, and the continuous expansion of the frozen surface, and that the forming time and frontal spreading speed should be divided into four types according to the time history.The experimental results show that the intersection time of measuring points increases with the increase of seepage velocity.Under the same freezing time, the smaller the seepage velocity, the lower the temperature of measuring points.The effect of seepage velocity u on the intersection velocity v_e of the second row of permafrost columns increases with the increase of u, while that of expansion velocity v_m of water front is the opposite, and affected by the group hole effect, v_m is greater than v_e.In the later period of freezing, the speed at which the water surface continues to expand v_u and expansion speed v_dof backwater surface decrease gradually with the extension of freezing time, and the length of upstream and downstream of horizontal frozen body tends to be constant.In this model test, through the regression analysis, the relationship between the second row of frozen soil column intersection time t_e, the water surface expansion speed v_m and the seepage velocity u is obtained.Meanwhile, the relationship between the water surface re-expansion speed v_u, the backwater surface expansion speed v_dand the freezing time t is obtained.Among them, t_e, v_m and t form a quadratic function relationship, and v_u and v_dare inversely proportional to the square of the freezing time t, respectively.In order to control the forming time and cost, and based on safety considerations, it is suggested that taking as the downstream length of horizontal frozen body, and according to the expansion law of horizontal frozen body, a formula for calculating the total forming time t_s is put forward.