大流速渗透地层人工冻结壁形成机制室内模型试验研究

Laboratory model test study on formation mechanisms of artificial frozen walls in permeable strata with high seepage velocity

为解决在大流速渗透地层人工冻结法施工过程中遇到的一系列工程问题,基于自主构建的大型水热耦合物理模型试验系统,对不同流速条件下的人工冻结温度场的发展规律进行室内模型试验研究。试验结果表明:当v=0和3m/d时,冻结壁在相邻冻结管的中间位置最先交圈,其交圈时间分别为740和840min;当v=6和9m/d时,冻结壁最早交圈位置由中间点向下游偏移50 mm,交圈时间分别增加至1 770和4 250 min;通过对试验数据进行拟合,得出渗流场作用下冻结壁交圈时间的预测公式,并计算预测出冻结壁交圈的极限流速为12.73 m/d。渗流场的存在导致冻结壁的厚度发展不均匀,当v=3,6和9 m/d时,冻结壁向下游扩展的半径与上游扩展半径的比值分别为1.17,1.21和1.81。对冻结过程进行分析后发现,当相邻冻结锋面之间的距离减小至临界值Lc后会产生群管效应,该效应会加快冻结锋面的扩展速度,缩短冻结壁的交圈时间;由于水流的对流传热作用会抵消部分群管效应,因此Lc会随着渗流速度的增加而减小;当流速小于3 m/d时,Lc=400 mm,当流速达到6和9 m/d时,Lc分别减小至154和130mm。本研究成果可为大流速渗透地层人工冻结孔的布置提供参考,也可为水热耦合数值计算模型提供验证依据。

In order to solve a series of engineering problems encountered in the artificial freezing process in permeable strata with high seepage velocity,laboratory model test for studying the development law of the artificial freezing temperature field under different flow velocity conditions was performed based on a self-built large-scale water-heat coupled physical model test system. The test results show that,when the seepage velocity is equal to 0 and 3 m/d,the earliest closure position of the frozen wall locates at the intermediate point between two neighbor freezing pipes and the closure time is 740 and 840 min respectively. When the seepage velocity is equal to 6 and 9 m/d,the earliest closure position of the frozen wall moves from the intermediate point to the downstream by 50 mm,and the closure time increases to 1 770 and 4 250 min respectively. By fitting the test data,a prediction formula of the closure time of the frozen wall under the seepage field was obtained,and the limiting velocity for the closure of the frozen wall was predicted to be 12.73 m/d. The seepage field causes uneven development of the thickness of the frozen wall. Specifically,when the seepage velocity is equal to 3,6 and 9 m/d,the ratio of the downstream expansion radius to the upstream expansion radius of the frozen wall is 1.17,1.21 and 1.81,respectively. It is also found from analyzing the freezing process that,when the distance between adjacent freezing fronts is reduced to a critical value Lc,the “group-pipe-effect” will occur,which will accelerate the expansion speed of the freezing front and shorten the closure time of the frozen wall. Since the convective heat transfer effect of the water flow offsets part of the “group pipe effect”,Lc decreases as the seepage velocity increases. When the seepage velocity is less than 3 m/d,Lc is 400 mm,and when the flow velocity reaches 6 and 9 m/d,Lc is reduced to 154 and 130 mm,respectively. The results of this study can provide reference for the arrangement of artificial freezing holes in the permeable formations with large flow velocity,and also provide verification basis for the hydrothermal coupling numerical calculation model.