"You want it very rigid and stiff, but from an electrical point of view you also want it to allow ions to flow through the material," says Dr Greenhalgh.
“材料要坚韧稳定，但从通电的角度看也需要让电离子顺利通过，”格林浩尔博士说。
When the tunnel is driven through stiff clay, the flow of water may be small enough to be removed by pumping.
当隧道穿越粘稠土质时，水的流量也许会小到用水泵就可以抽干。
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