"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。
2
When the tunnel is driven through stiff clay, the flow of water may be small enough to be removed by pumping.