The cement can negatively effect the water retenction of the clay, as does of course the temperature. The water needn't seep into the tunnel, just be moved away

The cement can negatively effect the water retenction of the clay, as does of course the temperature. The water needn't seep into the tunnel, just be moved away from it to adjacted areas. There's a nice article here which goes into more detail: https://www.ianvisits.co.uk/articles/cooling-the-london-underground-220/ https://www.ianvisits.co.uk/articles/cooling-the-london-unde...