A big restoration project I'm working on has a 2.2 mile tunnel under a big hill. By far the most difficult civil engineering challenge is the layers of soft clay it passes through.

Canals inevitably leak a bit. This type of clay expands when wet. You can imagine the resultant chaos as the entire tunnel is slowly crushed by the earth around it.

With modern engineering, there must be a way, but hydraulic forces in the soil are not to be underestimated.