Why depth charges are poor weapons against undersea tunnels

Could undersea railway tunnels such as the Channel Tunnel or Seikan Tunnel be attacked with depth charges?

Not effectively with ordinary depth charges; attacking a tunnel entrance with a large or guided bomb would be far easier.

The Channel Tunnel and Seikan Tunnel run through ground beneath the seabed rather than through exposed tubes in open water. Ordinary depth charges therefore cannot place their explosive force close enough to the tunnel structure. A weapon on the scale of an earthquake bomb might transmit a damaging shock, but that is a radically different requirement. The entrances are the practical weak points, where a guided bomb could block the route without having to penetrate the seabed.