I cant build tunnels,i cant even build road/rail up on small hill,I think the cause of the problem could be some texture mods for ground.
Your opinion...
What exactly happens if you want to build a tunnel? Which messages do you get? Can you provide a screen shot?
Some ideas what might be the problem:
> do you have a slope mod installed that alters the maximum slope a road, railway etc. can be buildt on?
> tunnels cannot be build in a place where the tunnel would dig under the game water level. So if your terrain's altitude is less then 15m above game water level, you cannot build a tunnel.