Trenches

[Salonen]

I have faced a problem. I am doing a pre-WW2 campaign which happens in 1930-1931 and that's why I use a lot of trenches in those scenarios. There is only one problem with trenches. They have altitude of -1 meter. That appears even if they are digged to the hills. I don't know any 31 meter deep trench. And when I place soldier and MGs there, they don't see anything. I know that the idea of trench is to be a good cover, but still. The enemy can approach without a fear of incoming fire. Anyone has an idea how to solve this problem?