By effectiveness I mean Protection from incoming fire - not a magic bonus to a units characteristics! The reason I want to mod this is that I keep seeing fully entrenched squads get unfeasibly heavy casualties. Their in a trench below ground level and only need expose head & shoulders briefly to shoot or spot. Under heavy fire a squad would duck below the parapet.
What Krabb is saying is there are not bonuses or anything to being in a trench. It is totally dependent on how the trench is located. They will duck down, but that is being suppressed. If you have infantry in trench and select ambush thing in the UI modifier panel they will pretty much do what you are after. But they are less effective. Enemy infantry can often walk almost right up to them. Fewer eyes looking for the enemy. But once engaged they do the peek and shoot. Other wise, you need to probably modify the infantry trench itself. Which is grey scale height map.
High explosive shells from larger guns have no issues blowing apart trenches. Its one reason why 75mm guns where seen as the minimum for infantry support work for tanks. What I do as soon as these guys start to get pounded bad is use the ambush order from the modifier panel as soon as fire gets to intense. Or perhaps make it easier to get suppressed I guess and a shorter time to recover from it. That would also maybe do it.