All I can say is building tenches into "enemy" territory before the game is underway is normal
the top trench was made from the German side of the line to the British side of the line and the bottom one was made starting on the Brit side of the start line
As well I ran a second experiment with a trench laid from the German side to the Brit side of the line and was able to remove the entire trench after it was laid and I used 6 different terrain types "erasing" it
HOWEVER..... once you start deploying troops the trench cannot be removed
