Help with using pathfinder/unite: mysterious path

please help me! I'm losing my mind over this.

I am trying to make the path of the letters in 'hanwell' trim to/flush with the edges of the bridge illustration, so that the letters only appear in the gaps between the birdge pillars.

I have drawn a bridge using the pen tool and shapes, all parts of it are all on the same layer.

In a new layer i then expanded some type selected both the type and the bridge and clicked 'unite' in the pathfinder box.

Weirdly, this nearly did what i wanted, but a strange horizontal line appeared, ruining it. Can anyone explain where this line has come from and how i can get rid of it?

To try and figure where this infuriating line was coming from, I tried getting rid of the text, selecting just the bridge and clicking unite, just to see if i could figure out where the line was coming from and it appeared again, which suggests to me that the problem is something to do with the bridge illustration and not the text. Can anyone help me? Pleaseeee