For a first real try to achieve a good level design, this looks great.
I wouldn't add any specularity because the directional light which is create by the sun, will bearly be visible inside a sewer. The normal map is a good idea to achieve that wet/slimmy look you normally think of in a sewer. Water would be a nice feature but if this sewer is located somewhere around the Middle East, I can't comprehense the dry thought if you know what I mean In overall, nicely done so far, I'll be waiting for a potential update

Cheers

Frazzle