The highway department is currently enlarging a tunnel, not to widen it but to... what? Is there a single verb that means to increase the vertical dimension of something? (For purposes of this question it does not matter whether they're doing that by modifying the floor or the ceiling.)
Raise is not correct because raising doesn't change size, only elevation. One can talk about raising the ceiling (which is how they are enlarging the tunnel), but that's not a single word. Heighten (suggested in an answer) seems questionable, e.g. from thefreedictionary.com "to make high or higher; raise".
If I were looking for an adjective this wouldn't be a problem; "tall" would suit. But it doesn't have a corresponding verbal form, and dictionary entries aren't giving me hints.