I've come to a little stumbling block in modelling/texturing, where I have a concrete bunker that meets a rock face, as if the underground bunker was created inside the rock, and is not quite complete.

Usually what I would do, is either vertex blend the concrete into the other surface (Wont look right because thats not how it would work in real life here), or just model the rock seperate and then hide the harsh transition with a dirt decal of sorts.

But to be honest, both methods are a tad odd, and I can't seem to find examples of anything on google, either?

Heres the sort of location I'm refering to from ETQW:



Now what I would do for this scene (as it would be an old, ruined area if done in the style I'm going for), is make the floor and cave wall a vertex blend of a dirt floor like surface, that blends into the rock as it creeps up the wall, and then using tiles, gradually break them up and seperate them as it gets nearer the cave wall. That works well, infact. However, the same can't be said the obvious transition from concrete wall to cave rock.

Any suggestions on what I could here?

mm, vertex blend might work, but the a hard edge (harsh transition between the concrete wall and the dirt wall might actually make some sense, it something like that existed on real life)
Still, you could always hide the transition with metal trims, dirt piles, decals, etc.

Anyways, if I had to do something like that, i would break it with some bricks, like this:

concrete poured around the rock? That would make it really easy to clip the crete into the rock.

That screenshot's from W:ET, not ETQW ;)

The key to this transition is actually having a hard edge between the two surfaces. You will need to make sure there's enough detail modelled into the lowpoly to make it look believable (jagged, broken silhouette where the rock meets the concrete).

A dirt decal on the wall where the two surfaces meet is probably gonna work pretty well, certainly better than a vertex blend which would just look soft, blurry and weird.