How strong are blocks in general - is adhesive necessary when building large structures?
Such structures in LEGOland parks are usually reinforced with a metallic structure hidden within the model. And they use glue. No wonder LEGO sold them away.

Some fans do use similar tricks, I've seen some make trees by embedding a wooden skewer in a trunk of 1x1 round bricks. Without that, the tree would collapse too often, and that's not even a large structure to start with. (Purists would use rigid 3mm LEGO hoses)

For a pure LEGO structure, in any case you would need to make sure not to rely on clutch power only, but I suppose you guessed as much already. As stack of 1x2 bricks won't hold, a length of interleaved 1x2 plates will be sturdier. Depending on the length, you'll need more layers, but that's about it.

I doubt adhesive would help that much — glue maybe, but let's not go there. 

There`s the question "How much weight can a handle of Technic pegs and beams support?" covering the principles for structures. You can find a mechanical stress study right here.