Get the site located for utilities, anything that comes up will fall on to the locating company(s). As for the Patio driveway etc, he can plainly see that they are there, it comes down to can he do the job himself or not?
But the last one, obstacles in excavation, that is going to fall on you, if he hits a boulder that is ten feet in diameter that he needs to remove, it could make something that costs him way more than his profit allows. I would be careful with what you allow him to get you on though, if it is just hard ground, then sorry, your problem, better luck next time. But something odd, like an old buried tank or something is not fair to charge him for.

. I will have all utilities marked, but PC even excluded damage to overhead electrical wires out by the street, (ours are not underground). How do I, diplomatically, address these issues? Moreover, can't we poke rods in the ground to check for underground boulders, etc?
) What I remember from the few contracts I've seen is the 'act of God' clause, which covers the contractor from war, pestilence, earthquake, flood, nuclear holocost and Biblical type stuff (ie, if it's completely out of his control, it ain't his fault your pool is messed up or isn't completed on time). The schedal of payments (cuts) is also a biggie and often effects the speed of the job completion (a couple of places I've worked went down the list of current jobs and worked as much as would get them the next 'cut' and then moved on to the next job on the list ... the place I'm with now tries to install the pool and get all but the final payment within a week - we throw ourselves at one job and get from dig to the owners able to swim in 3 -4 days). I think that GT's answer pretty much covers your ?, you can write in any clauses you want to and then haggle with the contractor which of your and his clauses will be deleted or ammended.

