It is important to qualify your bids to protect yourself in the event change orders are needed. When you enter into a contract, it is imperative that you read through it and make sure that everything that pertains to your bid, or part of the job, is qualified.
Any that needs to be qualified should be added somewhere in the contract as an exhibit.
For example, if you are a grading contractor and don't have a soils report, and don't know if you will hit rock, you need to have a rock clause to protect yourself.
You can do an all inclusive bid, in which case you are responsible for the work regardless of circumstances that arise, but be careful with that type of arrangement. The grading contractor would be responsible for dealing with rock in that case.
Construction industry veteran, Jeff Spencer, explains bid qualifications and gives several concrete examples of situations when bid qualifications were so important.
From ProfitDig - the world's easiest to use construction job bidding and estimating solution. Visit https://www.ProfitDig.com