Well obviously your main risks when there is a major scope change in a large construction project are cost risks.
You cannot control other cost risks if costs go out of control. The only solution is to ask for more funds for your project by re-negotating the contract. The client cannot expect a major change on the project without paying extra. You will also need to re-schedule the project and push the delivery date forward.
You should never process a major scope change without doing the above. You should also rebaseline the cost, the schedule, and the scope of the project once you have decided to process the changes.