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I am sorry to learn of this situation. It appears you have a breach of contract claim against the G.C. (and possibly the City). Unfortunately with these kinds of construction claims it is really necessary to review all of the paperwork and documentation to help you identify the exact claims, who is liable, whether you must participate in arbitration or if you can go straight to trial, and how to calculate your damages. This does require a local attorney to perform this kind of formal legal service for you, but fortunately once you get your rights figured out you can likely resolve the dispute with mediation rather than having to go to litigation or arbitration (you may not get the full amount you are owed, but you can recover something without having to invest in full blown litigation).
You can find local civil litigation attorneys using the State and local Bar Association directories, or private directories such as www.AVVO.com; www.FindLaw.com; or www.Martindale.com (I personally find www.AVVO.com to be the most user friendly).