I've had to "clean up the mess" several times in my career, when poor/lazy lawyering, incompetently written project scoping documents, and general incompetence finally caused or more often "forced" a change in "project management". And, it is never very pretty, and, as with this, can result in very costly "do-overs"... This incompetence of not specifying ballast cleaning, allowed the contractor to bid a cheaper contract. But, the costs of the "do-over" will often be many times that cost "savings" from the original bids, as will likely be the case here.

Incompetent/lazy, inexperienced, untrained, or unsupervised staff making decisions without "knowing", and/or "checking" with people who do, is more often the reason, than outright corruption. Although I've dealt with some of that, too.... Failure to higher competent and capable experienced "in-house" staff to oversee and monitor projects is a bad contractors wet dream come true... A lack of strong "in-house" staff and management is the number one cause of most "out-house" project outcomes... An old colleague used to observe in situations like this, that if you pay peanuts (often by hiring inexperienced and unsupervised "cheaper" staff to manage these contracts), you end up with monkey's, and with monkey's in charge, you'll get plenty of monkey business....

Some of us are old enough to remember when experience actually mattered....