Modern Continental Construction Co., the largest contractor in Boston’s $15 billion “Big Dig” road and tunnel construction project, filed for bankruptcy protection yesterday, one business day after it was charged with liability in the project’s 2006 collapse, Reuters reported. Modern Continental said that it faced debts of $500 million to $1 billion with an estimated 200 to 999 creditors. Its assets totaled $100 million to $500 million. The U.S. Attorney’s office on Friday charged Modern Continental in federal court with knowingly using the wrong epoxy to suspend concrete anchors that failed in the 2006 tunnel ceiling collapse that a crushed a car and killed a woman passenge. If convicted, the Cambridge, Mass.-based company faces criminal fines of up to $500,000 for each of 49 counts, or a total of $24.5 million.