When a party is unpaid on a public project, the general route to recovery is through filing a bond claim.  Governmental and public entities generally cannot be liened, so security for parties who furnished labor and/or materials to the project is not tied to the land or improvement itself, but rather, to a payment bond that the prime contractor is required to have posted.  It works in a similar manner to the "regular" mechanic's lien, but instead of encumbering the property, a bond claim is a claim on the money represented by the bond itself.  In some senses, a bond claim may even be preferable to a mechanic's lien, as it may not need to be recorded - just delivered.