By Ju-min Park | SEOUL SEOUL **rth Korea conducted a big live-fire exercise on Tuesday to mark the foundation of its military and a U.S. submarine docked in South Korea in a show of force amid growing concern over the **rth's nuclear and missile programmes.The port call by the USS Michigan, which is designed to carry ballistic missiles and cruise missiles, came as a U.S. aircraft carrier strike group steamed towards Korean waters in an effort to deter **rth Korea from a sixth nuclear test or more missile launches in defiance of U.N