There was no arresting explosion, no sudden, defining crack announcing the breakthrough. Only small rocks – occasionally a large slab of concrete – periodically tumbling down a crumbled wall 47 feet below ground level as “Big Alma,” the second of two tunnel-boring machines excavating San Francisco’s first new subway in nearly a half-century, slowly peered its cutterhead out Wednesday. A few dozen neon-vested, hardhat-fitted workers on the project — from the contractor’s foremen to the…