If you’ve been driving on I-35 through Central Austin, you may have noticed tall construction walls going up at certain points along the highway and wondered what’s going on behind them.Those barriers are designed to block the sound of machines digging deep holes called drop shafts for one of the largest tunnel projects in Austin history.The tunnel is so large that building it will cause the ground to vibrate along its 6.5-mile path down I-35 and East Cesar Chavez Street, creating uncertainty ab