Contractors are redoing the South Lawn helipad to accommodate a new supersized Marine One helicopter, with work proceeding around the clock ahead of a planned late-September visit by Xi Jinping. The National Transportation Safety Board and Federal Aviation Administration are investigating procedures during a Tuesday Marine One departure from the White House that did not halt airport traffic. The lawn slopes up to 20 degrees in places, and a 100-foot granite presidential seal was re-excavated last week.