Adjust the Datum for All Cross Sections
Jowenn Lua shows how to adjust the datum level for all your cross section to a fixed lowest elevation by employing the LinkOffsetElevation subassembly to an Assembly.
From the Autodesk AKN Civil 3D Screencasts
See Jowenn Lua's AKN page
As Jowenn notes in the notes of his post you might also want to employ a Placeholder or Dummy Baseline and Assembly with only the LinkOffsetElevation subassembly in a corridor to accomplish the same effect on the Section without adding the subassembly to all your Assemblies.