That appears to be a 3d solid extracted from a corridor. The easiest way to get a surface from the bottom link is to create a Corridor Surface using the correct link as the data source.
Another way is to Extract Corridor Solids, but extract a Link, not a Shape. This makes a "body", which is connected from station to station. If you explode that, you get Regions. Regions can be turned into Planar Surfaces (in the 3D Modeling sense, not Civil 3D Surfaces) Use _PlaneSurf command to turn each of the body sections into a planar surface. Use _Union command to join the planar surfaces into a single object.
Tim Corey
MicroCAD Training and Consulting, Inc.
Redding, CA
Autodesk Platinum Reseller
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