Editing Triangulation

You could also add a closed shape element as a Void to the terrain model.  A 'void' acts as an 'interior boundary' to trim triangulation from the terrain.  You can use the Add Features command to add a VOID or a DRAPE VOID to the terrain.  If you have a 3D element with elevations, you can use the VOID option.  If you have a 2D graphic which you want to drape onto the elevations of the terrain and use it to clip out the triangulation of the area, you can use DRAPE VOID.

https://communities.bentley.com/products/road___site_design/w/road_and_site_design__wiki/38762/video-add-interior-boundary-feature-ord

https://communities.bentley.com/products/road___site_design/w/road_and_site_design__wiki/15919/difference-between-a-hole-feature-and-a-void-feature

You could also add a closed shape element as a Void to the terrain model.  A 'void' acts as an 'interior boundary' to trim triangulation from the terrain.  You can use the Add Features command to add a VOID or a DRAPE VOID to the terrain.  If you have a 3D element with elevations, you can use the VOID option.  If you have a 2D graphic which you want to drape onto the elevations of the terrain and use it to clip out the triangulation of the area, you can use DRAPE VOID.

https://communities.bentley.com/products/road___site_design/w/road_and_site_design__wiki/38762/video-add-interior-boundary-feature-ord

https://communities.bentley.com/products/road___site_design/w/road_and_site_design__wiki/15919/difference-between-a-hole-feature-and-a-void-feature