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Creating a corridor assembly in Civil 3D involves defining the cross-sectional components (like lanes, shoulders, curbs, etc.) that will be applied to an alignment and profile to build the corridor model. Here's a step-by-step guide:

1. Open the Tool Palette
Go to the Home tab on the ribbon.
In the Palettes panel, click Tool Palettes (shortcut: CTRL+3).
2. Set the Assembly Tool
On the Tool Palettes, go to the Assemblies tab.
If you don't see it, right-click the tab area and load the Civil Imperial Subassemblies or Civil Metric Subassemblies based on your project standards.
Review the available subassemblies (lane, curb, ditch, etc.) in the palette.
3. Create a New Assembly
Go to the Home tab on the ribbon.
In the Create Design panel, click Assembly.
In the Create Assembly dialog box:
Provide a name for the assembly (e.g., "Urban Road").
Choose a layer for the assembly.
Specify the assembly type (e.g., Roadway, Railway, Channel, etc.).
Click OK.
4. Place the Assembly Baseline
After creating the assembly, click in the drawing area to place the baseline marker (the origin of the assembly).
5. Add Subassemblies
Select Subassemblies:
On the Tool Palettes, select the desired subassembly (e.g., Lane, Shoulder, Curb, Side Slope).
Attach Subassemblies:
Click on the assembly baseline marker and attach the subassembly to the left or right side.
Repeat this process for all required components to build your assembly.
6. Configure Subassembly Parameters
After placing a subassembly, you can adjust its properties:
Select the subassembly in the drawing.
Open the Properties palette (shortcut: CTRL+1).
Modify parameters such as:
Lane width
Slope or grade
Depths or material thicknesses
Save changes.
7. Verify the Assembly
Use the Preview option on the subassemblies or zoom in to verify that the cross-section matches your design intent.
8. Save and Use in Corridor
Once the assembly is complete, it’s ready to be used in a corridor:
Create a corridor by applying the assembly to an alignment and profile.
Tips:
Use Copy Subassemblies to replicate symmetrical sections on the opposite side.
Group your subassemblies logically (e.g., lanes, shoulders, ditches) to make it easier to modify later.
Save commonly used assemblies as templates for future projects.