An alignment on the plan and a design profile are still not a road. You also need a cross section: how wide the pavement is, what its crossfall is, how the cut and fill slopes run. Civil 3D combines all three into a 3D road model running the length of the route, called a corridor.
This article builds a Civil 3D corridor in two parts. First it assembles an Assembly, the typical cross section, from lane and side-slope pieces. Then it creates the corridor along the alignment and design profile, sets the existing ground as the target for the slopes, and reads the result. The alignment and profile come from Civil 3D profiles.
Corridor, Assembly and Subassembly
Three nested ideas:
| Concept | What it is | Example here |
|---|---|---|
| Subassembly | One piece of the cross section, with its own parameters | A 3.6 m lane at 2%; a side slope |
| Assembly | The typical section, built from subassemblies around a baseline | Two lanes and two side slopes |
| Corridor | The assembly applied repeatedly along the alignment at the profile grade | A 461 m 3D road model |
The big advantage is the dynamic link. Change the profile and the corridor rises or falls. Change a lane width in the assembly and the whole route follows. The Institute's online Civil 3D course spends two full sessions on it. Cross sections and earthwork quantities come from the corridor, as the chain in What is Civil 3D describes.
Step 1: Create the Assembly
On the Home tab, Create Design panel, click the arrow next to Assembly and choose Create Assembly.
| No. | Item | Meaning |
|---|---|---|
| 1 | Assembly button | Opens the assembly menu |
| 2 | Create Assembly | Creates a new baseline for the cross section |
| No. | Item | Meaning |
|---|---|---|
| 1 | Name | Assembly name, e.g. MAT CAT DIEN HINH |
| 2 | Assembly Type | Undivided Crowned Road: no median, pavement falling both ways |
| 3 | OK | Then pick an empty point to place the baseline |
A short red vertical line appears. That is the assembly baseline, where subassemblies attach.
Step 2: Attach the lanes from Tool Palettes
Subassemblies live in Tool Palettes. If it is closed, type TOOLPALETTES or click Tool Palettes on the assembly's contextual tab. The Civil Metric Subassemblies set has tabs by group: lanes, shoulders, medians, curbs, side slopes.
| No. | Item | Meaning |
|---|---|---|
| 1 | Basic tab | Basic subassemblies, enough for a simple road section |
| 2 | BasicLane | One lane with width, depth and crossfall |
| 3 | BasicSideSlopeCutDitch | A side slope that chooses cut or fill, with a ditch on the cut side |
Click BasicLane. The Properties palette shows the parameters of the piece about to be attached.
| No. | Parameter | Meaning |
|---|---|---|
| 1 | Side | Which side of the baseline: Right or Left |
| 2 | Width | Lane width, 3.6 m by default |
| 3 | Depth | Pavement depth, 0.2 m by default |
Below is Slope, the crossfall, −2.00% by default. Keep Side as Right and click the circle marker on the baseline. The right lane attaches.
Click BasicLane again, set Side to Left in Properties, and click the baseline marker again. The left lane attaches.
Step 3: Attach the side slopes
Click BasicSideSlopeCutDitch, then click the circle at the outer edge of each lane, once right and once left. Civil 3D takes the side from the attachment point.
| No. | Item | Meaning |
|---|---|---|
| 1 | Baseline | The assembly origin, which follows the alignment and profile |
| 2 | Right lane | BasicLane 3.6 m wide, −2% outward |
| 3 | Side slope | Two branches, Cut Slope and Fill Slope. In the corridor Civil 3D picks one at each station from the ground |
"Cut Slope Layout Mode" and "Fill Slope Layout Mode" are only the assembly's layout graphics; they do not appear in the corridor.
A wrong piece can be selected, deleted and attached again. To change an attached piece, select it and edit its parameters in Properties.
Step 4: Create the corridor
On the Home tab, Create Design panel, click the arrow next to Corridor and choose Corridor.
| No. | Item | Meaning |
|---|---|---|
| 1 | Corridor button | Opens the menu |
| 2 | Corridor | Opens Create Corridor |
| No. | Item | Meaning |
|---|---|---|
| 1 | Name | Corridor name, e.g. CORRIDOR TUYEN CHINH |
| 2 | Alignment, Profile, Assembly | Three cells in one row: TUYEN CHINH, design profile DUONG DO THIET KE, assembly MAT CAT DIEN HINH. Pick the design profile, not the existing ground |
| 3 | Target Surface | The surface the side slopes reach for: DIA HINH TU NHIEN |
| 4 | Set baseline and region parameters | Tick to open the detailed settings next |
| 5 | OK | Go to Baseline and Region Parameters |
Step 5: Regions, frequency and target surfaces
| No. | Item | Meaning |
|---|---|---|
| 1 | Baseline | The alignment and profile row, 0+000 to 0+461 |
| 2 | Region | A stretch using one assembly. The yellow exclamation mark means its targets are not complete |
| 3 | Set all Frequencies, Set all Targets | Set frequency and targets for every region at once |
Click Set all Targets to open Target Mapping. On the Surface tab both side slopes must point to the existing ground.
| No. | Item | Meaning |
|---|---|---|
| 1 | Surface tab | Target surfaces for subassemblies that search for ground |
| 2 | Right slope | Target Surface: DIA HINH TU NHIEN |
| 3 | Left slope | Target Surface: DIA HINH TU NHIEN |
Click OK, then Set all Frequencies to set the spacing between applied assemblies.
| No. | Property | Meaning |
|---|---|---|
| 1 | Along tangents | Spacing on tangents. 20 m by default, changed to 10 m |
| 2 | Curve increment | Spacing on horizontal curves, set to 5 m for smooth curve edges |
| 3 | Along vertical curves | Spacing on vertical curves, set to 5 m |
A smaller spacing gives a smoother but heavier model. For a short route, 10 m on tangents and 5 m on curves is enough.
Click OK in both dialogs. Civil 3D asks whether to rebuild; choose Rebuild the corridor.
Step 6: Read the new corridor
| No. | Area | What it shows |
|---|---|---|
| 1 | Start of route | The design profile is above ground: narrow green fill slopes, because fill heights are small |
| 2 | Across the small hill | The profile is almost 3 m below ground: wide red cut slopes |
The cross lines on the pavement are where assemblies were applied: denser on curves, sparser on tangents, following the frequency you set.
For a 3D view, select the corridor, click Object Viewer on the contextual tab, and choose the Conceptual style with a SW Isometric view.
Common Civil 3D corridor problems
"No sideslope intersect found" warnings. After building, the Panorama window may show this.
At those stations the slope ran out without meeting the target surface, usually because the station sits at the edge of the survey. Extend the survey data, or shorten the region to skip stations outside the terrain. On the sample route the end point lies right at the edge of the site and triggers exactly this.
The corridor lies flat at elevation 0. The existing ground profile was picked, or the Profile cell was left empty. Check the Baseline row.
No side slopes. Target Surface was not set. Reopen Set all Targets.
Jagged curve edges. The curve frequency is too large. Lower Curve increment.
A lane on the wrong side. Side was not changed before attaching. Select the lane, change Side in Properties, and rebuild.
Frequently asked questions
How does an Assembly differ from a Subassembly?
A subassembly is one piece such as a lane or a side slope. The assembly is the whole cross section built from those pieces around a baseline.
What if the section changes along the route?
Split the corridor into regions, each with its own assembly. For example, an urban stretch with curbs and sidewalks, and a rural stretch with earth shoulders.
Can I model local pavement layers?
The stock set covers lanes, shoulders, curbs, ditches and slopes. Multi-layer pavement uses GenericPavementStructure. Special shapes can be built in Subassembly Composer, which ships with Civil 3D.
How do I see cut and fill along the whole corridor?
Create a corridor surface from the top links of the corridor, then compare it with the existing ground in a volume surface. Civil 3D colours the result by cut and fill depth, which is a quick check before computing volumes station by station.
Why is my corridor slow to rebuild?
Every rebuild recomputes each applied section against the target surface. A dense frequency, a large terrain surface and many regions all add time. Keep the frequency as coarse as the design allows, clip the terrain to the corridor area, and switch rebuild to manual while you are still editing.
What comes after the corridor?
Create sample lines to cut cross sections from the corridor, then compute cut and fill volumes.
Related articles
The corridor is the fourth link after contour lines, Civil 3D alignment design and Civil 3D profiles. Autodesk's own steps are in To Create a Corridor.
Corridors with junctions, superelevation, multiple regions and pavement quantities are sessions 4 and 5 of the Institute's online Civil 3D course.