Junction Upgrade in Civil 3D

This post outlines a workflow for upgrading an existing junction using Civil 3D. The incoming and outgoing lanes of the junction are to be widened to accommodate HGV movements while maintaing the existing main road crossfall along the widened portion.

First lets take a look at the existing junction...

First create centreline alignments for both roads and create surface profiles for each sampling existing ground. Next use the junction design wizard to create our proposed curb return and offset alignments. We are only going to use the horizontal curb return geometry created by the wizard – untick the options to create curb return and offset alignment profiles... 

...and also the option to create the corridor.

A warning message appears to inform us that we are using dynamic surface profiles. Continue with the dynamic profiles.

The wizard creates offset and curb return alignments...

The curb return alignments rely on the offset alignments for their geometry. In order to be able to edit and stretch the curb returns to suit our design we will first have to stretch out the offset alignments to give ourselves enough room to work with.

This needs to be completed for both roads. Next we add turn in lanes to the junction to model the widenings. Select the junction marker and select edit curb returns from the ribbon. Tick to add widen lane for each of the incoming lanes for both curb returns.

Completed curb returns below:

Next we need to extend out the existing main road cross fall so that we can determine the levels along the widened curb return alignments. To do this we create a temporary corridor using the widenmatchslope subassembly. This will allow us to target the existing road surface at the crown and existing road edge and then continue this slope out wide enough to extend past the new widened section - giving us our proposed levels along the widened alignments.

First create the assembly:

The assembly properties are set as below:

Most of the values are set to zero - we are going to use corridor targets for the insert and sample points. We are not overlaying the existing road and we are not adding a widen lane with this assembly so these values are set to zero. The only value we set is the width for the widen portion, this is set at a value that will be wide enough to cover the widened area. Setting a number of values to zero like this will result in a strange looking layout assembly. The appearance of this is not important, it is the behaviour we are interested in.

Create a temporary corridor using this assembly, the main road centreline alignment and existing profile. In the target mapping set the insert point as the edge of existing road and the sample point as the existing road centreline for both the width and offset targets. Also target the existing ground surface.

The temporary corridor can be seen below:

Now create a surface on this corridor and set style to all off. To ensure that our corridor is extending the existing road cross fall correctly we can project the corridor surface onto our side road profile and check as below:

We then create a profile along each of the curb return alignments sampling both the existing ground and temporary corridor surfaces. This should look something similar to below:

We can see the existing ground surface profile in green and the corridor surface in purple. We need to manually draw our design profile using these as a guide. On the left in the profile above we can see where the NE curb return is running along the edge of the main road. The existing road surface is blank for a portion where we are outside the extents of the survey and then on the right we see where we are tying back into the side road. When drawing our design profile we can trace over the temporary corridor surface and tie back into the existing ground at the end of the curb return on the side road. See below:

Repeat this process for the SE curb return. Next we create a curb return assembly.

This will need to contain a subassembly with the ability to target horizontally and vertically. We then build our junction corridor using the curb return alignments as the baselines. The first pass of the NE curb return corridor below:

We need to set two corridor targets - the existing main road edge (surveyed 3D polyline) and the side road alignment and profile

These two targets can be added together - the default behaviour is that Civil 3D looks for the nearest one. Set these for both the horizontal and vertical targets.

Complete the SE curb return in the same manner and the finished corridor appears as below:

Found this useful? Need to learn more? Check out our Advanced Junction Design course HERE