This is another one of those things that is easy to do in Civil 3D when you are shown how. From experience I know a lot of people aren't aware that it can be done or think that it is difficult to do. The result is that a lot of people are exporting Civil 3D models to AutoCAD and hatching manually (madness!).
The code set style is what is used to control the appearance of your corridor in Civil 3D. It is a pretty big style and can look intimidating at first (it controls display in plan, section and 3D views, as well as labelling). This has been the most common thing that I have been asked from users in relation to corridor appearance - how to hatch a corridor in plan. Here's my corridor before, not exactly jumping off the page:
Select your corridor and go to Corridor properties on the ribbon. On the codes tab you will see the current code set style in use (3D Render Basic Plan in this case), we want to create a new one as below:
Before you create the new code set style, take a minute to look at the some of the ones currently available. You will notice that the style itself is broken down into Links, Points and Shapes - see image above. These L, P & S come from the subassembly you are using. Without digging into that too much, basically the L,P & S are parts of your subassembly. Points are joined by links and shapes are bound by links in your subassemblies. See below:
When we want to style our corridor in a particular way it is these elements of the subassembly in the corridor that we are styling effectively. When you create a new code set style you will see that the L, P & S are empty:
The quickest way to populate these to suit what is in your subassembly/corridor is to click Import Codes at the bottom of the dialog box and browse to your subassembly in your drawing and select it. You can then assign styles to the L,P & S, see below:
To hatch to corridor in plan we are only interested in links, and only in the Material Area Fill Style section. I am using the UKIE drawing template and picking some of the styles from that and assigning them to particular links in my corridor. Note that the Top link is generally common to most subassemblies so applying a style to this will likely colour your corridor all the one colour so best not to use it. If you are having trouble figuring out which link you should be applying a style to then hover ('hoover' if you are Dutch! ;)) over that link in your corridor and you will find the name of it in the tooltip that appears.
If you are not showing links in your corridor you can find the code by selecting your subassembly, right click and select properties and find the Top Link Code name as below:
The end result of editing the code set style is below: