In Civil 3D you want to create a dashed linetype as shown in this picture. The solution has several parts. The easy part, make a copy of the Double Line (Storm). I called my new style Double Line Dashed (Storm). On the Display Tab > Plan View toggle ON Centerline. Change the Linetype to DASHED. Set the Lineweight to 2.0mm. To see the spectacular result switch to a layout and set the viewport to 1:20 scale. Assign a plot style in Page Setup Manager. Toggle on Display Plot Style in the Page Setup Manager. Go back to the layout and look at your pipe. You may notice this produces rounded ends on your thick centerline but you want square ends. Set the Line End Style to Butt in the Plot Style to make square ends on the dashed linetype.