Sometime ago I thought how I can use a CAD model to carry out both Civil infrastructure and Mapping tasks. Now there have a file format called SDF, it supports entities which have Z-elevation information. It able to store spatial data and do ac-hoc database join and able to handle multi-class objects. Also this file format can be converted to another GIS native format (e.g. SHP, E00, MIF, ...). This is the file-based format.

For the testing, firstly I simply sketch lines by 3dpoly command. And then classify it to an object. Finally I try to export it to sdf, shp & e00. Also I try to create a buffering.

Left image: I just simply sketch the 3d polyline.
Right image: The entities have been classified to map3d object.

Left image: Change to perspective view. Dynamic annotation supports 3-D display.
Right image: Buffer created of 3Dpoly entity

Left image: From the perspective view, It shows the buffer is generated and displayed in 0 level plan view. The green circle shows the buffer is represented the vertical line.
Right image: In the display manage, it have 3 data connections (SHP, E00 and SDF). All is generated from Map3D. Only SDF can correctly show the objects which have Z-elevation.

This feature can be applied to drainage application. In reality each pipe should have their property (e.g. Z-elevation, Type, Material, Length, Use). So that we can make use of Map 3D to do the map and make use of Civil 3D to generate the elevation profile with existing surface.