Hi,
You can do this by creating a new custom elevation surface and setting the rasters elevation mode to the custom surface. It will then render at 0 elevation below the ground.
Steps:
1) Right click Elevation Surfaces > Add Elevation Surface Layer
2) Right click on raster > Elevation > Set features to On a custom elevation surface > Click OK
Note: You may need to enable "Navigate Underground" On the Ground which you can do by clicking Ground > Click on Elevation Surface Layer contextual tab and enable navigate underground.
thanks,
Andrew