Creating a 3D off road terrain

I am trying to create a 3D offroad terrain surface. I imported some sample terrain form the Google 3D warehouse and also imported some elements like stones and buildings. I am now trying to create a top surface model of the combination of terrain and elements. Can someone please suggest a way to do this?

take a look at his

not sure if this is too much, while its a more formal road the principles will apply to what you might want to do.
The initial part of laying the shape on the terrain could also be done with the sandbox tools…which in itself may be enough for you.

Also the sandbox tols can create level surfaces in the terrain for buildings.

Elements like rocks are best just added manually onto the surface where you need them.
To scatter rocks and elements more easily you could use a paid plugin like “Skatter”

Please post a screenshot of where you are at now and/or a reference image of what you’d like to get to.