Extracting .dxf contour data - problems...

The .dxf file created by my surveyor includes two sets of contour data. The contours generated by AutoCAD are in one-foot intervals, but they created two layers - major contours and minor contours. The major contours are in five-foot intervals, I believe so that they could easily use a heavier lineweight for the even 10-foot and 5-foot intervals which is a common practice in mapmaking.
I was able to import either the major or the minor contours, but not both; not concurrently nor one after another. If there's a way to do this I've not found it. I don't have AutoCAD and have tried with two free CAD applications (FreeCAD and LibreCAD) and neither of them were able to get the job done for various reasons, mostly unknown to me...