I don't THINK they have been excavated in modern times. They seem originally to have been just surface mined, not tunnelled, if my memory serves me.
I know it was thought that the flints they would have got from the pits were very small. Enough for arrow heads maybe?
THE CURSUS: The whole of the apparent Cursus is shown in this photo I put up:
http://www.themodernantiquarian.com/image.php?image_id=14499
Its base meets the South Downs Way. Like you say, a very odd path, hence the Cursus theory.