In original Minecraft, you, as a player, was about the size of two blocks one on top of the other (actual bounding box was smaller than two blocks). That allowed you to move very close to the wall, fall into 1x1 hole (1x1 means 1 block) and build tunnels that were 1x2 tall and you could easily navigate through them. In gridcraft version you seem to be blocked by blocks that should not block you, you get snagged on block corners that are kinda far away etc. It is tricky to play, I would recommend changing that to make it more in-line with original.
I can see there is a ticket already open regarding this. I’ve bumped it up. It is expected to be taken care of next week. Thank you for your report.
I have verified and confirmed the issue.
I’ve altered the code fragments responsible for collision checking.
It is now perfectly possible to create 1x1 holes and to drill 1x2 tunnels.
The change should become operational LIVE with the update which is scheduled for today.