Why does my PLA filament form a spiral shape and clog my extruder?
The shape you get is quite easy to explain. It's the shape of the lowest energy possible in your situation. Simple but it doesn't explain the issue... or does it?

It does. The filament cannot be put into the extruder as it becomes plugged. This leads us to some obvious explanations. You can read this post.

So how is that possible that there is enough room to form such corkscrew? My bet is you don't have teflon pipe inside the extruder heatsink. So filament goes into the heatsink and everything is ok until the heatsink itself warms up to the temperature when filament becomes soft, then there is no enough force to push the filament out of the nozzle so above the nozzle filament bends and forms the shape of the lowest energy as said.

Options to check:


Take your extruder apart. See if there is a teflon pipe. If its length is proper. 
Check if cooling fan is working well, if it's pushing air to the heatsing but not sucks from the heatsink.


[edit]

Looking closer to your photos I'm pretty sure you don't have such teflon pipe. Your spiral has flat external (virtual) surface... it looks exactly as it would be pushed into hot pipe with diameter of 4mm.

[edit2]
please take a look on the picture