Weed or strange bulb?

I have these all over my garden in Washougal, Washington. I have tried pulling them to no avail. Their roots look like a bulb of some sort. Please ID and advise how to dispose of - it spreads very fast.
Thanks!

 

my uneducated guess is it's an oxalis.

 

Oxalis deppei 'Iron Cross'

 

Oxalis tetraphylla (syn. O. deppei) was mentioned in a recent thread. Photos of that plant on the web look much like yours.

 

Thanks to all - I will look it up!