Without seeing a picture, I can only guess, but I think it may be nut grass. It doesn't do too much good to just pull it, because it leaves behind remnants what will regrow. I'm always reluctant to advise poisonous controls, but look at the information on the web and decide for yourself if it is nut grass, and if so, what you personally feel comfortable using.

A search at images.google for nut grass pulls up a few dozen pictures. They are interesting. I never knew of such a root.

It sounds like nutsedge to me. This is a pretty invasive plant. Look up Cyperus esculentus for more info. This has been called the world's worst weed. I hope that's not it.

For a good ID, post a photo on the Name That Plant forum. You will get your answer quickly. It would be helpful to have a photo that includes the tubers.

Here is a link that might be useful: Yellow nutsedge

Mulch the grass heavily, Grass clippings work better.When the grass grows next year try to pull out along with tubers and roots as early as possible..Any broken pcs. of roots in the soil will sprout again.
It seems to me it is Yellow nutsedge according to link provided by botanybob.
I have the same grass in some area of my back yard.I try to pull out most of it but still it appears every year.It is a big problem for farmers in countries like India, particularly in Monsoon.