Tree roots in garden

Large redwood trees send extensive root mats into my garden

Rather than fighting these roots, or trying to set up barriers blocking them, I'm trying to use them to add organic matter every year into the garden.  I dig a trench between the tree and garden, cut the roots and refill the trench with dirt.  Will add a drip line to then lead the roots back into the garden, so that I'll know where to cut them again next year.  This could add a great deal of organic matter to the garden each year. Only unknown is how far down I'll need to dig a trench to be sure all the roots are cut.

If anyone knows an easy way to cut tree roots about 3' deep, please post.