Breaking up thick & wide roots of Black Olive. 8 lb Maul or 3 1/2 lb Axe?

I had a 50 foot black olve cut down. The stump and surrounding area were ground with a stump grunder. I am doing sprinkler work and have so thick root masses. 4 inches thick 10 inches wide and some are just flat. Trying to hit them with a hatchet or hand ax is like hitting concrete.
I want to try to break them up at least to decompose faster. I driilled holes with stump rot in some holes, sugar in others and high nitrogen fertilzer in others.
I am thinking just cover em over and hope the decompose with the holes. Alternatively, I was thinking of a maul or big ax to break up these roots. Any suggestions? Forget fire as these roots seem impervious to fire. .
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A mattock, or a 5 or 6 pound axe (get the axe at a fireman's suppley store.) The mattock is probably the better choice. A maul would be almost useless.
Bob
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