arbor pro
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...And god help you if you hit a fiber line, they will bill you by the minute for lost revenue, you can tell them the home owner signed a waiver all you want but it will not help, if you think otherwise I seriously recommend that you talk to your insurance agent..
Around here, you're not going to hit fiber when you're only grinding 10" deep. If there is fiber in the area, it's going to be marked so even a blind man can see it.
The best solution is always to pass the liability onto the HO or utility. When in doubt, simply don't grind. Let the HO and utility fight it out. It's simply not worth a big repair bill and, as the contractor, you have to obligation to grind the stump if you're not comfortable with the situation.