There are better (and safer - both for you and the saw) ways to put in shallow pipes and wires. There are even special wire trenchers.
Look at this Easy Digging Guide to Trenching and Trenchers It covers homeowner and DIY trenching from hand trenching through small walk-behind trenchers.
If that link doesn't work, you can find it at EasyDigging.com in the Garden Guides section.
Good Luck - and be safe...
Greg
