You could try getting a piece of 1 1/2 - 2" EMT that's 8 to 10" wider than the sidewalk. flatten one end and then drive it under the sidewalk. then cut the flat end off. if you ream it a few times with your flex bit first it will be easier to drive under the sidewalk. That's the way we use to do it about 15 years or so ago.
If everyone is thinking alike, someone isn't thinking.
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