From other answers, we have an official source, and seem to agree that the symbol is intended to show a
"... person with a shovel digging."
Now, if it did show that, this question would not exist - we would understand the symbol. It's the job of a graphics designer to get that right.
To me, the most interesting question is
Why does the symbol not show a person with a shovel digging?
Looking at a scaled up image from a distance, I found out most of what went wrong:
The symbol does not depict "digging" - but an earlier version did!
Look at the symbol from bottom right, as if the lower right edge were vertical. The right-most part of the symbol now shows a small mound, just like you see on a construction site when someone is digging. I think the size matches very well.
What apparently went wrong is that the symbol - preexisting or designed for the purpose - was forced into the space available on the diamond-shaped sign, without making it too small.
That's pretty normal - but failed here badly, because the distorted mound image depends heavily on its orientation, which was changed.
I can not find the shovel this way, but I think with the recognizable mound, it is much easier to understand the symbol - maybe even good enough!