NYSDOT now specifies everything in metric. Can’t say about any other state.
3:30 (AKA “Beer-thirty”) is more usual. Half an our lunch (unpaid), but otherwise you got it.
This one requires an actual on-site inspection. WAG, maybe 4 feet, but I would need much more info for anything remotely approaching an accurate estimate.
and once the excavation starts, things might change. When the contractor rebuilt the road circle in front of my house, they dug out the base (it was poorly built initially) and rebuilt the road from the subbase up. When the workers dug down well past the 18inches they were required to dig out looking for solid ground, they finally came to a clay layer. I still remember the supervisor standing in the middle of this big hole where the road used to be, on “solid” ground, and hopping up and down. The entire clay layer oscillated like a trampoline. They went back to the county and got a change order to put down a cloth barrier. With that and a proper base the road has held up well for over ten years. Of course it is a lightly traveled residential street so that helps.