A trench in Austin stops being a trench the moment it reaches rock
Digging here is a rock problem far more often than it is a dirt problem, and the owner meets that fact as a number that moved after the crew arrived.
Shallow limestone and karst run under much of this area, with caliche in places. Setting a fence, chasing a line under a slab, replacing an irrigation main or planting anything with a root ball can turn into rock excavation without warning.
Nobody could have priced that accurately from the driveway. An owner who has never seen the ground here hears the revised number as an excuse, and the argument starts on the invoice rather than on the lot.
Publish the mechanism before you ever need it. A short clip of a machine working through rock in a yard, with a plain explanation of why this work is quoted as a range, settles the question in advance and gets sent to the next owner who asks.
West of the city the terrain compounds it. Slope, long driveways and access decide which machine can reach the yard at all, so in Lakeway or Bee Cave the equipment problem frequently costs more than the digging.