Every miner learns it on day one: never dig straight down. OSHA's excavation standard agrees with unusual enthusiasm — Subpart P exists almost entirely because vertical walls in soil eventually stop being walls. Cave-ins are among the most lethal events in construction, and a cubic yard of soil weighs about as much as a small car.
The core requirement: each employee in an excavation must be protected from cave-ins by an adequate protective system — sloping, benching, shoring, or a shield — with exactly two exceptions: excavations made entirely in stable rock, or excavations under 5 feet deep where a competent person finds no indication of potential cave-in (29 CFR 1926.652(a)(1)). 'The crew prefers it this way' appears nowhere in the exception list. Sloping and benching requirements are set by soil classification (1926.652(b)), which is why a competent person must inspect the excavation daily and after every rainstorm (29 CFR 1926.651(k)(1)).
Then there's the getting-out problem, which our vertical-shaft enthusiasts have made severe: trench excavations 4 feet or deeper require a stairway, ladder, ramp, or other safe means of egress within 25 feet of lateral travel for every employee (29 CFR 1926.651(c)(2)). A shaft with the digger at the bottom and no ladder is not an excavation strategy; it's a pre-dug incident report.
The lesson under the joke is the real hierarchy of excavation safety: classify the soil, protect the walls, guarantee the exit, inspect daily. Do those four and you can dig nearly anywhere — at a responsible angle.