When you mention a "perfectly graded trench," I mean, was it graded with the soil's saturation point in mind? Idk, maybe it's just me, but even the best grading can fail if the soil composition is wrong for wet conditions. What specific soil type are you working with that led to such a rapid collapse?

Sometimes the most practical jobs force you into ACCIDENTAL philosophy. I once spent a week recalibrating a support beam in the rain and started pondering the nature of temporary stability. My foreman just laughed and told me to pass the wrench, which was probably for the best.