Luna built the pressure model between the second hour and the fifth, on accounts paper, with a garrison surveyor checking her arithmetic. It is not a clever piece of work and she has said so every time anybody has praised it.
A wall holds pressure until it does not, and then it holds none at all.
Forty thousand animals arriving against a forty-kilometre frontage do not distribute themselves evenly along it. They pile at the places where the ground funnels them, and Orison's northern frontage funnels at four points because the land does rather than because anybody built it that way.
The model gave the first failure at the west culvert head, between the ninth hour and the eleventh.
It gave the second at the north gate itself, about two hours behind that. After the second one the model stops being useful, since a wall with two breaches in it has stopped being a wall.
