For a rigid 8' x 8' box culvert built below a county road, what should the walls be designed for?

The walls of a rigid box culvert need to be designed to withstand the at-rest pressure of the soil when it is constructed below a roadway. At-rest pressure refers to the lateral earth pressure that acts on the walls of the culvert when the soil is in a static condition, meaning the soil is not being disturbed or allowed to move.

In a situation where the structure is situated beneath a road, the soil weight interacts with the structure, leading to a pressure distribution that can be approximated as the at-rest condition. This is particularly important as it reflects the true behavior of the soil in its undisturbed state.

Designing for at-rest pressure ensures that the walls will be strong enough to withstand these static lateral pressures without taking into account additional movements that would increase those pressures, such as those considered in active and passive states. Active pressure occurs when the soil is allowed to move, which could reduce wall pressures, while passive pressure accounts for soil movement that can increase effective lateral pressures against the wall.

Additionally, while impact loads from live traffic are necessary to consider for overall structural integrity, they do not directly influence the necessary design of the culvert walls regarding soil pressure. Hence, focusing on at-rest pressure provides the foundational design consideration required to