Load On Ditch Conduit

In the case of a ditch conduit, the backfilling material has a tendency to consolidate and settle downward. This action plus the settlement of the conduit into its soil foundation, causes the prism of soil within the ditch and above the pipe to move downward relative to the undisturbed soil at the sides. This relative movement mobilizes along the sides of the ditch certain shearing stresses or friction forces which act upward in direction and which, in association with horizontal forces, create an arch action that partially supports the soil backfill. The difference between the weight of the backfill and these upward shearing stresses is the load which must be supported by the conduit at the bottom of the ditch. If it assumed that cohesion between the backfill material and the sides of the ditch is negligible, then the magnitude of the vertical shearing stresses is equal to the active lateral pressure exerted by the soil backfill against the sides of the ditch multiplied by the tangent of the angle of friction between the two materials.
This assumption of negligible cohesion is justified on two accounts. Even when a ditch is backfilled with cohesive soil, considerable time must elapse before effective cohesion soil, considerable time must elapse before effective cohesion between the backfill and the sides of the ditch can develop after backfilling. Also, the assumption of no cohesion yields the maximum probable load on the conduit. This maximum load may develop at any time during the life of the conduit as a result of heavy rainfall or some other action which may eliminate or greatly reduce cohesion between the backfill and the sides of the ditch.