When differential conditions of pipe support may occur, such as in transitions from manholes to trench or from hard to soft soils, a transition support region should be provided to ensure uniform pipe support. The soil next to the more rigid support is should be over-excavated to a depth of not less than 12 inches over a distance of 2 pipe diameters along the pipeline; over the next 2 diameters away from the rigid support, the depth of over-excavation is gradually decreased until it meets the normal trench depth. Pipe grade is then restored by the addition of granular material that is compacted. In the case of connections to manholes and buildings, the distance of over-excavation along the pipe length should be no less than required to reach undisturbed soil.
Pipe Support in Transition from (a) Rigid Support or (b) Rock Trench Bottom to Normal Trench Support