There may be differential settlement between a structure and the pipeline resulting in angular deflection of the joint. This creates no problem for the joint itself but when this movement is ?excessive? there is a shear force that can cause structural failure on the pipe, either shear behind the collar or from beam fracture of the pipe barrel. To prevent this, the first pipe in the line can be restricted in length. This is known as a 'rocker pipe'. The likelihood of differential settlement should be assessed and rocker pipes used as appropriate. In certain conditions where excessive differential movement is possible, for pipes ≥ DN750, it may be advisable to use multiple rocker pipes to avoid unacceptable angular deflection or shear force at the joint. Check with BPDA members
for size and availability of rocker pipes. Guidance on rocker pipes may be found in "Civil Engineering Specification for the Water Industry" and "Sewers for Adoption".