Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 12—Renovation of ConduitsFigure 177.—White HDPE pipe can reduceglare when using CCTV inspection equipment.Intermediate spacers (known as spiders) are placed at intermediate points betweenthe end spacers to provide additional support. The distance between the spiders is afunction of the pipe diameter and wall thickness. Large diameter, thick walled pipedoes not require the spiders to be as close as small diameter, thin walled pipe. Thejoints of the dual containment pipe are joined using the same heat fusion method, asused for joining single walled HDPE pipe.The annulus between the existing conduit and the outside pipe is grouted. However,the annulus between the inside and outside pipe of the dual containment piperemains open, even after installation of the end spacers and spiders, and uponcompletion of the heat fusion of the joints. This is one of the most desirable aspectsof this type of pipe. The open annulus allows downstream detection of any leakagefrom the inside pipe, while still having full containment protection provided by theoutside pipe. The end spacers and spiders are designed with openings to allow anyleakage to pass through the annulus between the two pipes and exit at a downstreamlocation. The dual containment pipe provides a redundancy and additional safetyfactors to the system. The inside pipe is rated at 75 percent of the normal burstingpressure due only to the inability to inspect the outside surface of the heat fusionjoints. For this reason, the inside pipe is typically designed for a 33 percent higherpressure than the outside pipe.When sliplining an existing CMP, the CMP is typically assumed to be corroded tothe point that it cannot be relied upon to provide any strength. With the dualcontainment HDPE pipe, both pipes can be designed for the full expected loading.This allows a factor of safety of at least 2 without relying on exterior conditions, suchas the existing CMP or the annulus grouting.313