Backdrop Manhole

Published: | Updated: September 17, 2020;

Definition - What does Backdrop Manhole mean?

A backdrop manhole is a drop shaft installed either externally or internally in a manhole in a sewer network. Backdrop manholes connect pipes of differing invert levels and allow for the pipe at a higher level to connect to the lower level pipe with minimum fall. Typically, backdrop manholes are used where the elevation difference is larger than 600 mm.

Backdrop manholes are constructed in two ways, depending on the required sizing of the pipe. Backdrop manholes can either be provided through a vertical drop using a downpipe or a gradual drop, which takes the form of a cascade or ramp. For pipes bigger than 450 mm, cascades are the preferred form of construction.

Figure 1: Backdrop Manhole Arrangements, Internal, External and Ramped/Cascaded (Source: KSP Cor)

If vertical pipes are warranted, the backdrop manhole is firmly anchored at the base using a 90-degree pipe bend in concrete surround. For vertical backdrop manholes, the T-branch located at the top inside the manhole contains a flap to prevent any splashing. Backdrop manholes are constructed to allow accessibility for cleaning, assessment, and maintenance.

Trenchlesspedia explains Backdrop Manhole

Backdrop manholes can be prone to defects from tree roots' ingress, blockage due to improper disposal techniques, waste build-up, and settlement. These elements can lead to fractures in the system or severe structural damage. In some cases, backdrop manholes are rehabilitated to improve its capacity and eliminate inflow/infiltration.

Methods of Repairing Backdrop Manholes

The severity and type of defects, the purpose of rehabilitation, manhole access locations, and pipe characteristics such as depth and length to be replaced are factors that dictate the best method of repairing and maintaining manholes. Trenchless solutions are becoming routine for repairs of manholes in wastewater systems.

Advantages of trenchless repair include minimal disruption to business and homeowners in terms of traffic disruptions and installation time, lowered probability of surface disturbances including excessive excavation and removal of trees and grubs, and reduction in air and noise pollution. Trenchless methods possess the capability to perform spot repairs and lining repairs that span between manholes.

If trenchless operations are employed in the backdrop manhole repair, comprehensive geotechnical and utility location exercises are required before undertaking the process. Extensive evaluations avoid costs due to unforeseen damage to utilities or equipment from the rocky soil.

Trenchless solutions such as pipe bursting improve the sewer system's hydraulic capacity as a larger diameter pipe is installed. Pipe bursting is suitable for installing sewer pipes of varying materials, including PVC, ductile iron, and polyethylene. Of all trenchless repair methods, pipe bursting yields the largest increase in hydraulic capacity. Other trenchless practices, such as sliplining, deform/reform, and cured-in-place liner (CIPP), are not best for improving pipes' hydraulic capacity.

Sliplining is advantageous for repairs in larger diameter pipes and does not require any flow diversion during the manhole repair. Deform/reform does not require any excavation for an inversion since existing manholes are used to insert the liner pipe. CIPP is used to repair a wide range of defects, including missing pipe segments, cracked pipes, and offset joints. CIPP is also useful in eliminating high inflow/infiltration rates.

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