Metronet Rail: Cross Track Culverts

Client: Metronet Rail

Project: Cross Track Culverts- Perival to Greenford

Value: £194k

Start: 2009

Completion: Ongoing

Scope:

Following reports of localised flooding, Metronet Rail undertook cleaning and CCTV
survey of twin 450mm diameter cross track culverts beneath the Central Line between Perivale and Greenford.

This location had an adjacent non electrified Network Rail track under which the culverts also passed.  Any works would have to be carried out under dual LUL /NWR constraints with operatives and management holding the relevant certification; and any plant meeting the requirements of both disciplines.

CCTV survey of the culvert had revealed structural defects to the pipe work which
could compromise the integrity of the track unless remedial action was taken.  The client explored a number of options including on line replacement and rerouting of flows, which for operational reasons were not deemed practical, and an option to slip line was taken.
 

The deformation of the damaged culvert dictated that the existing twin 450mm diameter pipes would be reduced to 250mm and 350mm id respectively. However it was calculated that works to improve flows either side of the track, together with the improved flow characteristics of the new pipes would be adequate to meet anticipated flows.

The scheme was designed and awarded to Clancy Docwra following a competitive tender process. The location of an existing manhole on the LUL side of the track, which was to be demolished and used as an access point, meant that works could be undertaken during traffic hours. This location also benefited from a privately owned road, which allowed access for grab lorries and materials to a point adjacent to the works.

The embankment on the downstream manhole was shuttered to prevent slippage, and protect the operational railway. The manhole, which was 4 meters deep, was then demolished and a working area created to enable the slip lining.

Three meter sections of screw fix HDPE pipe were inserted into the culvert, (2 x 27meteres) to the inlet on the Network Rail side of the track, the annulus between the old culvert and the new pipe was then grouted to fill the void and stabilize the new pipes and surrounding ground.  A new manhole was constructed on the downstream side and connected into an existing 600mm surface water outfall.  The embankment was reconstructed and profiled to match the existing.

On the inlet side minor works were undertaken to improve flow characteristics around the existing head wall, and a new trash screen fitted to secure the entrance to the culvert and prevent future blockages by debris.