From a dado line above walkway level, the cast-iron lining of the tunnel is entirely clad with vitreous enamelled steel sheeting consisting of a dark blue ceiling above and off-white wall panels. Suspender rods, bolting into holes provided in the flanges of the cast-iron rings, support a series of light steel trusses to which the ceiling panels are attached. A steel framework carries the wall panels, the joints of which are masked by stainless steel cover strips. All the concealed steel work is specially treated against corrosion by metallization and epoxy paint. The roadway faces of the walkway are faced with vitreous mosaic, the whole purpose of the tunnel lining and finishing being to provide hard, smooth surfaces resistant to traffic film and easily cleaned.
As anyone who visited the tunnel before the secondary lining was installed can testify, by the time it had been finally caulked seepage of water through the primary lining was negligible. Nevertheless, the ceiling trusses incorporate an asbestos cement drip shed. From this, any water that does not seep through and any condensation water is deflected behind the wall panelling where it is collected in gutters just above the horizontal axis line of the tunnel. From these gutters, the water is led down into the tunnel invert beneath the road deck by pipes, which pass behind the walkways.
The deepest portion of the tunnel beneath the river is not, as it may appear, truly level but is inclined towards the north on a gradient of 1 in 275. This means that any water collecting in the invert, either from the tunnel or from washing down operations, can flow naturally into a sump formed below the invert towards the north bank of the river. From this sump it is returned by flameproofed pumps to the surface where, after passing through oil interceptors, it is discharged into the local sewers.
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