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Prevent problems by regular inspection

Letters

Reports of the collapse of a new tunnel lining in Hull where the tunnel was situated in non-cohesive ground with considerable external pressure are worrying. In recent years large numbers of interceptor sewer tunnels have been constructed in similar circumstances. Many are situated in alluvial deposits below the groundwater table for a significant proportion of their length.

Other common features are single pass concrete linings relying on sophisticated gasketry for water tightness, and cyclic internal pressures caused either by storm surcharging or by their deliberate use for balancing storage. Often, the precise ground conditions supporting the lining were never seen during the construction phase where sealed face systems were used to excavate the tunnel.

Add a general lack of accessibility, exacerbated by a climate where health and safety considerations discourage man access for inspection and a picture of a potentially developing problem appears.

All segmental linings rely on uniform ground support for stability. Loss of support due to erosion of non-cohesive material behind tunnel linings suffering minor leakage produces deformation in the lining, which exacerbates the leakage problem and leads to an escalating cycle, culminating in collapse.

The solution lies in a programme of inspection rather than the 'out of sight out of mind' approach that prevails in many organisations which does not consider assets of such recent construction candidates for maintenance inspections.

HJ Orr-Ewing (M), Hydrock Consultants, Over Court Barns, Over Lane, Almondsbury, Bristol BS32 4DF

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