Ponts Mill Viaduct

Ponts Mill Viaduct is a four-span under bridge carrying the bi-directional Newquay branch line running between Par and Newquay in Cornwall. The existing viaduct was strengthened and refurbished to enhance its serviceable life as part of the mid-Cornwall metro scheme to introduce hourly trains to the route.

The original superstructure comprises four similar spans each consisting of steel riveted plate girders with k-bracing, transverse cross girders at main girder top flange level and longitudinal timbers. The substructure consists 2 No. abutments and 3 No. stone piers.

The strengthening works comprised replacement of the bridge k-bracing, cross girders and parapets with a new deck consisting of steel cross girders, rail bearers, deck plate and new parapets.

Scope of services

Nuttall Bowser undertook review of the outline design through comparison of the proposed outline design developed by our clients Crouch Waterfall and the previous structural assessment which had identified numerous under-strength elements. 

Nuttall Bowser provided CAT III calculations of the proposed strengthening and comparison of results with the designer. 

We undertook a constructibility and sequencing review of detailed design drawings to ensure all construction stages were adequately considered within design calculations - with particular focus to railway load restrictions during the various construction stages. 

Challenges

The structure was in a poor state of repair with some unknown geometric uncertainties arising from partial missing dimensionality from archive records.  The allowable window for construction works within the blockade prevented full aspect of works to be undertaken wheels free and solely within the blockade. 

Value added

Nuttall Bowser leveraged our 25 years experience in bridge assessments and repair designs for Network Rail to bring substantial knowledge in both structural analysis and detailing to ensure all prior under strength elements were adequately addressed whilst also ensuring all new and existing elements were adequate for the proposed loading. 

We undertook gap analysis of the existing records to ensure adequacy of information, in particular to the bearing arrangements and calculation of effective span, as there was an element of unknown to the bearing arrangement and bearing lengths.

Where web panel capacity was sensitive to condition we ensured consideration was given to the opportunity to add web plate strengthening should grit-blasting reveal lower web thicknesses than expected and identified alternate solutions, to full panel strengthening where web buckling limited assessed strength, through the provision of additional intermediate stiffeners.