A Case Study for Infrastructure Scour Management

Scour is the leading cause of bridge collapse worldwide, being responsible for compromising the stability of structures’ foundations. Scour and erosion can take place without prior warning and cause sudden failure. This study describes engineering measures and complications encountered during construction for a case study in the Scottish Borders (A68 Galadean Bridge). The bridge studied carries the A68 road across the Leader Water.

Transport Scotland’s structures crossing or near a watercourse are subject to a two-stage scour assessment following the Design Manual for Roads and Bridges (DMRB) BD97/12 Standard, ‘The Assessment of Scour and Other Hydraulic Actions at Highway Structures’. Structures identified at risk are monitored through Reactive Structures Safety Inspections following events likely to increase water levels. The most common form of monitoring includes visual inspections, however, monitoring sensors are being currently implemented and trialled at locations at high risk of scour.

Scour in the area was identified during a Reactive Structures Safety Inspection, following which a weekly scour monitoring regime was established, alongside further Reactive Structures Safety Inspections, until remediation measures were put in place.

Despite the bridge being constructed perpendicular to the Leader Water, meandering of the watercourse was detected upstream. Sediment transport was the cause of an island formation immediately upstream of the structure. Non-uniform flow and secondary, spiral currents, resulting from the formation of the bend were exacerbating scour and erosion in the area. The design of the remediation measures included the implementation of rock rolls alongside the affected riverbank. However, during construction, increased water levels resulting from thawing snow resulted in the collapse of a significant portion of the embankment supporting the structure’s abutment and the A68 road, prior to the realisation of the remediation measures. An emergency design revision was required and emergency measures had to be enforced.

The urgency of the works led to a two-phase approach being followed for the design and construction of the scour measures in the affected area. The first phase included the construction of a platform in front of the affected road embankment and the implementation of rock rolls to provide scour protection. The two-phase approach ensured the infrastructure at risk was protected from further deterioration while the reconstruction of the embankment was being designed.

The second phase of works included the reconstruction of the affected road embankment, for which the anticipated total scour depth was taken into account.

 

References:

Koursari E and Wallace S. 2019. Infrastructure scour management: a case study for A68 Galadean Bridge, UK. Proceedings of the Institution of Civil Engineers – Bridge Engineering, https://doi.org/10.1680/jbren.18.00062

 

Acknowledgements:

The authors would like to acknowledge Transport Scotland for funding this project.