The Jamieson's Bridge Underpinning project involved repairs to the three span continuous reinforced concrete T-beam structure, originally built in 1932 over a steeply incised gully in sandy materials, following major floods in 2004 causing extensive scour damage to one of the piers.
The flood waters had caused extensive damage to the bridge, previous repair works had been outflanked, supporting soils were scoured out to a depth of one metre beneath the spread footing of one of the piers, and consequently the pier and bridge deck dropped by approximately 120mm. BBO compiled an options report to investigate repair methods or potential replacement solution for the structure. Economics dictated that restoration rather than full replacement was the preferred solution in this instance.
BBO's selected design for the repairs involved underpinning using bored piles and installation of a drainage channel providing overall bank stability and scour protection to both piers. This repair solution resulted in restoration of the bridge to a state similar to what existed prior to the 2004 flood event, with the channel closely matched to the existing streambed path but with increased scour protection to both the northern and southern piers.
The repair involved a design philosophy that enabled the settled pier to be jacked back into position prior to being supported directly by the two new bored piles. The sheetpiled drainage flume provides long-term security against scour to the erodible bank soils and future protection against degradation of the stream bed.
The bridge was kept in service since the initial 2004 flood event by the installation of a temporary bailey bridge spanning over the settled zone in the deck. The bailey bridge was only removed once all the remedial works were completed and the repaired bridge signed off by BBO as once again fit for purpose.