Abstract
Bowen Bridge is the second road crossing of the Derwent River in Hobart. In addition to superstructure loading the bridge piers have been designed to withstand impact from 5000 tonne barges, this design feature being introduced principally as a result of the Tasman Bridge disaster in January, 1975. The bridge is supported on 13.3m diameter caissons, extending to 47m below river level to found on rock. This paper presents details of site stratigraphy, design and general arrangement of the foundations. Monitoring of two aspects of rock performance during construction is included in the paper; water pressure relief for cofferdam dewatering and rock mass stiffness for comparison with design assumptions. Water pressure relief in the rock at and below foundation level was required to permit construction of the caisson walls in the dry inside cofferdams. Pressure relief holes were drilled in the rock and piezometers installed to monitor pressures during and after dewatering. Rock mass stiffness is a significant parameter in assessing the response of the foundation to superstructure and ship impact loadings. A programme of rock coring and pressuremeter testing provided estimates of rock stiffness for design purposes. These estimates are compared with data from direct reading multi-rod extensometers installed up to 60m below river level. Details of the extensometer installations, together with assessments of rock mass stiffness from pressuremeter and extensometer records are presented (Author/TRRL)
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