New Ashton Arch – functional assessment of direct and indirect construction costs and evaluation of service life with respect to flooding risk.

Philip Ronné,Nadri Du Toit,Heinrich Van Wijk,Abe Newmark,M.G Alexander,H Beushausen,P Moyo,F Dehn

doi:10.1051/matecconf/201819906003

Abstract

The bridge crossing the Cogmanskloof River in the town of Ashton, South Africa, had a history of over-topping due to severe flood events. The poor flood resilience of the bridge was aggravated by the generally hydraulically inefficient openings, the number of substructure supports in the river course, and a high debris load during flooding. The strategically important tourist route had to be closed, while localized flood damage repairs were undertaken, with resultant adverse effects on the local economy. As part of a road safety improvement project between the towns of Ashton and Montagu, improvement of the flooding resistance at the Ashton river bridge was required. This paper documents the functional evaluation of economicand technical-assessments of the flooding risks for the existing retro-fitted bridge. A new tied-arch bridge was the selected structural form of the replacement structure – based on the assessment of the key service life and constructability criteria. The structural form of the Ashton Arch paid careful regard to the scenic location and historic character of the previous multiple arch bridge form.

Highlights

The project, conducted under the auspices of Western Cape Provincial Government’s Department of Transport and Public Works: Roads Infrastructure Branch, involved the reconstruction of parts of Trunk Road 31 Sections 2 and 3, between the Western Cape towns of Ashton and Montagu, South Africa
The greater Ashton-Montagu region, and Cogmanskloof Pass in particular, experienced substantial flood damage that resulted in multiple road closures and significant operational disruptions, with adverse impacts on the local economy
The high debris load in the Cogmanskloof River exacerbates the effects of flooding at the existing short span bridges

Summary

Project background

The project, conducted under the auspices of Western Cape Provincial Government’s Department of Transport and Public Works: Roads Infrastructure Branch, involved the reconstruction of parts of Trunk Road 31 Sections 2 and 3, between the Western Cape towns of Ashton and Montagu, South Africa. The greater Ashton-Montagu region, and Cogmanskloof Pass in particular, experienced substantial flood damage that resulted in multiple road closures and significant operational disruptions, with adverse impacts on the local economy. The high debris load in the Cogmanskloof River exacerbates the effects of flooding at the existing short span bridges These bridges have wide solid wall type piers, orientated at unfavourable skew angles relative to the sometimes variable flow direction. This has resulted in severe debris blocking of the hydraulic opening (Fig. 2) and resulting overtopping, at the river bridge in Ashton

End of functional life assessment of existing arch bridge

Physical and economic considerations of existing Ashton Bridge

Bridge replacement considerations

Precast beam and cast in place slab deck

Findings

Conclusions