Civil Engineering March 2021 | Vol 29 No 2

12 March 2021 Civil Engineering Struc tural Eng i neer i ng This original bridge structure has reached the end of its functional service life (Ronné et al., 2018) and will be replace with a new tied-arch bridge comprising a cable-sup- ported concrete deck which spans 110 m between bearings and arching ribs which rise 22 m above the asphalt road surface. The superstructure, which was com- pleted mid-2020, adjacent to the existing road-alignment, will be transversely jacked to its final position after completion of the substructure. This operation is currently scheduled for the second half of 2021. PROJECT IN CONTEXT The New Ashton Arch Bridge forms part of a broader construction project con- ducted under the auspices of Western Cape Provincial Government. This involves the reconstruction of parts of Trunk Road 31 Sections 2 and 3, between Ashton and Montagu in the Western Cape, which includes the historic Cogmanskloof pass. The 13 km long road section is also the start of the strategically important tourist ‘Route 62’. Typically, about 7 000 vehicles travel through Ashton and 3 600 vehicles through Cogmanskloof Pass toward Montagu every day. The historic Cogmanskloof Pass, a focal point of the project, stretches 6.5 km through a majestic landscape of towering rock formations and a colourful pastoral patchwork. The Cogmanskloof River flows adjacent to this section of the TR31 route for a significant distance and crosses the roadway at four discrete locations by means of bridge structures. The greater Ashton-Montagu region, and Cogmanskloof Pass in particular, has experienced substantial flood damage on several occasions over the years. This resulted in multiple road closures and significant operational disruptions, with adverse impacts on the local economy. The largest of these recent floods oc- curred in March 2003 (Figure 1). Less se- vere but still significant flooding also oc- curred during April 2005, July to August 2006, November 2008, and June 2012. The high debris load in the Cogmanskloof River exacerbates the effects of flooding at the existing short span bridges. Generally, these bridges have wide solid wall type piers, orientated at unfavourable skew angles relative to the sometimes variable flow direction. This has resulted in debris blocking the hydraulic opening as well as overtopping, particularly at the existing river bridge in Ashton. AIMS AND OBJECTIVES The original bridge over the Cogmanskloof River in Ashton was constructed in the 1930s and is shown in Figure 2. The original structural form was a five span, earth-filled, arch type super- structure, allowing single lane vehicular traffic. The substructure consisted of wall type piers and abutments, with a skew angle of 50° to the river. Around 1950 a substantial structural retro fitment was undertaken, modifying the superstructure to a cast in situ beam and slab configuration that maintained portions of the arch superstructure, including the arch-profile with related hydraulic opening configuration. The client’s brief involved improve- ment of flood resilience and overall safety of the road to acceptable modern stan- dards. Technical proposals were required to address the risk of frequent, and severe, flooding at the Cogmanskloof River Bridge in Ashton with due cognisance of the restrictive boundary conditions, Alton Ruiters Pr Eng ChiefEngineer–RoadContractsChiefDirectorate Transport and Public Works – Western Cape Government alton.ruiters@westerncape.gov.za Abe Newmark Pr Eng Executive – Team Lead: Bridge Engineering, Civil Infrastructure, EMEA AECOM abe.newmark@aecom.com Philip Ronne Pr Eng, IntPE (SA) Associate Engineer: Bridge Engineering, Civil Infrastructure, EMEA AECOM philip.ronne@aecom.com Heinrich van Wijk Pr Eng Senior Engineer: Bridge Engineering, Civil Infrastructure, EMEA AECOM heinrich.vanwijk@aecom.com Design and construction of a concrete tied-arch bridge in Ashton , Western Cape The New Ashton Arch Bridge is being constructed in the town of Ashton, Western Cape, on Trunk Road 31 Section 2 crossing the Cogmanskloof river. The bridge replaces an existing solid spandrel, multi-arch bridge that was built in the 1930s. Figure 1 Flooding at original Ashton bridge (2003)