Civil Engineering March 2021 | Vol 29 No 2

Civil Engineering March 2021 15 Construction challenges Basil Read (BR) was appointed as the main civil contractor and commenced with preparing the temporary works in mid-2016 (Figure 8). The first concrete for the permanent works was cast in late November 2017 and the deck, including the springing points and excluding the arch ribs, was completed on 25 April 2018. Shortly after completing this milestone the main contractor filed for business rescue, with the bridge partially complete. Post-tensioning of the deck continued by a specialist sub-contractor under the supervision and technical guidance of AECOM, prior to the appointment of a replacement contractor. After procurement of a new main contractor construction continued with the concreting of the arch ribs and wish bones before post-fitting steel anchor plates with stress bars. The post tensioning activity and anchor plate manu- facturing posed construction challenges which are elaborated on in this article. Contract termination On 15 June 2018, BR was placed under business rescue and, after subsequent ne- gotiations, the Western Cape Government (project owner) elected to terminate the contract with BR. An interim mainte- nance contractor was appointed to main- tain the site and facilitate the continuation of minor work by some sub-contractors, chiefly to maintain traffic accommoda- tion and safety measures for the site. A new tender was initiated for the completion of the outstanding construc- tion works. Haw & Inglis was awarded the new construction contract, with the partially completed works and components constructed by BR becoming free-issue materials supplied by the project owner. Post-tensioning of arch tie beams The deck of the tied-arch bridge comprises a grid of longitudinal and transverse post- tensioned beams. The transverse members are fitted with draped cables which assist with transferring the loads laterally to the tie-beams. The longitudinal members are also prestressed, although not for conven- tional reasons, but rather for countering the outward thrust action of the arch ribs. This is achieved by placing concentric tendons in the tie-beams. During tensioning of the first two longitudinal tendons, significant under- extensions, in comparison to theoretical values, of up to 30% were recorded. The post-tensioning activity was halted, and investigations undertaken to establish the cause of the under-extensions. Under- extensions usually indicate blockages in the duct or higher friction losses than expected in the design. A sensitivity analysis was carried out, using theoretically extreme variations in friction and wobble coefficients (van Wijk, 2019). It was however concluded that this did not fully explain the gross differences (Figure 10). Test tensioning without the anchor head was then performed on all the remaining tendons (Figure 11) and the evaluation of these tests concluded that the high axial stiffness on all tendons were similar. By inference it was also concluded that a blockage (e.g. grout leaks or installation defects) was unlikely to occur in all the ducts (confirmed by video footage, excerpt shown in Figure 12) and was therefore not the primary cause of the under-extensions. Figure 7 Data acquisition system Figure 8 Temporary works Figure 9 Casting the concrete deck