Water Journal : Water Journal May 2015
MAY 2015 water 33 Feature article asset management T his article highlights two case studies where Vinsi Partners (Vinsi) observed the failure of vertical stressing bars in reservoir walls while undertaking structural risk assessments of water assets. Had these failures not been identified and rectified, the consequences could have been catastrophic (Figure 1). Failure 1: Queensland reservoirs Vinsi was commissioned to undertake a structural risk assessment of 10 concrete reservoirs in Queensland. As part of the assessment the following was undertaken: • Visual inspection; • Review of water analysis testing; • Evaluation of structural engineering design drawings. During the visual inspection the failure of two vertical stressing bars was observed at one reservoir. Exactly when the failure had occurred, and how or why, was unknown. The initial concern was that the 50ML reservoir was located in and directly above a residential area. The consequences of failure could have been extreme. During discussions with the client there was anecdotal evidence to suggest other bar failures, and bars were required to be removed at two other reservoirs. A preliminary structural adequacy review utilising Finite Element Analysis (FEA) to allay immediate concerns was then undertaken. A monitoring regime was established, with contingency plans in place to lower water levels if required. This planning enabled a detailed site and materials investigation to be carried out to confirm design parameters, consider future remedial options and, ultimately, manage risk while maintaining normal operations. Failure 2: Tasmanian reservoirs Vinsi was engaged to undertake a study into observed defects and deterioration in over 25 structures including reservoirs, pump stations and wet wells in Tasmania. Utilising in-house concrete diagnostic testing and site investigation skills, appropriate remedial options and specifications were provided. During these investigations we learned of previous failures to vertical stressing bars in pre-cast wall elements at one of the reservoirs. The assessmenT It was established that the failure was caused by corrosion causing existing micro-cracks in the bar to increase. The corrosive medium for Stress Corrosion Cracking (SCC) was pitting (localised) corrosion at the base of pre-existing cracks of the bar leading to crack propagation and growth, or pitting corrosion causing crack initiation, propagation and growth. The combination of pitting corrosion and tensile stress can result in brittle and catastrophic failure of a bar. This can lead to a stressed bar being ejected vertically (to release energy) from the concrete wall panel. The design and construction methods employed to the reservoirs in question did not afford long-term corrosion protection to the bars, creating a very real failure risk. Armed with this knowledge, focused condition assessments of the reservoirs in Queensland and Tasmania were carried out. A visual survey, combined with select diagnostic testing (concrete, steel and water), ascertained deterioration mechanisms and appropriate service life cycles. Structural Failure ManageMent oF corroding reServoir inFraStructure Over the past five years Vinsi Partners has encountered a number of concerns in the design and construction of select water reservoirs. Brad Dockrill, Warren Green and Brett Eliasson present two case studies. Figure 1. Failed vertical stressing bar.
Water and CSG
Water Journal June 2015