CONDITION AND RISK ASSESSMENT OF GRIFFITH G4 SEWER RISING MAIN

摘要

Griffith City Council (GCC), in the NSW Riverina, has centralised their sewage treatment and has an extended network of series pumping and rising mains. The G4 SPS rising main is approximately 7.1 km long and was reportedly constructed in 2-3 phases from 1974 and 1991 from cast-iron and ductile Iron. The main travels alongside the main irrigation canal for a large proportion of its length, which also provides drinking water to the town. Previous failures, current low flow rates and high detention times, and high levels of hydrogen sulphide measured at the treatment plant inlet channel led GCC to initiate detailed condition and risk assessment of the main. KBR were engaged to review available operational and performance data, previous investigation reports, and undertake condition and risk assessment. External physical testing of the main including Broadband Electromagnetic scanning (BEM), physical inspection and spot ultrasonic testing at 30 representative and expected worst case locations. Bulk soil resistivity and transient pressure monitoring were also undertaken. Quantitative failure risk assessment and economic life modelling was undertaken by specialists WISER analysis. The findings validated the approach of point sampling with detailed scanning at regular intervals and high risk locations. The general rate of corrosion was found to be substantially lower than feared, but several critical points with significantly elevated risk or deterioration were identified due to local design features. A staged strategy was recommended involving condition monitoring, operational changes, and air valve relocation. The estimated cost of these measures over 5 years of approximately $100,000, providing a cost effective means of managing risk while deferring major rehabilitation or renewal works for a substantial period.