Quake danger to bridge pipeline

Earthquake proofing Tauranga's 82-year-old Matapihi railway bridge to take a sewer pipeline will cost more than if it was being brought up to modern standards for trains.
This curious situation emerged from evidence last week during hearings into the council's planned $106 million Southern Pipeline.
The risk of slinging the pipeline on to Matapihi rail bridge without major strengthening was highlighted by evidence that the bridge was unlikely to survive an earthquake larger than 5.5 magnitude on the Richter scale. Edgecumbe's disastrous earthquake of 1987 was 6.3 on the Richter scale.
Engineering evidence presented to the hearing showed that seismic standards required under the Building Act for a structure carrying a ``lifeline' pipeline were far stricter than normal earthquake criteria for railway and highway bridges.
The act leaves the council in a tough bargaining position on its cost-sharing negotiations with the bridge owner Ontrack _ even though the bridge was acknowledged as being of national transportation importance.
Ontrack's design standard for new bridges was a structure that could withstand a one-in-450-year earthquake, whereas the council's pipeline needed a structure that did not collapse in a one-in-2500-year quake and would continue to function in a one-in-500-year quake.
Ontrack engineering consultant Robert Park dispelled the idea that the bridge could not support the weight of the pipeline. He said the comparatively small additional load of the pipeline and walkway could easily be accommodated, once the trusses had been substantially modified to take the extra load.
Ontrack engineering and standards manager Walter Rushbrook did not disagree with the council's geotechnical investigation, which concluded that in a 5.5 magnitude earthquake, the bridge would suffer major damage from seismic liquefaction of weak soils under some piers.
However, Ontrack insisted that the bridge was "fit for purpose" for at least another 50 years.
Council's much stricter seismic requirements for a bridge carrying ``lifeline' structures called for the construction of 30 new piles to a depth of 40 metres below the seabed. Construction would take about two-and-a-half years.
The design also underpinned the existing piers and abutments and brought them up to modern design standards.
Construction needed to minimise disruption to the 25 to 30 trains that crossed the bridge each day. Access to the bridge during construction would be from Matapihi side, along an 11m widening of the causeway that would later take the pipeline.
The bridge was council's preferred option for the pipeline, although it has hedged its bets and also applied for a shallow submarine pipeline crossing about 50 metres upstream.
The Southern Pipeline will service the city's rapidly developing southern suburbs and prevent the hotspots in the sewerage system from becoming dangerously overloaded in heavy rain.
The planning hearing resumes on October 13.