By Mary Comerio.
September 12, 2010. Earthquake Engineering Research Institute.
The over-riding impression of the Darfield earthquake is one of serious liquefaction damage to homes (as well as schools and other low buildings) on soft soils and sand. Foundations are tilted, porches and rooms (esp. additions) are broken away, and floors are pushed up or down and separated, leaving doors and walls out of kilter. There have been 50,000 insurance claims to EQC and they expect 150,000. Many think the $4 bil NZ dollar estimate of losses will go up. The worst cases will have to be demolished, but there will be major planning and engineering questions about whether those with moderate damage (ie with lots of liquefaction but where the house is deemed habitable) will be able to repair, and whether they will be required to remediate the soil. The dilemma is that if such were required, it would apply to most of Christchurch.
There is also considerable damage to URM buildings which are historic structures. At a public meeting at City Hall, the mayor, and council (and the director of a newly opened recovery center–one stop shop for permits and advice) took questions from a packed audience. There is real concern about the loss of the historic fabric and about the economic impacts of a new requirement voted by the city that repairs will have to be done to 67% of code (as opposed to the former rule of 33%. The city has set up a fund to help owners with the differential cost and clearly, the city and building owners will have to set priorities on which buildings must be saved and which may need to be demolished. The impressive part of this meeting was the civil tone and sense of cooperation between the city and citizens.
Nonstructural damage is the other major issue: partitions cracked, ceiling tiles and lights, as well as contents toppled. We saw the clean up at the University of Canterbury and discussed the inspection process with their staff. Tall buildings had such damage on higher floors, but there was less than expected in low rise structures. Some think that the soft soils acted as something of a base-isolator–dispelling the energy. We were amazed to see a school which was closed because of the serious liquefaction impacts on the buildings, where not a chair had fallen over.
