A Look at Building Stock and Insurance Impacts by Tao Lai

March 30, 2011 by  
Filed under General Information, Structural

Outside of Christchurch’s Central Business District (CBD), the majority of buildings generally consist of low-rise wood-frame houses with brick veneer, which generally performed well under earthquake ground shaking. However, non-structural damage such as cracks in plasterboard and toppled chimneys was widespread.

A seismogram at Heathcote Valley, which is located one kilometer from the epicenter, recorded PGA (Peak Ground Acceleration) as high as 1.4 g. However, while no complete collapses were observed, fallen chimneys were widespread. In the hillside town of Lyttleton, about four km away from the epicenter, except for the collapse of old masonry buildings and a few poorly built houses, no major damage was reported.

   
Fig. 1 Typical chimney failure at Heathcote valley, and most brick veneer seems intact Fig. 2 Typical settlements in the hills of Lyttleton performed generally well though subjected to large ground shaking

 

 In Kaiapoi, about 23 km north of epicenter, where peak ground acceleration of 0.2g was recorded, many homeowners reported slight plasterboard cracks and minor foundation cracks because of uneven settlement. One homeowner had spent a good deal of money earlier to repair some minor settlement damage after the September 2010 earthquake, which had caused higher ground motion in the area (PGA of 0.34g), but, unfortunately, the same type of damage reoccurred after this February’s earthquake.

The surveyor on the team found the wood-frame construction in New Zealand is generally well constructed. The superstructure is firmly attached to the foundation through closely spaced anchorage and brick veneer, which is usually very vulnerable to earthquake shaking, has been effectively connected to plywood.

 
 
 
 
 
 
 

Fig. 3 Wood-frame construction in New Zealand is well constructed

 

Across Christchurch, which is situated on a very soft and deep alluvial deposit, particularly along the Avon riverbank and in east coastal areas, liquefaction was again the primary cause of severe damage to dwellings. Even within the CBD, liquefaction-related settlement and lateral spreading is quite common. At the intersection of Peterborough Street and Manchester Street, for example, some buildings settled as much as 1 meter, and the soil subsidence buried many cars.

     
Figure 4. Before Sept. 2010 (Source: Google) Figure 5. Feb. 23, 2011 (Source: Kam, WY) Figure 6. March 15, 2011 (Source: AIR)
This apartment settled significantly mostly after the Feb. 2011 earthquake

 

   
Figure 7. Cars are likely to be totaled after being buried by liquefaction- ejected sand Figure 8. Motorists negotiating a section of Fitzgerald Ave passing a dangerous break on the former northbound lane

 

Liquefaction along the Avon riverbank in eastern Christchurch was very severe. Although damage to houses was hardly perceptible from a distance, the saturated and graveled road along the river revealed the severity of liquefaction damage. The area’s disrupted sewerage service was also the result of liquefaction and it was reported that Christchurch is still without permanent sewarage systems.

 
Figure 9. Asphalt roads along Avon riverbank and in eastern Christchurch were severely cracked by liquefaction, and most of them will likely have to be rebuilt

At Courtney Drive in Kaiapoi, an entire dwelling was moved backwards about 1.2 meters by last September’s quake, and it was moved rearward another 0.6m by the February earthquake. Another dwelling nearby was ripped open 1.5m by the September rupture, and then pulled apart another 0.05m. The owner, a Japanese native who experienced the 1995 Kobe earthquake, was grateful that her family members were not injured, given that such a large fissure had opened right beside her bedroom.

The woman had decided to rebuild her house with reimbursements from the Earthquake Commission (EQC)  and her private insurer (plus some personal expenditures).  At present, a large portion of residents are living in temporary lodgings, and almost the entire community will have to be demolished and rebuilt. However, how long this will take is not yet known. Many people are concerned that if it takes too long to rebuild the community, many residents may be forced to settle elsewhere.

   
Figure 10. September 16, 2010 Figure 11. March 15, 2011
The fissure developed another few centimeter after the February 2011 earthquake

 

   
Figure 12. September 16, 2010 Figure 13. March 15, 2011
The dwelling drifted more than another half meter after the February 2011 earthquake

 

 The liquefaction-related damage to Christchurch was not completely new to New Zealanders. In 1996, a documentary (http://www.youtube.com/watch?v=NkTy6ogLDX8) had outlined the effects of a major earthquake in Christchurch’s eastern suburbs. The eight-minute clip reported that Christchurch’s “fatal flaw” was the land that it was situated on – “a soft, shaky sponge of river stones and silt half a kilometer deep.” The report continued, “In any decent quake, the Garden City will shake like a leaf, and liquefaction would cause sewers and drains to come apart, roads to break up and buildings to sink or tip over.”

In summary, because the magnitude of the February earthquake was much smaller than the one last September, its affected area was smaller. However, because the epicenter of the February event was located much closer to the city center and the city’s eastern shoreline , the shaking intensity in the CBD was much stronger and the area of liquefaction damage was wider and also extended to the more affluent Southeast Christchurch community.

While the exact number of dwellings that have been damaged remains unknown, a media outlet citing a government source has reported that about 10,000 homes are likely to be demolished and that some of the area is beyond repair and will have to be abandoned.

Based on this report, it is believed that the total number of damaged dwellings will be slightly lower than that caused by the last September’s earthquake (a total of 160,641 dwelling claims). However, the severity of the overall damage  is higher and would result in higher economic losses. Another significant observation concerns the new damage sustained by previously heavily damaged dwellings. Because the second earthquake followed the first one  in such a short period of time, before reconstruction or demolition had started, the final reconstruction cost will be counted only once. This could mean a significant financial relief for both EQC and private insurers.

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