Visit to site of massive liquefaction at Kaiapoi and Avondale; Downtown Christchurch destruction

Thursday 10-1

Day 5, we joined Mark Stirling (GNS Science) to visit Kaiapoi, a river town north of Christchurch that was heavily affected by liquefaction (seismic liquefaction is the result of uncompacted materials (usually sand) that is saturated becomes shaken and looses its strength). Kaiapoi is located in a low-lying area, near the coast, that is a former delta/estuary of the Waimakaririri River. Materials there include sands, silts and peats. The downtown is quaint with river walks, pedestrian bridges across the streams and neighborhoods that range from older, generally single story dwellings on established blocks to newly-constructed multi-story houses. 

[Just to note, we are not doing anything associated with the liquefaction analysis other than observing its effects. Much of the work documenting and analyzing the earthquake-induced liquefaction in Canterbury is being conducted by Canterbury University Department of Civil and Natural Resouces Engineering Associate Professor Misko Cubrinovski and his students. He presented impressive documentation of their work at the New Zealand Geotechnical Society Special Meeting we attended on 28 September. A link to the the geotechnical aspects of the event can be found at http://quake.canterbury.ac.nz]

The ground failure through this area was wide-spread and devastating. The recently-renovated New World supermarket (think scale of Safeway or Kroger) will have to be torn down. Several older downtown business buildings looked to be complete losses and were red-tagged. Those areas adjacent to the river had suffered liquefaction and also large scale lateral spreads. In some locations the extension cracks are large enough to stand and extend for 10‘s to 100‘s of meters parallel to the river bank. Many of the pedestrian bridges that cross the smaller tributaries to the Waimakaririri River were pleasant thoroughfares in town. The lateral spreads caused them to shorten and compress causing the bridges to become bent like pretzels. 

The largest impact to the area is likely the damage to the residential structures. Walking through the neighborhoods is eerie as houses are abandoned, many with contents emptied. Work crews are removing sand and silt that flowed up on the ground, in places to depths greater than 0.5 m thick. Remnants of sand volcanoes peak out around the foundations of houses, many of which are now tilted. Numerous garages and houses have become disarticulated. It appears that, in some cases, roads were built on gravel beds that floated during the liquefaction process; meanwhile, the houses along the road placed heavy loads on the soils and sank. The result is that we are walking along streets and looking into yards that have depressions of about 0.5 to 1 m deep centered around a sunken house. It became clear, from the discussions with geotechnical folks, that a big problem, aside from building on liquefiable materials, is that many of the foundations consist of unreinforced concrete slabs. Steel in the concrete would have kept the houses from self-destructing. That wouldn’t have prevented that subsidence problem though.

Another issue that was obvious as we first arrived in the neighborhood is that utilities are completely shattered by liquefaction. Port-a-loos (Porta-potties in US-speak) line the streets as all sewage lines were severed. Likewise, water and electrical lines have been disrupted. The infrastructure repair and replacement process will take a long time.

We also found neighborhoods that appeared almost unaffected by liquefaction aside from few, scattered sand spouts and cracks. We didn’t have construction plans for these neighborhoods but Mark Stirling was under the impression that some of the newer neighborhoods were built on reclaimed land that had been engineered. They seemed to perform well.

I left Kaiapoi knowing that there are communities in the Humboldt area that have similarities in construction practice and geologic setting. We have documented some liquefaction during recent earthquakes there but have not experienced the extensive, community-wide devastation that we observed in Canterbury. Certainly, a factor is the amount and duration of ground motion. The poorly consolidated, sandy materials were prone to liquefaction. It leads one to question whether these areas should be developed. The question that will be paramount to Kaiapoi for the future is what will be done with the town. Materials that are prone to liquefaction and have liquefied in the past, remain liquefaction hazards. Without substantial geotechnical engineering or structural foundation design, the town will likely continue to have these problems in future earthquakes.

We completed the afternoon by walking through downtown Christchurch in the area that received all of the international press with collapsed brick building façades and smashed cars. The main destruction occurred on Manchester Street, an area of older unreinforced masonry (URM) buildings near Victoria Square. The Kiwis are quite efficient in taking care of these hazards. Instead of letting a damaged building partially stand behind a temporary fence for months like I’ve seen happen in California they pulled the buildings down with diggas (excavators) within days of the earthquake. This has a two-fold effect, first, the regional council or city council, in conjunction with engineers, declares the building finished and it is destroyed. This eliminates future hazards (especially with the significant aftershocks that continue, even now, a month later). Second, there is a sense that the city is not going to sit on its hands but is ready to move forward. 

So, Manchester Street is now the site of vacant lots scattered between undamaged modern steel and concrete buildings and reinforced brick buildings. One thing we found as we worked our way into the city is that many of the brick buildings that seemed untouched by the earthquake had been retrofitted with steel. It seemed to work! Another thing we noticed, which is typical for most large earthquakes, is that for the most part the city appears untouched by the earthquake. You have to go looking for damage which is localized and not citywide. The media tries to make the devastation look widespread and total which is not the case. As we walked through Christchurch we were impressed with how vital the area looks as Spring is just kicking in with flowers blooming, fruit trees blossoming and people going about their everyday business.

We met with Kevin Furlong again last night for dinner and a beer. We had great discussions about New Zealand and western US tectonics. We were in the old town in a brewpub with brick walls (that had been retrofitted)…how soon we begin trusting the engineers and architects again… Thanks to Kevin for his generosity in time and resources as we invaded in the middle of an extremely busy time!
We just saw the GNS Science geodetics crew pull out of the motel to head back to Wellington. They said they got tons of data and now have days of post-processing. It was great getting a chance to see them again. Mark Stirling headed back yesterday. It has been a bittersweet return to New Zealand. It has been great to see all of my friends that Eileen, Ethan and I made when we lived here two years ago, unfortunately it had to be a damaging earthquake to bring me back. We’ll definitely be heading back soon.

We leave Canterbury to do some touring of the big South Island faults. I can’t bring Paul to New Zealand without getting him onto the Hope and Alpine faults. We’ve brought tents and sleeping gear so, we’re just going to do a route up the east coast to Kaikoura and then travel to Hanmer Springs, across the Alpine faut at Lewis Pass and onward to the West Coast. From there we’ll head to Hokitika, then across Arthur’s Pass again at the Alpine fault then back to Christchurch to depart on Monday.

We’ll keep blogging…