Understanding the Christchurch earthquake: building damage

There are many puzzles as to why some of Christchurch’s  buildings have survived the 6.3  earthquake of February 2011, and others have not. However,  a combination of observations plus document-searching does provide some answers.  In terms of magnitude, this was just a moderate earthquake, but the local effects on  Christchurch have been huge because of its proximity and shallowness. Even within Christchurch, location was of huge importance, but there is more to it than that.

It seems that we do indeed have very stringent building codes in New Zealand. Not only are the codes stringent, but they are enforced. However, in Christchurch we also have many old buildings which predate these regulations. The original building codes go back to 1935, but earthquake design really only started in 1965. The codes were further strengthened in 1976 with the concept of ‘controlled failure’, and strengthened again in 1984 and 1992. The codes focus on the standards that have to be met, rather than specifying precise building procedures. This is to encourage new and innovative building methods that can meet the required standards.

Having planned the building of a new house both in Australia (in the 1980s) and in New Zealand ( in 2001), I know that the building code is a lot stronger here in  New Zealand than in Australia. Our Queensland house was built on a hill-top and had a special cyclone rating, but I doubt very much whether it would have withstood the shaking that our New Zealand house has withstood. The Christchurch building inspectors made multiple visits at key times to our New Zealand home, particularly when the foundations were being laid. It is the stringent New Zealand building codes that make house building so expensive in this country. I have also looked with wonder at the simplicity of house construction in Uruguay, where earthquakes are essentially unknown.

The messages from the 7.1 ‘dress rehearsal’  earthquake on September 2010 seemed reasonably straightforward. Old houses and commercial buildings constructed in the early 20th century, or in some cases in the 19th century, and which relied on single or double brick for their structural integrity, disintegrated rapidly. Also, houses built on a concrete pad in areas adjacent to streams and waterways, were found to be susceptible to cracking of the pad linked to liquefaction of the soils.  

Unfortunately, a house with a concrete pad that has cracked and split is history. Each day over the last five months, as I have driven to work, I have looked with wonder at a multi-million-dollar modern home owned by a well-known sporting identity, which looks normal from the road but is going to be pulled down for exactly this reason. 

The conventional view on liquefaction seems to be that the damage is caused by subsidence of the buildings into the soil, but I don’t think it has to be this way. At least over my side of town (Halswell),  I saw evidence in the 7.1 September earthquake of a powerful wave. I liken it to an undergound tsunami, that travelled through the aquifer from the epicentre, and then burst upwards as it hit the underlying rocks of the Port Hills. Anywhere adjacent to a current or former spring was at risk. Even the underground fuel tanks at my local service station, filled with fuel just the day before, were thrust upwards by this force. Across the suburb, the silt and sand laden water continued to flow from some of the boils for several hours afterwards. In contrast, in parts of the low-lying Eastern suburbs I assume that the liquefaction damage was indeed through subsidence, as I have no evidence to argue otherwise. 

Liquefaction on Kilmore St ( Asher Trafford)

Liquefaction in park off Colombo St (Asher Trafford)

There is a somewhat historical  (probably about 1999) but generally informative assessment of the liquefaction risks in Christchurch at http://ecan.govt.nz/publications/General/solid-facts-christchurch-liquefaction.pdf  However, I think that if they were to write this document again it would say some things differently.  One small but perhaps important point is that at least in some places the water and sand continued to bubble up for hours after the earthquake and not just during the earthquake. Also, both  in September 2010 and again in February 2011, there has been some liquefaction in areas previously identified as low risk. Some places that escaped liquefaction last time have received it this time, and vice versa. It seems to me that we still have much to learn about liquefaction.

One of the key messages from the September quake seemed to be that as long as a building was away from the streams, rivers, or low-lying land with a high water table, then it could withstand the shock as long as it was of modern construction. Where the shaking or ground movement was severe, then houses on piles seemed to fare  somewhat better than those on concrete pads. Houses with modern timber structural framing  also stood up particularly well.  As for the brick chimneys on the older wooden houses, well they descended ungracefully to the ground.

With the February earthquake there now seem many more lessons to learn.  The devastation in  the CBD includes modern buildings built from the 1960s to the 1990s that were untouched by the larger but not so close 7.1 September dress rehearsal. At least up until the mid 1980s, the designs were non-ductile. In other words , they were built for strength, but with rigidity rather than the flexibility needed to  absorb the energy of a huge quake.  Modern design is not supposed to mean there will be no damage, but it is meant to prevent catastrophic failure that causes death.

Grand Chancellor Hotel with earthquake damage and tilting at 2 degrees

The story behind the failure of the 26 (some reports say 27) level Grand Chancellor Hotel, constructed in 1995, will take some time to emerge. Presumably it was built to the latest codes so as to absorb the energy without suffering structural failure. It is in Cashel St, several hundred metres from the Avon River. Clearly it suffered immediate damage, but it was only some hours later that it began to tilt. Perhaps the ground acceleration went beyond the design capacity, as there is no doubt the ground acceleration was extreme. Maybe the wonder is that more modern buildings have not collapsed, and perhaps we should give due credit for that.  A big unknown is how many of the modern buildings that apparently survived will still have to be pulled down. Semi official reports coming through are suggesting more than  500 buildings in the CBD will have to go.

Before and after shots of  the PGC building (see below) tell a story of a modern-looking building, although built in about 1963. That means it pre-dated the first earthquake design requirements of the 1965 code. Originally this was known as the ‘Drainage Board Building’. Eventually the Drainage Board morphed into Environment Canterbury. Whether or not PGC were actually the building owners in 2011, or simply the major tenants with naming rights, is not clear to me. [Update: The buildng was sold to private investors in 2009.] What is clear is that people on the upper floors had the best chance of escape, with many of those on the lower floors being crushed by the collapse of heavy concrete floors. This building is adjacent to the Avon River in  Cambridge Terrace.

 

Before and after shots (below) of the seven storey CTV building (built in the 1980s) show another  modern-looking building. [Update of 1 March: there are conflicting reports as to the date of construction. It may have been 1979.]  This building looks heavy but with light structural support in the corners. [Update of 1 March: There are reports that the structural strength was supposed to come from the ‘core’ built into the rear of the building. This core is still visible in the ‘after’ photo. The rest of the building was in effect suspended from this core with only light support in the corners.] The building is many hundreds of metres from the Avon River on the corner of Cashel and Madras Streets. It was built nearly ten years after the 1976 regulations [see update above] which  introduced the principles of  ‘controlled failure’ and may also have been subject to the 1984 regulations. Either way, there are questions to be answered as to why the failure was so catastrophic. This is the building where most of the deaths occurred, perhaps as many as a hundred or even more, including most of the foreign deaths at the Kings Language School. It would seem that some were killed by the ensuing fire after being trapped.

CTV building before and after

Christchurch Cathedral

 

The collapse of the cathedral is less surprising. In Christchurch we have been very attached to our heritage buildings, but now it is time to move forward and rebuild from the ground up. 

Christchurch Cathedral

 

Over on the eastern hills of Sumner, Redcliffs, and Mt Pleasant ,  it seems the shaking of the ground was so extreme that no practical building code could have saved many of the houses. However, on the western hills, where I live, the form of cladding seems relevant. Most of the houses have timber framing and this has stood up well. But whereas plaster cladding has survived, some of the brick veneer homes are looking a little naked, with timber and building paper now visible. As to whether they have retained their structural integrity underneath, that will require an engineer’s report.

As for the low-lying eastern suburbs, here too it won’t be until the engineers’ reports are available that the real picture will emerge. The liquefaction on these soils is much greater than  in September, and initial reports are of up to 150,000 tonnes of sand and silt needing to be removed, compared to about 30,000 tonnes last September. [Update: By early March some 180,000 tonnes had been trucked out.] A lot will depend on whether the concrete pads have shifted, cracked, and tilted.

What we do know for sure is that thousands of homes cannot be brought back to being livable.  And a lot of those that may be livable in the short-term, are still incapable of being permanently repaired.

As each day goes by, the task of rebuilding Christchurch city and suburbs seems to grow.  Yet only a few km away there is no damage. Earlier today I was out at Lincoln Uni, only 20 km from the city centre, and there the damage is minimal. In my Uni office, and in sharp contrast to the September earthquake, there is not one paper or book that has moved. This provides a reminder that the Christchurch earthquake really was a local event: short, sharp and vicious. Although Lincoln Uni does remain closed, that is because many staff and students live in the city. In any case, the Uni itself is being used as a base for rescue personnel.

(Photo credits: The first three photos in this post come from Asher Trafford. These are just three of many photos taken by Asher, who jumped on his bike and took some amazing photos around the city in the immediate aftermath. Other photos are sourced from Google and are currently uncredited.)