Residential, Lifestyle, Commercial and Industrial Property Valuers servicing the Hutt Valley

Lessons to be learnt from the 2010 and 2011 Christchurch Earthquakes

COMMENT RELATING TO THE CANTERBURY EARTHQUKE OF SEPTEMBER 2010.

It is likely to take quite some time for the long term effects of the Canterbury earthquakes to filter through; however from “Day One”, the earthquake was likely to have a profound impact, not only upon the Canterbury Region, but also upon the rest of New Zealand; in part, as it will cause us to contemplate the possibility of “the Big One” hitting in our area, and hopefully prepare for an event such as this.  I first wrote this comment in 2011; but the basis seems as relevant to today, so I have retained it on the web page, as a back ground piece.

I recently read a report on the “Performance of Residential Housing in the Canterbury Earthquake”; no surprises really.

However, most of us would probably had thought that “shaking induced damage” would have been the major cause of damage to buildings; in reality, the “shaking” caused only relatively minor damage (apart from chimney loss and damage occurring to a small number of older buildings constructed of un-reinforced structural brickwork). The main damage within the September 2010 Canterbury Earthquake came from “Liquefaction” (perhaps for many of us, a new word for our vocabulary); however, this form of damage was focused in particular areas that had certain soil strata, and the degree of  ground settlement varied greatly from “site to site”.

So what does this mean for the Hutt Valley’s housing stock?

Given the nature of the Hutt Valleys subsoil’s (much being “former” swamp and river bed laid down over many thousands of years), “Liquefaction” is potentially the biggest risk to some valley floor areas.  Whilst the Hutt Valley’s hillside suburbs could definitely be at risk from “slips”, much of the Hutt Valley’s hillside development has avoided the steeper escarpment slopes (such as those that have been built on in other centres, such as Wellington); however, we still do have hillside areas that have histories of “instability” (most notably, the slopes on the Eastern foreshore of the Harbour).

As expected, “structures sinking into soft soils” appeared to be more severe for buildings of “heavier construction” ie. those with brick cladding and heavy tile roofs (the Hutt Valley has its share of this style of construction). However, against expectations (questioning the construction trends of recent years, which saw the prolific use of “concrete slabs” in modern construction; used on the assumption that slab construction “allowed a better spread the buildings weight”); houses on “piled foundations” appear to have performed better than those on concrete slab foundations (perhaps partly as a consequence of the “timber ground floor construction” of “piled houses” better accommodating “differential settlement” of the foundation system). An important distinction however is that (to our understanding), the nature of slab foundations in the Hutt Valley and those built within Canterbury may have varied somewhat.  Historically, Canterbury Building Regulations did not require the use of steel mesh reinforcing within the floor slabs; whereas, in areas such as the Hutt Valley, it has been more common to use steel mesh to reinforce the whole slab (with this allowing better “bridging” of any weak points in the sub soils under the slabs; and more recently, “Pod construction”  with additional crossing beams to strengthen the slab construction have been used locally).  According to the report, the vast majority of modern brick veneers suffered very little, or no, damage; as modern (post 1970’s) brick veneer construction generally used thinner bricks, and “light weight” roof cladding (giving improved seismic performance).

The most common type of damage to older buildings (those more than 15 years old) was chimney collapse (this reportedly occurred in thousands of Canterbury houses). Although falling chimneys resulted in damage, often “piercing of the surrounding roof structure”, and damaging neighbouring property; the collapse of chimneys onto corrugated steel roofing often caused no further damage.  However, chimneys falling onto “concrete tile” roofing, often “fell through” the house (causing significantly greater risk to the occupants).

Comment initially written January 2011

 Comment updated late 2011: 

The events of the second and third major earthquakes (of 2011) which have devastated Canterbury, have left many of us speechless and shocked; whilst “liquefaction” was again an issue, it paled in comparison to the type of damage resulting from “shaking” caused by the much shallower epicentre (producing a level of destruction beyond that which most of us could have contemplated).

According to an NZPA release of 18 March 2011, GNS Science  stated that, four unfortunate factors combined to make the Christchurch’s 22 February earthquake so destructive; these included, the energy released in the faults rupture (reported to reach ground acceleration of four times that of Japan’s recent massive magnitude 9.0 quake), the direction it was released (which directed the energy released towards the city), the “trampoline effect” (where different layers of sediment moved upwards to different degrees, resulting in separation of the layers of strata), and the proximity of the fault to the city.

Comment updated 2014:

So how has the Canterbury Earthquakes affected the rest of New Zealand?

  • The insurance costs on our property have increased noticeably; as New Zealand is now perceived as a “higher risk” to both Re-insurers and local Insurers alike.
  • The method of insuring residential housing has changed, from one where insurers carried much of the risk, to a “nominated Replacement Cost approach”, which places the risk of assessing adequate cover back onto the individual.  I have spent a lot of time over the past year discussing with clients Replacement Cost matters, relative to insurance cover, and still feel that many of us have a long way to go to fully understand the risk decision, that is now ours as the individual insured parties – my approach in discussions is likely to be different to many, as my focus is to empower you with understanding what decisions might right for you, not merely suggesting that you must obtain Valuer’s advice on Replacement Cost.
  • Civil Defence awareness (it could happen to us) – Many of us have better equipped ourselves with food stocks and emergency “Civil Defence disaster” plans (however, after talking to a range of individuals and Civil Defence agencies, my own opinion is that we are still inadequately prepared, should a major disaster hit).
  • Central Government, Local Authorities and large portfolio property owners (such as churches) have escalated their review programmes for the seismic assessment and upgrading of “commercial buildings”; the cost effects of this are likely to be spread over a longer term, but already have had significant effects on local communities (particularly with the churches, many of whom have adopted significantly higher thresholds, than those set within legislation; and lead to the closure of numerous church buildings). Other sectors of the property owning business community are perhaps yet to show a similar response, although a number of properties may find themselves un-saleable or un-lettable, if compliance with statutory (or higher) seismic performance levels, are not achieved.

  

Lindsay Webb Valuations are Registered Public Valuers.

Servicing the Hutt Valley cities of:

                                Upper Hutt  and  Lower Hutt

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