House piles and earthquakes

Following the 2016 Kaikoura Earthquake I had a number of re-inspections of the sub-floor area of some houses to check the state of the sub-floor framing.

Fortunately in Nelson the motion of this earthquake was relatively gentle, despite the length of the shaking.

So what is the general state of the sub-structure of timber framed houses in Nelson?

These days there are considerable construction requirements to secure a new, or re-piled house to the ground. The depth of the piles, the fixing details of piles to bearers, and general sub-floor bracing. Houses built to the current building regulations are unlikely to fall off their foundations, except with the most severe earthquake.

Many older houses, even from 25 years ago, often do not meet current building structural standards. Some inspections show houses of this age are held in place by gravity, with little fixing or bracing, and that will prove inadequate if Nelson is subject to a strong earthquake. That should probably read when.

Houses with sub-standard sub-floor framing may be shaken from their piles. On a sloping site this may endanger lives.

Older houses may have met the standards of their time, and are not actually required to be upgraded to meet current, more stringent, requirements. Potential purchasers of a house should, however, be informed that if Nelson is subject to a strong earthquake, their house may be shaken from its piles.

Many Nelson houses require expensive remedial work to make them safe.

If a house is low to the ground access may only be through the floor, which means the carpet has to be lifted, holes cut in the floor, dirt removed, concrete poured, and the floor and covering subsequently reinstated, etc. It’s a messy operation. Good re-pilers use a jack with every pile so that means 60—80 holes may be required to support a house evenly.

Okay, that big earthquake may not happen in our lifetime.

Then again, if it does, yeah, it’s a significant issue.

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