Christchurch Houses

The houses in Christchurch are all built with timber framing, so they do not collapse in an earthquake. When they have brick or block cladding, it is just a veneer that does not support the roof trusses. So even if the cladding cracks or fall off, the timber framing holds the roof up.

Many of the roofs are corrugated iron or metal tiles, so they stand up to shaking well. Some have concrete tiles, which tend to shake off in an earthquake. I visited one yesterday, on which the top half of the tiles had slipped off.

In the eastern suburbs of Christchurch, the most serious problem is liquefaction. This occurs when the pressure of the earthquake forces water out of the earth, sometimes from 10 to 15 feet below. It brings find particles of sand to the surface. When the water runs away, large heaps of silt are left behind.

Under asphalt streets the water pressures cause huge bubbles to occure, so what was a smooth street is now a bumpy obstacles cause. The storm water drains have pushed up, because the access vents to stick six inches up above street level (see the photos below).

In the last twenty years, most houses have been built on a solid concrete pad, called a floating foundation. When the liquefaction occurs under the house it can cause serious problems. In some cases the concrete pad crack in half when one end of the house drops and the other rises. When this happens, the water and silt push up through the cracks and fill the rooms with mess.

So although the houses look okay from the outside, they can have seriously damaged foundations. If the cracking is too serious and the foundation is too twisted, the house cannot be prepared and will have to be demolished.

Older houses were built on short wooden or concrete piles. They cope with the liquefaction better, but often the earthquake shifts them off their piles. The house my daughter is renting was built this way. The house appears to have moved forward about three inches. It is still standing fine, but until a builder can get underneath and look, it is not clear whether the foundation is holding up okay.

Last night we saw the house below that had dropped about a foot into the ground, when the liquefaction washed out the earth under its concrete foundation.

Round the corner, power poles have dropped down into the earth when liquefaction occurred, and are now only half their previous height.